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RADIOACTIVE AND : SOURCES

The current worldwide expansion in the use of nuclear power for the generation of electricity H F Macdonald has stimulated a renewed interest in the problems Central Electricity Generating Board, of radioactive . It is generally Berkeley Nuclear Laboratories, Berkeley, accepted that at present provides Gloucestershire GL 13 9PB the only viable alternative source for the early part of the coming century as world reserves of fossil are gradually depleted (Yannacone 1974). However, if the full potential In the first of two articles on of nuclear power to provide a supply of energy radioactive from nuclear which is limited only by investment capabilities power we look at the nature and is to be realized, techniques must be developed origins of waste from present to cope with the radioactive wastes which arise and future installations at each stage of the nuclear cycle. Since some components of these waste materials represent a hazard to man for many thousands of or more, the problem of their secure containment presents scientists and engineers with a unique technological challenge. The fundamental principles of the manage- ment of radioactive wastes within the United Kingdom were established by the Radioactive Substances Advisory Committee (1959) in a White Paper entitled The Control of Radiocirtive Wastes, and may be summarized as follows: (a) To ensure irrespective of cost, that no member of the public shall receive a dose in excess of the limits recommended by the These principles are implemented through ICRP (International Commission on Radiological statutory instruments embodied in the 1960 Protection): Radioactive Substances Act, and subsequent (b) To ensure, irrespective of cost. that the whole legislation, and are administered by the Depart- population of the country shall not receive an ment of the Environment (Radiochemical Inspect- average dose of more than 1 Rt per person in orate) and by the Ministry of Agriculture, thirty years: and Fisheries and Food via ‘Authorizations’ which (c) To do what is reasonably practicable, having limit the discharge of radioactive wastes to the regard to cost, convenience and national import- ance of this subject, to reduce the doses far t An exposure of 1 R (roentgen) is approximately below these levels. equivalent to a dose in tissue of 1 rad or J kg-*

Physics in Technology January 1976 11 environment. In addition, the accumulation of such wastes at individual locations is controlled under the terms of ‘Nuclear Site Licences’ administered by the Health and Safety Executive (Nuclear Installations Inspectorate). Here we consider the nature and origins of the active wastes produced in each phase of the cycle. The dominant sources of Refined waste are associated with power reactor irradia- tion and the subsequent fuel reprocessing opera- tions. Various categories of nuclear waste can be identified from their method of production and potential hazard to man: this article examines the characteristics of waste arising from a range of Fuel fabrication and thermal and fast reactor designs and the prob- enrichment plant lems likely to arise in the disposal of obsolete nuclear plant. Alternative methods of exploiting Recycled nuclear energy, other than current or U and Pu/ based fuel cycles will also be con- sidered. The The nuclear fuel cycle comprises the full range of operations associated with the generation of Reprocessing plant Reactor electricity from nuclear power and is illustrated and waste storage irradiation schematically in figure 1. It commences with the and milling of the naturally occurring uranium (or ) bearing , prior to their concentration and refining for dispatch to the fuel fabrication plant. This forms the next stage Irradiated fuel of the cycle, in which uranium metal or U02 fuels are manufactured from the ore concentrates Figure 1 The nuclear firel cycle and encased within appropriate cladding materials to form the reactor fuel elements. In the case of advanced thermal reactor fuels isotopic enrichment of the fissile 235Uisotope, above normal levels only in the immediate which has a natural abundance of only 0.71:;, vicinity of the mine. is also carried out at this stage.? Next comes the Further radioactive wastes, principally the reactor irradiation and subsequent transport of 226Ra,are produced in the milling the irradiated fuel to the reprocessing plant. This of the ore which concentrates the uranium from latter stage completes the nuclear cycle with the a naturally occurring level of 0.1-1 *: up to about extraction of the unused uranium, and also the 90%, by weight. After crushing to reduce the size fissile plutonium produced during the reactor of the mined ore, the uranium is extracted by irradiation. These materials are then available acid or alkali leaching, combined with chemical for the fabrication of ne& fuels and recycle precipitation, or solvent extraction. through advanced thermal or fast These processes result in solid and liquid wastes, systems. known as , which contain the naturally Radioactive wastes are produced at each stage occurring active radium and are initially pumped of the nuclear cycle, beginning with the mining into settling ponds. Apart from the radioactive content of the tailings, care also has to be and milling operations. In the mining of uranium exercised in the control of chemically toxic agents ores quantities of natural radioactivity comprising present, notably traces of uranium and other radon gas and its daughter products are released, such as , copper and . and in underground installations these make it necessary to ventilate the mines. The con- Although the main of interest, %Ra, does not represent a significant radiological taminated ventilation air is rapidly diluted in the hazard, the residual solids ultimately require atmosphere where its nzRn content is detectable stabilization to minimize their long term dispersal in the environment (Eisenbud 1963). In the fuel fabrication phase of the nuclear t Although most reactor types employ enriched fuel, a substantial fraction of the world’s operating power fuel cycle the main hazard arises from the reactors employ fuel, for example chemical toxicity of uranium, the radiotoxicity of the reactors in the UK. Italy and , natural uranium being of secondary importance. and Candu reactors in Small quantities of uranium exist in the liquid

