Evaluation of Gamma-Ray Dose Components in Criticality Accident Situations

Abstract

Component analysis of gamma-ray doses in criticality accident situations is indispensable for further understanding on emission behavior of gamma-rays and accurate evaluation of external exposure to human bodies. Such dose components were evaluated, categorizing gamma-rays into four components: prompt, delayed, pseudo components in the period of criticality, and a residual component in the period after the termination of criticality. This evaluation was performed by the combination of dosimetry experiments at the TRACY facility using a thermoluminescent dosimeter (TLD) made of lithium tetra borate and computational analyses using a Monte Carlo code. The evaluation confirmed that the dose proportions of the above components varied with the distance from the TRACY core tank. This variation was due to the difference in attenuation of the individual components with the distance from the core tank. The evaluated dose proportions quantitatively clarified the contribution of the pseudo and the residual components to be excluded for accurate evaluation of gamma-ray exposure. Such the contribution sum increased with increase in the distance from the core tank and was estimated to be in the range of 16 to 40% of the total gamma-ray dose measured by the TLD at TRACY.

Retrospective Estimation of the Spatial Dose Distribution and the Number of Fissions in Criticality Accident Using Area Dosimeters

“…The uncertainty of 10-20% is allowable in radiation emergency medicine: its criterion is roughly 50% in 48 hrs and 25% in a week after the accident. 11) After the thorough investigation of the accident, the Monte Carlo simulation 10,12) under the condition at the time of the accident can remove the uncertainty from the first estimation.…”

“…As regards the dose range in criticality accidents, the previous criticality accident dosimetry at TRACY showed that the dose measured within a radius of $5 m from the radiation source was approximately in the range of 10 À15 -10 À17 Gy/fission. 9,10) The number of fissions was reported to be in the range of 10 15 -10 20 in the past sixty criticality accidents.…”

“…1) The possible dose range is thus estimated to be [1][2][3][4][5][6][7][8][9][10] À2 Gy for the smallest accident which was 10 15 fissions and 10 5 -10 3 Gy for the largest accident which was 10 20 fissions. Comparing the measurable range of the dosimeters to the possible dose range of the accident, it is recommended that the area dosimeters are placed so as to cover the area within a radius of $5 m from equipment likely to attain criticality (e.g., as will be shown in Fig.…”

“…Note that the values of both a and b become infinity where observed in the TRACY radiation field (e.g., from À1:4 to À2:1), 10) we may consider that the dose at the point P i or P j has been locally attenuated. The values of a and b for the spatial dose distribution in the area are obtained where the dose seems not to be locally attenuated (hereinafter called 'unattenuated area'), using only the combinations of i and j yielding values of b in the above range.…”

“…The unit dose D f can be determined by a Monte Carlo simulation taking account of the dose components of the prompt neutrons, the prompt gamma-rays and the delayed gamma-rays emitted in criticality accident situations, 10,12) based on the conditions of fissile materials which have been investigated. Before determining the D f value, the following values may be used as an initial guess of D f to obtain a first estimation of the number of fissions: ð21 AE 3Þ Â 10 À17 Gy/ fission for the sum of neutron and gamma-ray doses D n measured with the alanine dosimeter, ð13 AE 2Þ Â 10 À17 Gy/ fission for the gamma-ray dose D measured with the 7 Li 2 11 B 4 O 7 TLD, and ð8 AE 1Þ Â 10 À17 Gy/fission for the neutron dose D n (the difference in these two dose values).…”