Impact of Technology Characteristics on Transition to a Fast Reactor Fleet drhgfdjhngngfmhgmghmghjmghfmf 20 min total EDWARD Hoffman Bo Feng Ben Betzler, Eva Davidson, & Andy WOrrall Argonne National Laboratory Technical Lead for Transition Analysis Studies for the Systems Analysis and Integration Campaign Argonne National Laboratory Oak Ridge National Laboratory 3rd Technical Workshop on Fuel Cycle Simulation Paris, France, July 9-11, 2018

Overview Introduction Study objective Representative SFR and MSR systems Modeling Methodology Describe simplified modeling approach Results

Study Objective The focus of this study is on technology characteristics that are likely to be impacted by the choice of SFR or MSR Specifically those characteristics that will have the biggest impact on the supply of and demand for fissile material Inform R&D and design choices to enable a more efficient deployment of a large fleet of fast reactors under different scenarios Inform on the characteristics that will lead one technology to perform better than another and not try and predict which technology will ultimately perform better Assess our current understanding in this area

Fuel Cycle choice Fast Breeder Reactors Continuously Recycling U/TRU (EG24) Low enriched uranium (LEU) used as need Recycled fast reactor material utilized as soon as available Assume only constraint is a minimum recycle time (theoretical performance) Fast Reactors Liquid-Fueled Molten Salt Reactor (MSR) Solid-Fueled Sodium-Cooled Reactor (SFR) Recycle Time MSR - ~0 (longer times will look like SFRs) SFR – collocated (2-3 yrs) or centralized (7+ years) Breeding – Breakeven through maximum practical Discharged Recycle As Needed Fast Reactor Fleet LEU Recycled

Modeling and Performance Measures Because the study is focused on the impact of differences in technology characteristics and not detailed dynamic behavior of a specific scenario about the future, the modeling can be simplified Used the DYMOND code to model a few scenarios Acted as benchmark for a spreadsheet model used for rapidly modeling many scenarios Both were useful in calibrating user input for the other model The performance measure chosen was how much natural uranium (NU) and enrichment (SWU) is required No cost info, both should have similar waste without detailed designs, etc.

Spreadsheet Modeling MSR and SFR deployed with required startup inventory (fissile material needed at or very near deployment of new capacity) Initially only source of fissile material is LEU MSR and SFR breeding ratio is minimum to be reactivity breakeven or higher No additional fissile required after first fuel is recycled Breeding accounts for fissile quality difference: U-235 vs TRU (Pu-239) For the SFR, operating under the same conditions approximately 1.3 – 1.4 atoms of U-235 is equivalent to 1.0 atoms of Pu-239 The LEU system can have a fissile breeding ratio well below 1.0, but produce sufficient Pu-239 to have the same reactivity in the recycled fuel The LEU fuel will have significantly higher fissile (U-235) concentration than the steady-state fissile (primarily Pu-239) concentration Material is recycled as soon as assumed possible MSR is self-sufficient immediately (zero recycle time) SFR requires additional fuel reloads determined by the minimum recycle time For excess breeding scenarios, the system automatically balances once no more LEU is required Pu equivalence needed to estimate startup when there is no LEU case and the maximum equivalent breeding ratio. LEU fuel will have a much lower fissile breeding ratio, but in terms of reactivity of the recycled fuel, it can get near that of Pu because it needs to produce less.

Spreadsheet Modeling Single Unit Examples In order to explain the underlying behavior during transition, several examples of the fissile material flow are show for single units operating independently IFR: SFR with a 2 year recycle time such as an Integral Fast Reactor SFR - Central: SFR with a 7 year recycle time such as a system with centralized recycling MSR (y.yx): MSR that requires y.y times as much inventory at startup as and SFR Examples include 1.5, 2.5, and 3.5. Further study suggest that this range for well-designed commercial MSRs and SFRs ranges from 0.4 to 2.2 The spreadsheet integrates this behavior into a single system to calculate the equivalent fissile mass balance For reactivity breakeven systems with no constraints on recycling other than the minimum recycle time, this gives the exact answer since they are all effectively independent units (no net flow of fissile between units)

Basic Fissile Material Flow Pu-239 eq fissile (t/GWe) Pu-239 eq fissile (t/GWe) Note scale differences. Explain that for breeders the discharge will be greater than demand. Pu-239 eq fissile (t/GWe) Pu-239 eq fissile (t/GWe)

Integral Demand for a Single Unit For breakeven systems, MSR: all fissile demand filled at startup SFR: additional fissile demand filled for reloads based on minimum recycle time For breeder systems, MSR: Excess available immediately SFR: No excess available until minimum recycle time Simple constraints applied so there is no need for complex dynamic modeling Also no interest in cycle by cycle or other dynamic behavior, but approximate integral behavior Pu-239 eq fissile (t/GWe) Breakeven Explain Lines Pu-239 eq fissile (t/GWe) Breeder

Results These parametric calculations revealed 3 key technology characteristics that impact front-end requirements Startup fissile inventory, which includes all the fissile material in and out of the core for the MSR systems Recycle time, which determines the amount of material and timing for the SFR systems with the MSR effectively being a zero recycle time system Net breeding rate of fissile material, which account for system losses, isotopic evolution, etc. on a reactivity equivalent basis Each of these have many important underlying design characteristic such as power density, thermal efficiency, and others that combine to produce these key characteristics Given these uncertainties in the key characteristics that affect transition performance, it would be misleading to draw any general conclusions from the direct comparison of a few examples of specific SFR and MSR designs Additionally, the performance is sensitive to the assumptions about the future used in the particular transition scenario The approach to inform on this was to calculate the Equal Performance Line (EPL) for a range of scenarios Above the EPL, the SFR performs better and below the MSR does

Equal Performance Line SFR and MSR both breakeven, expand to 100 GWe in 20 years

Equal Performance Line SFR and MSR both breakeven, expand to 100 GWe in 20 years

Equal Performance Line Three cases: SFR and MSR both breakeven, SFR and MSR max Breeding, and SFR max breeding/MSR moderate breeding

Summary By using the dynamic systems tools as a benchmark, developed a simplified spreadsheet to run a large number of cases to study a very large design space efficiently Identified the most important features under a wide range of conditions Identified where uncertainty in the current designs is most important to front end requirements to transition to a large fleet The current range of designs being considered for MSRs and SFRs leads to a wide band of uncertainty in relative performance SFR systems with high power densities, high breeding rates, and short recycle times are needed to minimize front end requirements for transition MSR systems with high power densities, high breeding rates, and small fissile inventories outside of the core are needed to minimize front end requirements for transition A simple method was developed to generate a series of curves that can compare the relative performance of MSR and SFR systems Easily expandable to systems with more complex constraints Requires significantly more time to model and calculate EPL Note that when considering recycle of LWR UNF, it becomes far more complex. How much LWR UNF will you have available and how quickly and how large you grow your fast reactor fleet may lead to a large excess of LWR UNF or a large shortfall.

Thank you for your attention! ehoffman@anl.gov

Equal Performance Line Three cases: 40 year transition

Equal Performance Line Three cases: 40 year transition with 2% sustained growth

Equal Performance Line Three cases: 20 year transition with high burnup SFR fuel