Regulatory implementation of the occupational equivalent dose limit for the lens of the eye and underlying relevant efforts in Japan.

In April 2011, the International Commission on Radiological Protection recommended reducing the occupational equivalent dose limit for the lens. Such a new occupational lens dose limit has thus far been implemented in many countries, and there are extensive discussions toward its regulatory implementation in other countries. In Japan, discussions in the Japan Health Physics Society (JHPS) began in April 2013 and in Radiation Council in July 2017, and the new occupational lens dose limit was implemented into regulation in April 2021. To share our experience, we have published a series of papers summarizing situations in Japan: the first paper based on information available by early 2017, and the second paper by early 2019. This paper (our third paper of this series) aims to review updated information available by mid-2022, such as regarding regulatory implementation of the new occupational lens dose limit, recent discussions by relevant ministries based on the opinion from the council, establishment process of safety and health management systems, the JHPS guidelines on lens dose monitoring and radiation safety, voluntary countermeasures of the licensees, development of lens dose calibration method, and recent studies on exposure of the lens in nuclear workers and biological effect on the lens.

Radiation exposure to the lens of the eye for Japanese nuclear power plant workers.

In Japan, the radiation-dose limit for the lens of the eye was revised in April 2021. Consequently, for workers, the numerical values of the equivalent dose to the lens of the eye are equal to those of the effective dose. Radiation workers, radiation safety officers and licensees must comply with regulations related to radiation protection and optimize protection. The new guidelines on dose monitoring of the lens of the eye developed by the Japan Health Physics Society recommend for the dose to be estimated near the eye for accurate estimation, when the dose to the lens approaches or exceeds the management criteria. However, there is limited information regarding the non-uniform exposure of nuclear power plant workers. In this study, the dose equivalents of high-dose-rate workplaces and the personal doses of 88 workers were estimated at four Japanese commercial nuclear power plant sites (RWR: 3 units and BWR: 3 units) and the dose to the lens of the eye and the exposure situations of the workers were analyzed.

Report of IRPA task group on issues and actions taken in response to the change in eye lens dose limit.

In 2018, the International Radiation Protection Association (IRPA) established its third task group (TG) on the implementation of the eye lens dose limit. To contribute to sharing experience and raising awareness within the radiation protection community about protection of workers in exposure of the lens of the eye, the TG conducted a questionnaire survey and analysed the responses. This paper provides an overview of the results of the questionnaire.

Recent discussions toward regulatory implementation of the new occupational equivalent dose limit for the lens of the eye and related studies in Japan.

Purpose: In 2011, the International Commission on Radiological Protection (ICRP) recommended an equivalent dose limit for the lens of the eye of 20 mSv/year, averaged over defined periods of 5 years, with no single year exceeding 50 mSv for occupational exposure in planned exposure situations. Since then, there have been extensive discussions toward regulatory implementation of such a new occupational lens dose limit. This paper provides an overview of the recent discussions toward regulatory implementation and the current status of the studies related to radiation exposure of the lens and its effect in Japan. Conclusions: In Japan, the Radiation Council established a Subcommittee in July 2017 to discuss the feasibility of implementing the new occupational lens dose limit. In March 2018, the Radiation Council requested all relevant government ministries and agencies to take necessary actions toward implementation of the new occupational lens dose limit, considering a series of discussions made by the Subcommittee. According to the currently available information, the new occupational lens dose limit (20 mSv/year, averaged over defined periods of 5 years, with no single year exceeding 50 mSv) will be implemented into regulations in Japan, most likely in April 2021, albeit subject to change. In particular, there were extensive discussions about reduction of a dose limit and radiation control for workers who may exceed 20 mSv per year, such as the Fukushima Daiichi Nuclear Power Plant decommissioning workers and medical staff, and measurement of 3 mm dose equivalent. In order to underpin regulatory practice, epidemiological and biological studies on radiation effects on the lens and studies on lens dose measurements have been conducted in Japan, some of which have been funded by the Japanese Nuclear Regulation Authority.