12 Physics In Technology January 1976 mantling will take place, and the CEGB has given public undertakings that reactor sites could be returned to former use if required, although it is more likely that the sites would be reused for future CEGB developments.) In addition, small quantities of radioactivity arise in the form of low-level of gaseous and liquid which are routinely discharged from nuclear installations under the terms of their nuclear site licences. These effluents are associated with normal operational and maintenance activities but the quantities of radioactivity in- volved are negligible compared with the high- level reprocessing and reactor wastes mentioned above. The origins and nature of nuclear wastes During irradiation of fuel within the reactor several hundred are generated in the fission process, while others are produced via activation of the fuel and also reactor structural materials. These have half- lives ranging from a fraction of a second up to millions of years, and in undergoing may emit alpha, beta or gamma at up to a few MeV per disintegration. The aim of waste management activities is to contain these materials and limit their release to Irradiated firel in the storage pond of the the environment in order to minimize the hazards SGHWR. The glow that can be seen is to man from their transfer via air and Cerenkov radiation. (Photograph courtesy of or through various food chains. Tn considering UKAEA) waste management problems it is convenient to identify in turn three main classes of radio- wastes from fuel fabrication plants, while some , namely fission products, iso- processes involve potential exposure of operators topes and activation products. to alpha-emitting dusts. However, the commercial FISSION PRODUCTS value of uranium ensures that all operations are A highly readable account of the neutron physics carried out with only minimal losses, and the of the fission process was presented in an earlier amounts present in waste discharges are not article in this magazine on the evolution of nuclear normally detectable above the natural back- fuels (Boltz 1973), and only a brief outline will ground levels (Hill 1973). be given here. Fission product isotopes, together All of the activities considered so far are with the which sustain the chain re- naturally occurring and in general their levels in action within a reactor core, are the products of the waste materials produced in the manufacture interactions involving a neutron and a fissile of reactor fuels do not present any major heavy metal atom and their mass distribution problems. Indeed, it is only in the reactor irradia- follows the known double-humped fission- tion and subsequent phases of the nuclear fuel yield curve, peaking at mass numbers in the cycle that significant quantities of radioactive range 85-1 10 and 130-150. The primary products waste materials are produced. This situation is of the fission reaction are normally short lived slightly modified in systems involving the recycle and rapidly decay via beta and gamma emissions of recovered uranium and plutonium isotopes, through one or more longer lived isotopes to a and these will be discussed later where stable species. Frequently the radionuclides of appropriate. In the following sections the greatest radiological significance are the fourth or emphasis is placed upon the artificial radio- fifth members of one of these decay chains, as in nuclides generated during power reactor opera- the case of 90Sr and I3lI, which occur in the tions, many of which appear as the main con- decay schemes at the foot of the next page. taminants in fuel reprocessing wastes. Others are The build-up of activity within the fuel during retained within the reactor pressure vessel, or in a reactor irradiation and its decay following dis- shielded waste storage facilities on the site (Myatt charge of the fuel from the reactor, are thus 1974), and only constitute a waste disposal prob- determined by the combined effects of several lem when the reactor is finally dismantled at the tens of chains of the type illustrated above. On end of its life. (At least some degree of dis- discharge the fuel is stored at the reactor for up

