SCOPE 50 - Radioecology after Chernobyl

2.1 INTRODUCTION

This chapter examines specific data which are available as a consequence of various planned and accidental releases of radioactivity to the environment. The case-studies presented do not comprise an exhaustive treatment of all incidents involving radioactive discharges, but include examples considered to be of particular significance, especially with regard to elucidation of environmental pathways. Releases associated with waste management operations, due to plutonium production and fuel reprocessing activities, are examined in connection with planned discharges. A number of case-studies relating to non-routine releases, arising from nuclear warhead production (Kyshtym) and reactor operations (Windscale, Three Mile Island, Chernobyl) are then examined chronologically in detail, and information relating to the re-entry of nuclear powered satellites (SNAP 9A and Cosmos 954) is presented. Another significant potential source of radioactivity in the environment exists due to the practice of ocean disposal of packaged radioactive waste. Further discussion of this issue appears in Chapter 5.

2.2 SELLAFIELD

The Sellafield complex, located on United Kingdom's west coast in Cumbria (see Figure 2.1), is the largest of British Nuclear Fuel's (BNF) sites and has been involved with the reprocessing of spent fuel since 1952. The current reprocessing plant at Sellafield began operation in 1964, replacing the earlier plant, its main function being to reprocess fuel from Magnox reactors. A new fuel handling plant concerned with the receipt, storage and decanning of Magnox fuel was commissioned in 1985 (primarily to increase the storage capacity and throughput of spent fuel for reprocessing), the year in which the site ion exchange plant (SIXEP) began operation. BNF has also built a large vitrification plant (for the conversion of highly active waste into solid glass blocks) and store at Sellafield. The enhanced actinide removal plant (EARP) and thermal oxide reprocessing plant (THORP), a new facility to reprocess fuel from more modern nuclear power stations in Britain and overseas, are scheduled for commissioning in the early 1990s. Apart from reprocessing and waste management activities, Sellafield is also the location of the world's first industrial-size nuclear power station, Calder Hall.

Figure 2.1 The concentration of ¹³⁷Cs (Bq kg^-1) in filtered water from the Irish Sea, April 1987 (from Hunt, 1988. Copyright © British Crown copyright, 1988.)

Considering releases of radioactivity to the environment, discharges from Sellafield are subject to authorization (by appropriate UK regulatory bodies), inspection, monitoring and assessment. The authorization is kept under review, subject to advice from national and international organizations, with due account of any discharge reductions achieved. Six main categories of effluent from reprocessing operations at Sellafield may be identified; pond purge liquours (from pond storage of spent fuel) and arisings from dissolution and solvent extraction processes which include medium active salt-free concentrate, medium active salt concentrate, highly active effluents, low active effluents and spent solvent (Horsley and Howden, 1990).

Another source of environmental activity arises from the disposal of materials originating from within controlled areas and the immediate working environment of personnel on site (which have not been in contact with process materials), comprising solid low-level waste. These wastes are disposed in shallow trenches at Drigg, 6 km from Sellafield.

With regard to the aquatic environment, activity concentrations in the Drigg stream, into which the solid low-level waste disposal trenches are presently drained, have not, on average, exceeded 5 per cent of the authorized limits (1.5 x 10⁵, 2.0 x 10⁶ and 1.0 x 10⁹ Bq m^-3 respectively for alpha, beta and ³H: Donn, 1990). Moreover, whilst most (over 99 per cent) of the radioactivity in waste generated from fuel reprocessing operations is, in fact, presently stored on site, there are routine discharges of low-activity liquid waste (containing a variety of radionuclides) into the north-east Irish Sea from Sellafield. This occurs via pipelines which terminate 2.1 km beyond the low water mark. The fuel element storage ponds and the reprocessing plant are responsible for the most significant liquid radioactive discharges. The peak beta release from Sellafield for any year was 9 x 10¹⁵ Bq in 1975, a peak in the alpha release occurring in 1973 with a discharge of 1.8 x 10¹⁴ Bq (mostly in the form of plutonium). The introduction of new plant (e.g. SIXEP and the salt evaporator) has caused discharges to the aquatic environment to decrease considerably in recent years. During 1988 discharges from the Sellafield pipelines involved the release of 8.1 x 10³ Bq total beta activity and 2.1 x 10¹² Bq total alpha emitting radionuclides (British Nuclear Fuels, 1989a). Further reductions in discharges will be achieved through EARP, and the stringent effluent standards of THORP will lead to only a limited contribution to site liquid discharges of isotopes which are radiologically significant.

Radionuclides discharged to the aquatic environment may be accumulated by aquatic plants and animals, which may subsequently form human foodstuffs, terrestrial livestock feeds or crop fertilizers. Important pathways of exposure arise from fish and shell-fish consumption. Previously, harvesting of Porphyra used in the manufacture of laverbread, was significant; however in recent years this has not been harvested in the immediate vicinity of Sellafield. Additional pathways to the terrestrial environment (and humans) result from grazing farm animals in intertidal zones, from the inhalation of water vapour and gases which arise from the water or from external gamma-ray exposure over sediments. Radiocaesium and transuranic radionuclides are considered the more important with regard to contributors to exposure from discharges to the aquatic enivronment (Hunt, 1988). The concentration of ¹³⁷Cs in filtered water from the Irish Sea in April 1987 is shown in Figure 2.1 . Monitoring programmes have revealed that the decline in discharges is associated with an approximately parallel decrease in radionuclide concentrations in foodstuffs.

