Tadashi Yamasaki

Measurements of Thoron Concentration by Passive Cup Method and Its Application to Dose, Assessment

A new type of passive integrating cup monitors was developed by using a 50 mm radius stainless steel hemisphere to measured indoor radon (222Rn) and thoron (220Rn) concentrations. By placing a pair of cup monitors with an air exchange opening of diameter 5 mm and four openings of diameter 20mm at a 20cm distance from wall during about three months, the concentrations of both gases could be assessed from the a track densities on the cellulose nitrate (CN) films. The 222Rn and 220Rn concentrations were surveyed with the cup monitors in the different types of dwellings around Nagoya in Japan over three years. The 220Rn concentrations were rather high in the dwellings with soil wall, and the mean 220Rn concentration was 160±12 Bq·m−3. The 220Rn exhalation rate from wall and the 220Rn diffusion were evaluated from the distribution of 220Rn concentrations in the dwellings. The results of the surveys have also clarified the relationship between the 220Rn concentrations a t a 20cm distance from wall and the 220Rn...

Feasibility Study on a Simple Method of Retrospective Neutron Dosimetry for Reactor Internals and Reactor Vessel

Nondestructive and simple retrospective dosimetry method was developed. Mercuric iodide or cadmium zinc telluride detector was used as a detector to measure 54Mn, 58Co, and 60Co that were created in reactor internals and vessels. Detector is surrounded by tungsten collimator which shield background gamma rays and detect gamma rays from desired measuring point. Photopeak count rate of this detector were calculated using the MCNP-4C code. This response was calculated for each reactor internal geometry that was irradiated by unit neutron flux which energy spectrum was calculated by the TORT code. Neutron fluence is calculated using precalculated response, measuring time, decay time and reactor power history. We can also get Displacement per Atom (DPA), hydrogen and helium production simultaneously using neutron spectrum precalculated in reactor vessel. Because the time and cost of this method are much less than the sampling method, we can measure at many positions and even get a contour map.

High sensitive airborne radioiodine monitor

Airborne radioiodine monitoring includes a problem in that commercial radioactive gas monitors have inadequate sensitivity. To solve this problem, we designed a highly sensitive monitoring system. The higher counting efficiency and lower background made it possible to perform the low-level monitoring. The characteristics of the system were investigated using gaseous (125)I. The minimum detectable activity concentration was 1 × 10(-4)Bq cm(-3) for 1 min counting, which is one tenth of the legal limit for the radiation controlled areas in Japan.