Individual monitoring for internal exposure of workers in FDNPS

The decontamination and facing process at premises (outdoors) of TEPCO's Fukushima Daiichi Nuclear Power Station (referred to as FDNPS) has made it possible to work in standard work clothing without a full face mask in more than 96% of the site, but high levels of contamination from melted nuclear fuel still remain in the reactor buildings. In the FDNPS, since Cs-137 is the dominant radionuclide in most of the workplace, all workers who enter the controlled area are monitored individually for internal exposure by whole-body counter (for screening purposes). From 2024 or later, the gradual removal of fuel debris is planned, which will increase work inside the reactor building. To ensure the radiation safety of workers in such special operations, it is essential to establish an internal dose monitoring system that properly uses a combination of in-vivo and in-vitro bioassay. This article discusses monitoring methods by calculating the radionuclides that affect dose during an internal exposure event based on the FDNPS workplace measurement data. In addition, the approach for quality assurance of individual monitoring in the FDNPS will be presented.