Supplemental study on dose control for a criticality accident

Tokaimura criticality accident is considered as a precious material for nuclear emergency response study. In the previous report 'A Study on Dose Control for JCO Criticality Accident Termination' (JAEA-Technology 2009-043), we discussed how to control the dose received during the termination work of the criticality accident. We reevaluated the dose rate at work place based on the dose rate measurement data ranging around 40 to 100 m from the criticality accident point, and compared it with the dose rate calculated based on the worker's dose received. They matched within 60% to 80% accuracy. In this paper, we focused on the difference of the way in which dose rate attenuates between within 100 m from the source point and beyond 100 m and discussed the validity of using log-log plotting / semi-log plotting of dose rate - distance relation in order to extrapolate the dose rate at work place near the criticality accident point. In addition, we studied on the effect of the number of dose rate measurement data to be used for extrapolation. We recommend that about 10 mSv which is a half of 20 mSv annual dose limit should be used as worker's dose control target for the high neutron dose field work to ensure enough safety margin considering the following three points; 1. annual dose limit for workers, 2. dose received before, 3. measurement error. (author)