Hence, it is apparent that radionuclide pathways may be varied and in considering dose to humans, as an indicator of the radiological impact of Sellafield releases, it is important to take into account the different contribution of radionuclide concentrations in various foodstuff tissues. The mechanisms by which doses are received by members of the public, as a result of discharges from the Sellafield plant, are continuously reviewed. Radiation exposure of the general public from Sellafield discharges as a consequence of fish consumption is primarily due to radiocaesium whereas a wide range of radionuclides contribute to radiation exposure of shell-fish consumers. Since the radiation dose to fish and shell-fish consumers varies according to the product of the mass of the foodstuff consumed and the concentration of radioactivity contained, a range of annual doses may be expected. Thus, a `critical group' approach is applied, based upon the identification of groups of individuals exposed to the highest radiation doses. The committed effective dose equivalent to the local sea-food consuming critical group was just under 4 x 10^-4 Sv in 1988, which is within the International Commission on Radiological Protection's (ICRP) recommended principal dose limit of 1 x 10^-3 Sv y^-1 for members of the public. In recent previous years exposures are considered to have remained within this principal limit, whilst over a period of a few years prior to this exposures exceeded 1 x 10^-3 Sv y^-1, although remaining within the ICRP subsidiary dose limit of 5 x 10^-3 Sv y^-1. Since dose-rates greater than 1 x 10^-3 Sv y^-1 did not occur over a sufficiently long period to exceed the 1 x 10^-3 Sv y^-1 average lifetime exposure, the dose limitation objectives of ICRP will be achieved. For typical fish-eating members of the public the average dose in 1988 was around 1 x 10^-5 Sv.

As noted above, radiation exposure of the public may also arise as a result of the uptake of gamma-emitting radionuclides by intertidal sediments. At present these gamma dose rates are primarily attributable to caesium. It is found that radioactivity is most readily absorbed by fine-grained muds and silts (such as are often found in estuaries and harbours) than by coarser-grained sands (e.g. on open beaches). Monitoring of the period of time spent in intertidal areas at various locations has been undertaken to identify members of the public subject to the highest external exposures. This group comprises boat-dwellers located along the intertidal area of the coastline boardering the north-east Irish sea, who received lower radiation doses than the critical group for sea food in 1988 (British Nuclear Fuels, 1989a).

Considering non-aquatic pathways, the release of treated ventilation air from the process plants and the Calder Hall reactors is primarily responsible for atmospheric discharges, leading to low concentrations of radioactivity in the air and in foodstuffs such as milk and vegetables as a result of deposition onto soil or vegetation. Radiation doses to the critical group due to discharges via these atmospheric pathways were less than 2 x 10^-4 Sv in 1988. The critical group which is most affected in the case of gaseous discharges has been indentified as children drinking milk from local farms (British Nuclear Fuels, 1989c). Further studies have revealed no excess airborne radioactivity (and accordingly a negligible dose impact) arising from the Drigg site. Direct radiation from the plants, rather than from discharged waste, is mainly due to gamma radiation. This direct radiation mostly affects a small group of people near Sellafield, who received an estimated dose of less than 2 x 10^-4 Sv in 1988 (British Nuclear Fuels, 1989a).

In conclusion, routine discharges from the entire nuclear industry actually account for less than 0.1 per cent of the radiation received by the UK population: The UK collective dose commitment (truncated at 500 years) due to Sellafield discharges represented less than 50 per cent of the total collective commitment for the nuclear fuel cycle in 1986 (Berry and Coulston, 1989).

2.3 CAP DE LA HAGUE

Cogéma's Cap de la Hague centre, located on the Normandy coast of France (see Figure 2.2), commenced reprocessing spent fuel from commercial graphite/gas reactors in 1966. The complex was completed 10 years later by a high-activity oxide head-end facility to provide for reprocessing light water reactor (LWR) fuel. Almost 3000 t of LWR fuel has been reprocessed at the plant's Usine Plutonium-2 (UP2)/400 facility to date (Rougeau and Lallement, 1990), with commissioning of the UP3 facility, which has a nominal capacity of 800 t year^-1, during 1990. Work is also being undertaken to increase the capacity of UP2 to 800 t year^-1. The capacity of Cap de la Hague is intended to reach 1600 t y^-1 by 1992. With regard to the transportation of spent fuel, since 1975 over 5000 t of LWR spent fuel has been shipped to this complex, from western Europe and Japan, without incident (Aycoberry and Rougeau, 1988).

Figure 2.2 Location of Cap de La Hague and the distribution of ¹³⁷Cs in the locality in 1983 (from Guegueniat et al., 1988; reproduced by permission of Elsevier Science Publishers, Ltd).

The amount of waste arising from reprocessing spent graphite/gas fuel is decreasing and being replaced by that due to LWR fuel reprocessing. Considering waste management techniques at Cap de la Hague, high-level radioactive wastes generated from reprocessing operations undergo a period of on-site storage, prior to incorporation in borosilicate glass (through vitrification techniques), further interim storage, with eventual deep geological disposal. Depending on the specific nature of lowand intermediate-level wastes various containment procedures may be employed utilizing, for example, bitumen, cement or polymer resins, or metal boxes or casks (Beniston et al., 1987; Rougeau and Lallement, 1990). Shallow land (near-surface) disposal (e.g. at Centre de la Manche) is then used for containerized short-lived wastes, whereas long-lived wastes will be disposed in deep geological formations. Another technique employed, for low-activity liquid and gaseous effluents, is environmental dispersal.

Considering routine discharges of low-level radioactive wastes from Cap de la Hague, one of the principal releases concerns liquid effluents discharged to the aquatic environment. The release of liquid effluents is subject to very strict controls, with specification of maximum permissible activities. Various effluent treatment processes may be utilized (based upon e.g. techniques such as co-precipitation and ion exchange) before discharge to the sea occurs. Statutory analyses are undertaken prior to the release of liquid wastes to assess the specific activity of radionuclides and the total activity of the release.

