Space Reactor Radiation Shielding Design

With the deepening development of the deep space detection works, more and more stringent requirements are needed of the power supply system. The space nuclear fission reactor system(referred to as space reactor system) is being highly valued by the industry which is because of its significant advantages, such as its high power ratio, long life, small volume, not affected by light and so on. At the same time, considering the radiation generated by the space reactor system can add adverse effects to the radiation sensitive devices’ life and performance in the system; In view of the manned case, will also pose a threat to the health of the workers, which will in turn affect whether the whole detection task can be completed smoothly or not. So the space reactor radiation shielding design is of great significance.The process of the deep space exploration can be mainly divided into two stages: the spacecraft propulsion stage and the long terms of detecting stage after the spacecraft landing on the planet’s surface, This paper focuses on the latter situation. Radiation types generated by nuclear fission reactor mainly include the alpha rays, beta rays, gamma rays and neutrons. The energy of alpha rays and beta rays is relatively low, Therefore we mainly consider the protecting of neutrons and gamma rays during the shielding design process of the space reactor system.After the deep space detection system landing on the planet, the overall degree of shielding largely depends on the core’s layout. Taking the moon as an example, when the space reactor system launched, the core can be embedded in the lunar regolith, still fixed on the Lander or installed to the surface of the moon, etc.; The United States has accomplished a large amount of calculation about this in its currently research system named “FSP-The Fission Surface Power System”, which is aimed at the detection of the moon base. At last the United States decided to adopt the core buried layout, because in this pattern the lunar regolith can be used to reduce a large amount of the leaked neutron in the radial direction of the core, which can help to reduce the weight of the shield, shorten the distance between staff and the core, shorten the length of the cable needed in the overall layout significantly, so it is obvious that this scheme can effectively reduce the overall weigh of the space reactor system which is need to be delivered from earth to other planets.In this paper refers to the core buried layout, which is adopted in the United States’ FSP system, using MCNP program carried on many simulation calculations on the shielding design part of one space reactor system which is used to detect the lunar base and is cooled by heat pipes. In addition, set the radiation shielding target as: within the whole life cycle of the space reactor system, the radiation limits accepted by the equipments are 2.5 x 1014 neutrons/cm2(equivalent to 1MeV Si displacement damage) and 5Mrad(dose of gamma rays); Staff along with the space reactor system who stays about one hundred meters far from the core, the radiation dose limit is 5rem/a. The achievements in this article are mainly listed as follows:1) Using the MCNP program, complete the core’s fine structure modeling work of the space reactor system we focused, calculate the effective multiplication factor keff of the core is between 0.95~1.05. obtained the leakage energy spectrum of neutrons and photons above the core, etc.2) Using MCNP program to calculate the average leakage energy of neutrons and photons above the core, respectively 0.86 MeV and 1.09 MeV. Under both energy. 8 kinds of commonly used space of shielding materials’ attenuation coefficient values were calculated. Choosing B4 C as the neutron shielding materials, and selecting W as photonic shielding materials after comparing the calculated results.3) Shielding calculations of the heat pipe penetrate space reactor are made, found that the highest count is at the center position of the truncated shielding bottom., about three orders of magnitude higher than the target limit. The results can’t get improved by other attempts, so give up this kind of design scheme.4) Calculation results of the heat pipe bending space reactor shows that, the highest count place is at the bottom edge of the truncated shielding. By increasing the bottom radius of the truncated cone, joining moderating materials at the top of the truncated cone, joining W and other methods, determine the shielding configurations for roughly:The cavity height is 90cm; truncated cone’s total thickness is 50 cm, From inside to outside, the materials are: 2cm W, 9cm B4 C, 1cm W and 38 cm B4C; Truncated cone bottom radius is 66cm; The core is surrounded by a layer of B4 C, which thickness is 11cmIn this scheme, the highest count of the cumulative neutron flux and cumulative gamma dose were 2.26x1014 neutrons/cm-2(equivalent to 1MeV Si displacement damage) and 2.60Mrad(gamma rays), this result can well satisfy the shielding requirements.5) Radiation levels received by staffs has been calculated, the total dose was 11.92mSv/y, about 1/5 of the target limit.6) Simple thermal analysis of B4 C and lunar regolith around the core has been made, to make sure that the final design of the space reactor’s shielding structure can avoid the temperature of the lunar regolith become too high to cause many other potential dangers.Space reactor power supply system has a broad application prospect in deep space detection work, but at present there is no open literature about domestic studies on space reactor shielding design. The research results made in this article can provide a good reference and with great help to the space reactor system’s design and the optimization of the shielding part in our country in the future.