Fig. 3-1 TRACY core tank The core tank of TRACY has an annular shape that is 7.6 cm in inner diameter, 50 cm in outer diameter, and is 2 m in height. A 10 wt%-enriched uranyl nitrate solution is fed into the tank as fuel. TRACY can create criticality accident situations up to 1×1018 fissions.

Fig. 3-2 Schematic diagram of gamma-ray components in criticality accidents of solution systems The gamma-rays are classified into four components from the viewpoint of dosimetry: (1) prompt component gamma-rays from fission and neutron-capture reactions during criticality, (2) delayed component gamma-rays from decay of fission products (FPs) during criticality, (3) pseudo component gamma-rays, or a neutron absorbed dose registered erroneously in gamma-ray dosimeters as a gamma-ray absorbed dose during criticality, and (4) residual component gamma-rays from FPs and neutron-activated materials after the termination of criticality.

Fig. 3-3 Spatial distribution of dose proportions of gamma-ray components in a criticality accident situation This graph shows a typical result evaluated at TRACY using a 7Li211B4O7 TLD. The total absorbed dose of the gamma-ray components attenuates almost in inverse proportion to the square of the distance from the core tank. The dose proportions of the gamma-ray components vary with the distance because of perturbation of the radiation field by peripheral equipment. The contribution of the pseudo and the residual components to be excluded for accurate evaluation of the gamma-ray absorbed dose during criticality was estimated to be in the range of 16 to 31% of the total gamma-ray absorbed dose.

When a criticality accident occurs, the large number of neutrons and gamma-rays from the explosive chain of fission reactions cause serious, often fatal, exposure to workers in proximity to the accident scene. Studies on neutron and gamma-ray dosimetry in such criticality accidents of solution systems have been performed in JAERI using the Transient Experiment Critical Facility (TRACY, Fig. 3-1). Our recent study ascertained that component analysis of the gamma-ray absorbed dose on the basis of gamma-ray emission behavior in criticality accidents was necessary for accurate gamma-ray dosimetry. The gamma-rays observed in criticality accidents of solution systems are classified into four components, as shown in Fig. 3-2. Previous studies, however, accounted for only the prompt component gamma-rays. These studies assumed the delayed, the pseudo, and the residual component gamma-rays were negligible. This assumption is incorrect, and such evaluations were found not to provide precise dosimetry. For accurate criticality accident dosimetry, dose proportions of the above four gamma-ray components were first evaluated. In this evaluation, the prompt, the delayed and the pseudo components during criticality were classified by computational analyses, and the residual component after the termination of criticality was experimentally determined. The gamma-ray absorbed doses were then verified to be correctly evaluated by comparison between the analytical and experimental results. Lithium tetra borate (7Li211B4O7) thermoluminescent dosimeters (TLDs), whose sensitivity to gamma-rays is equivalent to that of human muscle, were placed around the TRACY core tank in the experiment to examine the spatial variation of the dose proportions. This evaluation confirmed that although the prompt component provided the majority of the total absorbed dose, the other components should not be neglected, and that the dose proportions varied with the distance from the core tank (Fig. 3-3). The component analysis of the gamma-ray absorbed dose could quantitatively clarify the contribution of the dose components, which should be excluded, for accurate evaluation of gamma-ray exposure in criticality accidents and thus improve the accuracy of criticality accident dosimetry.

Reference H. Sono et al., Evaluation of Gamma-ray Dose Components in Criticality Accident Situations, J. Nucl. Sci. Technol., 42(8), 678 (2005).

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