SCOPE 50 - Radioecology after Chernobyl

1.1 INTRODUCTION

Calculations of the transport of radionuclides through the atmosphere usually start with a source term. The source term provides the initial conditions from which the atmospheric pathway models begin. These initial conditions include a description of the isotopic composition and quantity of the released radionuclides, their physical and chemical forms and the spatial and temporal distribution of the release. In this chapter we examine the basic physical processes that create these nuclides, a description of the physical and chemical properties of significant radionuclides, and the systems and processes that result in their release into the environment.

1.2 BASIC PROCESSES: FISSION, FUSION AND NEUTRON ACTIVATION

The principal physical processes for artificially creating radionuclides are nuclear fission, nuclear fusion, and neutron activation. Radionuclides also include isotopes produced by natural processes. These include residual nuclides created before the Earth was formed, their daughter products and naturally produced radionuclides. We focus here on artificially created nuclides which dominate accidental releases. Later we shall examine natural radioactivity.

1.2.1 NUCLEAR FISSION

Neutron-induced nuclear fission is by far the most important source of artificially created radionuclides. This process produces the energy released in contemporary nuclear reactors. Fission also dominates the release of radionuclides in nuclear weapon explosions. The nuclides produced in fission and their decay products constitute a spectrum of hundreds of species with a range of half-lives that extends from fractions of seconds to billions of years. A primary fission product is usually the starting point of a sequence of decays, each daughter nuclide tending to have a half-life longer than its parent, until a stable nuclide is reached. Only some of these fission nuclides are potentially significant to biological systems, because of their physical and chemical properties, and the quantities produced (fission chain yield). The actual distribution of fission yields depends on the fissionable nuclide (e. g. ²³⁵U, ²³⁸U, ²³⁹Pu) and the energy of the neutron initiating the fission. A representative fission yield distribution for the isotope ²³⁵U for a thermal spectrum of neutrons (thermal reactors) as well as for 14 MeV neutrons (from nuclear fusion of deuterium and tritium) is presented in Figure 1.1. The mass number distribution of the fission products is asymmetric, with the high yield isotopes having mass numbers that range from approximately 85 to 105, as well as from 130 to 150. The fission yields at the peaks are typically around 6 or 7 per cent, which includes the yields for the important isotopes strontium-89 and 90, iodine-131, and caesium-137. The total fission yield for all the isotopes in Figure 1.1 adds up to 200 per cent, reflecting the fact that two fragments are produced in each fission event. The relatively high fission yield of ¹³⁷Cs and ⁹⁰Sr, their half-lives of about 30 years, and the fact that caesium is chemically similar to potassium, and strontium is chemically similar to calcium, all combine to select these two radionuclides as potentially very significant to humans. Similarly, iodine-131 with its large yield, half-life of 8 days, and its affinity to the thyroid gland, is also very significant to humans.

Figure 1.1 Percentage fission yield from thermal and 14-MeV neutron-induced fission of uranium-235 (Lamarsh, 1966; reproduced with permission of Addison-Wesley Publishing Company).

1.2.2 NUCLEAR FUSION

A second source of radionuclides is nuclear fusion. Here two nuclei of small atomic mass are combined (fuse) in an exothermic nuclear reaction. One such fusion reaction, important in the hydrogen bomb, is when deuterium and tritium combine to yield an alpha particle and a neutron.

In this reaction, 80 per cent of the 17.6 MeV of energy that is released is in the form of kinetic energy of the neutron (14.1 MeV). The initial reactant nuclei must collide at high energy (temperature) in order to overcome the coulomb repulsive force. Hence, to initiate a thermonuclear explosion, contemporary fusion weapons have a fission atomic bomb trigger. However in the laboratory, this reaction is much used for the production of fast neutrons.

1.2.3 NEUTRON ACTIVATION

A third source of man-made radionuclides is produced by the neutron activation process. This process can be important in both nuclear reactors and nuclear weapons. One reaction is:

This occurs when neutrons escaping from a weapon detonation interact with atmospheric nitrogen. The product nucleus, carbon-14, is also produced naturally in the atmosphere by the same reaction, from neutrons emanating from cosmic ray reactions. Many radionuclides produced in nuclear reactors come from the neutron activation processes. For example:

   
After a second beta decay, the ²³⁹₉₃Np becomes ²³⁹₉₄Pu. This is the principal reaction 94 process for producing plutonium-239. Further neutron absorptions produce 

Another important neutron activation product is caesium-134. This radionuclide is not a direct fission product, but is produced by neutron activation of the stable fission product caesium-133. Hence, caesium-134 is produced copiously in reactors, but hardly at all in a weapon explosion. Measuring the ratio of caesium-134 to caesium-137 in a sampling of radioactivity is an important test to determine if the source of a radioactivity release is a nuclear reactor accident or a nuclear weapon explosion. Other important isotopes created by neutron activation in fission and fusion reactor structures are ⁵⁵Fe and ⁶⁰Co.

1.2.4 TIME DEPENDENCE OF RADIOACTIVE DECAY

Calculations of radionuclide transport that include quantitative estimates of activity concentrations and dose commitments must include radioactive decay with time. Figure 1.2 compares the overall time decay of the activity generated by the detonation of a 1 Mt fission weapon, with that present in a power reactor, a spent fuel storage pond, and a fuel reprocessing plant with a 10 year and 30 day high level waste storage facility. The relative contribution of each source depends on the time scale being considered. One can consider a hypothetical situation where the radioactivity from each source is spread over a square 1 km wide and examine the gamma dose rate in the centre at a height of 1 m. In the first few days, the higher activity of the nuclear weapon debris would dominate over the gamma radiation of the reactor. Likewise, gamma radiation levels from a light water reactor (LWR) are greater than that of 10 years worth of stored spent fuel for about one year after reactor shutdown. Subsequently, the spent fuel would be relatively more radioactive. Similarly, the gamma radiation from 10 years of spent fuel is greater than the radioactivity of a 1 Mt fission weapon after about two months because of the greater abundance of long-lived gamma emitters in the spent fuel.

Figure 1.2 Gamma-ray dose rate area integral versus time after shutdown or detonation (Chester and Chester, 1976). The dose rate area integral is a measure of the radioactive inventory. (From Chester and Chester 1976; copyright 1976 by the American Nuclear Society, La Grange Park, Illinois).

1.3 ISOTOPES OF INTEREST 

1.3.1 INTRODUCTION

The artificial radionuclides synthesized by humans range over all groups of the periodic table, and those released into the environment show a wide spectrum of behaviour. The RADPATH study focuses upon the more abundant and toxic isotopes.

Table 1.1 Radioactive nuclides, decay modes and half-lives

3H	ß	12.26 y	132mI	ß+IT	83 min
7Be	EC	53.29 d	133I	ß	20.8 h
14C	ß	5370 y	133Xe	ß	5.25 d
24Na	ß	15 h	134Cs	ß	2.065 y
40K	ß	1.25 x 109 y	136Cs	ß	13.1 d
60Co	ß	5.37 y	137Cs	ß	30.17 y
85Kr	ß	10.72 y	137mBa	IT	2.5 min
87Rb	ß	4.9 x 1010 y	140Ba	ß	12.76 y
89Sr	ß	50.52 d	140La	ß	40.3 h
90Sr	ß	29 y	141Ce	ß	32.5 d
91Y	ß	58.8 d	144Ce	ß	284.4 d
95Zr	ß	64 d	147mPm	ß	2.62 y
95Nb	ß	53 d	210Po		138.4 d
99Tc	ß	2.13 x 105 y	210Pb	ß	19.4 y
103Ru	ß	39.24 d	222Rn		3.82 d
106Ru	ß	372 d	231Pa		3.27 x 104 y
110mAg	ß+IT	249.8 d	232Th		1.41 x 1010 y
115mCd	ß	44.6 d	235U		7.04 x 108 y
125mTe	IT	58 d	237Np		2.14 x 106 y
125I	EC	59.9 d	238U		4.46 x 109 y
127Sb	ß	3.84 d	239Pu		2.41 x 104 y
127Sb	ß	2.1 h	240Pu		6537 y
129I	ß	1.6 x 107 y	241Pu	ß	14.4 y
131I	ß	8.04 d	241Am		432.2 y
131mXe	IT	11.92 d	242mAm	+IT	141y
132Te	ß	78.2 h	244Pu		8.1 x 107 y

Principal nuclides include isotopes of the elements caesium, strontium, ruthenium, cerium, iodine and plutonium. Isotopes of other elements such as silver, tellurium, zirconium, niobium, lanthanum, technetium and americium, are included but not treated in great detail. Table 1.1 lists the radioactive nuclides with their half-lives in minutes, hours, days and years, and their principal modes of decay. Almost all simultaneously emit gamma or X-rays of many energies; those in the list which do not are ³H, ¹⁴C, ⁸⁷Rb and ⁹⁰Sr (all pure beta emitters). Most are fission products, some result from neutron activation, the first three are cosmogenic as well as human in origin, and six are primordial (see Sub-section 1.4.5).

The deposition, migration and uptake of radioactivity by living organisms is largely determined by the chemistry and physical form of the active species. The significant radionuclides are now considered individually.

1.3.2 CAESIUM

¹³⁷Cs (t_1/2 = 30 years) is a high-yield fission product, and stored highly-active waste and low level discharges generally contain large proportions of this isotope. Neutron activation of the fission product ¹³³Cs generates another active caesium isotope, ¹³⁴Cs (t_1/2 = 2 years), but since its production is proportional to the square of the neutron flux in the reactor, it is generally in a lesser proportion. Other sources of radiocaesium are nuclear weapons tests, and accidents such as those at Windscale and Chernobyl. The caesium released to the atmosphere at Chernobyl was associated with particles of a mean geometrical equivalent diameter of 0.51.0 µm. In rainwater it was mostly in colloidal form, with only a minor fraction as ionic.

The addition of ¹³⁴Cs or ¹³⁷Cs to soils as soluble compounds results in about 85 per cent rapidly becoming bound with a small fraction (about 15 per cent) remaining available for plant uptake. The availability of caesium in soils is related to the mineral composition. The subsequent movement and distribution is greatly influenced by the form of the overlying vegetation. It is believed that more than 95 per cent of ¹³⁷Cs remains in soluble ionic form upon the addition to seawater as a soluble compound. In freshwater, however, a large proportion of the total is expected to bind to suspended and deposited sediments.

Caesium is an alkali metal of Group I with properties similar to those of potassium and rubidium. As with all alkali metals, most of its salts are highly soluble in water. Despite the chemical similarities between caesium and potassium, they are not metabolized identically, and although potassium is often used to model caesium environmental transport, there can be more similarities between calcium and caesium. 

1.3.3 STRONTIUM

Strontium is an alkaline metal of Group II, and its metabolic pathways tend to follow those of the other Group II elements calcium and radium. Radiostrontium is of environmental importance because of its tendency to concentrate throughout the volume of mineral bone in animals. The isotopes ⁸⁹Sr and ⁹⁰Sr are primary fission products with half-lives of 50.52 days and 28.8 years. Radiostrontium is generated in nuclear weapons explosions and is present in routine releases from nuclear facilities.

In both marine and freshwater environments strontium behaves primarily as a soluble element. In aquatic ecosystems ⁹⁰Sr is of greater importance than ⁸⁹Sr. On adding ⁹⁰Sr to seawater as a soluble compound, less than 1 per cent associates with suspended particulates or colloids. In freshwater, although the association with suspended particulates may be greater, it is unlikely to exceed 10 per cent of the total. 

1.3.4 RUTHENIUM

Ruthenium, a transition metal, can exist in four valence states and forms numerous complexes. As a fission product it usually remains in fuel as a refractory, although at Chernobyl rapid oxidation formed the volatile oxides RuO₃ and RuO₄, which then condensed to aerosols or `hot particles' of nearly pure ruthenium. The median diameter for particles bearing ruthenium in atmospheric aerosols was just less than 1 µm (ApSimon and Mahadeva, 1989). The primary ruthenium fission products are ¹⁰³Ru (t_1/2 = 39.3 days) and ¹⁰⁶Ru (t_1/2 = 368 days), discharging to the environment from fuel reprocessing and weapons detonations.

In soils, ¹⁰⁶Ru distribution and retention is largely dependent on the form (cationic, nitrosylruthenium complexes, anionic) in which it is applied. Following aerial contamination of plants by ruthenium, 10 per cent retention may be expected. Reprocessing operations give rise to the production of nitrosyl-nitrato and nitrosyl-nitrito ruthenium complexes leading to nitrosylchloride in seawater. In discharges, particulate ¹⁰⁶Ru is bound to ferric hydroxides. Ionic ruthenium is in very low proportion in water, but up to 25 per cent may be present in colloids and as much as 60 per cent in particulates. A large proportion of ruthenium uptake in aquatic plants and animals is attributed to surface adsorption.

1.3.5 CERIUM

Cerium is the second of the lanthanide group of metals, rare in nature, but with two radioactive isotopes that are important fission products. The principal sources of ¹⁴⁴ Ce (t_1/2= 284 days) and ¹⁴¹Ce (t_1/2 = 32.5 days) is fuel reprocessing. Other sources are from nuclear weapons tests and accidental releases, such as occurred at Chernobyl. At Chernobyl emitted fuel fragments contained cerium in concentrations close to the composition in the core. Fuel fragments carrying ¹⁴¹Ce were not found to be sensitive to precipitation.

More than 95 per cent of ¹⁴⁴Ce added to soil is rapidly bound to exchange sites on clay minerals and to soluble, insoluble and colloidal organic complexes. It is likely that a very small amount of ¹⁴⁴Ce will be absorbed following deposition to the above-ground parts of plants. The addition of ¹⁴¹Ce or ¹⁴⁴Ce to seawater leads rapidly to 70 per cent becoming associated with suspended sediments, over 25 per cent with colloidal material, and less than 5 per cent remaining in soluble ionic forms. There is marked association of cerium radioisotopes with particulate or sedimented materials in natural conditions. In living organisms cerium exhibits similar uptake and transfer behaviour as other lanthanides and actinides, and has a limited similarity with caesium and strontium.

1.3.6 IODINE

Iodine, a halogen of Group VII, is rare but widespread in Nature. Radioactive isotopes of iodine, ¹²⁹I and ¹³¹I, are produced by nuclear weapons and reactor operation. Their half-lives are 1.57 x 10⁷ years and 8.04 days. Around 6 per cent and 2 per cent respectively of the activity in boiling water reactors (BWR) and pressurized water reactors (PWR) liquid effluents can be due to ¹³¹I. Several chemical and physical forms of iodine were released in the Chernobyl accident (particulate iodine, elemental iodine and organic iodides), the major fraction being present in gaseous form as elemental iodine and volatile organic iodides.

