Influence of artificial radionuclide deposited on a monitoring post on measured value of ambient dose rate

In Japan, monitoring posts (MPs) are placed around nuclear facilities to measure ambient dose rate. When it exceeds 20 μSv/h at an accidental release of radionuclides, it is stipulated that inhabitants should temporarily relocate (OIL 2) within one week. It is judged by the measured value of the dose rate by MPs. However, if artificial nuclides deposit to a MP, it is considered that the measured value will be affected. Increasing the measured value by the artificial nuclides, it is considered that the dose rate of the surrounding environment, that is, the net dose rate is overestimated. The purpose of this research is to evaluate exactly the dose rate even if the radionuclides deposit to the MP, in order to adequately evacuate inhabitants. In order to research a ratio of the dose rate from roof surface of MP and ground surface, we simulated just horizontal ground and a MP. To calculate the dose rate, a Monte Carlo calculation for artificial radionuclides was done. The roof surface and ground surface were used as source areas. Artificial radionuclides were uniformly distributed there. And, the dose rates in the NaI crystal region of the NaI detector was obtained. Next, a ratio R of the dose rate from the roof was obtained by the dose rate of the roof surface and ground surface. Result of the calculation, R was 0.44, which was constant value. Even if time elapsed from an accident, R did not change. When R is 0.44, the net dose rate is 0.56 of the measured value. If the net value is between 11 μSv/h and 20 μSv/h, the measured value may indicate 20 μSv/h which is the reference value of OIL 2. However, because of being simulated only the flat ground surface and a MP, R = 0.44 is an overestimated ratio. In the future, we aim to raise the lower limit of 11 μSv/h, assuming a more realistic environment. (author)