Preliminary Assessment of Two Alternative Core Design Concepts for the Special Purpose Reactor

Abstract

The Special Purpose Reactor (SPR) is a small 5 MWt, heat pipe-cooled, fast reactor based on the Los Alamos National Laboratory (LANL) Mega-Power concept. The LANL concept features a stainless steel monolithic core structure with drilled channels for UO2 pellet stacks and evaporator sections of the heat pipes. Two alternative active core designs are presented here that replace the monolithic core structure with simpler and easier to manufacture fuel elements. The two new core designs are simply referred to as Design A and Design B. In addition to ease of manufacturability, the fuel elements for both Design A and Design B can be individually fabricated, assembled, inspected, tested, and qualified prior to their installation into the reactor core leading to greater reactor system reliability and safety. Design A fuel elements will require the development of a new hexagonally-shaped UO2 fuel pellet. The Design A configuration will consist of an array of hexagonally-shaped fuel elements with each fuel element having a central heat pipe. This hexagonal fuel element configuration results in four radial gaps or thermal resistances per element. Neither the fuel element development, nor the radial gap issue are deemed to be serious and should not impact an aggressive reactor deployment schedule. Design B uses embedded arrays of heat pipes and fuel pins in a double-wall tank filled with liquid metal sodium. Sodium is used to thermally bond the heat pipes to the fuel pins, but its usage may create reactor transportation and regulatory challenges.An independent panel of U.S. manufacturing experts has preliminarily assessed the three SPR core designs and views Design A as simplest to manufacture. Herein are the results of a preliminary neutronic, thermal, mechanical, material, and manufacturing assessment of both Design A and Design B along with comparisons to the LANL concept (monolithic core structure). Despite the active core differences, all three reactor concepts behave similarly and retain the same ex-core features and characteristics. While INL has developed Designs A and B, LANL is evaluating a Hot Isostatic Pressed reactor configuration to overcome the drawbacks of the monolithic core design.

Key Nuclear Data Impacting Reactivity in Advanced Reactors

“…Figure 9. Cross section view of a heat pipe reactor with a hexagonal lattice of fuel cells (fuelred; heat pipe -yellow) surrounded by a reflector (blue), steel structure (green), and shield (gray) (Sterbentz et al 2018). .............................................................................................. Figure 35.…”

“…Depending on the design, the temperature of the coolant in the heat pipes is 620-730°C, and the fuel temperature is only slightly higher than that of the coolant and under 800°C. Diagrams of a potassium-cooled UO2 fuel heat pipe reactor are provided in Figure 9 and Figure 10 (Yan et al 2020, Sterbentz et al 2018, Maoioli et al 2019, Matthews et al 2019, Clark et al 2020.…”

“…Hexagonal lattice structure of fuel cells in a heat pipe reactor (fuel -red; heat pipeyellow, sodium pool -blue)(Sterbentz et al 2018). ...................................................................... …”

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“…Design A is a 5-MWt microreactor concept developed at INL, which is an alternative proposed in lieu of the LANL's Megapower core concept [16]. Figure 11 shows the layouts of the core and the fuel element of Design A.…”

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“…These reactors come in a wide variety of configurations and physical attributes, e.g., neutron spectra, coolants, moderators, and relevant feedback mechanisms. Designers and researchers have proposed a plethora of reactor designs: pebble bed reactors (PBRs) both of the high-temperature gas-cooled (HTGR) and fluoride-salt-cooled varieties, 12,13 prismatic HTGRs with hexagonal unit cells and cylindrical fuel slugs, 14 microreactors (MRs) with annular fuel in hexagonal unit cells, 15 hexagonal assembly fast 16 and mixed spectrum reactors, 17 hexagonal 18,19 and plate fuel 20 nuclear thermal propulsion systems, and thermal 21 and fast open-pool type liquid fuel molten salt reactors (MSRs). 22,23 These designs also come with a variety of control mechanism geometries: control rods placed inside 14 or outside the reactor core, 12 control drums, 15,18,20 and cruciform control materials are often used.…”