Physics in Technology January 1976 13 associated with the fuel processing. The decay of the activity of a typical thermal reactor fuel is illustrated in figure 2, with the decay of the fission product isotopes being characterized by the beta and gamma curves. The former curve follows the decays of 1LLCe/''4Pr up to about four years cooling time, and there- after the decay of the bone-seeking 9"Sr isotope. The gamma decay curve is more complex, having a large number of contributing isotopes during the first few years following fuel discharge, but at longer cooling times it is dominated by the decays of '"'Cs/ '"'"'Ba. The beta and gamma decay curves show that the fission product activities in reprocessing wastes fall by almost two orders of magnitude over a period of 100 h \ \ -3 ; years, and beyond this time continue to fall with IO \ a halflife of about 30 years, determined by the a \ \ \\ \\ and '"'Cs decays. - The ultimate fission product activity of re- Separated waste- - - -1. processing wastes arises from a few very long lived nuclides, such as lZ9I with a radioactive half- 10-~ life of 1.6 X 10' years, but their activities are Cooling time (yr) many orders less than that of the more hazardous I IO IO0 nuclides mentioned above. An assessment of the relative hazards of the more active fission pro- Figure 2 a. /3 and decay c-itrves for natural ducts is presented in table I, in which the isotopes rrrcmirim Magnox reactor fuel irradiated to are listed in groups of similar halflife. The 3500 M W d t-l at 2.7 M W t-' hazards, expressed as the volume of air or water in which the activity produced per unit generation to three months, prior to transport to the repro- (m"/GW(e)) must be diluted to yield the cessing plant. This greatly reduces its radioactive maximum permitted concentrations for members emissions by allowing the decay of the majority of the public recommended by the ICRP (1960, of the shorter-lived isotopes, particularly those of 1964), are largely independent of reactor type, the noble gases krypton and xenon, and also the apart from minor variations associated with volatile element . The chemical repro- differences in fission yields of the various heavy cessing of the fuel normally takes place within a metal isotopes. For example. the 9"Sr hazard in a year of discharge from the reactor, when the ?'jU fuelled thermal reactor is 2-3 times greater remaining fission product isotopes are separated than that for a plutonium-fuelled fast breeder re- from the unused uranium and the plutonium actor, while in the case of In6Ru and the rare isotopes produced during the reactor irradiation. earth nuclides the fast-reactor hazards are greater Thus only those isotopes having halflives of a by a similar factor. However, beyond the first few hundred days or more make a significant few years after discharge of the fuel from the contribution to the activity of the wastes reactor the dominant fission product hazard is Decay products from fiwioti reactions 90Br + IOKr 90Rb 90Sr 90Y 90Zr (1.6 s) (33 s) -(2.9 min) - (243 yr) - (64h) -(stable)

\ / (11.8 d) I3lSn 1311 (3.4 min) - (3.05 d) \ 131XeI (25 min) (stable)

