Annual Report

46. Tokai-mura Criticality Accident: Whole Body Distributions of 45Ca and 32P in Bone of the Victims Exposed to High-Dose Neutron Irradiation

Yoshito Watanabe, Masae Yukawa, Kiriko Miyamoto, Hiroshi Takeda, Yoshikazu Nishimura, Makoto Akashi, Toshiyasu Hirama, Hisamasa Joshima and Fuyuki Kouno

Keywords: Tokai-mura criticality accident, neutrons, bone, 45Ca, 32P

In the criticality accident in Tokai-mura on 30 September 1999, two workers received especially high dose of neutrons and -rays over their bodies. We found evidence indicating the inhomogeneous exposures of neutrons to their bodies by measuring the whole body distribution of 45Ca and 32P generated in bone matrix through neutron capture reactions of 44Ca + n > 45Ca and 31P + n > 32P.

Small pieces of bone (0.5-3g wet) were taken from fourteen parts of the body after the death of the two workers. Calcium and phosphorus in the samples was purified separately for the measurements of 45Ca and 32P activities with a low-background -ray spectrometer (Pico-beta, Fuji Electric Co., Japan). There was a high positive correlation (r=0.90) between 45Ca and 32P concentrations in the bone samples of one worker (worker A). This assured us that both the analyses of 45Ca and 32P were suitable for the estimation of thermal neutron fluence entering bone.

In worker A, the concentration of 45Ca varied widely among the different parts of the body (Fig.15 ). The highest concentration was observed in the right side of the anterior rib and the iliac bone. The bones in the trunk and thighbone showed higher concentrations of 45Ca. On the other hand, the lower and upper ends of his body, such as the frontal bone, finger bone and toe bone had lower concentrations. In the comparison between the right and left parts in the anterior rib and the iliac bone, the right showed much higher concentrations than the left. This suggested that the worker received a higher dose of neutrons at the frontal right position around the waist and chest, and that the dose decreased with the distance from the central part of the body.

In the other worker (worker B), there was only a small variation of 45Ca concentration among the parts of the body. The concentration in the ribs was lower, and the concentrations in the frontal bone, finger bone, iliac bone and thighbone was higher. He seemed to have gotten higher doses of irradiation in the face, hands and waist.

The 45Ca concentration ratio of worker B to worker A in the iliac bone was about 0.5. The value agreed with the worker B / worker A ratio of the estimated average doses for the whole body from lymphocyte number, chromosome abbreviation and 24Na activity in the blood. This suggested that the dose to the abdomen should contribute mainly to the estimation of the average dose from the parameters in the blood. The analyses of 45Ca and 32P in bone gave us valuable information for the estimation of the dose received by particular organs, which were critical for the health conditions of the workers.

Fig.15. 45Ca concentrations in bone samples from fourteen parts in the bodies of worker A (left) and B (right). Circles indicate the concentration levels in the samples from anterior parts, and squares indicate the levels from posterior parts. Activities are normalized to the date of the accident.

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