Takeshi Ichiji

Calculation procedure of potential alpha energy concentration with continuous air sampling

A continuous potential alpha energy concentration monitor was developed to estimate the lung dose for inhalation of radon progeny. A silicon semiconductor detector was used as a detector. The build-up method was used and alpha particles emitted from 218Po, 214Po, 212Bi, and 212Po were detected. As 218Po and 212Bi have alpha particles of nearly the same energy, three detecting channels were set up. Counts corresponding to each nuclide were sent to a printer every 30 min. For the purpose of determining the potential alpha energy concentration of radon progeny continuously, a proper calculation procedure was investigated in detail. With this method, 218Po concentration and potential alpha energy concentration of radon progeny could be continuously obtained. The potential alpha energy concentration based on this procedure agreed well with that calculated from individual radon progeny concentration. When the measurement was done at 30-min intervals, the minimum detectable concentrations of 218Po concentration and equilibrium equivalent radon concentration were 0.3 Bq m(-3) and 0.15 Bq m(-3), respectively. The monitor can be used not only to estimate the lung dose but also to analyze environmental behavior of radon progeny.

Verification of screening level for decontamination implemented after Fukushima nuclear accident.

The screening level for decontamination that has been applied for the surface of the human body and contaminated handled objects after the Fukushima nuclear accident was verified by assessing the doses that arise from external irradiation, ingestion, inhalation and skin contamination. The result shows that the annual effective dose that arises from handled objects contaminated with the screening level for decontamination (i.e. 100 000 counts per minute) is <1 mSv y−1, which can be considered as the intervention exemption level in accordance with the International Commission on Radiological Protection recommendations. Furthermore, the screening level is also found to protect the skin from the incidence of a deterministic effect because the absorbed dose of the skin that arises from direct deposition on the surface of the human body is calculated to be lower than the threshold of the deterministic effect assuming a practical exposure duration.

Current situations and discussions in Japan in relation to the new occupational equivalent dose limit for the lens of the eye

Since the International Commission on Radiological Protection recommended reducing the occupational equivalent dose limit for the lens of the eye in 2011, there have been extensive discussions in various countries. This paper reviews the current situation in radiation protection of the ocular lens and the discussions on the potential impact of the new lens dose limit in Japan. Topics include historical changes to the lens dose limit, the current situation with occupational lens exposures (e.g., in medical workers, nuclear workers, and Fukushima nuclear power plant workers) and measurements, and the current status of biological studies and epidemiological studies on radiation cataracts. Our focus is on the situation in Japan, but we believe such information sharing will be useful in many other countries.

Verification Test of Clearance Automatic Laser Inspection System for Surface Contamination Measurement

Recently, a clearance automatic laser inspection system (CLALIS) has been developed for clearance measurement of scrap metals and concrete debris. It utilizes three-dimensional laser scanning, gamma-ray measurement and Monte Carlo calculation, and its outstanding detection ability has been verified. In Japan, when an object is removed from a radiation-controlled area, the activity level must be lower than the surface contamination density standard of 4 Bq/cm2 for beta and gamma emitters, which is one-tenth of the surface contamination density limit. According to the clearance inspection report published by the Nuclear Safety Commission of Japan, the activity level of waste must be compared with not only the clearance level but also the surface contamination density standard for clearance inspection. To demonstrate that CLALIS can also be used for the measurement of surface contamination, a verification test was carried out using actual metal waste samples of various shapes, sizes and activity levels at Kashiwazaki-Kariwa Nuclear Power Station of Tokyo Electric Power Company. As a result, it was clarified that CLALIS gives a conservative value for surface contamination compared with the conventional GM survey meter measurement. This because the activities of metal waste samples were estimated using the total count rate, a fixed average surface area of 100 cm2 and the conservative source position assumed in the Monte Carlo calculation for the calibration factor. In a nuclear power plant, the actual judgment of whether an object can removed from a radiation-controlled area is based on whether the result of surface contamination measurement is lower than the detection limit, which is significantly lower than 4 Bq/cm2 . According to this criterion, CLALIS provides an almost identical judgment to the GM survey meter, which means that CLALIS can be used as a rational clearance monitor to carry out clearance level and surface contamination inspections in a single radiation measurement. The prospective detection limit for CLALIS at nuclear power plants is also discussed and compared with that for the conventional surface contamination monitors.Copyright