14 Physics in Technology January 1976 that associated with gOSr,in terms of potential wastes by a slow build up due to growth d the leakage to both the atmosphere and the aqueous alpha emitter zrlAm from beta decays of 211Pu. environment. In the separated wastes remaining after uranium ACTINIDE ISOTOPES and plutonium recovery the main contributors to Apart from initiating fission reactions. the the alpha curves following the decay of zrCm neutrons generated within a reactor core may are its daughter 2'8Pu, together with the interact with a heavy metal atom to produce an- "'Am present in the fuel at the time of separation, other heavy metal nuclide. Such capture reactions which in the case of the data presented in figure 2 are accompanied by the emission of gamma has been taken as 300 days post discharge. radiation or further neutrons, while the resultant Unlike the fission product activities and heavy metal nuclide may undergo further neutron hazards per unit generation of electricity, the captures, or decay with the emission of alpha, equivalent parameters for the actinide isotopes beta or gamma radiations. Thus, during a power show a marked dependence on reactor type. On reactor irradiation successive progressing from low burn-up uranium-fuelled and decay processes result in the build up of a thermal reactors to more advanced thermal or wide range of actinide isotopes, in addition to plutonium-fuelled fast breeder reactor designs, the fission products. the main effect is an increased production of the In the case of uranium-based reactor systems higher transplutonium nuclides, notably *"Cm a broad spectrum of transuranic nuclides is and ?"Am, which manifests itself as a net in- present in the discharged fuel, the most import- crease in the alpha emissions relative to the fission ant being the fissile which product beta and gamma decays. This effect is are recovered during reprocessing for use in the associated largely with the higher fuel burn-up fabrication of advanced thermal or fast breeder in these systems, but other factors such as the reactor fuels. Also present are higher transuranics, degree of neutron moderation and fuel enrich- particularly the isotopes of and ment can play an important role (Clarke et a1 , which constitute the dominant source of 1975). The build-up of the curium and americium alpha activity and decay heating in the repro- isotopes responsible for the alpha decays in re- cessing wastes following uranium and plutonium processing wastes occurs via two virtually separation. The relative contributions of the independent routes, one controlled by the zrlPu fission product and actinide isotopes to repro- activity within the fuel and the other by neutron cessing wastes are shown in figure 2, in which the captures in ?4LP~as shown below. total and separated-waste alpha-decay curves are As an example of the variability of actinide plotted together with the associated fission production, the build-up of Wm in a range of product beta and gamma curves. At cooling times typical thermal and fast-reactor fuels is shown in up to about two years the alpha decays are figure 3. This clearly demonstrates the increased dominated by 'Tm, followed in the unseparated curium production in the higher burn-up thermal

Routes to the build-up of Cu and Am isotopes

241"nlAln (26 min)

243Am 0 (7370 yr) \ 1 'I 'I 2'4AmI (16 h) (10.1 h)

24rcmip- **Cm 163 d) (18.1 yr)

Physics in Technology January 1976 15 c reactor systems, as well as illustrating the the basis of uptake via inhalation outweigh those dependence of the 2**Cm build-up on the *"Pu associated with the fission product isotopes, parti- content of the fuel through the characteristic cularly at longer cooling times. However, in terms shape of the fast-reactor curves. These show an of accidental discharges to the aqueous environ- enhanced curium production during the early ment, would dominate the hazard index for stages of the fuel cycle, due to the small nrlFu the first few decades following reprocessing, this content of the unirradiated plutonium fuel, in period decreasing with the enhanced production contrast to the uranium fuelled thermal systems of the higher transplutonium isotopes in advanced where significant build-up of the plutonium thermal and fast breeder reactors discussed isotopes occurs only in the later stages of the above. cycle. In addition, this effect is more marked for ACTIVATION PRODUCTS the long shelf-life fast-reactor fuel due to the The third class of radionuclides generated within formation of *+IAm from decay of 2'1Pu during power reactors, activation products, arise from storage of the fabricated fuel. interactions between neutrons and atoms within Finally, activity and hazard values for the more the , moderator or structural materials. important actinide isotopes present in Magnox They are thus the products of capture reactions, reactor fuel are also given in table 1. In the un- but unlike the do not in general form separated fuel the actinide hazards are dominated complex capture and decay schemes, decaying by the istopes of plutonium, but following the instead directly to stable species with the emission recovery of these during reprocessing the of beta and gamma radiations. Activation pro- dominant hazards associated with the wastes arise duct inventories are strongly dependent on from 211Am and 2"Cm, or at longer cooling times reactor type, even on detailed design features of the latter's 23sPu daughter. The data presented in individual reactors of a given type, and they may table 1 show that the actinide hazards derived on originate from trace impurities having large

Table 1 Fission product, actinide and activities and hazards for thermal power reactors. (a) Indicates values based on mean burn-up fuel irradiated to 3500 MW d t-I at 2.7 MW t-' und cooled for 100 d. (b) indicates representative values based on gas or water cooled reactors (see text)