The absolute amount of activity discharged annually from Cap de La Hague is, in fact, much lower than from Sellafield. It should also be noted that the composition of nuclides released from the two plants differs. At Cap de la Hague, besides tritium, the principal radionuclides released include; ruthenium, caesium, strontium and antimony. Routine discharges into coastal waters made by Cap de la Hague, up to 1985, involved total releases of approximately: 1.019 x 10¹⁶ Bq, ³H; 4.905 x 10¹⁵ Bq, 106Ru; 9.4 x 10¹⁴ Bq, ¹³⁷Cs; 7.55 x 10¹⁴ Bq, ⁹⁰Sr; 3 x 10¹² Bq ²³⁸Pu + ^239/240PU and 1 x 10¹⁵ Bq, ¹²⁵Sb (Pentreath, 1988). The total discharge of caesium isotopes from Cap de la Hague is some 3 x 10¹³ Bq annually. The distribution of activity in the Channel due to ¹³⁷Cs is shown in Figure 2.2. Since the mid-1970s, however, the ⁹⁰Sr contribution to Cap de la Hague's efluents has exceeded that due to ¹³⁷Cs, and peaks in the discharges of ⁹⁰Sr are found to occur every four to five years. The activity ratio for ⁹⁰Sr/¹³⁷Cs has, in recent years, been greater than 1.0, and in 1983 the ratio was of the order of 12. A maximum in the release of ¹³⁷Cs from Cap de la Hague occurred in 1971 whereas a maximum release of ⁹⁰Sr occured in 1983. 

Discharges from the reprocessing operations of the Cap de la Hague plant give rise to contamination effects in the North Sea, contributing to concentrations of plutonium isotopes, americium and curium. The contribution of caesium isotopes contained in Cap de la Hague's liquid effluents is, however, small. Examination of transuranic elements (e.g. plutonium and americium), in 1987, revealed very low activity concentrations in the southern part of the North Sea, which is attributed to the fact that the dominant fractions are buried in sediments in the discharge area of the Cap de la Hague plant (Nies, 1990). The ¹³⁴C s activity is too small to be detected at a distance, although the ratio ¹³⁴Cs/¹³⁷Cs is significantly higher for Cap de la Hague than for Sellafield (Guegueniat et al., 1988). Considering the ¹³⁷Cs distribution in the North Sea, values in the range 2.712 Bq m^-2 were found in 1987 (Nies, 1990).

Regarding gaseous effluents, the dominant nuclide is ⁸⁵Kr, which gives rise to a dose of 3 x 10^-4 Sv at the skin (for 800 t of fuel reprocessed) within the maximal fallout area of Cap de la Hague's stack (Lafaille, 1980). In the terrestrial environment, it is considered that releases of gaseous effluents. may give rise to contamination at the site, as a result of the passage of radionuclides into the subsoil (Lafaille, 1980). The gaseous effluents released by the stack and the plant's dozen secondary outlets are, therefore, monitored continuously. No difficulties are experienced in the implementation of this standard monitoring procedure.

With regard to non-routine releases of radioactivity, on 6 January 1981 an incident involving the ignition of solid waste in an air-cooled, steel-lined, concrete radioactive debris storage silo occurred, leading to the escape of radioactive vapour from the silo. It was announced by Cogéma that the radioactivity did not exceed a fifth of the permissible dose, although the following day Cap de la Hague's Hygiene and Security Committee revealed that in parts of the site radiation reached maximum levels. In addition, a claim was made by the unions (which was undisputed by Cogéma) that at the plant's medical centre, situated some 200 m from the main road, levels reached a value 10 times higher than those permitted in the plant (Lloyd, 1981). It was, nevertheless, concluded that the incident did not constitute any serious risk to the employees or the local population.

A working group on Irradiated Fuel Management, which was chaired by Professor Castaing of the French Academy of Sciences, has undertaken a review of the exposure of Cap de la Hague's personnel to ionizing radiation, and the impact of radioactive waste discharged during normal operation, which does not reveal any serious reasons for concern (ATOM, 1983). Hence, by considering personnel exposure statistics, since no significant incidents have occurred, an indication of the safety performance of Cap de la Hague may be obtained. The maximum dose in 1975, of 5.09 x 10^-3 Sv, decreased to 1.56 x 10^-3 Sv in 1986, which may be compared with the maximum allowable dose of 5 x 10^-2 Sv (Aycoberry and Rougeau, 1988).

2.4 KYSHTYM

The possibility of the occurrence of an accident involving the release of radioactive material in the eastern Urals region of the Soviet Union during the late 1950s was first widely publicized by Zhores Medvedev in 1976. It was suspected that a release had occurred in the Kasli area of Chelyabinsk province. Examination of Soviet radioecological literature revealed a series of studies in which the location of large experimental areas remained unidentified, but which involved the investigation of soil and biota typical of the Kasli area (Medvedev, 1979; Auerbach, 1990). In response to concerns expressed with regard to the occurrence of such an accident, Soviet specialists issued reports to the International Atomic Energy Agency (IAEA) in 1989. Subsequently excerpts from a previously classified 1974 Soviet report were published, from January 1990, in Energiya, and additional papers about the accident were presented in 1990 at the Commission of the European Communities (CEC)/Intemational Union of Radioecologists (IUR) Luxembourg Seminar. However, Trabalka and Auerbach (1991) note that a self-consistent data set is not presently available to permit an authoritative analysis of the accident.

It has recently been confirmed that on 29 September 1957, at 16.20 local time, a chemical explosion took place at Kyshtym in a storage tank containing 250 m³ of high-level radioactive waste, generated as a result of plutonium production operations. The explosion has been attributed to the ignition of acetatenitrate concentrate following the failure of a crude cooling system. This is reported to have led to the release of 7.4 x 10¹⁷ Bq of activity to the atmosphere. Whilst most of this release was deposited on the ground close to the site of the explosion, a plume of finer particulates (containing about 10 per cent of the activity released) was carried to a height of 1 km. This radioactive cloud was transported to the north and north-east towards the towns of Kamenets-Ural'skij and Tyumen (see Figure 2.3). The largest contribution to the total activity of the release (accounting for over 60 per cent) was due to ¹⁴⁴Ce+¹⁴⁴ Pr. Additional contributions were from; ⁹⁵Zr + ⁹⁵Nb (over 20 per cent), ⁹⁰Sr + ⁹⁰Y (over 5 per cent), ¹⁰⁶Ru + ¹⁰⁶Rh (over 3 per cent) and ¹³⁷Cs (over 0.03 per cent). Detection of ⁸⁹Sr, ¹⁴⁷Pm, ¹⁵⁵Eu and plutonium in the release has also been reported (Nikipelov, 1989). The principal source of radiation dose to biological surfaces over the first year after the accident was ¹⁴⁴Ce+ ¹⁴⁴ Pr, whilst ⁹⁰Sr was the principal contributor to long-term exposure.

