Studies on Shielding and Activation Levels in a Concrete-Iron-Concrete Sandwich Structure Wall in the CMUH Proton Therapy Facility

The shielding performance and activation susceptibility of a sandwich wall in the proton therapy facility of China Medical University Hospital were investigated in an integrated manner using FLUKA Monte Carlo simulations. The 2-m-thick partition wall between two adjoining treatment rooms had a three-layered structure, which comprised a 0.2-m-thick iron layer sandwiched between two layers of 0.9-m-thick concrete. In comparison with that of a concrete wall of the same thickness, the shielding performance of the concrete-iron-concrete wall was marginally better, further reducing the transmitted dose rate by approximately a factor of 2 against secondary neutrons generated through proton bombardment. This study also investigated radioactivity levels from long-lived radionuclides (3H, 22Na, 54Mn, 55Fe, 60Co, 134Cs, 152Eu, and 154Eu) that are primarily induced in concrete or iron by neutrons. The specific activities of 54Mn, 55Fe, and 60Co in the middle iron layer were considerably higher (by factors of 75, 25, and 5, respectively) than those in the neighboring concrete. However, as for clearance levels, the index value of the iron layer was lower than that of the neighboring concrete because of the presence of fewer types of long-lived radionuclides in iron. Under irradiation scenarios considered in this study, the residual activity levels of the sandwich wall do not exceed those of a full-concrete wall, and the indexes at various depths estimated at 5 years cooling following a 20-y operational period comply with clearance criteria.