A Neutronic Feasibility Study on a Small LEU Fueled Reactor for Space Applications

Transactions of the Korean Nuclear Society Autumn Meeting Pyeongchang, Korea, October 30-31, 2014 A Neutronic Feasibility Study on a Small LEU Fueled Reactor for Space Applications Hyun Chul Lee , Hong Sik Lim, Tae Young Han Korea Atomic Energy Research Institute, 989-111 Daedeok-daero, Yuseong-gu, Daejeon, Korea *Corresponding author: [email protected] 1. Introduction critical reactor mass and the corresponding reactor size for a homogeneous reactor geometry were investigated A power supply system of a spacecraft plays a key with the combinations of various fuel types, moderator role in deep space exploration missions. Possible materials, and reflector materials. The effect of the core energy sources for a spacecraft in the solar system are heterogeneity on the reactor effective multiplication solar energy and nuclear energy. However, the solar factor was also investigated. A neutronic feasibility energy density is too low to provide sufficient power study including a launch accident scenario analysis was required for exploring beyond Jupiter. For example, also conducted for small space reactors with a control solar energy density near Saturn is as low as 1% of that rod system and coolant pipes in the core. All near the Earth. Therefore, the only practically calculations were performed using a Monte-Carlo code, applicable option for the power supply of a spacecraft McCARD [7] with continuous energy ENDF/B-VII.0 exploring beyond Jupiter or out of the solar system is cross-section libraries. nuclear energy. Nuclear power supply systems for space application can be classified into three categories 2. Sensitivity Study on a LEU Fueled Small Space [1]. The first one is a radioisotope power system for Reactor heating or low electric power. Their thermal power covers the W-range. The second one is a small fission 2.1 Minimum Critical Mass of a LEU Fueled power system for a spacecraft electric power supply, Homogeneous Reactor whose thermal power ranges in kW. The third one is a large fission power system for electric propulsion or The small fission power system, KRUSTY, which direct thermal propulsion. being developed by the NASA and LANL is a very A small nuclear reactor with a thermal power of compact reactor with a fast neutron spectrum because it 5kWth is being studied at the Korea Atomic Energy utilizes HEU fuel without a moderator. On the contrary Research Institute (KAERI) as a possible electric power in this study, the focus was placed on the LEU fuel and supplier for a deep space probe. Many small fission thus the thermal reactor is very favorable from a reactors for space applications have been developed neutronic point of view. since the SNAP-10A reactor launched in 1965 [2]. Almost all nuclear fuel types developed thus far, Recently, the United States (US) National Aeronautics such as UO2, UC, UN, UH3, and U-metal, were 235 and Space Administration (NASA) and the Los Alamos considered and the U enrichment was assumed to be National Laboratory (LANL) have been developing a 19.95% in this study. Metal hydrides were considered small fission reactor, KRUSTY, with a fast neutron as a moderator since hydrogen is the best moderator to spectrum for deep space mission, where highly enriched make a thermal reactor compact. The moderator uranium (HEU) is used as fuel [3]. On the other hand, materials considered are lithium hydride (LiH), alkaline other researchers have also surveyed a thermal reactor earth metal hydrides (MgH2 and CaH2), ZrHx, and YHx. concept with low enriched uranium (LEU) for space Water (H2O) and hydro carbon materials such as applications [4,5,6]. One of the main concerns in terms polyethylene or oils were excluded because they are not of a space reactor design is the total reactor mass as feasible for a high temperature operation. Figure 1 well as the reactor size including the reflector. In the compares the absorption cross-sections of the metal KRUSTY reactor design, 93 w/o enriched U-10Mo and elements contained in the moderator materials. It was 7 BeO were used as the fuel and reflector materials, assumed that Li in LiH is enriched to 99.99w/o to respectively. The total mass of the KRUSTY reactor avoid high neutron absorption through a tritium 6 including the reflector was only 122.1kg and the outer production reaction of Li. Natural Ca has a relatively 7 radius of the reactor was as small as 16.5cm. Kugo large absorption cross-sections compared to Li or concluded that they could achieve a reactor mass as low natural Mg. Natural Y has a about seven-times larger as 500kg with a combination of 20 w/o enriched UN thermal absorption cross-section than natural Zr. fuel, a YH1.5 moderator and a Be reflector