High Fidelity Thermal Simulators for Non-Nuclear Testing: Analysis and Initial Results

Shannon M. Bragg-Sitton _, Ricky Dickens _,David Dixon 2tNASA Marshall Space Flight Center, Nuclear Systems Branch/ER24, MSFC, AL 358122North Carolina State University, Raleigh, NCTel: 256.544.6272, E-Mail: Shannon.M.Bragg-Sitton@nasa.govAbstract - Non-nuclear testing can be a valuable tool in the development of a space nuclearpower system, providing system characterization data and allowing one to work through variousfabrication, assembly and integration issues without the cost and time associated with a fullground nuclear test. In a non-nuclear test bed, electric heaters are used to simulate the heat fromnuclear fuel. Testing with non-optimized heater elements allows one to assess thermal, heattransfer, and stress related attributes of a given system, but fails to demonstrate the dynamicresponse that would be present in an integrated, fueled reactor system. High fidelity thermalsimulators that match both the static and the dynamic fuel pin performance that would beobserved in an operating, fueled nuclear reactor can vastly increase the value of non-nuclear testresults, With optimized simulators, the integration of thermal hydraulic hardware tests withsimulated neutronie response provides a bridge between electrically heated testing and fuelednuclear testing, providing a better assessment of system integration issues, characterization ofintegrated system response times and response characteristics, and assessment of potentialdesign improvements' at a relatively small fiscal investment. Initial conceptual thermal simulatordesigns are determined by simple one-dimensional analysis at a single axial location and atsteady state conditions; feasible concepts are then input into a detailed three-dimensional modelfor comparison to expected fuel pin performance. Static and dynamic fuel pin performance for aproposed reactor design is determined using SINDA/FLUINT thermal analysis software, andcomparison is made between the expected nuclear performance and the performance ofconceptual thermal simulator designs. Through a series of iterative analyses, a conceptual highfidelity design can developed. Test results presented in this paper correspond to a "first cut"simulator design for a potential liquid metal (NaK) cooled reactor design that could be appliedfor Lunar surface power. Proposed refinements to this simulator design are also presented.I. INTRODUCTIONAt the NASA Marshall Space Flight Center (MSFC)Early Flight Fission Test Facility (EFF-TF) electric heatersare used to simulate the heat from nuclear fuel to testpotential space fission power and propulsion systems. Toallow early utilization, nuclear system designs must berelatively simple, easy to fabricate, and easy to test usingnon-nuclear heaters to closely mimic heat from fission. Inthis test strategy, specialized electric heaters are used tosimulate the heat from nuclear fuel, allowing one todevelop an understanding of individual components andintegrated system operation without the cost, time andsafety concerns associated with nuclear testing. Electricheaters have been used for this purpose in numerousterrestrial and space reactor test and developmentprograms. 1-5The thermal simulators (heaters) developed atthe EFF-TF have been applied in a variety of space reactorconcepts for power or propulsion applications, includingheat pipe, direct gas, and liquid metal cooled reactorsystems. 6-9To accurately represent the fuel, the simulatorsshould be capable of matching the overall properties of thenuclear fuel elements rather than simply matching the fuelelement temperatures during nominal operation. Inaddition to matching the total core power and core powerprofile (axial and radial), this includes matching thermalstresses in the pin, effective radial pin conductivities, andtransient pin characteristics (affected by the effective radialpin density and heat capacity) during both static anddynamic test conditions: