Challenges in Prismatic HTR Reactor Physics Javier Ortensi R&D Scientist - Idaho National Laboratory www.inl.gov Advanced Reactor Concepts Workshop, PHYSOR 2012 April 15, 2012
Outline HTR reactor physics characteristics HTR modeling challenges PMR characteristics PMRs modeling challenges PMR validation Conclusion
HTR Reactor Physics Characteristics (1/3) Due to dispersed coated particles (fuel) Flux gradients across compact/pebble are small both in energy and space unlike LWRs Energy self-shielding less important than in LWRs, but still treated Very adaptive to various fuel cycles: LEU, MOX, Deep Burn, Pu, Th Can reach high burnups (>120 GWD/MTU) Efficiently burns Pu, produces 20% less Pu that LWRs Coolant is decoupled from neutron moderation due to time scales for conduction vs convective heat transfer High temperature gradients between fuel and moderator during transients
HTR Reactor Physics Characteristics (2/3) Reactor control via control rods or coolant mass flow rate Great reactor stability. Temperature reactivity coefficients (HTR-10) Fuel, immediate (~ -2x10-5 Δk/k/ o C) Moderator, delayed, sec - minutes (~ -16x10-5 Δk/k/ o C) Reflector, very delayed, >minutes (~ 8x10-5 Δk/k/ o C) Most transients are slow Strong Doppler shuts down the reactor DLOFC (LOCA)
HTR Reactor Physics Characteristics (3/3) PMR Core PMR Block Fresh Burned # collisions to thermal 95.9 96.1 96.2 22.4 # collisions while thermal 174.0 42.1 37.2 14 PWR Assembly Distance to thermal (rms) 44 cm 45 cm 45 cm 15 cm Distance while thermal (rms) 34 cm 22 cm 21 cm 4 cm Thermal utilization 0.91 0.984 0.986 0.89 Resonance escape 0.64 0.53 0.54 0.63 % Elastic_scattering reactions 35 68 70 67 % Bound_scattering reactions 64 31 29 28
HTR Modeling Challenges Double Heterogeneity (DH) of the fuel Resonance self shielding and interference effects Specially low lying resonances Graphite cross sections Heterogeneity at the core/reflector interface Anisotropies
Double Heterogeneity Effects for various fuels UO 2 (1.5% Δρ) TRUO 1.7 (14.0% Δρ) UC 0.5 O 1.5 (2.3% Δρ) DH treatments Dancoff (COMBINE) Reactivity-Equivalent Physical Transformation (RPT) PW Disadvantage Factors (SCALE) Hebert (APOLLO-1, DRAGON-4) Sanchez-Pomraning (APOLLO-2, HELIOS-2)
Resonance Self Shielding and Interference Energy dominates over spatial self-shielding effects Fine group structure in cell / lattice calculations is important HELIOS-2 177 group library showed swing during depletion New 335 group library based on SHEM structure developed HELIOS-2 Fine Group Library => 177 335 Reactivity swing [0-120 GWD/MTU] ~250 ~100 RR a (U-235 near 27.55 ev) -53-2 RR a (Pu-240 near 1.9 ev) -132 1 RR a (U-238 near 9.5 ev) -99-21 RR nufiss (U-235 near 27.55 ev) 62 5 RR nufiss (U-235 near 0.625 ev) -68 2 Work by Chuck Wemple (Studsvik)
Graphite Cross Sections How well do we know s(α, β) for graphite, specially at high temperatures? Bound elastic scattering dominates the full core calculation (~67% of all reactions) How well do we know the 12 C capture cross section? Currently two distinct groups of measured thermal 12 C(n,γ) 3.86mb vs 3.53mb Large amount of graphite in the core Difference worth up to ~1.5% Δρ C. Shull, neutron scattering using Diffractometer at Graphite Reactor, ORNL
