Evaluation of energy spectrum around structural materials in radiation environments

https://doi.org/10.1016/j.radphyschem.2019.108493Get rights and content

Highlights

  • Oxidizing radiolysis products affect structural steels within their reachable region.

  • Primary radiations are converted to secondary ones with lower energies in the region.

  • The energy spectra of secondary radiations depend on the thickness of steel plate.

  • The energy spectra depend on spacing between the primary radiation source and plate.

  • The energy spectra at the front and back sides of plate become different each other.

Abstract

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.

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 Epav and Eeav at the front side were nearly independent of the SUS304 thickness. On the other hand, the Epav and Eeav 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.

References (22)

  • A.O. Allen

    Radiation Chemistry of Water and Aqueous Solution

    (1961)
  • H.A. Bethe

    On the stopping of fast particles and the creation of positive electrons

    Proc. R. Soc. London, Ser. A

    (1934)
  • K.F. Eckerman

    Availability of nuclear decay data in electronic form, including beta spectra not previously published

    Health Phys.

    (1994)
  • ESTAR program < https://physics.nist.gov/PhysRefData/Star/Text/ESTAR.html...
  • Farhataziz

    Radiation Chemistry Principles and Applications

    (1987)
  • S. Hanawa

    ECP measurements under neutron and gamma ray in in-pile loop and their data evaluation by water radiolysis calculations

  • W. Heitler

    The Quantum Theory of Radiation

    (1954)
  • J.H. Hubbell

    “Tables of X-ray Mass Attenuation Coefficients and Mass Energy-Absorption Coefficients from 1 keV to 20 MeV for Elements Z = 1 to 92 and 48 Additional Substances of Dosimetric Interest”, NISTIR 5632

    (1995)
  • N. Itoh
    (1988)
  • JIS G 0577

    2014, “Methods of Pitting Potential Measurement for Stainless Steels

    (2014)
  • JIS G 0578

    2013, “Method of Ferric Chloride Tests for Stainless Steels

    (2013)
  • Cited by (2)

    • Characterization of bremsstrahlung and γ-rays of fuel debris

      2022, Radiation Physics and Chemistry
      Citation Excerpt :

      The energy spectrum of bremsstrahlung produced by electrons is continuously distributed from the maximum energy of electrons incident on an object to zero, and the radiation yield increases as the atomic number, Z, of the atoms contained in the object increases (SELTZER et al., 1985). Since fuel debris contains high-Z elements, such as uranium, it is assumed that bremsstrahlung will be generated with higher photon intensity and average energy than that emitted from contaminated water, which has been a problem in the past (Matsumura et al., 2020). Therefore, it is important to consider bremsstrahlung in the evaluation of fuel debris, unlike conventional spent fuel.

    View full text