while Lithium hydride (LiH) and alkaline earth metal Nishiyama estimated the reactor mass as low as 460kg hydrides (MgH2 and CaH2) have a very low thermal when they combine 20 w/o enriched UO2 fuel, a H2O conductivity compared to ZrHx or YHx. Magnesium moderator, and a Be reflector. hydride (MgH2) requires hydrogen pressurization for In this paper, a feasibility study on a small space high temperature operation due to the very low reactor with LEU fuel is presented. First, the minimum hydrogen decomposition temperature. Zirconium Transactions of the Korean Nuclear Society Autumn Meeting Pyeongchang, Korea, October 30-31, 2014 hydride (ZrHx) is a well proven moderator for a small Figure 2 illustrates the simple reactor geometry used thermal reactor for a space application since SNAP- in estimating the minimum critical reactor mass. The 10A utilized HEU as a fuel and ZrHx as a moderator. core is a homogeneous cylinder with a radius of rc and a Although the hydrogen to zirconium ratio, x, of ZrHx height to diameter ratio (H/D) of 1.0. The core is used in the SNAP-10A reactor was 1.68 to 1.83 [8], a surrounded by a reflector with a thickness of δ. The lower value of 1.5, was used in this study for core was assumed to be filled with a homogeneous conservatism. Beryllium (Be) and beryllium oxide mixture of fuel and moderator. For a given set of (BeO) are also well proven reflector materials and are reflector thickness, δ, and a moderator to fuel volume considered candidate reflector materials in this study. ratio fm=Vm/Vf, the critical core radius can be determined by adjusting the core radius. Once the critical core radius is determined, the critical reactor radius (R=rc+δ) and the critical reactor total mass (mtot) including the reflector can be calculated. (a) Top View (b) Side View Figure 1. Comparison of Absorption Cross-sections of Some Figure 2. A Homogeneous Simple Reactor Geometry Metal Elements Figure 3 shows R and mtot as a function of δ and fm Table I summarizes the properties of fuel, moderator, for a UO2-CaH2 core with a Be reflector. The minimum and reflector materials investigated. Lithium hydride critical reactor mass of 169.8kg was achieved with (LiH) and calcium hydride (CaH2) were chosen as δ=6.94cm and fm=9.01 and the critical reactor radius moderator for the ceramic fuels (UO2, UC, and UN) with these parameters was found to be 23.46cm. Figure because their hydrogen decomposition temperatures are 4 shows R and mtot as a function of δ and fm for UO2- relatively high. The uranium hydride (UH3) fuel LiH core with a Be reflector. The minimum critical requires hydrogen pressurization to prevent hydrogen reactor mass of 70.1kg was achieved with δ=0.00cm release during high temperature operation due to its low and fm=16.23, which means that a bare core is the best hydrogen decomposition temperature. In spite of the in terms of the reactor total mass when UO2 and LiH low hydrogen decomposition temperature, magnesium are used as a fuel and moderator, respectively. The hydride (MgH2) was considered as the moderator for large difference in the minimum critical reactor mass of the UH3 fuel in addition to LiH and CaH2 under the these two cases is attributed to the fact that the thermal condition that the core was pressurized. A porosity of absorption cross-section of natural Ca is much higher 7 3% was considered for the fuel and moderator materials than that of Li as shown in Figure 1 and the fact that manufactured by sintering or powder pressing. the density per hydrogen of CaH2 (1.70/2=0.85g/cm3) Zirconium hydride (ZrHx) and Yttrium hydride (YHx) is higher than that of LiH (0.78 g/cm3). The cases with were chosen as the moderator for the U-metal fuel. A UC and UN fuel showed a very similar trend to the UO2 system temperature of 1000K was assumed. fueled cases. Table I. Fuel, Moderator, and Reflector Materials Density H. Decomp. Material Form (g/cm3) Temp. (°C) UO2 Powder 10.97 - UC Powder 13.63 - UN Powder 14.30 - UH3 Powder 10.95 ~400 U-metal Metal 19.10 - LiH Powder 0.78 ~900 MgH2 Powder 1.45 ~300 CaH2 Powder 1.70 ~800 ZrH1.5 Metal Hydride 5.60 ~800 YH1.5 Metal Hydride 4.20 ~800 Be Metal 1.85 - BeO Powder 3.01 - Figure 3. Critical Reactor Radius and Total Reactor Mass for a UO2-CaH2 Core with a Be Reflector Transactions of the Korean Nuclear Society Autumn Meeting Pyeongchang, Korea, October 30-31, 2014 Figure 4. Critical Reactor Radius and Total Reactor Mass for Figure 7. Critical Reactor Radius and Total Reactor Mass for a UO2-LiH Core with a Be Reflector a UH3-MgH2 Core with a Be Reflector Figure 5, 6, and 7 show R and mtot for a UH3-CaH2 Figures 8 and 9 show R and mtot for the U-ZrH1.5 and core, UH3-LiH core, and UH3-MgH2 core with a Be U-YH1.5 cores with a Be reflector, respectively.

Load more