In most soils it may be assumed that 10 per cent of an addition of ¹³¹I in soluble form remains available for uptake. Most elemental and organic iodine deposited on plants is directly absorbed by the foliage, whereas only about 10 per cent of particulates or inorganic compounds is absorbed by plants from solution. Iodide appears to be absorbed from water more effectively than iodate.

In the atmospheric dispersion of iodine, appreciably higher dry deposition of elemental, rather than particulate, iodine usually occurs, with dry deposition of the dominant organic iodide (methyl iodide) some two orders of magnitude lower than that of elemental iodine. In comparison with organic iodide, rapid washout of particulate and elemental iodine occurs. Iodine concentrates in the thyroid, absorption of the soluble form occurring through the skin, lung and alimentary tract.

1.3.7 PLUTONIUM

Neutron activation of uranium fuel in reactors leads to the production of the plutonium isotopes ²³⁸Pu (t_1/2 = 87.7 years), ²³⁹Pu (t_1/2 = 2.4 x 10⁴ years), ²⁴⁰Pu (t_1/2 = 6.6 x 10³ years) and isotopes of heavier elements with smaller abundances. The volatility of plutonium is very low and no significant releases occur during reactor operation, but environmental releases may occur from severe reactor accidents, as at Chernobyl. Further sources are controlled releases from research facilities, production facilities, experimental reactor operations, waste disposal and nuclear weapons testing.

In soils, the chemical and physical behaviour of plutonium is not well known. It seems that physical mechanisms dominate vertical transport as the chemical availability is rather low. In most soil types, soluble plutonium is absorbed rapidly. Plutonium concentrations in plants depend on the species, the vegetation type, age and status, soil pH, cation exchange capacity, mineral and organic composition and physico-chemical form, and levels change with the duration of contamination. Redox, pH and organic ligands determine the solubility in water. Plutonium has a complex chemistry and most chemical forms are relatively insoluble, tending to adsorb onto sediments. At least four oxidation states Pu(III), Pu(IV), Pu(V) and Pu(VI) exist in aqueous environments, with the ions in a hydrated form. The solubility of plutonium in natural waters is low and mainly as Pu(V) or Pu(VI). The absorbed forms are usually in the Pu(III) and Pu(IV) state. Ionic, particulate, colloidal and pseudo-colloidal forms of plutonium are present in the water column. Over 90 per cent of plutonium is efficiently scavenged and removed from the water column within coastal environments in the short term where plenty of sediment is available. It is less efficiently scavenged and removed from the water column in mid-oceanic waters.

1.3.8 SILVER

Silver is a transition metal, rare but widespread in nature. In nuclear reactors it is both a minor fission product and an activation product. The radioactive nuclides of interest are ¹¹⁰Ag (t_1/2 = 25 seconds), ^110mAg (t_1/2 = 250 days) and ¹⁰⁸Ag (t_1/2 = 2.4 minutes). It is assumed that when silver enters soil a large proportion is rapidly adsorbed on the mineral fraction or complexed with insoluble organic compounds. Less than 2 per cent of the total in soil can be expected to be in soil solution.

1.3.9 TELLURIUM

Tellurium is an amphoteric element of Group VI, chemically related to sulphur and selenium. It is a minor fission product with five significant active isotopes and daughters: ^126mTe (t_1/2 = 109 days) decays to ¹²⁷Te (t_1/2 = 9.3 hours) plus ¹²⁷I (stable), ^129mTe (t_1/2 = 33.6 days) decays to ¹²⁹Te (t_1/2 = 73 minutes) plus ¹²⁹I (t_1/2 = 1.6 x 10⁷ years), ¹³²Te (t_1/2 = 78.2 hours) decays to ¹³²I (t_1/2 = 2.3 hours). ¹³²Te has been measured in marine environments after nuclear detonations, mostly in particulate (including colloidal) form. Tellurium in the environment arises from accidents such as the Chernobyl release.

1.3.10 ZIRCONIUM

Zirconium is a refactory transition metal and is a major fission product. Nuclides of interest are ⁹³Ar (t_1/2 = 1.5 x 10⁶ years) and ⁹⁵ Zr (t_1/2 = 65 days). ⁹⁵ Zr is widespread after weapons explosions and is a major gamma source in the mixed fission products from fuel reprocessing. Zirconium release to the environment also occurred during the Chernobyl accident from the extensive spreading of fuel fragments carrying such non-volatile elements. Zirconium is found in soil systems after nuclear detonations and near reprocessing plants. In most soils 95 percent of the ⁹⁵Zr entering the system is rapidly adsorbed or precipitated. In water, over 95 per cent is expected to be bound to colloids, organic acids and suspended particles, leaving less than 5 per cent in solution.

1.3.11 NIOBIUM

A daughter product of ⁹⁵Zr is ⁹⁵Nb (t_1/2 = 35 days) which is released from reprocessing plants and nuclear weapons tests. An indication of ⁹⁵Nb chemistry in soil is provided by zirconium, although uptake of niobium to terrestrial plants occurs more readily than with zirconium. The behaviour of ⁹⁵Nb in water resembles that of ⁹⁵Zr but with a higher proportion bound by particulates, colloids and soluble organic complexes. The accumulation factor for sediment is higher for niobium than for zirconium.

1.3.12 LANTHANUM

Lanthanum, a transition metal element, is a fission product contributing to the longlived radiation around Chernobyl. The principal active isotopes are ¹³⁸La (t_1/2 = 1.1 x 10¹¹ years) and ¹⁴⁰La (t_1/2 = 40.3 hours). Lanthanum has a very low degree of availability in soils.

1.3.13 TECHNETIUM

Technetium, a transition metal element, no longer exists naturally on Earth as there is no stable isotope. It is a primary fission product, with the principal isotopes ⁹⁹Tc (t_1/2= 2.1 x 10⁵ years) and ⁹⁷Tc (t_1/2 = 2.6 x 10⁶ years). Releases to the environment have occurred from weapons explosions and fuel reprocessing plants. The element is most stable as the very soluble pertechnetate anion. The form most likely to occur in fallout entering surface soils is Tc(VII). Soil type affects the extent to which technetium is fixed and the nuclide is very mobile in plants, transfer from roots to shoots and into developing fruit and seed tissues occurring readily. When entering waters as the pertechnetate ion, it can be assumed, in the absence of published data, that Tc remains mostly in a soluble form (Coughtrey et al., 1983). It is assumed that on release of  ⁹⁹Tc to the aquatic environment it remains available to all organisms. 

1.3.14 AMERICIUM

Americium is an actinide element of which the most important isotope, ²⁴¹Am (t_1/2 = 432.2 years) forms from the decay of ²⁴¹Pu. It has been detected around waste disposal sites, operating power plants, and as a result of world-wide fallout. The isotope ²⁴²Am (t_1/2 = 16 hours) also occurs in weapons fallout. A further product of weapons tests is ²⁴³Am (t_1/2 = 7.4 x 10³ years) but this isotope has not been detected in the environment. Oxides and hydroxides of Am(III) are relatively insoluble. The most likely chemical species of americium in soils is Am(III). The concentration factor of americium in vegetation depends on both plant and soil factors. Americium is generally assumed to exist in waters as the Am(III) oxidation state, and in freshwater and marine environments the soluble fraction of americium has been found not to be in the form of cations, but as negatively charged complexes. Americium hydroxide, resulting from the rapid hydrolysis of americium in solution, is insoluble in both freshwater and marine environments. These insoluble products of hydrolysis quickly form particulates or adsorb onto suspended particulates. Soluble complexes with organic ligands may maintain americium in solution. The association of americium with particulate material and sediments, controls its behaviour and distribution in the aquatic environment. In sediments, americium concentrations are generally highest where particle sizes are smallest. Americium deposits primarily on endosteal surfaces, and additionally on periosteal bone surfaces.

