Windscale fallout from

Many lessons were learnt during the period 1955-1965 by analysis of dispersion and fallout from bomb tests and also from the Windscale accident, but some of these had been forgotten by 1986 when the Chernobyl accident happened, so no apology is made for describing some work which is now 30 years old. 

Plutonium does not exist as a naturally occurring element with the exception that small amount of Pu generated in the natural reactors (the Oklo natural reactor in Gabon). The primary input of Pu to environment is fallout from nuclear weapon tests and accident release (e.g., the Windscale accident in 1957 and the Chernobyl accident in 1986). Three of the most common of the 15 known Pu isotopes are Pu (94%), °Pu (6%), and Pu (0.4%). Pu has a half-life of 24,000 yr. Its (X-dccay results in production of (Tj/2 = 700 million yr). Pu has a half-life of 6560 yr, emits a particles and decays to U (T1/2 = 20 million yr). Due to (he long half... 

The different pattern of fallout is mainly related to the particle size of the aerosol in the circumstances. The cumulative fallout of 137Cs in Cumbria from distant weapon tests reached a peak of 4 to 7 kBq m-2 (depending on annual rainfall) in 1964. Thus a few farms within about 1 km of the Windscale site received two to four times as much fallout of 137Cs and 90Sr from the oxide particles as they did subsequently from weapons tests. At the nearest large settlement, Seascale, which is 3 km from the Windscale site, the fallout in oxide particles and in bomb debris were of similar magnitude. 

The deposition of 137Cs in the direction of maximum fallout SSE of Windscale is shown in Fig. 2.5. There was a maximum at 5 km downwind, where the plume from the 130-m stack reached the ground, and a second maximum at 20 km, where there is high ground. There was little rain during the passage of the plume. The ratio 1311/137Cs in deposited activity was 50 1 (Booker, 1962). 

In the UK, rainfall of 10-20 mm on 2-4 May gave heavy fallout in the coastal region of Cumbria and Galloway (Clark Smith, 1988). By a coincidence, there was particularly heavy deposition in the hills 20 km SSE of Sellafield, which were also in the path of the emission of October 1957. In this area, there was about 12 kBq m-2 of 137Cs from the Windscale accident (Fig. 2.5), 6 kBq m-2 from distant bomb tests, 1955-65, and 20 kBq m-2 from Chernobyl. Clark Smith estimated the total fallout of 137Cs over the UK at 300 TBq. This compares with about 10 TBq from the Windscale accident and 1000 TBq from weapons tests. 

Table 2.10 shows a comparison of the fallout at Seascale, Cumbria, from the Windscale accident and the fallout in the Munich area from Chernobyl, together with estimates of the effective dose equivalent received by people in those areas at the relevant time (Crick Linsley, 1982,1983 Doerfel Piesch, 1987). The doses at Seascale are extrapolated to 50 a from the accident to allow for long-term contributions, mainly from 137Cs. The doses at Munich are for the first year only, and should be increased by about 50% to give the life-time dose (Clarke, 1987). 

Seascale, Cumbria and Leeds after the Windscale accident (Booker, 1958 Burch, 1959) and near Seascale and Harwell after Chernobyl (Fulker, 1987 Cambray et al., 1987). The values are lower than the theoretical curve, especially those derived from the Chernobyl fallout, which mostly occurred in heavy rain. As another example, in the Karlsruhe district of Germany, the fallout of 131I in the first few days of May 1986 was 10 kBq m-2. The peak activity in the milk of cows feeding outdoors was 47 Bq l-1 (Doerfel Piesch, 1987), giving a normalised Ci max) of only 5 x 10-3 m21-1. 

If vg is taken as 3 mm s-1, then a dosage of 1 Bqs m-3 of 131I in air gives 3 x 10-3 Bq m-2 fallout. The ratio of thyroid doses from milk compared with inhalation are then as shown in Table 3.9. The calculated ratio of 20 for adults compares with an increase of about 10 in the thyroid activity of Leeds residents attributed to consumption of milk in the days after the passage of the plume of activity from the Windscale accident (Fig. 3.7). The transfer factor fallout/milk in the Leeds area was lower than that assumed in the calculations leading to Table 3.8... 

In Cumbria, after the Windscale accident, a ban on the sale of local milk was made effective within 3 d. If vg for1311 in the area of maximum fallout downwind of Windscale was 3 mm s-1, then the measured deposition of 1 x 106 Bq m-2 corresponds to an air dosage of 3 x 10s Bqs m-3. From Table 3.7, this would give a theoretical thyroid dose to a child of 40 mSv which corresponds to the lower end of the range given in the first row of Table 3.10. If there had been no ban, and if the transfer from fallout to milk had followed the theoretical curve A in Fig. 3.6, then from Table 3.8 a child drinking 0.92 1 of milk per day would have received a thyroid dose of 1.4 Sv. 

Knapp also sought to deduce the maximum concentration C/(max) of 131I in milk resulting from the fallout, using records of tests in the 1960s where gamma dose and C/ were both measured, Garner s (1960) data on transfer from cattle feed to milk, and Booker s (1958) measurements after the Windscale accident. 

On 19 May 1953, a bomb of 32 kT yield (code name Harry) was exploded at the Nevada Test Site. A gamma dose rate equivalent to 27 milliroentgen per hour (230//Gy h 1) at 1 d was measured at St George, Utah, 200 km from the test site. Assuming no chemical fractionation of fission products, the fallout of 131I at St George was 1.0 MBq m 2. This is almost identical to the fallout of 1311 in the zone of maximum deposition about 10 km from Windscale (Chamberlain, 1959). Relative... 

Public interest in radioactive aerosols began in the mid-1950s, when world-wide fallout of fission products from bomb tests was first observed. The H-bomb test at Bikini Atoll in 1954 had tragic consequences for the Japanese fisherman, and the inhabitants of the Ronge-lap Atoll, who were in the path of the fallout. In 1957, radio-iodine and other fission products, released in the accident to the Windscale reactor, were tracked over much of Europe, and these events were repeated on a much larger scale after the Chernobyl accident. 

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