A study on dose evaluation for Tokaimura criticality accident termination

Verification of dose control method for Tokaimura JCO criticality accident was performed. Personal dose estimation for Tokaimura criticality accident termination was performed based on measurements of neutron and gamma ray doses taken before the work commenced, but the personal dose for the workers as a result of the termination work was found to be approximately 50 times higher than the previous estimation in which the dose rate at the work point was estimated by the extrapolation of semi-log plot of the distance - dose rate relation. For this report, we reevaluated doses based on the extrapolation of log-log plot of the distance vs. dose-rate relation using close range measurements from 40 meters to 100 meters, and the results were found to match personal doses with an accuracy of between 60-80%. Therefore, the work can likely be done safely by taking into account three factors - [1] annual dose limits for workers, [2] dose to the workers from sources other than the pertinent work, [3] measurement error - and adopting 10mSv, or 1/2 of the annual value of 20mSv, as a dose guideline for tasks such as dose evaluation in dose fields with high levels of neutron radiation. It is also in accordance with the ICRP recommendations. (author)