1.4 PROCESSES RELEASING RADIOACTIVITY INTO THE ENVIRONMENT

Radioactivity is released into the environment from a variety of systems and processes. We examine here the more important systems that lead to the significant creation and release of radioactivity. Man-made sources include nuclear weapons and the nuclear fuel cycle. The fuel cycle includes mining, milling, fuel enrichment, fabrication, reactors, spent fuel stores, reprocessing facilities and waste storage. Natural radioactivity includes primordial natural series decay products and cosmogenically generated isotopes.

1.4.1 NUCLEAR WEAPONS: FISSION, FUSION, ACTIVATION AND FUEL DISPERSAL

Modern nuclear weapons produce radioactivity from the fission, fusion and neutron activation process. The type and composition of a nuclear device markedly affects the kinds of radioactivity produced, while the location and size of detonation determine the quantity of radioactivity released to the biosphere. In a modern fissionfusionfission weapon, the fission trigger may be fuelled either with highly enriched uranium or weapons grade plutonium. The fission trigger initiates the second stage fusion reactions. Fast neutrons from fusion then initiate the third stage fission. The third stage fission is normally fuelled with depleted uranium (²³⁸U). Most of the fission yield usually emanates from this uranium. Table 1.2 contains approximations of the source term for these three processes. Most of the radio-

Table 1.2 Sources of radioactivity in nuclear explosions

Source		Isotope of interest	Strength of source
1.	Fission fragments	Mass numbers 70160	2.9 x 1023 fragments per kiloton of
			fission. Activity at 1 hour
			= 1.5 x 1010Bqa
2.	Fusion reaction products	Principally tritium (31 H)	Depends on weapon design.
		from reaction	Magnitude 104 Ci/kilotonb 1023
			to 1024 atoms 31H/kiloton. Fusion
			energy release can vary from 0%
			to 99% of total
3.	Neutron activation	Depends on explosion	2 x 1023 neutrons liberated in fission
		location and bomb	and fusion per kiloton
		case material.
		A principal isotope is
		carbon-14 from
		atmospheric nitrogen
4.	Fissile materials	Plutonium-239	Depends on weapon design. Comes
			from non-fissioned weapons
			material plus neutron activation
			of uranium
a) Distribution and evolution of radioelements after nuclear explosion, Dupuis, M.J. (1970) Bulletin of lnformation,
Science and Technology, 149, 41-52.
b) Ericksson, E., Tellus, 17, 118-130 (1965).

activity of interest comes from fission. An estimate of the activity of a specific isotope produced from fission energy release per kiloton of TNT explosive yield can be obtained as follows

(1.1)

Here A is the activity of the isotope, is the decay constant, N is the number of atoms of the isotope produced in the explosion, t_1/2 = is the half-life in seconds, and Y is the fission chain yield (the fraction of the particular isotope out of all the isotopes and their daughter products produced in fission). For example, ¹³⁷Cs has a half-life of 9.5 x 10⁸ seconds and a chain yield of 0.062 (6.2 per cent), resulting in an activity A of 6.7 x 10¹² Bq/kT. The corresponding results for ¹³¹I and ⁹⁰Sr are A(¹³¹I) = 4.4 x 10¹⁵ Bq/kT and A(⁹⁰Sr) = 6.3 x 10¹² Bq/kT. While the yields used in the above examples apply to thermal fission of ²³⁵U, they can also be used as an approximation for the activity yields from fast neutron-induced fission of ²³⁸U in nuclear weapons, and also for the yields from nuclear weapons fuelled with ²³⁹Pu. From these, it is easy to estimate the isotopic activity releases from Hiroshima (~15 kT), and from all the atmospheric weapons tests conducted between 1945 and 1963 (approximately 450 MT energy yield, of which approximately half was from fission, and half from fusion). Table 1.3 provides a comparison of the release of selected isotopes from weapon detonations and the Chernobyl and Windscale accidents.

The neutrons produced in fission and fusion can induce nuclear reactions that produce radioactive isotopes. One example of this neutron activation process, discussed earlier, is the reaction with atmospheric nitrogen producing carbon-14. Another is the production of iron-55 resulting from neutron activation with the bomb assembly materials ( 10¹³ Bq per kilotonRotblat, 1981). For ground or near ground detonations, the escaping neutrons can induce activity in the ground, water or structures in the vicinity of the explosion. Some of the more important isotopes produced in the earth and in buildings are aluminium-28, manganese-56, and sodium-24. For explosions in salt water environments, sodium-24 and chlorine-38 can be important.

During a nuclear explosion, a considerable amount of un-fissioned uranium and plutonium is dispersed, constituting an additional injection of radioactivity into the atmosphere. In addition, neutron activation of uranium-238 can produce a considerable amount of additional plutonium. Rotblat (1981) suggests that a 10 Mt bomb with 5 Mt from fission can produce 250 kg of plutonium through activation. This suggestion, which should be considered speculative, is considerably more than the plutonium used in the trigger.

In almost all cases, the hazard of fallout of fission products from weapon explosions dominates over the neutron induced activity. The dispersed nuclear fuel normally poses a much smaller hazard, except in the case of a very low yield explosion, or in the case of an accidental non-nuclear explosion of a nuclear weapon.

Table1.3 A comparison of radioactive releases from nuclear detonations and nuclear reactor accidents

	Radioactivity released (PBq)a
Nuclide	Hiroshima	Weapon tests	Chernobyl	Windscale
137Cs	0.1	1500	89	0.044
134Csb			48	0.0011
90Sr	0.085	1300	7.4	0.00022
133Xe	140	210 0000	4400	14
131I	52	780 000	1300	0.59
a Decay corrected to 3 days after shutdown or detonation.
b 134Cs is produced in reactors by neutron activation.
Gudiksen et al., 1989; reproduced by permission of the Health Physics Society.

1.4.2 MINING, MILLING, ENRICHMENT AND FUEL FABRICATION

The nuclear fuel cycle includes mining, milling, enrichment processes, reactors, spent fuel storage, fuel reprocessing and radioactive waste storage.

In uranium mining, the principal effluent is radon-222. Release rates are about 1 GBq per ton of ore with a content of 1 per cent uranium oxide (U₃O₈). World production of mined uranium ore is about 40 kt per year. For the fuel needs of current design thermal reactors, this is equivalent to a release of about 20 TBq per gigawatt-year of reactor operation.

The extraction of the uranium from ore during milling typically leaves tailings which are a further source of atmospheric releases. Airborne emissions from a mill processing 2 kt of ore per day is roughly 1 GBq for ²³⁸U, ²³⁰Th, ²²⁶Ra and ²¹⁰Pb. For ²²²Rn, it is larger by a factor of a few thousand.

