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Tokai JCO Criticality Accident

Report [15]11/5/99 18:10

Calculation On The Scale Of The JCO Criticality Accident

On October 20th , 50cc of the uranium solution left in the precipitation basin was taken as a sample and was handed over to the Japan Atomic Energy Research Institute to be analyzed. Some of the results were released during the fourth JCO Criticality Accident Investigation Committee held on October 29th. Below is a table based on that information. ( indicates that it was below the amount detectable.)

Japan Atomic Energy Research Institute (as of Oct. 20)

Fission Products

Group A (Bq/ml)

Group B (Bq/ml)

zirconium 95

2.2E+05

2.6E+05

molybdenum 99

4.3E+04

4.6E+04

ruthenium 103

1.8E+05

2.0E+05

cerium 143

-

-

cerium 144

6.9E+04

-

strontium 89

-

-

iodine 131

1.9E+05

2.2E+05

cesium 137

1.5E+03

-

barium 140

5.3E+05

6.0E+05

Uranium Concentration

278.9 (g/l)

279.3 (g/l)

The radioactive concentration of the solution was analyzed by two groups for the purpose of cross-checking. Based on the above data, CNIC attempted a trial calculation on the scale of uranium fission at the time of the accident.

By assuming that 16kg of uranium was put into the precipitation basin, we calculated the total amount of each nuclide in the precipitation basin at 4:30 a.m. on October first using the average of the results of group A and B. Thus we only made calculations on ones that have data from both group A and B.

Using the results from the above calculation, we then calculated the number of fissions for each nuclide based on the fission yield data of each nuclide.

The Average of the Data Obtained From the Analysis of the Sample Solution From the Precipitation Basin

Nuclear Material

Half Life

(A+B)/2 (Bq/ml)

In the precipitation basin

Adjusted for attenuation at 4:30a.m. on Oct. 1

Number of fission

zirconium 95

64.0d

2.4E+05

1.4E+10

1.7E+10

2.2E+18

molybdenum 99

66.02h

4.5E+04

2.6E+9

3.5E+11

2.0E+18

ruthenium 103

39.4d

1.9E+05

1.1E+10

1.5E+10

2.5E+18

iodine 131

8.04d

2.0E+05

1.2E+10

6.2E+10

2.0E+18

barium 140

12.79d

5.6E+05

3.2E+10

9.3E+10

2.2E+18

It can be assumed from the above analytical data that the number of fissions for the entire accident was 2.5 (+- 2) E+18. This figure corresponds to the fission of 1mg of uranium 235.

This figure is very close to the lower figure of the amount earlier calculated by CNIC. However, this result was obtained from a sample that was taken from the top of the solution left in the precipitation basin because an equipment installed to stir the solution failed to operate, and thus the content of the solution was uneven. Since fission products would have settled at the bottom of the precipitation basin, in reality the amount of uranium 235 that under went fission may be some what higher.

On November 4th, the Science and Technology Agency released the analytic results of the scale of the accident and the effects to the area residents.

Based on the results from analyzing the sample solution taken form the precipitation basin, the STA assumes that the entire nuclear fission number resulting from the criticality reaction is 2.5x10E+18, and the weight of uranium 235 that underwent fission was about 1mg. This figure is extremely close to the above calculation conducted by CNIC. Even by simply comparing the nuclear fission number from accidents of the past, the scale of this accident at Tokai-mura JCO plant lines in next to the accident (4E+19) at the reprocessing plant in Idaho state of the United States in 1959.

Furthermore, the STA admitted that the area residents and the environment was seriously affected by the neutrons released by the accident. In addition it was revealed that during the 18 hours it took to contain the criticality, a person could have been exposed to 160mSv of radiation should he/she was at the boundary of the site which is about 80 meters away from the conversion building where the accident took place. This is indeed 160 times the amount of annual dose limit for the general public. This result can be interpreted that a person at a place 350 meters away from the conversion building would have been exposed to twice the amount of the annual dose limit of 1mSv per year. This result makes it clear that the health of the area residents had to have been affected.

CNIC has been asserting from the following three points that this accident should be rated level 5 based on the guide lines of the international nuclear event scale:

  1. The scale of nuclear fission
  2. The release of rare gas, iodine, and other gaseous radiation into the environment.
  3. The exposure of area residents and employees caused by massive release of neutrons.

The findings and conclusions drawn by the STA that were released on November 4th have indeed confirmed that the accident was a scale of level 5.



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