LEU Conversion of the University of Wisconsin Nuclear Reactor Paul Wilson U. Wisconsin-Madison Russian-American Symposium on the Conversion of Research Reactors to Low Enriched Uranium Fuel 8-10 June 2011 Moscow
Overview Introducing the UWNR Conversion analysis: neutronics, thermal-hydraulics & accident analyses Methods Challenges Outcomes Lessons learned/opportunities Current status 9 June 2011 P.Wilson/UWNR LEU Conversion 2
About the UWNR 1MW TRIGA-type pool reactor Converted MTR with 4 pin square bundle Training as primary mission Irradiation services Neutron research 3 full time staff 1 part-time operator 3-6 student operators 12-20 hours/week 9 June 2011 P.Wilson/UWNR LEU Conversion 3
UWNR History January 1961 First critical (10 kw) December 1964 Power upgrate 250 kw November 1967 TRIGA conversion - 1 MW uprate & pulsing capability June 1979 Completed conversion to GA FLIP TRIGA HEU September 2009 Initial criticality with GA 30/20 TRIGA LEU fuel March 2011 Renewed facility license R-74 issued by U.S. Nuclear Regulatory Commission 9 June 2011 P.Wilson/UWNR LEU Conversion 4
Conversion History 1987 First notification of intent to convert October 2005 Confirmation of conversion schedule September 2007 Began in-house conversion analysis August 2008 Submitted conversion report to USNRC February 2009 Requests for Additional Information from USNRC June 2009 Order to convert from USNRC September 2009 First LEU fuel loaded September 2009 First criticality with LEU fuel 9 June 2011 P.Wilson/UWNR LEU Conversion 5
Neutronics Analysis Methods MCNP5 with ENDF/B-VII data (UW) Full 3-D core model including axial and radial details REBUS coupling for burnup analysis (ANL) 3 radial x 5 axial burnup zones per fuel pin Some confirmatory analysis with HELIOS (UW) 2-D deterministic analysis with detailed geometry 1-D diffusion approximation 9 June 2011 P.Wilson/UWNR LEU Conversion 6
Neutronics Analyses Most analyses repeated for beginning of life (BOL), middle of life (MOL) and end of life (EOL) Power distribution (for T/H analysis) Total pin power/core power distribution Axial/radial distribution in maximum power pin Shutdown margin as a function of burnup Reactivity parameters Delayed neutron fraction, prompt neutron lifetime, control element worth, prompt temperature coefficient 9 June 2011 P.Wilson/UWNR LEU Conversion 7
Neutronics Analysis Challenges Lack of data for robust HEU benchmarking of model/bias determination Operational data Material compositions Large CPU resource requirements for some analysis Modest existing capacity with reactor analysis 9 June 2011 P.Wilson/UWNR LEU Conversion 8
Neutronics Analysis Outcomes Calculated system reactivity too high with like-for-like replacement Change from symmetric 23 bundle + 10 reflector configuration (H) To symmetric 21 bundle + 14 reflector configuration (X) Slightly reduced core lifetime Additional bundles provided for future reactivity boost 9 June 2011 P.Wilson/UWNR LEU Conversion 9
Neutronics Operational Experience Real system substantially less reactive than calculated ~0.7% k/k (~$1) reactivity discrepancy from fuel (including axial reflectors) ~0.7% k/k (~$1) reactivity discrepancy from graphite reflectors Planning near-term core shuffle From X-configuration (21F + 14R) to +-configuration (21F + 6R) 9 June 2011 P.Wilson/UWNR LEU Conversion 10
Thermal Analysis Methods RELAP5/Mod3.3 Single channel with highest power channel 20 axial nodes 15 in active fuel region 27 radial nodes 21 in fuel Two channel model for pulsing Hot channel + rest of core RELAP point reactor kinetics model Temperature coefficients from MCNP5 analysis Two channel model of LOCA Three phases with different water levels Axial conduction in fuel 9 June 2011 P.Wilson/UWNR LEU Conversion 11
Thermal Analyses Results for high-power channel (assuming no cross-flow) Steady state Low power pulse (transient & maximum) High power pulse (transient & maximum) Flow rate Temperatures Maximum fuel centerline/clad Axial/radial profile Minimum Departure from Nucleate Boiling Ratio (MDNBR) 9 June 2011 P.Wilson/UWNR LEU Conversion 12
Thermal Analysis Challenges Sensitivity to gap thickness Discrepancy between critical heat flux correlations Uncertainty in natural convection heat transfer models Appropriate air-cooled temperature safety limits for U-ZrH 2 fuel in SS clad 9 June 2011 P.Wilson/UWNR LEU Conversion 13
Thermal Analyses Outcomes Small changes in steady-state operation Average pin power increases with fewer pins Improved definition of fuel temperature Limiting Safety System Setting Pulsing operation from 1 kw Max fuel temperature ~727 o C (MOL) Pulsing operation from 1.3 MW is within technical specifications Max fuel temperature ~1000 o C < 1150 o C (EOL) LOCA fuel temperatures < 700 o C (MOL) 9 June 2011 P.Wilson/UWNR LEU Conversion 14
Accident Analysis Methods ORIGEN calculation of fission product inventory Analytic analysis of release fractions and Gaussian plume model MCNP5 calculation of radiation doses 9 June 2011 P.Wilson/UWNR LEU Conversion 15
Accident Analysis Methods Fission product release from Maximum Hypothetical Accident Cladding failure in high power pin (25 kw) after continuous full power operation With & without intact pool & operative ventilation system Reactivity insertion (see T/H analysis) Loss of cooling accident Fuel temperature Radiation dose from exposed core 9 June 2011 P.Wilson/UWNR LEU Conversion 16
Accident Analysis Outcomes No changes due to LEU conversion Within regulatory limits under all variations to Maximum Hypothetical Accident More detailed understanding of radiation dose Time dependent behavior Spatial distribution 9 June 2011 P.Wilson/UWNR LEU Conversion 17
Lessons Learned/Opportunities Capacity building Neutronics analysis performed by post-doc now working at Thai regulator on TRIGA licensing Widespread upgrade in UWNR staff analysis capabilities Ongoing experimental research to better understand natural circulation heat transfer in TRIGA-relevant conditions 100% fresh core provides wide variety of benchmark data for improving analysis capabilities 9 June 2011 P.Wilson/UWNR LEU Conversion 18