Evaluation of energy spectrum around structural materials in radiation environments
Introduction
Since metallic structural materials in nuclear reactor are placed in the environment of radiations, their corrosion and degradation occur. These radiation effects depend on the quality of radiation around the materials. The radiation quality such as linear energy transfer (LET) is determined by the type and energy of radiation. The quality further depends on the kind and thickness of structural material. Generally, the effect has been evaluated based on the dose given by primary and incident radiation. But the incident radiation interacts with the materials and then its quality changes. So, it is important to take this change into consideration for the evaluation of effects.
Radiations that affects the materials at the loss-of-coolant accident (LOCA) are mainly gamma (γ)- and beta (β)-rays emitted from 137Cs, 90Sr, and 90Y which are major radionuclides (Okumura et al., 2013) as seen in Fukushima Dai-ichi Nuclear Power Station (1F) (Nagaishi et al., 2014). γ-ray is a type of electromagnetic waves with monochromatic energies and interacts with matter through photoelectric effect, Compton scattering, and electron pair production to generate secondary electrons as well as scattered photons. On the other hand, β-ray (minus) as an electron is emitted with a continuous energy distribution. The electrons are fairly influenced by atomic coulomb fields in matter because they are very light compared to other charged particles (e.g. proton). Therefore, β-ray strongly interacts with matter, and is quickly decelerated and absorbed by matter. When γ- and β-ray interact with matter, the primary radiations become secondary radiations of photons and electrons with lower and continuous energy so that the quality of radiations depends on matter. Therefore, it is important to evaluate the quality of secondary radiations around the structural material for evaluating its radiation effects as well as the quantity.
In the research field of radiation chemistry, primary yields (G-values) of radiolysis products of water are well known to strongly depend on LET (or stopping power) (Allen, 1961). The secondary radiations of lower and continuous energies give higher LET than the primary one before interacted. So, the G-values for the secondary radiations would be different from those for the primary one. Currently in works on the radiation effects of materials, they have been examined with various thickness, but not with a common thickness. The material of different thickness would give the different energy spectra of secondary radiations especially around the material.
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, the energy spectra of secondary photons and electrons at the front and back sides of plate were simulated with changing the thickness of plate, and the spacing between the source and plate by using a Monte Carlo calculation code of the Particle and Heavy Ion Transport Code System (PHITS) (Sato et al., 2018). In these simulations, the following four items were examined and discussed.
- (1)
Analysis of energy spectra of secondary radiations of photons and electrons generated from SUS304 and water by their interaction with γ-rays from 137Cs (3.1.1)
- (2)
The estimation of average energies of secondary radiations generated from SUS304 and water by their interaction with γ-rays from 137Cs (3.1.2)
- (3)
Analysis of energy spectra of secondary radiations of photons and electrons generated from SUS304 and water by their interaction with β-rays from 90Sr and 90Y (3.2.1)
- (4)
The estimation of average energies of secondary radiations generated from SUS304 and water by their interaction with β-rays from 90Sr and 90Y (3.2.2)
The main component of structural materials such as stainless steel (SUS) is iron. So SUS304 was used as a typical material in this work.
Section snippets
Determination of calculation condition
In this subsection, three parts important for calculation were determined at fast: (1) thickness of observation region, (2) spacing between radiation source and SUS304, and (3) observed surface area of SUS304.
In the first part, the thickness of observation region was determined in the followings. In water radiolysis, hydrogen peroxide (H2O2) is formed as a molecular product and generally acts as an oxidizing agent for the corrosion and degradation of structural materials (Hanawa et al., 2010).
Energy spectrum with γ-ray irradiation
Fig. 3a shows that the spectra of secondary radiations of photons and electrons were calculated at the front and back sides of SUS304 with changing the spacing and thickness indicated in Fig. 3b. The shapes of spectra were irrespective of the existence of SUS304. In order to observe the shape changing clearly, the spectra were shown at the fixed thickness of SUS304 (1 mm) in Fig. 4. However, the shapes significantly changed with the spacing.
At the front side of SUS304, a photon peak under any
Conclusion
The energy spectra of secondary radiations of photons and electrons at the front and back sides of SUS304 plate in water were simulated by using a Monte Carlo calculation code of PHITS.
In the case of 137Cs γ-ray (662 keV), the and at the front side were nearly independent of the SUS304 thickness. On the other hand, the and at a constant 1 mm thickness decreased from 326 ± 2 keV and 226 ± 37 keV at 1.0 mm spacing to 157 ± 19 keV and 121 ± 38 keV at 0.0 mm, respectively.
Acknowledgements
I am grateful to M. Fujita and S. Abe for useful technical advices and discussions on the PHITS simulation.
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