Particulars of the choice of radiation protection for planetary-surface nuclear power plants

Abstract

The results of computational studies on choosing radiation protection for planetary-surface nuclear power plants are present. Protection on the base of a 0.4-1.5 MW(t) YaEU-100 thermionic space reactor was considered for a Martian nuclear power plant and a 0.36 and 0.6 MW(t) YaEU-25 reactor was considered for a lunar reactor. The mass/size characteristics of the radiation protection were obtained for different arrangements of the nuclear power plant on the planet -directly on the surface with protection delivered or an embankment consisting of local soil and in a shaft prepared beforehand.The computational investigations of the choice of radiation protection for planetary-surface nuclear power plants which were performed at the Physics and Power Engineering Institute are presented. Protection based on the 0.4-1.5 MW(t) YaEU-100 thermionic space reactor was considered for a Martian nuclear power plant and 0.36 and 0.6 MW(t) YaEU-25 and -50, respectively, reactors were considered for a lunar nuclear power plant. Different variants of radiation protection were considered for both planets: natural or artificial burial of the nuclear power system, embankment of the nuclear power plant, and a radiation protection block consisting of materials delivered from Earth.The characteristics of radiation transfer and the choice of an optimal configuration for the radiation protection were analyzed using the domestic codes RAPID [1] and KASKAD [2] (two-dimensional geometry). The final (verification) calculations of the radiation fields in the reactor, shielding, and surrounding space (atmosphere and soil on Mars and the Moon) were performed with the MCNP code [3] using ENDF/BVI constants [4].Protection for a Martian Nuclear Power Plant. The radiation protection must provide on a scientific base 1 km from the nuclear power plant a yearly dose of reactor radiation not exceeding 2 cSv (Fig. 1). The angle in the direction of the base protected from the nuclear power plant radiation was 90°.The particulars of radiation transfer in the Martian atmosphere, whose density is approximately 100 times lower than that on Earth and which is 95% carbon dioxide, should be noted [5]. The Martian atmosphere, which essentially does not attenuate the direct reactor radiation, does strongly scatter it, which makes it impossible to use shadow protection. Calculations performed to determine the influence of the radiation scattered in the Martian atmosphere on the radiation environment showed that a hemisphere with radius at least 11 km makes a substantial contribution to the yearly equivalent dose. For an unshielded reactor, the fraction of the radiation scattered in the atmosphere is 16% for neutrons and 2% for photons ( Table 1). The radiation scattered by the ground also makes an appreciable contribution. Since the dose of the scattered radiation from an unprotected reactor exceeds the prescribed levels, circular shielding is required. Shaping the protection could substantially decrease its mass. Figure 2 illustrates the need for shaping. This figu...