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.

EVALUATION OF EYE LENS DOSES OF INTERVENTIONAL CARDIOLOGISTS.

The effective dose of medical staff members, especially interventional radiologists and cardiologists, is classified as a relatively high level. We measured the dose for interventional cardiologists by using optically stimulated luminescence dosimeters (OSLDs). However, this quantity is not the same as Hp (3). In experiments, the dose at the eye-lens position of a phantom were measured using OSLDs and thermoluminescence dosimeters (TLDs). A conversion factor from dose measured by using TLDs to OSLDs was estimated from these results. In addition, the eye doses of interventional cardiologists in clinical situations were measured, and the effect of eyewear on the eye-lens dose was discussed.

Development of lung and soft tissue substitutes for photons.

The use of solid tissue substitutes is a well-accepted and common practice in dosimetric studies and in the production of counting standards for radiological protection. However, only a few solid tissue substitutes simulating a particular body tissue with respect to a set of physical characteristics are commercially available. Hence, we have developed polyurethane-based tissue substitutes simulating soft tissue, muscle, muscle-adipose mixture tissue (90% muscle + 10% adipose), brain, cartilage, larynx, thyroid, trachea, liver, kidney, skin and lungs. Tissue substitutes for photons were formulated using the basic data method together with an equation for calculating the optimum relative mass of corrective additives. The tissue substitutes were formulated to be phantom materials in the photon energy range of at least 8 keV-10 MeV. In particular, they were designed to match the body tissues with linear attenuation coefficients for low photon energy (13.6, 17.2 and 20.2 keV from 239Pu) and to have the same mass densities as the tissues. The tissue substitutes developed in the present study were examined for the photon transmissions using 16.6 keV KX rays from 93Nb(m). The experimental transmission curves of the tissue substitutes were found to be consistent with those derived from data on the body tissues in ICRP Publication 23. It was found that the developed tissue substitutes are suitable to the corresponding body tissues defined by ICRP.

Characterisation of radionuclides formed by high-energy neutron irradiation.

The physicochemical properties of radionuclides suspended in the air are important parameters in order to evaluate internal doses due to the inhalation of the airborne radionuclides and to develop the air-monitoring system in high-energy proton accelerator facilities. This study focuses on the property of radioactive airborne chlorine (38Cl and 39Cl) and sulphur (38S) produced in Ar gas by irradiation with high-energy neutrons. As a result of the irradiation of a mixture of Ar gas and dry air, 38Cl and 39Cl existed as non-acidic gas and 38S was present as acidic gas. Furthermore, it has been found that in the high-energy neutron irradiation of aerosol containing Ar gas, the higher the amount of radioactive aerosols, the lower will be the amount of radioactive acidic gas.

Vertical profile of tritium concentration in air during a chronic atmospheric HT release.

The vertical profiles of tritium gas and tritiated water concentrations in air, which would have an influence on the assessment of tritium doses as well as on the environmental monitoring of tritium, were measured in a chronic tritium gas release experiment performed in Canada in 1994. While both of the profiles were rather uniform during the day because of atmospheric mixing, large gradients of the profiles were observed at night. The gradient coefficients of the profiles were derived from the measurements. Correlations were analyzed between the gradient coefficients and meteorological conditions: solar radiation, wind speed, and turbulent diffusivity. It was found that the solar radiation was highly correlated with the gradient coefficients of tritium gas and tritiated water profiles and that the wind speed and turbulent diffusivity showed weaker correlations with those of tritiated water profiles. A one-dimensional tritium transport model was developed to analyze the vertical diffusion of tritiated water re-emitted from the ground into the atmosphere. The model consists of processes of tritium gas deposition to soil including oxidation into tritiated water, reemission of tritiated water, dilution of tritiated water in soil by rain, and vertical diffusion of tritiated water in the atmosphere. The model accurately represents the accumulation of tritiated water in soil water and the time variations and vertical profiles of tritiated water concentrations in air.