The resulting terrestrial distribution of radioactivity may be considered to comprise two phases associated with initial radioactive fallout and wind migration (Ternovskij et al., 1989). Deposition of virtually all radioactivity from the plume occurred within 11 hours along a > 300 km path from Kyshtym (see Figure 2.3) leading to the contamination of an area which extended over some 2 x 10⁴ km² (Trabalka and Auerbach, 1991). Effects of wind resuspension with regard to redistribution of contaminants were most apparent during the first few days after the accident, but did not lead to significant changes in the trace boundaries. By 1958 wind transfer caused the redistribution of less than 1 per cent of the original radioactive  fallout, the formation of the radioactive trace being essentially completed. The stability of the geographical distribution of contamination was demonstrated by further measurement and sampling programmes.

Figure 2.3 Location of Kyshtym area and radioactive plume (from Dickson, 1989).

The distribution of radioactive fallout in the environment was subsequently affected by biogeochemical processes. Radionuclides were rapidly redistributed through the soil profile due to vertical migration and leaching processes. In the spring following the accident most activity (9095 per cent) was concentrated in the turf, with 0.51.5 per cent in living plants and 510 per cent in the mineralized portion of the soil. Maximum radionuclide movement was observed in turfy podzol soils with minimum movement occurring in leached chernozem soils. A rise in the accumulation of ⁹⁰Sr in natural herbaceous plants 612 years after the contamination event was apparent, due to vertical migration from turf into the mineralized portion of the soil, with a subsequent decrease in accumulation. Root uptake became more important over this period, the contribution of ⁹⁰Sr from soil by this route increasing to 95 per cent in the last decade, from 10 to 20 per cent in the year after the accident. The ⁹⁰Sr contamination levels in the above-ground biomass of trees fell, over a period of 2025 years, to 0.02 and 7 per cent respectively for pine and birch, compared with 8090 per cent in 1957, as a consequence of biogeochemical migration processes.

As a result of the release, over the two year period following the accident effects of radiation damage were apparent to varying degrees, depending on the radiation dose, in both trees and herbaceous vegetation (Trabalka and Auerbach, 1991). All pine trees, within some 20 km² in which the dose to needles exceeded 3040 Gy, perished by autumn 1959, whilst doses in excess of around 200 Gy to herbaceous vegetation and meristem buds of birches (involving an area of some 5 km²) killed these plants and trees. Lasting effects arising from radiation exposure of wildlife have not been recorded, due to the potential for migration of animals from less affected or uncontaminated areas to replace those lost due to acute exposure, whilst the survival and fitness of exposed populations is not expected to be significantly affected as a consequence of sub-lethal effects. Regarding farm animals, loss of animals exhibiting acute radiation sickness symptoms is reported to have begun within 912 days from those sites nearest the accident. However, at greater distances from the accident no mortality was apparent in farm amimals over a six-month period, whilst upon removal from such contaminated areas no diffences in comparison with control animals were found.

With regard to the aquatic environment, half periods for the elimination of radionuclides from lake water varied from 1 to 24 days for ¹⁴⁴Ce, and from 780 to 1100 days for ⁹⁰Sr. Contamination densities in the more highly contaminated rivers ranged from 4 x 10³ to 28 x 10³ times higher than pre-accident levels immediately after the accident. However, radioactive decay, absorption by bottom deposits and natural migration led to reductions by a factor of 150 in river contamination by 1958, causing a decrease in the contamination level in lakes by a factor of 2030. Over a 25 year period the ⁹⁰Sr concentration in lake-water within the contaminated region decreased by a factor of 30. Similar reductions of 2030 per cent were apparent in the upper layer of bottom deposits (00.05 m), with reduction by a factor of 35 in fishes (Ternovskij et al., 1989).

During the autumn/winter seasons following the accident fish received doses of 40 Gy in the most heavily contaminated lakes. Nevertheless, although diminished reproduction was apparent in the vulnerable herbivorous fish species (e.g. carp and goldfish) a few years after doses to eggs exceeded lethal levels (in excess of 10 Gy), by 1960 no ecological effects were observable. No effects on plankton, invertebrates or aquatic plants have been detected.

Immediate actions to mitigate the consequences of the accident involved the evacuation of 1154 people from four villages, three of which were located in the most affected zone (> 37 T Bq km^{2 90}Sr), within 710 days. An additional 9580 inhabitants located in areas with progressively lower levels of activity were moved out over a period of 250670 days. Thus, in total almost 11 000 people were evacuated from a 700 km² area in which contamination exceeded 74148 GBq km^{-2 90}Sr. Additional measures taken to mitigate the circumstances of the accident included: decontamination of portions of agricultural land, monitoring of agricultural produce (with the rejection of produce exceeding accepted norms), controlled agricultural production through specialized state farms with introduction of restrictions on the use of contaminated areas, reorganization of agriculture and forestry. Besides ploughing 200 km² of land at the head end of the radioactive cloud using standard procedures during 19581959, deep ploughing (to a depth exceeding 0.5 m) was carried out on 62 km² of land during 19601961 (Nikipelov, 1989; Medvedev, 1990). Whilst quantities of grain, potatoes, meat, milk, vegetables and other produce were rejected as unfit for consumption, effective control over all produce was not ensured due to administration problems and delays in the introduction of such measures. The implementation of a hydrological diversion and isolation system for various purposes which may include remediation of contamination from the 1957 accident has been noted (Trabalka and Auerbach, 1991). An area of 170 km² which was most highly contaminated following the accident has, nevertheless, remained unsuitable for human habitation, agriculture or forestry and is designated for research purposes as a radioecological reserve.