Fuel fabrication involves further processing of the uranium ore from the mill. These processes include purifying, conversion to uranium hexafluoride (UF₆), isotope enrichment of ²³⁵U, conversion to metal fuel, and fabrication of fuel elements. Solid wastes produced in these fuel fabrication processes are trivial when compared to the mining and milling stages. Emissions of radionuclides in fuel fabrication are also relatively small.

1.4.3 REACTOR OPERATION

Nuclear reactors are copious producers of radioisotopes and the principal source of radioactivity in the fuel cycle. Each fission event produces two radioactive fission fragments, each of which undergoes radioactive decays, eventually producing a stable isotope. Hundreds of radioactive isotopes are produced. In addition, the intense neutron fluxes within the reactor create radioactive transmutations through neutron absorptions producing additional radioisotopes (neutron activation).

The actual inventory within a reactor core at any time depends on the type of reactor and its operating history. The primary factor is the power level as a function of time. The principal variable that determines the inventory of the longer-lived isotopes is the integrated power level (or energy output, sometimes called the burnup). The abundance of the shorter half-life isotopes approaches an equilibrium level proportional to the reactor power level. This can be understood qualitatively by a simple model of a single isotope that is being produced at a rate P, and simultaneously decaying with a decay constant and a half-life t_1/2

(1.2)

(1.3)

We see that an equilibrium level (P/) is approached within a few half-lives (87.5 per cent of equilibrium level in 3 half-lives). If the reactor stops (P = 0), then a simple exponential decay (e^-t) ensues. Hence, in a typical reactor situation where the fuel elements are removed after about 3 years, we find the shorter-lived isotopes (half-lives of less than 1 or 2 years) approaching or reaching their equilbrium levels, and the longer-lived isotopes (half-lives > 5 years) building up their abundances in rough proportion to the time-integrated power level.

Table 1.4 contains an inventory of selected radioactive isotopes at 1 year and 5 years after startup, for a typical reactor operating at a power level of 3000 MW thermal (about 1000 MW electrical). Table 1.4 also contains the inventory for the same reactor after 1 year of continuous operation, for two different times after shutdown. One can see that power reactors normally contain many billions of GBq of activity. We have not listed in the table other isotopes that could be important for contamination problems in the very short term, or in the very long term, but have selected those deemed potentially important for human exposure.

In the process of normal operation of nuclear reactors some gaseous, liquid and solid wastes of low and intermediate activity are produced by fission and activation generated nuclides causing contamination of various materials, or due to releases from fuel or cladding surfaces. The type and amount of waste is determined by factors such as the reactor type, its design features, operating conditions and the integrity of the fuel.

Considering the generation of airborne radioactive wastes, three main particulate aerosol sources may be identified: emission of activated corrosion and fission products; involatile elements from radioactive decay of gases; and adsorption of fission-produced volatile radionuclides on suspended material present. Gaseous radioactive waste arises from the generation of volatile radionuclides, the most important of which are the halogens, noble gases, ³H and ¹⁴C. Filters are employed to remove particulate activity from contaminated gases and building ventilation air prior to discharge to the atmosphere via exhaust stacks. A combination of charcoal and particulate filters is being used to remove radioactive iodine from gas effluents. Decay-storage methods are used to reduce the quantities of noble radioactive gases discharged to the environment since these are mostly short-lived.

Gas-cooled reactors generate less liquid waste than reactors which are water-cooled and moderated, with significantly lower volumes of such waste. Virtually no liquid concentrates are generated by heavy water reactors (HWR), which operate mainly with once-through ion exchange techniques (Efremenkov, 1989). Active liquid wastes produced at nuclear power plants (PWR and BWR type) generally contain soluble and insoluble radioactive components (fission and corrosion products). Maintenance activities on plant piping and equipment generate liquid wastes from decontamination operations. In order that the bulk volume of aqueous waste may be released to the environment or recycled, waste treatment processes are employed. These treatments are based on four main techniques; evaporation, chemical precipitation/flocculation, solid-phase separation and ion exchange.

Table 1.4 Production and decay of important radionuclides produced in a reactor

Fission product		A (production)		B (decay)
Isotope	Half-life (months)	1 year	5 years	100 days	1 year	5 years
89Sr	1.7	4200	4300	1100	36
90Sr	350	160	740	160	150	110
91Y	1.9	5400	5500	1600	64
95Zr	2.2	5500	5600	1900	110
95Nbb	1.2	5400	5600	3200	240
103Ru	1.3	3400	3400	660	8.2
106Ru	12	240	470	200	120	7.80
115Cd	1.4	0.66	0.066	0.13
125Teb	1.9	3.8	15	4.3	4.0	1.40
127Sb	0.13	87	87
131I	0.27	2800	2800	0.44
131Xeb	0.40	28	28	0.16
132Te	0.11	4100	4100
132Ib	0.003	4100	4100
133Ib	0.029	720	720
133Xe	0.18	6100	6100
136Cs	0.44	5.8	5.8	0.033
137Cs	360	120	570	120	120	110
140Ba	0.43	5700	5700	26
140Lab	0.056	5700	5700	29
141Cc	1.1	5300	5300	530	1.10
144Ce	9.4	3000	4900	2300	1200	30
147Pmb	31	540	1800	530	440	150
Columns marked A represent the activity PBq ( l015dps)a after selected periods of continuous operation of
a reactor at a power level of 3,000 megawatts thermal. Columns marked B represent the activity at
specific times after shutdown or removal from a reactor that had been operating for 1 year
aCalculated using fission product yields.
bDaughter product.
IAEA, 1974; reproduced by permission of IAEA, Vienna.

1.4.4 SPENT FUEL STORES, REPROCESSING, AND WASTE DISPOSAL

Most of the world inventory of the longer-lived radionuclides exists in spent fuel storage ponds, reprocessing plants and high-level waste storage facilities. Typically, commercial power reactor fuel elements are removed from the reactor after 3 years, and are kept in storage in `swimming pools', usually on-site, allowing for shorter-lived activity to decay. In principle, after cooling sufficiently, the fuel elements are then shipped to reprocessing plants for the extraction of the uranium and plutonium. After extraction, the remainder is called high-level waste, and would normally be sent to a suitable waste storage facility. In practice, because of a limited reprocessing capacity, only about 5 per cent of spent fuel from commercial power reactors is reprocessed, in the UK at Sellafield, and in France at Cap de la Hague and at Marcoule. The remaining 95 per cent is kept in storage, usually near the reactor site. Japan has just begun operation of a reprocessing facility. The percentage for reprocessing from military production reactors is much higher. This limited reprocessing capability has the effect of confining these isotopes within the spent fuel, hence limiting them as a potential environmental source term. The noble gases (Kr and Xe) and iodine gas are however released under normal reprocessing conditions.

The current world total of nuclear generated electric power capacity is about 300 GW, and the annual production of nuclear electric energy is about 200 GW-year (the difference resulting from reduced operating levels and downtime). Hence, based on the inventory in a 1 GW reactor (Table 1.4), it is clear that the quantities being stored, and the rate of buildup worldwide, are enormous.

