Development of emergency monitoring and prediction code systems in Japan

ABSTRACT

In 1980 a research and development programme for emergency monitoring and prediction code systems was started in Japan. An aerial radiation survey system (ARSAS) using a helicopter has been developed. This system is equipped with three of NaI(TI) scintillation detectors to cover the wide range of radiation level from background up to higher than 1 R/h from gaseous effluents. Using this system several experiments have been conducted in order to clarify environmental radiation characteristics and to calibrate the system in radiation fields, e.g. natural, 60Co and137Cs. Experiments for tracking a 41AR plume and evaluation of the source intensity released from nuclear facilities showed that this system performs well. For the monitoring of source intensity released from a nuclear plant, an exhaust-stack monitor which enables the easy estimation of exposure in the environment has been developed. This stack monitor can present source information in a form suitable for external exposure evaluation and indicate an average gamma-ray energy of the mixed radioactive rare gases. For the real-time prediction of environmental radioactivity and dose resulting from accidental releases, a code system, SPEEDI, has been developed. SPEEDI consists of codes of a statistical wind prediction model (WEADUS), a three-dimensional mass consitent wind field model (WINDO4), a transport and diffusion model employing a random-waltk method (PRWDA), and a model for calculating gamma dose and thyroid dose (CIDE). The validation study for computing models in SPEEDI was carried out with the experimental results which had been obtained form a series of gas tracer experiments on atmospheric diffusion in a flat coastal terrain and around an isolated mountain mass. This study showed that SPEEDI gives useful information on environmental radioactivity and dose levels around the site of a nuclear accident (AU)