Activity Hazard index (m3/GW(e)yr) Nuclide Halflife (Ci/GW(e)yr) Wafer Air

9'Y 58 d 10' 3.3 x 10" 10'6 Fission g5Zr 64 d 1.5 x 107 2.5 x 10" 1.5 X 1OI6 product 1311 8.05 d 4.4 x 103 2.2 x 109 1.5 x 1013 isotopeda) Io6Ru 369 d IO' 10" 5 x 1016 l3CS 2.1 yr 1.1 x lo6 1.2 x 10" 2.8 X 10'j 'Te 284 d 2.8 x 107 2.8 X 10" 1.4X 10" 14:pm 2.6 yr 7.8 X 10% 3.9 x 10'0 3.9 x 1015 9USr 28.5 yr 2.9 X lo6 7.3 x 10" 7.3 x 10'6 "1 1.6 x 107 yr 1.4 35x 106 2.3 X 10" 1S7Cs 30.1 yr 3.7 x 106 1-9X 10" 7.4 x 10'5 lsrEu 16 yr 6xlo4 3 x 109 6 X 10" 85Kr 10.73 yr 3.4x 1oj __ 1.1 x 10'2 jH 12.3 yr 3 x 104 10' 1*5x10"

23spu 86.4 yr 9.4 x 103 1.9 x 109 1.3 X 10" Actinide za9pu 2.4 x lo1yr 3.7 x IOr 7.4~109 6.1 x 1017 Isotopeda) **UPU 6580 yr 3.2 X 10' 6.4 x 109 5.3 x 10" Z41PU 14.3 yr 2.8 X lo6 1.4 X 10" 9.3 x 1017 "'Am 433 yr 54x 103 1.5 x 109 2.9 X 10l6 242Cm 163 d 1.2 x 105 6X lo9 3 x 10'6 244Cm 18.1 yr 1.7 X lo" 24x 107 5.7 x 1014 3H 12.3 yr 10-1oj 3 x 103- 5 x 107- Activation 3 x 107 5 x 10" productdb) ly: 5760 yr 50 6X 10' 5 X lo8 fiAr 1.83 h 105 I 2.5 x 10l2 To 5.3 yr loj-106 3 x 109- 3 x 1014- 3 x 10'0 3 x 1015

16 Physics in Technology January 1976 end of its life. This operation is likely to be post- poned for several years following reactor shut- down in order to allow decay of the shorter-lived radioactive species; thus, it is only the longer- lived activation products which are of interest in IO6 the context of the waste management of obsolete n L nuclear plant (Bainbridge et a1 1974). For this h n reason "Co. with its penetrating gamma "U emissions, is one of the more radiologically 3 U significant activation products and typical 105 > activities and hazards are included in table 1. At 5 cooling periods up to a few years the 6oCovalues .-# are comparable with those of the fission product ,-5 cl and actinide wastes, but beyond this time the U importance of the cobalt isotope is much reduced due to the relatively longer radioactive halflives io4 .-5 L of these other nuclides. 2 Cobalt is also an important constituent in the materials arising in water cooled reactors, where Tois produced via acti- io3 vation of dissolved impurities and suspended Fuel irradiation CG Wd/) particulate corrosion products (crud) present 0.1 I IO IOC in the coolant. Deposition of activated crud around the reactor circuit is a major source of Figure 3 Curium-242 build up in thermal and operator doses during routine operation and fast breeder reactors. A-fuel stored for 4 yr, maintenance of both light water (LWR) and heavy B-fuel stored for 1 yr water (SGHWR or CANDU) moderated plant, necessitating periodic decontamination of the neutron absorption cross sections, as well as from system. In these circumstances Vo is activation of bulk materials in and around the accumulated in the ion exchange resins and other reactor core which are subject to neutron irradia- spent filtration media which must be stored in tion. Thus, while it is difficult to generalize about the active waste facilities at the reactor site. the activation product contributions to nuclear Of the other longer lived activation product4 wastes, it is possible to discuss a few represent- which are normally considered in waste manage- ative examples characteristic of a range of reactor ment studies, and with halflives of types. 12.3 and 5760 years respectively, are worthy of One of the most important activation products mention and typical parameters for these isotopes arising in nuclear power reactors is the 5.3 year in power reactor systems are also given in table halflife isotope 6oCo,formed by thermal neutron I. The "C values are based upon the build-up of captures in "CO or fast neutron captures in 6oNi. this isotope within the moderator of gas These reactions occur in the various used cooled reactors, while the wide range of tritium as fuel cladding materials, in components of fuel values quoted occurs because this isotope may assemblies and also in core support structures. arise from a variety of sources, including the For example, in the advanced gas cooled reactor fission process itself. The dominant activation (AGR) one of the major sources of Tois the sources of tritium are neutron captures in lithium activation of trace impurities in the impurities in graphite for gas cooled reactors, or fuel pin clad, while for Magnox reactors it occurs in coolant additives in light water reactors in the cobalt-rich springs used to locate the and captures in the case of - clad fuel elements systems. Both isotopes decay via weak beta within the fuel channels. In the former case the emissions and thus do not give rise to the gamma cobalt activity is returned with the fuel for re- shielding problem associated with the 6oCodecays, processing and thus appears in the wastes their main radiological significance being as accumulated at that stage of the nuclear cycle, potential sources of internal irradiation due to either as solid active waste or in liquid form uptake by man following accidental leakage of depending on whether mechanical or materials to the environment. decladding techniques are employed. Finally, one short-lived activation product In contrast, the Magnox fuel retaining springs which may be included under the heading of are removed prior to fuel transport and are re- nuclear waste is "Ar, which occurs in the tained as solid active waste at the reactor site. biological shield cooling air of the earlier Magnox This waste, together with the activated core reactors. This isotope has a halflife of 1.83 h structures and moderator, remain to be disposed and is produced by of of when the reactor is finally dismantled at the naturally occurring , as well as argon im-

