Evaluation of Gamma-Ray Dose Components in Criticality Accident Situations

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.