In order to evaluate three-dimensional distributions of radionuclides with high precision from gamma-scanning profiles of adsorption vessels used for decontamination of radioactive water performed at severe accidents, gamma scanning of the submerged demineralizer system vessel at the Three Mile Island Unit 2 (TMI-2) accident was simulated in the axial and radial directions of real and cylindrical-shaped vessels using a Monte Carlo calculation code [Particle and Heavy Ion Transport Code System (PHITS)].

In the axial simulation, the true distribution of radioactive 137Cs in the zeolite packed bed of the vessel was successfully evaluated when a correction function derived from a virtual constant distribution of 137Cs was applied to the reported gamma-scanning profile. In the radial simulation, the virtual disk-formed and shell-formed sources of 137Cs displaced in the packed bed were clearly observed from the top and bottom views of the vessel. This new radial gamma scanning indicates that the radial localization of 137Cs could be well observed by measuring the gamma ray from the top view of the vessel during storage. Further, the radial gamma scanning from the side view and whether or not the radial localization of 137Cs can be confirmed in the normally existing gamma-scanning room were examined.