Physics in Technology January 1976 17 purities in the CO2 coolant of Magnox reactors isotope has a natural abundance of less than 1 ":, . and AGRS. Dose rates due to routine argon dis- The situation may be improved by the develop- charges close to power reactors are generally ment of fast breeder reactors, which use the fissile small compared with those due to natural back- plutonium isotopes produced by neutron captures ground sources (UNSCEAR 1972). while the in the more abundant 238Uisotope, and ultim- hazard index for "Ar shown in table 1 is also of ately have the potential of regenerating their fuel little radiological consequence. independently of the thermal reactors. However. The examples outlined here indicate that acti- both thermal and fast reactors employing vation products may be accumulated at the uranium or plutonium fuels are characterized by reactor site or as part of the waste arisings from the production of long-lived radioactive wastes of the irradiated fuel reprocessing. In the former the type discussed earlier and these require instance they are an important consideration in isolation and containment for many hundreds or the context of the dismantling and disposal of thousands of years beyond the useful lifetime of obsolete nuclear plant. However, in the case of the reactor systems in which they originate. reprocessing wastes the activation product con- An alternative to these conventional fuel tributions to both activities and hazards are cycles, which is attractive from both the energy relatively insignificant compared with those from resource and waste disposal viewpoints, is the the fuel itself, and in particular those of the long- development of thorium-fuelled advanced thermal lived actinide isotopes. or fast breeder reactors. Although the naturally occurring 2H2Thisotope is not readily fissionable, Alternative nuclear options when subject to reactor irradiation it undergoes Power reactors based on the thermal fission of neutron captures to form the fissionable 2'rW 235U provide a relatively inefficient means of isotope in accordance with the capture and decay exploiting the world's uranium reserves, since this scheme opposite.