A survey of radioactive effluents from post-reactor sources has been evaluated by UNSCEAR (1988a). There, detailed inventories are kept of effluents from reactors, ponds, reprocessing plants and waste storage facilities. The isotopes of principle concern from these sources are: ³H, ¹⁴C, ⁶⁰Co, ⁸⁵ Kr, ⁹⁰Sr, ⁹⁵Zr, ⁹⁹Tc, ¹⁰⁶Ru, ¹²⁹I, ¹³¹I, ¹³⁴Cs, ¹³¹Cs, ¹⁴⁴Ce,¹⁴⁷Pm, ²³⁷Np and the plutonium isotopes.

Reprocessing of spent fuel permits its separation into three basic components; uranium, plutonium and waste materials (fission products, actinides, cladding materials). Waste products account for only a small percentage of the total weight, the remainder being unused uranium and plutonium. Hence reprocessing enables resource conservation and ensures security of supply.

Since residual uranium, fission products and actinide elements are contained in the fuel upon discharge from the reactor, a period of on-site storage (in reactor ponds) is necessary to allow the decay of short-lived radioactivity prior to transport for reprocessing. The fuel is transported to reprocessing plants in specially designed containers (or `flasks') which, in accordance with regulations based on International Atomic Energy Agency (IAEA) guidelines, are capable of withstanding the severest of accidents.

All industrial-scale fuel reprocessing plants have employed the Purex separation process, a solvent extraction method based on tri-n-butyl phosphate (TBP), or variants of it, following its development and use in the USA in 1954. After cooling, the first stage of reprocessing involves decanning, i.e. the removal of the fuels' cladding. More refractory fuel, ceramic UO₂ canned in stainless steel or zirconium alloy, is used in reactors with high power densities, e.g. PWR, BWR and AGR. This canning material cannot be easily cut or stripped from the fuel; hence chopleach systems are used, whereby a shear is used to chop whole fuel assemblies. The chopped sections of fuel and cladding fall into a basket which is suspended in a dissolver containing nitric acid, effecting the removal of the fuel from the pins, leaving the cladding (or `hulls'). The hulls are then withdrawn and may be treated prior to storage and disposal as intermediate-level waste. A series of TBP extraction and stripping cycles may then be used for the separation of the fuel into its constituent parts prior to eventual uranium and plutonium product finishing.

Uranium and plutonium constitute at least 96 per cent and up to 1 per cent respectively of the spent fuel, and may be recovered by reprocessing via extraction into the solvent. Almost all fission product activity remains in the aqueous phase, forming highly active waste which is stored following removal of any entrained solvent.

With regard to sources of low-level gaseous radioactivity associated with reprocessing activities, these arise principally from fuel-element shearing and dissolution processes. Considering radioiodines, most ¹³¹I will have decayed as a result of storage procedures. In view of the amount which remains, and due to the presence of ¹²⁹I in reprocessing wastes, special chemical treatment is still required to remove these radionuclides. There has been no significant exposure due to stack releases of ⁸⁵Kr, moreover ³H releases are small relative to amounts in liquid form. After treatment to minimize radionuclide concentrations, low-level liquid wastes, such as water from cooling ponds, may be discharged to the environment.

Leakage from spent fuel storage sites has been relatively low; there are extensive data on effluents from the major fuel reprocessing plants, with Sellafield and Cap de la Hague being major contributors. Information available regarding military facilities is more limited, but recent disclosures indicate major releases have occurred in the past from this source. Similar comments can be made about the high-level waste storage facilities. New information is now being released about a major accidental release in 1957 from a high-level waste storage tank at a military site in Kyshtym in the USSR. In size of activity this accidental release is second only to Chernobyl. New information from Hanford, Washington, in the USA, describes sizeable releases that occurred in the 1940s and 1950s from plutonium production reactor operations, fuel reprocessing and waste storage facilities.

Depending on the nature of the waste, various combinations of techniques may be used to achieve reductions in the volume and in the mobility of the radioactive content. Cutting, shredding and crushing treatments may be used to achieve volume reductions or as pre-treatments before compaction or incineration of solid low-level waste. Compaction may be used to reduce the volume of the waste, although this method does not improve waste properties with regard to longer-term management. Incineration, of suitable waste, enables very high volume and mass reduction, final packaging in containers for storage and disposal requiring no further conditioning treatments.

Intermediate-level wastes from reprocessing activities (such as cladding material, contaminated equipment, spent treatment materials) may be stored pending encapsulation and ultimate disposal. Encapsulation strategies have been developed for intermediate-level wastes, the nature of the waste determining the precise method of encapsulation used: for example, in-drum grouting may be used to encapsulate wastes such as hulls or swarf, whereas the addition of dry cement powder to materials like waste slurries may be employed.

The most significant fraction of wet-solid waste generated from the operation of nuclear power plants arises from spent ion exchange resin, with additional contributions to this category of waste from filter media and sludges (from pre-coated filters used to process liquid waste and decontamination corrosion products removed from liquid waste). Prior to final disposal of wet solids arising from liquid waste treatment, these must be converted to solid products using methods such as cementation, bituminization or incorporation into polymers.

Highly radioactive waste, in the form of a liquor, arises from the primary separation stage of fuel reprocessing. This high-level waste may be concentrated, by evaporation, for storage in cooled tanks prior to eventual disposal. It is desirable that such liquid wastes should ultimately be converted into solid form for storage and disposal. Hence, vitrification processes, based essentially on either metallic or ceramic melter techniques, are being employed for the conversion of high-level waste into borosilicate glass (Baehr, 1989).

⁸⁵Kr is a fission product of half-life 10.4 years and its main source is fuel reprocessing. It is produced in nuclear reactors to the extent of 4.38 x 10¹⁰ Bq per MWd(e) of fuel irradiated, but is mostly contained within the fuel rods and is released to the atmosphere during reprocessing of the fuel (for extraction of ¹³⁹Pu and other transuranics). Its total inventory in the global atmosphere is indicative of the amount of fuel of given irradiation reprocessed. ⁸⁵Kr is a rare gas and hence is not scavenged like aerosols. Unlike ¹⁴C as CO_2, its solubility in water is low. Hence it is a very useful tracer for studying air mass exchange. Since the only sink for this tracer is radioactive decay, it distributes itself rather uniformly within the hemisphere. It is therefore used for studying air mass exchanges across the subtropical fronts and across the equator.

Table 1.5 puts some perspective on the size of these sources and the collective dose to the public. Mine and mill tailings and fuel fabrication dominate. The dose commitment in the table is integrated over 10 000 years; the dose in one human generation is quite small.

1.4.5 NATURAL RADIOACTIVITY

1.4.5.1 Primordial and natural series: the human release of natural radionuclides

While we focus in this book on the environmental dispersion of man's synthetic radionuclides, human technology also releases pre-existing natural radionuclides which otherwise would remain trapped in the Earth's crust. Most TENR (`technologically enhanced natural radiation' Gesell and Prichard, 1975) such as the mining and use of phosphatic fertilizers, monazite processing, uranium mining, and slag and pumice building construction, results in little emission of activity to the global atmosphere other than by incidental radon release (Eisenbud, 1987). For the direct atmospheric release of radioactivity, the burning of fossil fuelscoal and oildominates; all other anthropogenic emissions of natural activity to atmosphere such as ore smelting, deep-well hydrothermal schemes and mineral waters are negligible in comparison.

Natural radionuclides are present in the Earth's crust either because they are primordial, with half-lives comparable with the age of the Earth, or because they are continually generated by the decay of long-lived precursors.