Fuel / Reflector Interface (1/2) Large # of neutrons slow down in the reflectors and return to the active core bypassing resonance region Reflectors have large effect on the power shape within the first 36 cm of active core -> affect local peaking and fuel burnup Need better spectrum to generate cross sections Traditional approaches for whole core calculations 0.03 In-line spectrum correction (PBR) Use large # of coarse groups 0.025 New approach Supercell calculation Flux/Lethargy 0.02 0.015 0.01 1.9 cm 5.6 cm 7.5 cm 11.3 cm 13.2 cm 16.9 cm 24.4 cm 28.2 cm 30.1 cm 33.8 cm 35.7 cm 39.5 cm 0.005 0 1E-03 1E-01 1E+01 1E+03 1E+05 1E+07 Neutron Energy [ev]
Core Anisotropies PBRs have an upper plenum just above core PMRs have CR holes (some more than others) Can be easily treated with anisotropic diffusion coefficients Derived from MC computations Analytic Benoist method (check applicability to size) E. Larsen approach
www.inl.gov PMR Specific Issues
PMR Characteristics (1/2) 2.74 m 1.60 m 7.9 m 2.97 m 2.0 m 0.9 m
PMR Characteristics (2/2) Parameter Value Unit Block Pitch 36 cm Block Height 79.3 cm Fuel Holes 210 Coolant Holes 102 / 6 LBP Locations 6
PMR Modeling Challenges Fuel/reflector interface Knowing precisely the fuel location can also be painful In-line 1-D transport corrections not optimal for 2-D effects Use large numbers of groups Still might need Surface Discontinuity Factors (SDF) Burnable Poison (BP) effects CR influence (deeply inserted at BOEC) Thermal Gradients Validation
Coarse Group Energy Structure Matters 26 23 9 1.96E+07 2.00E+07 1.40E+07 7.41E+06 3.68E+06 1.83E+05 3.33E+06 1.35E+06 9.61E+02 7.07E+05 5.00E+05 1.76E+01 1.16E+05 1.11E+05 3.93E+00 1.86E+04 6.74E+04 2.38E+00 3.48E+03 9.12E+03 1.28E+00 1.59E+03 3.67E+02 8.25E- 01 7.49E+02 4.00E+00 1.30E- 01 2.76E+02 1.50E+00 1.33E+02 1.10E+00 6.23E+01 1.05E+00 3.17E+01 9.72E-01 1.35E+01 8.50E-01 8.30E+00 5.00E-01 5.11E+00 4.00E-01 2.33E+00 3.50E-01 1.29E+00 3.00E-01 6.25E-01 2.50E-01 3.53E-01 1.80E-01 2.10E-01 1.40E-01 1.20E-01 1.00E-01 7.65E-02 5.00E-02 5.55E-02 2.00E-02 1.05E-02 # Groups Keff pcm 26 1.01107 25 1.01102-5.2 23 1.02344 1223.5 9* 1.00938-167.3 Cancellation of error is predominant Need a robust group optimization method Minimize reactivity swing during depletion *GA experimentally validated
PMR Reflector Effects (1/4) QUESTION: Domain necessary to decouple a block? Compute 1 GRP cross sections while increasing domain size until it stops changing ANSWER: beyond core boundary Ultimately need a full core calculation for optimum spectrum Supercell (large colorset) could be a very good approximation
PMR Reflector Effects (2/4) QUESTION: How do the pin powers between a block with similar neighbors (reflected BC) and one with dissimilar compare? Use supercell calculation with reflector and burned fuel (No BP) Once burned (no burn) Reflector
PMR Reflector Effects (3/4) Factor of 2.1 across block diagonal Power shift during burnup helps Leads to uneven TRU production
PMR Reflector Effects (4/4) Note: missing global power shape & TH effects (future work) Potentially worse near inner reflector Increase the domain size to Improve cross sections Improve compact power reconstruction Obtain surface discontinuity factors Inline corrections lattice code difficult but possible Large numbers of groups in both lattice and transport Spatial, angular, and energy unfolding techniques Albedo B.C.
PMR BP Effects (1/3) Major influence within 11 cm on the reflector side Factor of 1.85 across block diagonal Less on the burned side, but power shifts to other regions
PMR BP Effects (2/3) How does adding BPs affect the supercell neighbors? Differences are larger in neighboring blocks due to asymmetry
PMR BP Effects (3/3) Potential ways to handle Add spatial dependent sink term to the balance equation to account for strong absorbers (HEXPEDITE - M. Ferrer, A. Ououag, A. Bingham) Isolate BP with a special mesh (INL-PRONGHORN, INSTANT, RATTLESNAKE) Consider effects on neighbor during depletion How do we ensure consistent reaction rates? SDF SPH
PMR with Control Rods (1/3) CR hardens the spectrum across the block Forces similar spectra on both sides
PMR with Control Rods (2/3) Depression region ~ 17 cm Power shift significantly higher than BP due to asymmetry Power shape similar to no CR at higher burnups
PMR with Control Rods (3/3) Must do depletion in the presence of CRs Controlled cross sections via SC or larger domain Use GET to ensure consistent reaction rates Triangular mesh for CR treatment makes life easier INSTANT, PARCS, PRONGHORN
Thermal Gradients At steady state core mid height high power location ΔT~100 from fuel to moderator ΔT~125 moderator to coolant ΔT~300 across a block near inner reflector Neutronics / themal fluids coupling is an active area of research
PMR Validation Challenges No abundant validation data Best available & applicable to current LEU and TRISO design VHTRC (critical facility, temperature data) HTTR (criticals & depletion) VHTRC is preferred initially because is clean and simpler neutronics issues HTTR issues Control Rods (CR) Axial heterogeneity (-1.5% Δk/k) Block orientation (BP & CR) Axial streaming effects (1.8% Δk/k)
Conclusion (1/2) HTRs are very stable and versatile reactors Extremely safe due to strong temperature feedback and high thermal inertia Long migration lengths in HTRs create unique challenges in modeling Graphite cross sections are an open question Fine and course energy group structures are important for selfshielding effects (both energy and spatial) Core anisotropies can be treated with current methods Validation of PMR difficult due to lack of data
Conclusion (2/2) PMR Supercells can be useful to treat reflectors, BPs, and CRs with Improved cross sections & compact reconstruction data Obtain SDFs or SPH factors PMR cross section preparation Ensure that spectrum is consistent in lattice and core locations Must include CR branches How to treat BP effects for neighbor blocks? PMR Core modeling Triangular mesh preferable Isolate BP and CR regions OR spatially dependent cross sections? Can we optimize shuffling schemes? What is logistically possible (rotations)?
OECD/NEA MHTGR-350MW Benchmark Main Focus on coupled neutronic thermal fluids time dependent behavior Full 3-D specification Based on MHTGR-350 MW EOEC design 4 parameter cross section tabulation in 26 energy groups Temperature and fluence dependent thermo-physical properties
Acknowledgements Work partially supported by the U.S. Department of Energy, Assistant Secretary for the Office of Nuclear Energy, under DOE Idaho Operations Office Contract DE-AC07-05ID14517.
ADDITIONAL SLIDES
PMR Reflector Effects - Isotopics 0.1 0.09 0.08 30 GWD/MTU 60 GWD/MTU 100 GWD/MTU 120 GWD/MTU U-235 Concentration 0.07 0.06 0.05 0.04 0.03 0.02 0.01 0 18 16 14 12 10 8 Compact Position 6 4 2 0
PMR BP Effects - Isotopics
PMR Control Rods Effects - Isotopics
Fuel / Reflector Interface Variation of 1 GRP Macro Cross Sections For an annular core same fuel across width Using middle block as reference (most resembling lattice physics) Fresh Fuel Block 1 Block3 Fission 30.4% 26.1% Capture -21.6% -22.0% Burned Fuel Block 1 Block3 Fission 35.4% 30.2% Capture 3.8% 4.4%