Although average external exposure doses preceding evacuation are reported to have reached 0.17 Sv, with effective dose equivalents of 0.52 Sv (1.5 Sv to the gastro-intestinal tract) it should be noted that significant uncertainties are associated with these figures (Trabalka and Auerbach, 1991). It is suggested by Nikipelov (1989) and Buldakov et al. (1989) that these doses may be doubled to account for the non-uniformity of contamination density and exposure conditions. Inhabitants of the area exhibiting measurable contamination located outside the most contaminated zone were estimated to have received effective radiation dose equivalents of about 110 per cent in excess of that due to natural background radiation over the 30 year period following the accident. No significant radiation effects have been revealed by studies, which have been undertaken over the past 30 years, of evacuated and resident exposed population's.

2.5 WINDSCALE ACCIDENT

The first substantial publicized release of radioactive material from a nuclear reactor accident occurred in October 1957 at the UK Atomic Energy Authority's Windscale Works, Sellafield, Cumbria, the location of which is shown in Figure 2.4. The accident to the Windscale No. l reactor (an air-blast cooled, graphite moderated, reactor designed for the production of military plutonium) was caused by overheating during a procedure to release Wigner energy (Nuclear Engineering, 1957; Command 302, 1958). This led to can failure, resulting from an exceptional temperature rise, uranium oxidation and subsequently a graphite fire which affected some 150 fuel channels. In order to contain the fire the fuel channels adjacent to those which were red-hot were discharged, thereby creating a fire break. An attempt to extinguish the fire using CO₂ proved ineffective; thus on 11 October water was injected into the core, and was applied for 24 hours until the core was cold. Recent assessments have revealed that at the time of the accident the reactor was also being used as an irradiation facility to produce ²¹⁰Po from bismuth.

The Windscale accident resulted in the release of radioactivity, mainly isotopes of the noble gases and volatile elements, into the atmosphere from the 125 m reactor stack. The release took place over the period from approximately noon on Thursday 10 October, 1957, to noon on Friday 11 October (Command 302, 1958). Crick and Linsley (1982, 1983) estimated that the principal nuclides released during the accident were; ¹³⁷Cs (2.2 x 10¹³ Bq), ¹³¹I (7.4 x 10¹⁴ Bq), ²¹⁰Po (8.8 x 10¹²Bq), ²³⁹Pu (1.6 x 10⁹ Bq), ¹⁰⁶Ru (3 x 10¹² Bq), ⁹⁰Sr (7 x 10¹⁰ Bq), ¹³²Te (4.4 x 10¹⁴ Bq) and ¹³³Xe (1.2 x 10¹⁶ Bq). Dunster et al. (1958) estimated that about 3 x 10¹² Bq of ⁸⁹Sr was also released. Chamberlain and Dunster (1958) concluded, from radioautographic examination of filter paper and other fallout-exposed surfaces, that most of the radioiodine was in gaseous form or absorbed on very small particles. There was found to be little dissemination of particulate uranium oxide, the amount of uranium disseminated being considered negligible (Chamberlain, 1981; Chamberlain and Dunster, 1958).

Figure 2.4 Time integral of air concentration of ¹³¹I Bq days m^-3 (from Stewart and Crooks, 1958; reproduced by permission of UKAEA. Copyright © 1958 Macmillan Magazines Ltd). 

The meteorological conditions over the period of the accident were examined by Crabtree (1959) and Stewart and Crooks (1958) who found that this information provides a qualitative explanation of the deposition and air concentration data. Analysis of airborne dust samples collected on routine industrial monitoring filters located in the UK and Europe enabled the passage of cloud, the direction of which was ultimately south-easterly, to be traced. The cloud was detected in Mol, Belgium, at 19.00 GMT on 11 October and at Frankfurt, Germany, and Sola, Norway, on 12 and 15 October respectively. The highest activity in the portion of the cloud which reached Belgium, Holland and Germany was found to be only 6 per cent of the London value (Crabtree, 1959). Results from long-range measurements of the time integral of ¹³¹I air concentration are provided in Figure 2.4. This demonstrates that the radioactive plume avoided north Wales but diffused to cover a wide front in the southern regions of the UK.

Considering deposition, radioactive dust was deposited on grass mainly by turbulent diffusion and impaction because little rain fell while the cloud passed over England (Stewart and Crooks, 1958). The mean value of the deposition velocity for ¹³¹I in northern and southern England were 0.30 cm s^-1 and 0.11 cm sec^-1 respectively. The deposition velocities of ¹³⁷Cs and ¹⁰³Ru were less than 15 per cent of that of ¹³¹I. Marked deposition of ¹³¹I on high ground was observed (Chamberlain and Dunster, 1958; Dunster et al., 1958), the pattern of strontium on grass in the Windscale area being found to be virtually the same as that for ¹³¹I on grass. The disappearance of ¹³¹I and ¹³⁷Cs from grassland after the Windscale accident was examined by Booker (1958) revealing field loss half-lives, T_b, of 13 and 10 days respectively for the two nuclides. It was noted that the field loss of ¹³⁷Cs and other nuclides from grass after the accident appeared to slow down after mid-November 1957 (Booker, 1958).

Whilst the fallout after the accident of 1957 comprised mainly dry deposition of sub-micrometre particles, some larger particles were found near the reactor (Chamberlain and Dunster, 1958). Ground surveys of the deposition of particulate matter in the vicinity of the Windscale works revealed that deposition extended for some 4 km south-south-east from the site. Particles in the size range 20500 µm were found with total beta activities for individual particles of 37(4.81 x 10³) Bq. Such particles were not considered to contribute appreciably to public health problems.

With regard to contamination of aquatic systems, analysis of drinking-water samples collected from reservoirs and streams revealed that concentrations of radioiodine and other radionuclides were not found to exceed the maximum permissible concentration permitted by International Commission on Radiological Protection (ICRP).

