Evaluation of energy spectrum around structural materials in radiation environments

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Structural materials such as stainless steel (SUS) in nuclear reactor are exposed by the environment of ionizing radiations so that their corrosion and degradation take place. These radiation effects depend strongly on the quality of radiation (energy, kind) around the materials. The metallic materials with various thickness have been examined in previous works on the radiation effects, but not with a common thickness. The different thickness would give the different radiation quality, leading to the different results of radiation effects on the materials. In this work, when radiation sources of 137Cs, 90Sr and 90Y were assumed to be put in the front of a plain SUS304 plate as a typical material submerged in water, energy spectra of secondary photons and electrons at the front and back sides of plate were simulated with changing the thickness of plate, and spacing between the source and plate by using a Monte Carlo calculation code of PHITS.In the case of 137Cs gamma-ray (monochromatic 662 keV), the energy spectra at the front side was smaller than those at the back side due to the existence of plate. Then the dependence of spectra on the plate thickness was observed more clearly at the back side than at the front side. In the cases of 90Sr and 90Y beta-rays (maximum 546 and 2284 keV), the energy spectra were clearly different from those in the case of gamma-ray. At the front side, the energy and flux of electrons were higher than those of photons. Also, the energy spectra of electrons and photons were not dependent on the plate thickness. At the back side, the energy and flux of electrons were much lower than those of photons reversely. It was clearly shown how the energy spectra of photons and electrons varied with the incident radiation type, the spacing, and the thickness.The average energies of secondary radiations were further estimated, and the changes of incident radiation to the secondary ones were discussed especially in terms of the lowering of radiation energy.