Nuclear and Emerging Technologies for Space, American Nuclear Society Topical Meeting Richland, WA, February 25 – February 28, 2019, available online at http://anstd.ans.org/ SENSITIVITY STUDIES OF THE TUNGSTEN VECTOR ON THE PERFORMANCE OF A LEU NTP ENGINE Matt Krecicki, Dan Kotlyar 770 State St NW/Suite 3-58, Atlanta, GA, 30313 mkrecicki@gatech.edu, dan.kotlyar@gatech.edu This paper explores the sensitivity of the criticality to the tungsten vector for a nuclear thermal propulsion (NTP) core, utilizing low enriched uranium (LEU) fuel. Tungsten ceramic metal composite (cermet) fuel is required due to the extremely high temperatures achieved in the core. However, tungsten has a non-negligible thermal neutron absorption cross section. This requires the tungsten to be enriched to contain mostly 184 W to maintain a critical configuration. The results of this study show that the core favors a harder spectrum, as expected, to reduce the parasitic absorption in tungsten. In addition, inaccurate tungsten vector definition (or relatively high manufacturing tolerances) can have a detrimental effect on the prediction of criticality, i.e., above 1,000 pcms. I. Introduction A near optimal core design has be established by previous work done at Georgia Institute of Technology 1 . The design achieved a specific impulse, denoted as ISP, of ~900s, 40 klbf of thrust, and a thrust-to-weight ratio of 4. This design is based on cermet fuel elements with 95% enriched 184 W. This imposes a potential problem because the cost of enriching a thousand pounds of tungsten to 81.1% 184 W was estimated to be 79,800$ per pound 5, 6 . Reducing the required enrichment of tungsten from 81.1% to 60.0% would save 45,000$ per pound 5, 6 . Therefore reducing to the required enrichment while maintaining criticality would be a significant benefit to the feasibility of NTP systems. I.B. NTP Core Design Description Cermet fuel elements are the optimal choice because of their chemical compatibly with hot hydrogen, and high melting point 1 . However, the natural composition of tungsten contains five isotopes, detailed in Table 1, which are characterized by large thermal neutron absorption cross sections, shown in Figure 1. One strategy to decrease the parasitic absorption of neutrons in tungsten is to middle enrich the tungsten vector to contain mostly 184 W. Middle enrichment is a process used when the middle atomic mass isotopes of a material vector is enriched. In the case of tungsten 184 W and 183 W are the middle isotopes. The process involves using two enrichment cascades, one to remove the heavier isotopes, 186 W, and another to remove the lighter isotopes, 182 W. This can be accomplished by using a double cascade enrichment scheme, which has been investigated for enriching molybdenum for use as a structural material in light water reactors 2 . Tungsten 180 W has been ignored in the calculation of the total cross section, and in the vector definition due to it only being a trace element, and its capture cross section having a similar behavior to the other isotopes. 5 . TABLE 1. Natural Tungsten Vector Isotope Weight Percent, % Thermal Neutron Absorption Cross Section, barns 180 W 0.12 60 182 W 26.50 20 183 W 14.31 11 184 W 30.64 2 186 W 28.43 35 Total Macroscopic Cross Section is 1.05 cm -1 Fig. 1. Tungsten Isotopic Capture Cross Sections 1 The cermet fuel contains 19.75% enriched UO2 particles, which are embedded in a tungsten matrix with 6 mol% ThO2 introduced as a stabilizer. A schematic view of the cermet fuel element is presented in Figure 2, where the red and blue regions represent the fuel and hydrogen coolant channels respectively. The LEU design requires to introduce moderating elements to achieve a critical core configuration. The moderating elements moderate fast neutrons, and heat the hydrogen gas before it enters the fuel elements. The H2 flows through the supply channel