BNL-114869-2018-IR Phenomena Important in Modeling and Simulation of Molten Salt Reactors Manuscript Completed: April 23, 2018 Prepared by: David J. Diamond,1 Nicholas R. Brown,2 Richard Denning,3 and Stephen Bajorek4 1Brookhaven National Laboratory Upton, NY 11973-5000 2Pennsylvania State University University Park, PA 16802 3Consultant Columbus, Ohio 43220 4Office of Nuclear Regulatory Research U.S. Nuclear Regulatory Commission Prepared for: George Tartal Office of New Reactors U.S. Nuclear Regulatory Commission ABSTRACT The U.S. Nuclear Regulatory Commission (NRC) is preparing for the future licensing of advanced reactors that will be very different from current light water reactors. Part of the NRC preparation strategy is to identify the simulation tools that will be used for confirmatory safety analysis of normal operation and abnormal situations in those reactors. This report advances that strategy for reactors that will use molten salts (MSRs). This includes reactors with the fuel within the salt as well as reactors using solid fuel. Although both types are discussed in this report, the emphasis is on those reactors with liquid fuel because of the perception that solid-fuel MSRs will be significantly easier to simulate. These liquid-fuel reactors include thermal and fast neutron spectrum alternatives. The specific designs discussed in the report are a subset of many designs being considered in the U.S. and elsewhere but they are considered the most likely to submit information to the NRC in the near future. The objective herein, is to understand the design of proposed molten salt reactors, how they will operate under normal or transient/accident conditions, and what will be the corresponding modeling needs of simulation tools that consider neutronics, heat transfer, fluid dynamics, and material composition changes in the molten salt. These tools will enable the NRC to eventually carry out confirmatory analyses that examine the validity and accuracy of applicant’s calculations and help determine the margin of safety in plant design. The study makes use of the limited amount of experience with such concepts, for example, from the 8 MWt Molten Salt Reactor Experiment at Oak Ridge National Laboratory in the 1950s and 1960s, and recent conceptual design studies. Since the latter studies are so preliminary, the present work focused on the primary system, and did not consider in detail systems for secondary or tertiary heat removal, decay heat removal, and reactor cavity cooling, nor operation of emergency shutdown equipment. This study will facilitate decisions in the future, as to required simulation tool capabilities and what research is necessary before such tools can be successfully used. The identification of modeling needs for liquid-fuel MSRs was carried out by subject matter experts, using a process frequently used by NRC to elicit information; namely, the Phenomena Identification and Ranking Table (PIRT). However, since that process would normally require knowing specific details about a reactor design and the sequence of events during accidents and since MSR designs are varied and not yet well defined, the process used was considered a “pre-PIRT” and the panel considered what might happen generically if there were changes in reactivity in the core or changes in temperature. The panel met after reviewing the available information on design details and postulated normal and abnormal operation. They defined phenomena that would need to be modeled and considered the impact/importance of each phenomenon with respect to specific figures-of-merit (e.g., peak power, fluence, flow velocity, temperature). Each figure-of-merit reflects a potential impact on radionuclide release or loss of a barrier to iii release. The panel also considered what the path forward might be with respect to being able to model the phenomenon in a simulation code. Subsequent to the pre-PIRT meeting, this report was written to capture the information from the panel and add other relevant material. The tables of phenomena, their impact and a proposed path forward for filling in the technology gaps were generated separately for neutronics and thermal-hydraulics for thermal spectrum and fast spectrum liquid-fuel reactors. There is already considerable information in the literature on the simulation of solid-fuel MSRs. The tables give a sense of what are the most important phenomena and of those, which ones are the least well understood and hence will require the most effort to be modeled in the future. The neutronic phenomena are broken into two categories: basic nuclear data, and material composition. The latter is particularly important for liquid-fuel MSRs and is discussed extensively in the report because material inventory and distribution may be continuously changing due to the addition of fissile material, the removal of fission products and/or certain actinides, and chemical interactions and radiolysis possible in the molten salt. The thermal-hydraulic phenomena are broken into categories representing different systems/components: fuel salt, core materials, primary pumps, and primary heat exchanger. In addition to discussing the specific phenomena in each table, the report considers more generally the issue of inventory control and distribution, and experimental and other research needs and provides a summary of recommendations related to these aspects. It is expected that as an MSR design nears its final stages and is submitted to the NRC for review, a PIRT panel will re-examine the information in this report and will identify what additional work must be done to have the desired simulation capability. iv TABLE OF CONTENTS ABSTRACT ..................................................................................................................... iii LIST OF FIGURES ......................................................................................................... vii LIST OF TABLES ........................................................................................................... vii ACKNOWLEDGEMENTS ............................................................................................... ix ACRONYMS ................................................................................................................... xi 1 INTRODUCTION ................................................................................................... 1-1 1.1 Background ..................................................................................................... 1-1 1.2 Objective ......................................................................................................... 1-2 1.3 Methodology.................................................................................................... 1-3 1.4 References ...................................................................................................... 1-4 2 DESCRIPTION OF MOLTEN SALT REACTORS ................................................. 2-1 2.1 General Design Features ................................................................................ 2-1 2.2 Thermal Liquid-Fuel Molten Salt Reactors ...................................................... 2-1 2.3 Molten Salt Fast Reactors ............................................................................... 2-8 2.4 Solid-Fuel Molten Salt Reactors .................................................................... 2-10 2.5 References .................................................................................................... 2-12 3 SIMULATION SCENARIOS .................................................................................. 3-1 3.1 Introduction ..................................................................................................... 3-1 3.2 Liquid-Fuel Molten Salt Reactors .................................................................... 3-3 Normal Operation ..................................................................................... 3-3 Reactivity Changes ................................................................................... 3-3 Increase/Decrease of Temperature .......................................................... 3-5 Other Scenarios ........................................................................................ 3-5 3.3 Solid-Fuel Molten Salt Reactors ...................................................................... 3-5 Generic Information .................................................................................. 3-5 Overcooling Events .................................................................................. 3-8 Inadvertent Control Element Movement ................................................... 3-9 Primary Loop Break .................................................................................. 3-9 Loss of Forced Flow ................................................................................. 3-9 Station Blackout ...................................................................................... 3-10 3.4 References .................................................................................................... 3-10 4 IMPORTANT PHYSICAL PROCESSES FOR MSRs ............................................ 4-1 4.1 Introduction ..................................................................................................... 4-1 4.2 Thermal Spectrum Liquid-Fuel MSRs ............................................................. 4-1 Neutronics Phenomena ...........................................................................