CHALLENGES WITH THE CONVERSION OF THE MITR

Transcription

1 Russian-American Symposium on the Conversion of Research Reactors to Low Enriched Uranium Fuel Moscow, Russia CHALLENGES WITH THE CONVERSION OF THE MITR T. H. Newton, Jr Director of Reactor Operations MIT Nuclear Reactor Laboratory, Cambridge, MA USA

2 MIT Reactor Licensed power: 6 MW License renewed for 20 years November Aluminide fuel 15 plates/assembly 93% enriched Al clad with longitudinal fins

3 Fins on cladding surface Fins increase heat transfer surface area

4 The MIT Reactor Core 4

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6 LEU Conversion Challenges and Opportunities We have used the LEU conversion process to improve our capabilities where possible: Improve neutronic and thermal-hydraulic modeling Optimize LEU fuel/reactor design for adequate flux to experiments and fuel cycle length Improve our understanding of fuel oxide growth and heat transfer coefficient though measurements

7 Challenge: Accurate burnup modeling and benchmarking Opportunity: Develop upgraded models and compare to recent core measurements

8 Neutronic model improvements Extensive review of models core structure and dimensions Cross-section libraries updated Homogenized volume fractions and discrete structures updated Two initial HEU cores modeled and compared favorably with measurements Initial Keff Control blade worths (25 cases) Physics parameters

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10 Neutronics Code Development Graphical user interface Handles and burns HEU, LEU, and mixed geometries In-core experiments can be modeled Can model all fuel movements, flipping, rotating, and fuel storage Optional criticality blade position search Can track and plot Mass of isotopes Power distribution

11 Burnup modeling No standard refueling for the MITR Twelve recent cores modeled Showed good agreement with measured beginning- and end-of-cycle control blade positions as well as between different models

12 Challenge: Accurate and reliable thermalhydraulic calculations Opportunity: Develop new models including fin effects, develop test measurements

13 Thermal-Hydraulic Studies Models modified to include fins Used for steady state and loss-of-flow analysis LEU design core has higher margin to ONB and lower peak cladding temperature with loss of flow

14 Challenge: Maintain HEU performance for experiments with LEU fuel Opportunity: Optimize fuel design to maximize heat transfer and experimental neutron flux

15 Optimization studies Because of compact core, the only feasible LEU fuel is monolithic U-Mo Flux optimization focused on maintaining HEU equivalent fast flux to in-core materials experiments and thermal flux to ex-core facilities Neutronic and thermal-hydraulic studies show optimum design for power increase: More plates/assembly Thinner fuel and cladding

16 Fuel Design Optimization UAl x (HEU) Monolithic U-10Mo (LEU) Enrichment 93% 19.75% Fuel Density (gu/cm 3 ) Number of plates per assembly Fuel thickness 0.76 mm 0.51 mm Cladding Thickness 0.38 mm 0.25 mm Operating Power 6 MW 7 MW Cycle length days days

17 Challenge: Safety analysis parameters not well known, particularly for finned cladding Opportunity: Develop measurements

18 Safety analysis measurements Finned channel friction pressure drop information needed Flow experiment built and measurements made Finned channel correlation developed Need to determine adequacy of Onset of Nucleate Boiling correlation for finned channels Oxide distribution in finned cladding not known

19 MITR boiling flow loop Objective: to measure ONB for LEU channel geometry channels to validate the Bergles-Rohsenow ONB correlation for finned channels Facility being built, operational later this year 19

20 Oxidation Current burnup limit of 1.7E21 fissions/cm 3 is based on even 50 m aluminum oxide distribution on cladding Studies at INL suggest there may be no burnup limit for monolithic UMo LEU fuel Actual oxide distribution unknown, particularly within finned region

21 Measurement of oxide thickness Use eddy current probe for fin tip measurement Measurement of thickness and compare with operational history Evaluation of belowfin areas Replication using acetate films Selected element to be shipped to INL for evaluation

22 Other Challenges Gamma heating outside of fuel significantly less for LEU Less heat load on D 2 O reflector and shield system Some in-core materials experiments rely on gamma heat for temperature control, may need redesign Mechanical stress from heavier loading Analyses indicate no problem

23 Challenge: How to introduce first-of-akind fuel Opportunity: Transition core

24 Future mixed core evaluation Current practice is for three new (HEU) elements at a time Because of lack of operating experience with U-Mo fuel, LEU will be introduced into HEU core New LEU to be introduced into core in B ring Move to peaked positions (A and C rings) as burnup progresses Analysis of other transition options Reactivity and power peaking will determine feasibility

25 Mixed core analyses Power peaking generally a bit higher in new LEU elements Steady state HEU and LEU margins to onset of nucleate boiling decrease with increasing number of LEU fuel elements in the transitional core May need partially burned HEU elements in reserve as we transition to full LEU core 25

26 Future plans Preliminary safety analysis report in preparation for conversion With fuel developers, determine fuel manufacturing tolerances and analyze effect on engineering hot channel factors Run neutronics model up to present core and use for routine fuel management calculations

27 Summary LEU Fuel design will allow increase to 7 MW, giving HEU equivalent fluxes to experiments Fuel cycle length for LEU will be longer than HEU Neutronic and thermal-hydraulic models have been upgraded, benchmarked and provide state-of-theart accuracy Measurements have been or are being made to determine: Finned channel friction factor Distribution of oxide on cladding Onset of nucleate boiling correlation

28 Conclusion Challenges are also opportunities: The process of LEU conversion should be used to improve things!

29 Acknowledgements Thanks to : NNSA-GTRI for financial support The fine folks at ANL and INL for their hard work and technical expertise NAS/RAS for organizing and supporting this symposium