Global Threat Reduction Initiative Safety Analysis of the MIT Nuclear Reactor for Conversion to LEU Fuel Erik H. Wilson, Floyd E. Dunn Argonne National Laboratory Thomas H. Newton Jr., Lin-wen Hu MIT Nuclear Reactor Laboratory 5 th International Symposium on Materials Testing Reactors Columbia, Missouri - October 22-25, 2012 Work Sponsored by: Defense Nuclear Nonproliferation The submitted work has been created by UChicago Argonne, LLC, Operator of Argonne National Laboratory ( Argonne ). Argonne, a U.S. Department of Energy Office of Science laboratory, is operated under Contract No. DE-AC02-06CH11357. The U.S. Government retains for itself, and others acting on its behalf, a paid-up nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government. Work supported by US Department of Energy, Office of Global Threat Reduction, National Nuclear Security Administration (NNSA).
MIT Reactor (MITR) overview MIT Reactor LEU Conversion- Neutronic Modeling MIT currently operates an HEU (93% 235 U) research reactor @ 6MW Conversion to LEU (<20% 235 U) planned Conversion strategy U-10 wt %Mo fuel under development with U density > 15 g/cc 7 MW operation; maintain flexible fuel utilization Neutronic Modeling Benchmark / Methods Reactor fuel cycle Coupling to LEU conversion: HEU LEU Conversion Safety Basis Fuel Development Demo Irradiations Fuel Fabrication 2 MITR LEU Conversion
MITR Reactor Core tank & pool (break-away view): Reactor Facility: Fueling of Core Region
MITR Reactor ~ Flexible Fuel Loading Degrees of Freedom Rhomboid-shaped fuel element 27 fuel positions Three rings (A,B,C) Experimental loops Flexible fuel shuffling Rotate &/or flip Core Tank Elements reside in-core over C-8 B-5 several years A-2 C-9 C-7 C-10 B-6 B-4 C-11 C-12 B-7 B-8 A-3 A-1 B-9 B-1 C-14 C-15 C-1 Fuel Element C-6 B-3 B-2 C-2 C-5 C-4 C-3 Control Blades (6) Coolant Entrances (6) Core Structure Regulating Rod
MITR Reactor Core layout (top view): MITR fuel element: 6 cm wide
MITR Reactor Core layout (top view): MCNP Model (cross-section): 50 cm diameter
MITR Neutronic Modeling Methods Reactor transport model- MCNP5 Geometry Explicit geometry per specifications Materials Uses fuel loading information from fabricator Fully isotopic ENDF/B-VII Trace isotopes and impurities Model benchmarking of original fresh and early cores 1-4 Model benchmarking of depleted cores 179-190 MIT s MCODE MCNP coupled ORIGEN DEpletion ORIGEN2.2 or ORIGEN-S Constant power irradiation steps in each core using historical operating powers Depleted at critical control position Code to code comparisons with REBUS-MCNP and REBUS-DIF3D Core Depletion-Power distribution discretization 1k independently depleted materials 30k regions for power
Model Benchmarking vs. HEU Measured Data Historical startup physics & retrofits: Cores 1-4 Recent depleted MITR: Cores 179-190 Deviation from critical modeled at measured control positions " " " " MITR HEU Core BOC k-eff EOC k-eff 1 0.997-2 1.002-4 1.004-179 1.000 1.007 180 1.000 1.006 181 1.003 1.008 182 1.003 1.008 183 1.003 1.002 184 1.002 1.008 185 1.004 1.009 186 1.005 1.010 187 1.004 1.010 188 1.004 1.009 189 1.003 1.006 190 1.004 1.006 Average dk/k 0.3% 0.7% Uncert. 1-σ 0.2% 0.2%
Model Benchmarking vs. HEU Experiments Component reactivity worth Fuel element worth (A/B/C-rings) Model Measured -0.2% ± 0.2% Non-fuel element worth Measured 0.6% model 0.57% ± 0.04% Control blade calibrations Travel 53 cm (21 inches) Several cores Multiple blades Cd and borated steel
Model Benchmarking vs. HEU Experiments Heavy water reflector dump Measured 7.0% model 6.9% ± 0.2% Experiment D2O shutter worth measured 55 pcm model 53 ± 6 pcm Void worth in a single coolant channel Displace water in 60 cm channel Displace water in 15 cm of channel
Modeled Critical Control Position (cm withdrawn) Core Fuel Loading Sequence for LEU Safety Analysis Historical Core Sequence depleted with MCODE LEU model of an analogous 12 core sequence: Optimized regions for depletion and power distribution Regions Plate Division Fuel Axial Division Fuel Plate Lateral Division Total per (LEU) Core Geometry Each plate discrete Optimization Study 1 to 6 6 groups Current LEU Safety Analyses Power Depletion Shape 18 Continuous 6 to 32 6 18 Continuous 1 to 4 1 4-100 ~ 10000 864 31104
Peak Clad Temperature (ºC) Lateral Heat Conduction into Side Channel How many stripes is enough for power distributions? Power peaks at edge of plate Peak of sixteen stripes (0.3 cm) ~20% higher than four stripes (1.3 cm) How much does heat conduction help? Four stripes, no lateral conduction is conservative vs. sixteen with conduction Heat flux peaks at edge Peak of sixteen stripes (0.3 cm) ~20% higher than four stripes (1.3 cm) 12 Stripe
