COMSOL-based Simulations of Criticality Excursion Transients in Fissile Solution Christopher J. Hurt *,1 , Peter L. Angelo 2 , Ronald E. Pevey 1 1 University of Tennessee, Department of Nuclear Engineering 2 Y-12 National Security Complex, Safety Analysis Engineering *Corresponding author: P. O. Box 2008, Oak Ridge, TN 37831-6399, churt2@utk.edu, cjh@ornl.gov Abstract: Simulation of criticality accident transients offers the ability to confirm understanding of critical configurations, bound accident scenarios and aid comprehensive emergency planning. Computational ability to recreate excursion power histories in fissile solution is sought due to the predominance of fissile solutions in process criticality accidents [1]. Applicable solution transient physics methodologies are developed for neutron kinetics, heat transfer, and radiolytic gas transport. This work is the first step in validating the use of COMSOL [2] as a flexible and powerful computational tool to simulate criticality excursions. Previously, characterization of excursion power history profiles has been limited to hand-calculations and simulation tools with 1- D/0-D physics treatments for specific geometries [3, 4, 5, 6]. This work demonstrates COMSOL’s capability for an accurate, generalized approach to excursion simulations for nuclear safety applications. Keywords: SILENE, neutronics, multiphysics, transport. 1. Introduction COMSOL Multiphysics is used to develop coupled physics for radiolytic gas transport, neutron kinetics, and heat transfer for the purpose of simulating criticality excursion power histories. This simulation tool is exhibited for two cases: a documented criticality benchmark experiment, SILENE LE1-641 [3] and a theoretical configuration. 1.2 Brief Criticality Excursion Background A critical, or supercritical, configuration of fissionable material is said to be present when the fission chain reaction is self-sustaining. That is, when the ratio of fissions, or neutrons, from one generation to the next (called the effective neutron multiplication factor, k eff , per Eq. 1) is equal to or greater than unity. Reactivity, ρ, is the measure of the deviation of k eff from unity, or a critical state, (see Eq. 2). Reactivity is often expressed in terms of dollars and cents, where a dollar is equal to the delayed neutron fraction, β eff . A criticality excursion occurs when this chain reaction experiences exponential growth until a “spike” occurs in the power history, due to a natural or external negative reactivity feedback effect.           - (1)       (2) For a fissile solution excursion, there are several multiphysics phenomena which naturally serve as the reactivity feedback to dampen a power excursion. The first feedback considered in this study is the rise in temperature, which affects both the density and nuclear properties of materials, and the second the phenomena of radiolysis, where gas bubbles are formed due to fission particle tracks. 1.3 SILENE LE1-641 The SILENE LE1-641 benchmark was part of a series of criticality experiments performed at the SILENE reactor at the Valduc Laboratory Critical Experiment Facilities in France. The SILENE reactor is an annular, cylindrical tank with an outer chamber for fissile solution and an inner chamber for control rod insertion. Figure 1 shows a picture and diagram of the SILENE reactor. The SILENE LE1-641 benchmark consists of a 93% enriched uranyl nitrate (UN) solution, in which reactivity is inserted via the rapid withdrawal of a control rod. This results in a 2$ reactivity insertion over the first 20 seconds of the transient.