The determination of levels of strontium isotopes in samples of produce such as potatoes, cabbage, kale, turnips and lettuce taken from within and outside the restricted area around Windscale is reported by Dunster et al. (1958). Results from farms at 3.2 and 12.8 km south-east of the Windscale site revealed concentrations in turnips of 5.55 and 1.48 Bq ⁹⁰Sr g^-1 Ca, and in potatoes of 4.81 and 0.93 Bq ⁹⁰Sr g^-1 Ca, respectively. Lower concentrations were found in other samples such as cabbage and kale. None of the levels measured represented any danger.

Considering transfer of radionuclides to animals, the level of ¹³¹I in the thyroids of unselected sheep were examined by Van Middlesworth (1958), who observed that 10 days after the Windscale accident the average level was 5.18 x 10²5.55 x 10² Bq g^-1 for London sheep thyroids, compared with concentrations averaging 37 Bq g^-1 during August/September 1957. Comparing results from sheep slaughtered in London with those of sheep slaughtered in Tennessee reveals that the former received less ¹³¹I from Windscale than the latter received from distant nuclear weapons tests.

It was apparent, from radiation surveys and air sampling activities, that by the evening of Friday 11 October, no external radiation hazards or significant inhalation problems existed. However a possible marginal level of milk contamination was recognized on the basis of gamma radiation survey data. Measurements to analyse the passage of radioiodine and radiostrontium activity into milk from contaminated pasture land indicated that contamination was due essentially to ¹³¹I the release having involved a preferential radioiodine content rather than a normal distribution of mixed fission products. The highest measured ¹³¹I level during the survey was 5.18 x 10⁴ Bq l^-1 in a sample collected on 13 October 1957 from a farm 16 km from Windscale, lying in the direct path of the plume. Further studies of milk were undertaken elsewhere, for example in Cardiganshire revealing a value on 13 October less than 1 per cent of the highest level found in the Windscale area. Consequently, restrictions on the consumption of milk in regions where the radioiodine activity exceeded levels of 3.7 x 10³ Bq l^-1 were introduced, involving over an area of about 500 km² by 14 October. Derestriction of all regions in this area had occurred by 23 November. A survey of ¹³⁷Cs in milk during late October 1957, when gamma spectrometry analysis was feasible due to the reduction in ¹³¹I activity, revealed a similar pattern to that of ¹³¹I However, the concentration Of ¹³⁷Cs in the area of maximum concentration corresponded to a level of 3.7 x 10²  Bq l^-1.

A retrospective assesment of the wider implications of the Windscale accident was recently undertaken by the National Radiological Protection Board (Crick and Linsley, 1982, 1983). The collective thyroid dose and collective effective dose equivalent were calculated to be 2.6 x 10⁴ and 2 x 10³ person-Sv respectively in Europe. 

2.6 THREE MILE ISLAND

The first nuclear power plant accident occurred in 1979 at the US Three Mile Island Unit 2 (TMI-2), a 900 MWe pressurized water reactor, located in Pennsylvania. The events leading to this loss-of-coolant type accident are attributed to the combined effects of equipment malfunctions and operator error. The sequence commenced when feedwater pumps supplying the steam generators tripped or shut down at 4.00 a.m. on 28 March 1979 (Kemeny, 1979) with consequent automatic shutdown of the steam turbine and generator, and reactor. Increases in the temperature and pressure of the reactor coolant resulted, due to the heat from decay fission products in the core, causing a pressure relief valve to open (as designed); however this subsequently failed to close automatically. Provision of cooling water via three emergency feedwater pumps did automatically occur; however two valves inadvertently remained closed, thereby preventing water from reaching the reactor, with loss-of-coolant occurring due to the pressure relief valve which had remained open. In order to cool the system high-pressure injection pumps then automatically began to operate but these were deactivated by operators, who were unaware of the continued water release occurring through the open relief valve. A temperature in excess of 2273 K was reached as water turned to steam, causing cladding failure and melting of some 50 per cent of the fuel, thereby releasing fission products to the reactor vessel and coolant system (Gerusky, 1988). More than two hours after the accident began it was realized that the pressure relief valve (through which coolant drained to a tank in the reactor building with subsequent pumping, upon overflow to a sump, to an auxiliary building) had remained open, and a manually operated backup valve was closed, thus preventing further coolant loss. Eventual cooling of the core was achieved subsequently, via the operation of the highpressure injection pumps, bringing the accident under control.

Very little radioactivity was actually released to the environment, despite the transport of a large amount of radioactive material from the core into the containment, since reactor vessel and containment structure remained intact. The amount of radioactivity released into the environment was of the order of 10¹⁷ Bq, and consisted mainly of the noble gases ¹³³Xe, ^133mXe and ¹³⁵Xe. The detection of a relatively small amount of radioiodine, well below levels apparent during preceding nuclear weapons fallout episodes, was also reported. It has been calculated that less than 1.11 TBq of ¹³¹I was released in the course of the accident.

Environmental monitoring undertaken in the locality of TMI following the accident included analysis of samples of milk, air, water, produce, soil, vegetation, fish, and river silt and sediment. These studies revealed that little or no radioactivity was present in the environment apart from the noble gases, and it has been concluded that the accident did not contribute significantly to increased levels of radioactivity in local foodstuffs. Increased environmental radionuclide concentrations were observed only due to ¹³¹I in cow's milk, goat's milk, on-site non-drinking water, on- and off-site air and ¹³⁷Cs in fish following the accident (Gerusky, 1988).

The highest doses received as a consequence of the accident were considered to be to people living within a 3.2 km radius of the plant. An estimated dose of some 0.5 Sv was received by one person located on the islands in the vicinity during the first few days of the accident, with 260 people, mostly located on the east bank of the local river, receiving 0.20.7 Sv. The estimated collective dose to the population within an 80 km radius of the TMI plant was 20 person-Sv (President's Commission on the Accident at Three Mile Island, 1979). The exposure of nearby residents due to subsequent controlled venting of ⁸⁵Kr present in the reactor building to the atmosphere was considered to be trivial.

