Measurement of Neutron and Gamma-Ray Absorbed Doses Under Criticality Accident Conditions at TRACY Using Tissue-Equivalent Dosimeters

To evaluate neutron and gamma-ray absorbed doses in human bodies at criticality accidents, two kinds of tissue-equivalent dosimeters, a polymer-alanine dosimeter and a thermoluminescent dosimeter (TLD) made of ⁷Li₂¹¹B₄O₇, were applied to dosimetry experiments with ~10% enriched uranyl nitrate solution at the Transient Experiment Critical Facility (TRACY) in the Japan Atomic Energy Research Institute. For the experiments, five transient operations were conducted to simulate criticality accidents by varying the conditions of reactivity addition. Very high doses from both neutrons and gamma rays were successfully measured in the range of 1.5 to 1600 Gy by using polymer-alanine dosimeters. The gamma-ray doses were able to be determined in the range of 1 to 900 Gy by using ⁷Li₂¹¹B₄O₇ TLDs. In addition, it is confirmed that the doses are proportional to integrated power during transient operations although the conditions of reactivity addition are different. Since the sensitivity of ⁷Li₂¹¹B₄O₇ to gamma rays is almost the same as that of alanine, the neutron doses are easily evaluated without any complicated correction by subtracting the gamma-ray doses obtained by the ⁷Li₂¹¹B₄O₇ TLDs from the sum of neutron and gamma-ray doses by the polymer-alanine dosimeters. As a result of computational analyses by the MCNP4B code, it is also found that calculated doses agree with measured ones within 95% confidence intervals and the MCNP4B is applicable to the evaluation of neutron and gamma-ray absorbed doses during the transient.