Primordial radionuclides can be divided into non-series and series. Of the twenty or so natural non-series nuclides (radionuclides without progenitor on Earth and whose daughter is a stable isotope), only ⁴⁰K (t_1/2= 1.25 x 10⁹ years) and ⁸⁷Rb (t_1/2 = 5.0 x 10¹⁰ years) are appreciable sources of activity, with average concentrations of 17 pCi and 2 pCi per gram of crustal rock (630 Bq and 74 Bq per kilogram).

The three significant series radionuclides, ²³¹U (t_1/2 = 4.5 x 10⁹ years), ²³²Th (t_1/2 = 1.5 x 10¹⁰ years) and ²³¹U (t_1/2 = 7.1 x 10⁸ years), each decays down a chain of short-lived radioactive daughters, the uranium series, the thorium series and the actinium series. The latter series is less important because ²³⁵U now forms only 0.72 per cent of natural uranium.

The overall activity of a series radionuclide in a given rock depends not only on the total mass present but on how near the series is to radioactive equilibrium; that is, on how many of the short-lived highly active daughters are present. With the thorium series (parent ²³²Th) equilibrium is established in less than a century. The actinium series (parent ²³⁵U) requires half a million years, and the uranium series (parent ²³⁸U) over a million years. Thus on geological timescales thorium is always at or near equilibrium and hence has a constant activity per unit weight of thorium, whereas uranium is often considerably out of equilibrium because of the long period needed to recover after changes in the local chemistry or the physical environment.

Two factors in particular perturb the equilibrium. Elements above radium in the decay chain (i.e. precursors of radium) are soluble in local sulphate-rich groundwaters and insoluble in chloride-rich, while radium and its daughter elements have the reverse character. In addition, the uranium series has ²²²Rn as a member. This chemically unreactive gas has a half-life of 3.8 days, giving time to diffuse away from the parent or to transport in solution before decaying down the successive 11 members of the chain. The inert-gas members of the thorium series (²²⁰Rn) and the actinium series (²¹⁹Rn), on the other hand, both have half-lives of seconds, not long enough to allow significant movement.

Hence a uraniferous deposit and especially a porous sedimentary depositwith a given uranium content can vary greatly in total activity per unit mass, and much of the activity can migrate to large distances from the parent isotope. In contrast, the activity of the thorium daughters mainly remains associated with the parent element, and is calculably constant with mass. The activity of the non-series ⁴⁰K is of course always closely proportional to the total potassium present.

Radioactivity associated with oil and gas fields

Most of the world's oilfields have formed over organic shales, whose uraniferous phosphate content can raise their uranium concentrations to 1000 ppm or more. The underlying porous sandstones contain brine, which continuously dissolves the ²²⁶Ra (t_1/2 = 1620 years) and its daughters being generated by the uranium in the shale, carrying them to the oil and gas reservoirs above. As the uranium series is the major source of activity, individual geochemical circumstances make it impossible to predict the activity of the gas or oil even when the uranium content of the underlying beds is known.

The daughters of ²²²Rn have half-lives of only seconds or minutes until ²¹⁰Pb (t_1/2 = 22.3 years) is reached, succeeded by ²¹⁰Po (t_1/2 = 138 days). With radium, these become the major contaminants of petroleum, and they also deposit in plant and pipelines from the radon swept out from a natural gas reservoir.

Initially the oil or gas from a new well is dry but soon water mixes with the outflow, forming an emulsion or an aerosol. This formation water contains many ions in solution, notably Group IIA cations Ca, Sr and Ba, with sulphate and carbonate anions. Temperature and pressure changes can then deposit a dense adherent scale in which radium (also Group IIA) and its daughters will co-precipitate. Most of the world's mature fields have operational problems from radioactive scale, with activities ranging from less than 1 Bq to over 15 000 Bq per gram, requiring health physics supervision, approved plant recovery procedures and active waste disposal. The main disposal route is to sea, and very few statistics of scale tonnage and activities are available. The most complete are from the UK sector of the North Sea, a field of exceptionally low activity. There, at least 10 per cent of the installations form scale and sludge above 1000 Bq g^-1 in amounts ranging from 1 tonne to over 100 tonnes per year. Extrapolating globally, with some thousands of installations, 24 times the production (1988), and in general higher activities, a total ²²⁶Ra scale discharge of 3.6 to 7.2 TBq (100200 Ci) per year can be estimated.

Nuclides not depositing from the brine as scale are discharged in solution to seas or rivers. Activities in US production waters range from 3700 to 41 000 Bq m^{-3 224}Ra, and up to 64 000 Bq _M-3 226Ra, and up to 14 800 Bq m^{-3 224}Ra. The mean of 172 German brines was 8800 Bq m^-3 with a maximum of 286 000 Bq m^-3 while Russian brines have been measured at 250 000 Bq m^-3. Worldwide, again one can only guess. In the North Sea 0.85 tonnes of formation water is discharged per tonne of crude oil produced (1989). Assuming this to apply globally, the 1988 world oil output of 3.0 x 10⁹ tonnes was accompanied by 2.1 x 10⁹ tonnes of water containing 2.121 TBq (57570 Ci) of ²²⁶Ra plus similar amounts of ²²⁴Ra and ²²⁸Ra.

Table 1.6 exemplifies the wide range of radon activities passing into natural gas distribution systems (the average figures are from UNSCEAR, 19774). From the field outputs the apparent annual discharges to the atmosphere are deduced. The last column assumes all wells within the country to have equal production rates at the quoted average radon concentration, which is unlikely. No activities are quoted for the world's largest producer, the former USSR (770 x 10⁹ m³ in 1988), nor for the other 61 countries whose natural gas production statistics are known. World total output in 1988 was over 1900 x 10⁹ m³, which if averaging 1000 Bq m^-3, as seems possible globally, implies an annual ²²²Rn release of 1900 TBq (50 000 Ci).

Accompanying the radon will be small amounts of pre-formed ²¹⁰Pb and ²¹⁰Po, present as aerosols. For North Sea gas, with radon averaging 35 Bq m^-3, ²¹⁰Pb and ²¹⁰Po each amount to 0.010.05 Bq m^-3 at the land terminal, but fields with higher radon concentrations or less-effective filtering may emit more.

Table 1.6 Some natural radon gas activities and outputs

	222Rn			Gas output		Activity released
Source	Range  (Bq m-3)	Average  (Bq m-3)		(109 m3 y-1)		(Ci y-1)		(TBq y-1)
North Sea (UK)	3040	35		45		43		1	.6
North Sea (ND)	401700	74		88		241		9
West Germany	1004000	200		54		270		10
United States	18554 000	1300		473		16 600		600
Canada	15020 000	8700		98		23 000		900
Nigeria	4060	67		3		5		0	.2
Indonesia	5592

As the world's primary fossil fuel, 4.6 x 10⁹ tonnes of coal are burnt each year (1987) with about 70 per cent used in large-scale power generation. All coals contain radionuclides of the uranium and thorium series, with the uranium as well as the thorium more nearly in secular equilibrium than in oil and gas fields. The equilibria established in the coal are destroyed in combustion as the chemical elements distribute according to their volatility over the range of ash particle sizes.