Figure 4 a,0 and decay curves for "'U and Th/*jjU HTR firel (irradiarion 80 GW d t-I ut 100 MW ?-I. a-6.67% 235U,b4$, ThiUj3U

a b

10-1

n L z1 10-2 n " 3 U .3 I U OI IO-^ .-C 4 m aJ r h m U 0"

-10-5 Cooling time (yr) I IO IO0 I IO 100 I I I I I

IS Physics in Technology January 1976 232Th (1.41 x 101oyr)

232Pa (3.24 X 10' yr) (1.3 d)

(1.59 X lo5yr)

Captitre arid decay scheme for '"Th

The production of 2a3U,and to a lesser extent for enriched VJ and th~rium/~Wfuels in a 2sTJ,from This analogous to the build up of high temperature gas cooled reactor (HTR) shown plutonium isotopes via conversion of the 238Uin in figure 4 (Clarke et a1 1975). natural uranium-based fuels and the fission pro- The main contributors to the alpha activity duct wastes generated during power reactor of thorium based fuels are 228Th,212U, 233U and irradiation are very similar for the conventional ??'U,or, in the case of the waste materials remain- uranium and thorium fuel cycles. However. an ing following thorium and uranium separation, important difference in the latter case is the "'Pa and 2'8Pu. However, the activity and hazard virtual absence of the build-up of the higher values for these actinide isotopes, derived on the transuranic nuclides responsible for the long- same basis as those presented in table 1, are small term alpha-active wastes associated with the 2"sU compared with those of the longer lived 90Srand or plutonium fuels. Instead, the actinide isotopes "'Cs fission product isotopes, as illustrated by the produced during reactor irradiation are pre- data for a thorium fuelled thermal reactor given cursors of the naturally occurring radioactive in table 2. Thus the thorium fuel cycle offers the series and tend to decay via alpha or beta and possibility of reduced long-term waste heating gamma emissions. This results in a much reduced and hazard commitments, but this is at the level of alpha activity in the fuel reprocessing expense of a shelf life problem associated with wastes, particularly if the unconverted thorium the fabricated 2iW fuel. Unlike the case of 2i1Pu and uranium isotopes are separated for recycle, discussed earlier, this arises not from decay of as can be observed from the curves the , but is due to the presence of

Table 2 Fission product arid actinide activities and hazards for a Th/233Ufuelled thermal reactor. Valites are based on mean hitrn-lip HTR fitel irradiated to 80 GW d t-' at 100 MU/ t-' and cooled for 100 d

Haljlifc Activity Hazard index (m?/GW(e)yr) Nitclide (years) (Ci/GW(e)yr) Water Air

gflSr 28.5 2.7 x 106 6.6 X 10" 6.6 X 10lG Fission 1291 1.6 x 107 1.1 2.7 X IO6 1.8 x 10'0 product I R7C 30.1 2.6 X IO6 1.3X IO" 5.2 x 1015 isotopes(") 15$Eu 16 1.4X 1Oj 7xloQ 1.4 X IO'j 85Kr 10.73 2.4 x 105 - 8 x IO" 3H 12.3 2x104 6.7 X lo6 IO"

228Th 1.91 2x103 2.8 X 10' 1 0'6 A ctinide mpa 3.24 X 10' 3.9x IO' 4.4 x 10' IO'S isotopedo) 232U 72 6.1 X 10' 7.7 x I 09 6.8 x IOl5 233U 1.59~105 2.2 x IO' 5.5 x 109 5.5 x 1015 ?%U 2.47 X 1 O5 9.6 X 102 2.4 X 10' 24x 1014 2%pu 86.4 6.3 1.3 X 10' 9 x 1013

