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.

NRC Multiphysics Analysis Capability Deployment (FY2021 - Part 2)

“…Several heat pipe microreators with hexagonal cladding have been proposed in recent years [3,4]. In these designs, the fuel cell is surrounded by a gap that can present a significant resistance to heat transfer in the case of a heat pipe failure.…”

SCALE Modeling of the Fast Spectrum Heat Pipe Reactor

“…Although the geometry of this SCALE model is as close to the INL reference design as could be inferred, as described by Sterbentz et al [21], there are a few key material and modeling differences. The reference document gives the isotopic number densities for each material used in the model.…”

“…With a range of potential operating temperatures listed in the reference document [21], a uniform temperature distribution was initially used. All fuel material temperatures were set at a constant 1,000 K, regardless of position in the core, whereas all other materials were modeled at 950 K. The fuel temperature of 1,000 K was chosen because it is near the upper limit operating temperature provided in Sterbentz et al [21]. This uniform temperature case was used to generate a power profile that was then given to the MELCOR team at SNL.…”

Coupled Multiphysics Simulations of Heat Pipe Microreactors Using DireWolf

“…Many of the heat pipe values displayed in Table II are drawn from previous design parameters and experimental measurements. [17][18][19][20] The material properties used in this analysis, included in Table III, utilize approximate values to provide comparison to any future reference of this core. Note that due to homogenization of the mesh resulting in no-fuel/monolith or moderator/ monolith gaps, the Young's modulus for all materials is set to be a tenth of the monolith strength to prevent overestimation of displacements from fuel swelling or moderator thermal expansion.…”

“…Moreover, one of the anticipated accident scenarios for heat pipe-cooled microreactors is a cascade failure of heat pipes initiated by a failure of a heat pipe as a consequence of loss of working fluid due to gross mechanical failure or manufacturing defects. 17 To illustrate the ability of DireWolf to capture such a transient, an overpower transient at 19% was simulated with and without a manual "failure" of a heat pipe between the center and the edge of the assembly.…”