2.7 CHERNOBYL

The most serious accident to have occurred in the history of nuclear reactor operation, was in April 1986 at the Unit 4 reactor of the Chernobyl nuclear power plant (NPP) located in the Ukraine, USSR (see Figure 2.5). This reactor was one of the 15 Soviet RBMK (Reactor Bolshoi Moschnosti KipyashiyLarge-power boiling reactor) type, featuring a graphite-moderated light-water-cooled system in which vertical zirconium alloy pressure tubes contain the fuel pins. Cooling water flows, under high pressure, upwards through these tubes and boils, producing steam which is used to drive turbines.

The accident at Chernobyl occurred as a result of a power excursion during an experiment to test the ability of turbine generators to supply energy for a limited period, in the event of an external power failure, until standby diesel generators could be brought into operation. During procedures intended to reduce the reactor power to the required 7001000 MW _th (thermal) level, there were strong violations of operating procedures (including the over-riding of all in-built safety systems) and the reactor was brought to low power. Operator mis-handling caused the reactor power to reach 30 MW _th^; consequently the control rods were withdrawn from the reactor (beyond safe limits) to increase the power (to 200 MW _th). Inherent RBMK design weaknesses result, in such low-power regimes, in a positive void coefficient, i.e. increased neutron flux and power resulting from the conversion of water to steam voids in the coolant circuits, producing more heat and steam voids. The fact that additional circulation pumps were started, in order to permit the experiment to be repeated if required, caused flow limits to be exceeded, leading to flow cavitation and noticeable vibration. Hence an uncontrollable growth in power, resulting from rapid void formation in the coolant flow in a large part of the core combined with the positive void coefficient, began on 25 April 1986 at 21.23 GMT (26 April, 0 1.23 h local time), the reactor power reaching 100 times normal full power within four seconds. This power excursion led to fuel rupture, succeeding explosions causing the movement of the 1 x 10³ t reactor cover plate, which cut all reactor cooling channels, and exposure of the core to the environment. Ejection of material containing spent fuel, enriched with noble gases and volatile nuclides of iodine, tellurium and caesium, into the atmosphere occurred during the initial explosions, with a continuing release due to the subsequent fire in the graphite moderator. Efforts to suppress radionuclide releases and to prevent further fission, involved covering the core with more than 5 x 10³ t of boron, dolomite, sand, clay and lead. This caused the emissions to be decreased substantially for a few days; however the blanketing of the core led to increased temperatures and a further release peak from 1 to 5 May. Emissions were terminated on 6 May upon pumping a nitrogen coolant through tunnels constructed under the core.

Figure 2.5 Calculated (solid lines) and actual (dashed lines) distribution of radiation levels near Chernobyl on 29 May, 1986 (in µ Sv h^-1). (From Israel et al., 1987.)

At the end of the 10-day release period following the accident some 2 x 10¹⁸ Bq of activity, of condensible radioactive fission and transuranium activation products, was present in the environment, representing some 34 per cent of the core inventory. The composition of fission products in the release resembled that in the fuel, excepting the preferential release of the more volatile nuclides. Besides a total release of noble gases, some 20 per cent of iodine available in the core was released (6.7 x 10¹⁷ Bq), with releases of around 10 per cent of the inventory of caesium (1.9 x 10¹⁶ Bq ¹³⁴Cs, 3.7 x 10¹⁶ Bq ¹³⁷Cs) and about 3 per cent of rare earths and actinides.

Of the fraction of the core ejected to the atmosphere, 0.30.5 per cent was deposited on the site, 1.52 per cent was deposited within 20 km, the remainder being dispersed beyond 20 km. The deposition pattern close to the site was dominated by gravitational settling of coarser material released over the first five days. With regard to dispersal beyond the Chernobyl region, during the first day the radioactivity became segmented, with sections heading toward Scandinavia and central Europe, and easterly across Asia to Japan, the North Pacific and the North American continent. The plume reached Scandanavia on 27 April, where the highest levels of deposition outside the USSR were found. Radioactivity became incorporated in a developing frontal precipitation system the following day, causing localized deposition peaks (greater than 1 x 10⁵ Bq m^-2) in parts of central Scandinavia. The release spread south-westward across Europe from Poland, and a section then turned northward to the United Kingdom, other material travelling eastward across the Soviet Union and southward to Turkey and Greece, much material eventually moving across the North Sea and Scandinavia thereby leaving Europe (ApSimon et al., 1988, 1989). By mid-May levels of the order of 1 x 10^-3 Bq m^-3 were observed in Hong Kong and North America. Transport of the release to the southern hemisphere was not apparent. A component of Chernobyl fallout within the Soviet Union and various European countries (e.g. Sweden, Poland, Greece) comprised hot particles. Two basic categories of hot particles have been identified: those comprising mixed fission products (originating from the fuel) and those associated with a few elements only (such as Ru, Ce and Sr isotopes). An important feature of the observed contamination pattern was the patchy nature of the deposition of radioactive material, which was often concentrated in a few small areas within countries. This was found to be highly correlated with local variations in amounts of rainfall during the passage of the plume (ApSimon et al., 1988).

As a consequence of the accident a 30 km radius zone around the power plant was evacuated during the first few days following the release. An area of more than 2 x 10⁴ km² outside this zone has been designated as contaminated. Radiation levels in the Chernobyl region shortly after the accident are shown in Figure 2.5. Agricultural land comprised about a quarter of the contaminated territory, consequently restrictions have been placed on food production (a total ban on produce where ¹³⁷Cs contamination levels exceed 1.5 x 10⁴ Bq km^-2) and agricultural techniques have been altered on land contaminated to intermediate levels. Various measures to reduce contamination at the reactor site were also implemented, including activities such as collection and disposal of loose fuel fragments, removal of topsoil, application of special solutions to effect decontamination and the containment of the damaged Unit 4 within a concrete `sarcophagus'. Extensive monitoring of radioactivity, especially within and near the 30 km zone, has been undertaken by Soviet scientists: caesium isotopes generally dominate persistent radiation levels, with contributions from Sr, Pu, Zr, Nb, Ru and La isotopes.

Considering ecological damage within the Soviet Union, it has been reported that deciduous trees were considerably less affected than pines (the most susceptible species to radiation damage and mutation) although, in the first year, over-sized leaves were observed in species such as oak. There is no evidence that the effects observed in Pinus or any other species involve the genome. With regard to small mammals and birds, the recovery of these has been recorded (Clough, 1990; Sokolov et al., 1990).

Contamination of river water and the Kiev reservoir in the vicinity of the accident was due initially to jets of volatile and aerosol products, with subsequent possible contamination as a result of precipitation washing radionuclides from the surface. The total amount of radioactivity in the water was calculated to be 7.4 x 10¹⁵ Bq, with less than 0.05 per cent attributable to ⁹⁰Sr (Israel and Petrov, 1988). A system of protective and filtering dams was established on waterways feeding the Kiev reservoir during the winter of 1986/1987 in order to preclude the possibility of increased contamination from spring flood influxes. Contamination levels were not found to increase above those of the previous autumn, the level of ¹³⁷Cs falling by a factor of 20, to 1 Bq Bq l^-1, in the period July 1986May 1987. It has been concluded that during the period following the accident contamination levels did not exceed prescribed limits acceptable for drinking.

Considering collective effective dose commitments, the collective whole-body dose to the 1.35 x 10⁵ Soviet evacuees is estimated to be 1.6 x 10⁴ person-Sv. In the European Soviet Union the collective effective dose equivalent is in the range 2 x 10⁵2 x 10⁶ person-Sv, whilst in the European Community the value is 8 x 10⁴ person-Sv (Clarke, 1989).

2.8 THE HANFORD SITE, WASHINGTON STATE, USA

The Hanford Site is located in a rural region of south-eastern Washington State in the USA. The Site, which is operated by the US Department of Energy (DOE), was acquired by the US government in 1943 for the construction and operation of facilities to produce plutonium for the atomic weapons programme. From 1945 through 1960, Hanford's primary mission was the production of plutonium for national defence and management of the wastes generated by associated chemical processing operations. The current mission no longer includes defence, but rather waste management, environmental restoration, advance reactor development, and research and development.

The Hanford Site is underlain by a thick unconfined aquifer contained in alluvial and glaciofluvial sediments and a series of confined aquifer systems isolated by a sequence of basalt flows. The following discussions pertain to the unconfined aquifer because essentially all contamination resulting from liquid waste disposal occurs in this aquifer. The depth to groundwater ranges from less than 1 m near the Columbia River to approximately 105 m in the centre of the site about 15 km from the river. The thickness of the aquifer ranges from less than 1 m where the basalt rises above the water table to more than 200 m.

Recharge to the unconfined aquifer is both natural and artificial. The vast majority of recharge occurs from operational liquid-waste disposal, which has created groundwater mounds near the 200 Area location (Graham et al., 1981). These mounds have altered the aquifer's local groundwater flow pattern, shown on Figure 2.6, which is generally from the recharge areas in the west to the discharge area, primarily the Columbia River, which is hydraulically connected to the unconfined aquifer. A representation of the water table at the Hanford Site is constructed and reported on at least annually.

Operations on the Hanford Site have resulted in large volumes of waste water being discharged to the ground (and soil column) through cribs, ditches and ponds. These discharges have contaminated the groundwater and have greatly influenced groundwater flow and contaminant movement in the unconfined aquifer beneath the Site. Discharge of waste water to the ground at Hanford began in the mid-1940s and reached a peak in 1955. After 1955, discharge to the cribs declined because of improved treatment of waste streams and deactivation of various facilities.

Approximately 23.7 billion litres of liquid effluent were disposed to the ground in the 200 Area during 1988. This value is indicative of the magnitude of previous years' discharges to ground. Approximately 0.8 billion litres of liquid effluent were disposed to ground in 1989. Radionuclides inventoried in the waste streams in recent years include, ³H, ⁹⁰Sr, ¹⁰³Ru, ¹⁰⁶Ru, ¹¹³Sn, ¹²⁹I,¹³⁷Cs,¹⁴⁷Pm, U (total), ²³⁸Pu, ^239/240Pu, ²⁴¹Am, gross alpha, and gross beta.

Figure 2.6 Water table map with location of wells. (From Jaquish & Bryce, 1990.) 

2.9 SOVIET SPACE REACTOR RE-ENTRY

The re-entry of the nuclear reactor powered Cosmos 954, a Soviet satellite, occurred on 24 January 1978 causing radioactive debris to be spread over a 1000 km path from Great Slave Lake to Barker Lake upon its disintegration over Canada (Tracy et al., 1984). At this time the reactor core was estimated to contain some 3.11 TBq of ⁹⁰Sr, 181 TBq of ¹³¹I and 3.18 TBq of ¹³⁷Cs. The retention of 75 per cent of the original material in the upper atmosphere has been reported. Samples of air, water and foodstuffs did not reveal any detectable contamination.

2.10 SNAP-9A RE-ENTRY

Satellite nuclear auxiliary power (or `SNAP') units utilize the decay heat of radionuclides to supply electrical energy for space satellite equipment. The re-entry of a navigational satellite containing such an isotopic power unit (known as SNAP 9A) occurred on 21 April 1964, over the Indian Ocean, after failure to attain orbital velocity. The heat of re-entry caused volatilization of the SNAP 9A device which contained 629 TBq of ²³⁸Pu. The deposition of around 95 per cent of the plutonium intially injected was estimated to have occurred by the end of 1970. The device's re-entry resulted in a 50 year dose commitment of approximately 0.36 Sv to the respiratory lymph nodes of the world's population due to exposure between 1961 and 1968 (Shleien et al., 1970).