Fhysics ;n Technology January 1976 19 2RiUin the fuel. The decays of this isotope several orders of magnitude lower than those of ultimately lead to the build up, on a timescale of the more significant fission product or actinide a few months, of the high energy gamma emitting wastes in a corresponding fission reactor (Hall isotope ?"*Tl,which makes it necessary to employ et a1 1975). shielded fabrication and handling techniques for Thus although significant quantities of radio- the unirradiated fuel (Weissert and Schileo 1968). active wastes will be produced in current thermal Another nuclear alternative based on the fission and future advanced thermal or fast breeder re- process, which has been developed recently as a actors up to the year 2000 AD, it is possible to means of space propulsion, is the gaseous-core anticipate the amounts and composition of these reactor. In this system the solid core of fuel arisings and ensure that adequate waste manage- elements with recirculating liquid or gaseous ment provisions are made to deal with them. coolant of conventional reactor designs is re- These centre largely around the development of placed by gaseous UF, which is recirculated techniques for the isolation and containment of through a large-cavity reactor vessel. The absence the long-lived fission product and actinide of neutron absorbing fuel cladding and core isotopes which are reviewed in a companion support structures enables high neutron energies article in this magazine. In addition, alternative and fluxes to be attained. and this to more nuclear options are available which offer the favourable fission to capture cross section ratios possibility of fission reactors with more favour- in the gaseous-core system, with the result that able radioactive waste characteristics, while there is a greater tendency for the long-lived ultimately could provide an actinide isotopes to undergo fission reactions. The essentially unlimited source of energy with the reactor thus exploits the principle of induced production of only minimal quantities of long- in order to minimize the lived wastes. build up of actinide wastes by converting them to Acknowledgment shorter lived fission product isotopes (Paternoster et a1 1974). The gaseous-core reactor is currently This article is published by permission of the at the advanced prototype construction phase in Central Electricity Generating Board. the US and offers the possibility of commercial power generation with relatively low actinide References waste production towards the end of the century. Bainbridge G R, Bonhote P A, Daly G H, Detilleux E and Krause H 1974 Atom. Energy Rev. 12 Finally, looking ahead into the next century, 145-60 the development of fusion reactors could provide Boltz C L 1973 Phys. Technol. 4 223-39 a viable alternative to fission systems, which is attractive on both economic and environmental Clarke R H, Macdonald H F, Fitzpatrick J and Goddard A J H 1975 Annl. Nucl. Energy 2 451-66 grounds (Steiner and Fraas 1972). One of the most promising fusion reactor designs currently Eisenbud M 1963 Environmental Radioactivity (New York : McGraw-Hill Books) under consideration is based on the deuterium- S, S, R tritium D-T fuel cycle, with the energy released Hall R Blow Clarke H, Tozer B A and Whittingham A C 1975 Proc. of CEGB Research D-T in the reaction being extracted by absorbing Department Symposium on Long Term Studies 24 the 14.1 MeV neutrons produced in a liquid-metal October 1974 published as CEGB Report lithium blanket. The system is characterized by RD /B/N3299 a complete absence of fission product and Hill J 1973 Firel and the Environment, Proc. Inst. actinide isotope production and the only radio- of Firel Conf., Eastboirrne 26-29 November 1973 active wastes generated are activation products ICRP 1960 Report of Committee II on Permissible resulting from neutron interactions in the lithium Dose for Internal Radiation (Oxford : Pergamon) blanket region and its associated structural com- - 1964 Recommendations of the ICRP (as Amended ponent materials. The former result in the pro- 1959 and Revised 1962) (Oxford : Pergamon) duction of tritium, which is extracted for use as Myatt A R 1974 Proc. 7th Znt. Congr. of the SFRP, fuel, thus the system is a breeder reactor, while VI1 - SFRP/9 the activation product wastes generated in the Paternoster R, Ohanion M J, Schneider R T and containment structures depend markedly on the Thom K 1974 Trans. Am. Nucl. Soc. 19 203-4. materials employed. These are likely to be the Radioactive Substances Advisory Committee 1959 refractory metals niobium or vanadium, which The Control of Radioactive Wastes Cmnd. 884 possess the necessary corrosion resistance to ( : HMSO) liquid lithium, and the long-lived activation pro- Steiner D and Fraas A P 1972 Nrtcl. Safety 13 353-62 ducts of interest are 931nNband 94Nb with half- UNSCEAR 1972 Ionising Radiations: Levels and lives of 13.6 and 2 X 10' years respectively. Effects Vol. I: Levels (New York: United Nations) These arise from neutron activation of the bulk Weissert L R and Schileo G 1968 Fabrication of niobium or, if vanadium is used, of trace niobium Thorirrm Firel Elements, AEC Monograph (Hins- impurities. However, the hazard levels associated dale, Illinois: American Nuclear Society) with these niobium isotopes, and also the tritium Yannacone V J 1974 Energy Crisis-Danger and inventory, of a fusion power reactor are typically Opportrtnity (New York: West Publishing)

20 Physics in Technology January 1976