Radionuclides which have been measured in coal are ²³⁸U, ²³⁴U, ²³⁴Th, ²³²Th, ²³⁰Th, ^22aRa, ²²⁸Th , ²²⁸Ac_, ²²⁶Ra ²¹⁴Pb, ²¹⁴ Bi, ²¹²Pb, ²¹²Bi, ²¹⁰Pb, ²¹⁰Po, ²⁰⁸Tl and ⁴⁰K. Table 1.7 summarizes the range of activities for a wide variety of coals (Corbett, 1980; UNSCEAR, 1977a). The figures are roughly consistent with secular equilibrium. On burning, the non-volatiles remain in the ash, which for UK coals averaging 16 per cent implies a concentration by five or six times. Within the ash there is a partitioning between bottom and fly-ash with Th, U, and Ra fairly evenly distributed but ²¹⁰Pb and ²¹⁰Po enhanced in the finer fly-ash particles by as much as a further factor of six. 

Table 1.7 Radionuclide concentration ranges in coal and ash (Bq kg^-1)

	Coal	Bottom ash	Fly-ash
238U	931	56185	70370
234U	19	92	160
232Th	919	59	81174
228Ra	620	1878	63130
228Th	120	5681	15130
226Ra	725	20166	85281
210Pb	1026	21185	521813
210Po	41	13185	196466
40K	26130	230962	233740

New coal-fired power stations in the United Kingdom now nominally retain 99.3 per cent of the fly-ash and at this rate the radioactive emission from a 1000 MW plant using coal of 16 per cent ash content would be:

²¹⁰Pb and ²¹⁰Po    5.3 x 10⁹ Bq y^-1

Th, U and Ra         5.3 x 10⁸ Bq y^-1

If all the world's coal-fired electricity generation were from such stations, the total annual emission would be over 1000 times the above, i.e. a total of 85 TBq (2300 Ci), with 5 TBq (140 Ci) of ²¹⁰Pb and ²¹⁰Po and 0.5 TBq (14.3 Ci) as Th, U and Ra. But few other countries achieve so low a fly-ash emission; parts of North America rarely exceed 97.5 per cent retention and most of the world falls far short of modern European and US standards. Indeed, it is probable that the majority of `Third World' power stations use conventional cyclone separators only, releasing as much as 10 per cent of their fly-ash to the atmosphere. Worse still, in brick and pottery manufacturing aerosol retention world-wide is usually zero, and it has been suggested that the total release of ²¹⁰Pb and ²¹⁰Po from these industries could exceed that from power generation (Peute et al., 1988). The global radon emmission is of course in direct proportion to coal tonnage consumed, i.e. 75 TBq (2030 Ci), since 100 per cent of this volatile gas is released whatever the fly-ash retention. But the total of thorium, uranium, radium, lead and polonium is likely to be at least ten times the above figures, i.e. 50 TBq (1400 Ci) of ²¹⁰Pb and ²¹⁰Po and 5 TBq (140 Ci) Th, U and Ra, totalling with the radon 130 TBq (3570 Ci) per year.

The global perspective

It is also of interest that one consequence of fossil-fuel burning, the Suess effect, decreases atmospheric specific radioactivity. The 6.7 x 10⁹ tonnes of carbon released annually as carbon dioxide from coal and oil contains no ¹⁴C, and its dilution into the atmospheric 6.6 x 10¹¹ tonnes of carbon (containing 62 tonnes = 1.1 x 10⁷ TBq or 285 MCi of ¹⁴C at equilibrium) implies simplistically a reduction of atmospheric ¹⁴C concentration by 1 per cent per year. In actuality, because of ocean and biosphere involvement, there was a decrease in atmospheric ¹⁴C concentration by 34 per cent from this cause between 1850 and 1950 (Aitken, 1978). After that a rise of at least 50 per cent was seen from nuclear weapons testing, but since 1970 the fall probably continues.

Table 1.8 Relative emissions calculated as radiation energy flow, (J s^-1)

1.4.5.2 Cosmogenically produced isotopes

Radioactivity is easily measured in very small quantities and hence serves as a very useful tracer for atmospheric circulation.

Radioisotopes used as atmospheric tracers are both of natural and man-made origin. Amongst natural sources are cosmogenic and terrestrial origin isotopes whereas man-made sources comprise fission and activation products from nuclear weapons tests and the nuclear fuel cycle including nuclear power stations and fuel reprocessing.

Cosmogenic radioisotopes are produced by cosmic ray bombardment of the Earth's atmosphere. The important cosmic ray produced isotopes are ³H, ⁷Be, ¹⁴C, ²²Na, ³²p_, ³³p and ³³S. These isotopes are produced by spallation reactions with the atmospheric nitrogen, oxygen and argon, by high-energy cosmic rays, mostly protons and neutrons. Spallation reactions are those in which nuclei are split into smaller nuclei due to the very high energy of the bombarding cosmic ray particles. Table 1.9 lists some of the cosmic ray produced isotopes, their production modes and range of concentrations observed in the air and rainwater. Production is maximum in the upper atmosphere at high latitudes where cosmic ray particles are maximum. Consequently, these isotopes are good tracers for studying stratospherictropospheric exchange processes. Detailed calculations are available on the expected concentrations of these isotopes at various latitudes and altitudes (Lal et al., 1958).

The second type of natural isotopes which can be used as atmospheric tracers are radon and thoron and their decay products. The soil always contains varying amounts of uranium-238, thorium-232, uranium-235 and their daughter products. The radon (²²²Rn), thoron (²²⁰Th) and actinon (²¹⁹Rn) gases formed in the decay of uranium-238, thorium-232 and uranium-235 series, respectively, escape through the pores of the soil and diffuse into the atmosphere.

Actinon, although present in the atmosphere, is in such small quantities that it is not of much significance. Even for radon and thoron, their emanation over the oceans is negligible in comparison to that over the land due to the low radioactivity content of sea waters and lower emanation rate of radon and thoron as compared to continental areas. The daughter products of the gases, radon and thoron, are solids and condense on the fine aerosols always present in the atmosphere forming radioactive particulates. Since these radioactivities, both gaseous and particulate, orginate from the soil, their concentrations generally decrease with height and at high altitudes only the longer-lived isotopes are found in measurable concentrations. The behaviour of these radioactive materials in the atmosphere thus can be used for the study of the atmospheric circulation processes of the lower atmosphere.

 Fission and activation products dispersed as global fallout from nuclear tests are the most important, and to a lesser extent are those from reactor accidents and other releases from peaceful uses of nuclear energy such as fuel reprocessing. Dispersal patterns of these isotopes depend on the radioactivity release site and height, magnitude or amount of release and meteorological and other factors of the release event. High-yield tests push the bomb cloud into the stratosphere. In the case of low-yield tests, the dispersal of the radioactivity is by tropspheric wind systems.

Table 1.9 Radionuclides produced by cosmic rays

Primary	Atmospheric	Production	Approximate concentration range
radio-nuclide	half-life	mode	Air (Bq/103 m3)	Rain (Bq/103 l)
3H	12.3y	14N(n, 12C)3 H	0.0200.200	2004000
7Be	53 d	Spallation (N and O)	0.08030	4004000
10Be	1.6 x 106 y	Spallation (N and O)		4 40 x 10-5
11C	20.3 min	Spallation (N and O)
14C	5730 y	14N(n, p) 14C		40
18F	1.8 h	18O(p, n)18F
		Spallation (Ar)
22Na	2.6 d	Spallation (Ar)	(2 8) x 10-4	0.0040.04
24Na	14.7 h	Spallation (Ar)	(2 8) x 10-3	440
28Mg	20.9 h	Spallation (Ar)		0.21.20
31Si	2.6 h