MITR LEU Power Peaking Locations by Core Loading Blade 4 Blade 3 C-8 C-9 C-7 C-10 B-5 C-6 B-6 B-4 C-5 Blade 5 BOC& EOC 188 BOC 187 EOC179,180,187 EOC BOC 187-8 EOC179,187 C-11 188 C-12 C-13 B-7 B-8 A-3 A-2 B-3 Fresh C-4 A-1 BOC 179-180,185-6 B-9 B-2 B-1 C-3 C-14 C-2 C-15 BOC183-4,189-90 EOC180,183-6,189-90 Fresh, BOC179,180,183-85,189-90 EOC183-5,189-90 C-1 BOC&EOC 186 Blade 1 Core Structural Wall Blade 6 BOC&EOC 181-2 BOC&EOC 181-2 Peak locations Exclusively in plates adjacent to reflector Edge of plate Peak stripes All-fresh A-ring Depleted core peak stripes found in plate adjacent to reflector edge of plate EOC > BOC stripe power Hot Spot Core Tank Hot Stripe Blade 2
Peak Power Profiles from Pseudo-equilibrium Peak location of all cores Core 189 BOC in depleted element Peak stripe of all cores In same depleted element, Core 189 EOC 100 Heat flux from foil surface (W/cm2) 80 60 40 20 0 Core 189 EOC Core 189 BOC 0 10 20 30 40 50 60 Distance from bottom of fuel (cm)
Peak Power Profiles from Pseudo-equilibrium Power distributions Edge of plate more limiting Stripe size accounts for heat conducted at edge of fuel into unfueled region Peak stripe of all cores Peak stripe 13% higher than a plate average 100 Heat flux from foil surface (W/cm2) 80 60 40 20 0 Core 189 EOC Stripe 1 Core 189 EOC Stripe 2 Core 189 EOC Stripe 3 Core 189 EOC Stripe 4 0 10 20 30 40 50 60 Distance from bottom of fuel (cm)
Power History of a Plate through MOL Power Peak Power lower in beginning of fuel plate life Element loaded into core interior Flatter power shape Plate becomes peak stripe of all cores in 4 th core loading of the element Plate loaded next to reflector Stripe heat flux increases by 63% in this limiting depleted element 100 Heat flux from foil surface (W/cm2) 80 60 40 20 0 Core 179 BOC (plate in core interior) Core 189 EOC (plate at reflector) 0 10 20 30 40 50 60 Distance from bottom of fuel (cm)
Power History of a Plate through Middle of Life Power Peak Peak power in MITR occurs in middle of life When fuel moved from core interior to fuel ring adjacent to reflector Peak location increases in power by 43% in MOL Peak stripe averaged power BOC EOC while in core interior (initial fuel loadings) EOC >> BOC (moved next to reflector when depleted) Control blade motion shifts power out towards D2O reflector by EOC Peak stripe increases by 63% in MOL to 2.1 x core average
MIT Reactor LEU Experimental Performance HEU performance maintained with the use of monolithic U-10Mo LEU fuel and an upgrade in operating power from 6 MW to 7 MW LEU in-core fast flux 1 x 10 14 n/(cm 2 s) LEU ex-core thermal flux 5 x 10 13 n/(cm 2 s) LEU uses about 30% fewer elements LEU Core Flux (cm -2 s -1 ) In-Core Irradiation (A-ring) Twelve-inch (30 cm) Beam Port Two-inch (5 cm) Pneumatic Facility Fission Converter Window Below-core Thermal Beam Facility Energy >0.1 MeV <0.4 ev <0.4 ev <0.4 ev <0.4 ev LEU Fresh 22 Element 109% 103% 103% 102% 102% LEU 185 BOC 107% 103% 104% 104% 103% LEU 185 EOC 107% 101% 104% 104% 101% LEU 186 BOC 107% 103% 106% 106% 103% LEU 186 EOC 108% 103% 106% 107% 104% LEU 187 BOC 108% 103% 106% 105% 103% LEU 187 EOC 107% 102% 105% 106% 103% LEU 188 BOC 107% 103% 105% 104% 103% LEU 188 EOC 106% 101% 103% 103% 100% LEU 189 BOC 107% 104% 106% 105% 104% LEU 189 EOC 106% 102% 104% 104% 102% LEU 190 BOC 107% 103% 105% 106% 103% LEU 190 EOC 106% 101% 103% 104% 101% 18
MIT Reactor LEU Conversion- Neutronic Modeling LEU conversion analyses summary Initiated by benchmarking neutronic models for HEU LEU and mixed HEU/LEU cores power analysis performed Incorporate depletion representative of fuel cycle All core loading state points fuel power history over life are of interest EOC of a depleted fuel plate most challenging for LSSS Transient and accident analyses are being conducted Continuing ability to optimize fabricability/performance of fuel element design Fuel Development HEU LEU Conversion Safety Basis Demo Irradiations Fuel Fabrication 19 MITR LEU Conversion
Acknowledgements The authors would like to thank: NNSA Office of Global Threat Reduction Convert Program for financial support The fine folks at MITR, ANL and INL for their hard work and technical expertise Nick Horelik for his excellent efforts in MCODE-FM development which were utilized in this work 20