Source Range Detection Response at Watts Bar Nuclear Plant, Unit 1 John Ritchie, Tennessee Valley Authority Background Watts Bar Nuclear Power Plant, Tennessee Valley Authority, www.tva.gov, 2018. Isometric Section of a Tritium-Producing Burnable Absorber Rod, Pacific Northwest National Lab, TTQP-1-015, Rev. 19, 2012. 2 Background (cont’d) • Multiple TPBAR Irradiation Increases – Nearly 40% of all TPBARs Irradiated in the Last two cycles. – Nearly 25% of all TPBARs in Currently Operating Cycle • Tritium Demand Expected to Increase – Outcome Uncertain from 2018 Nuclear Posture Review • Impacts of TPBARs Loading Increase – Only Loaded in Fresh Assemblies – Increased Enrichment Necessary – Low Discharge Burnup is Expected 3 Source Range Detector Response • Tech Spec Requirements for Minimum Counts – Must Demonstrate while Subcritical – 0.2 cps for SRD Declared Operable • Intrinsic Source Term – Driven by Spontaneous Fission, Subcritical Multiplication, Delayed N and Alpha,N Reactions – A Function of Assembly Burnup and Enrichment • Fixed Source Term Driven by Secondary Sources. – Typically Loaded in Second or Third Row – Response Drops Exponentially Every Row Balance Between Counts and ICRR 4 Impacts on Intrinsic Source Term • Average Enrichment N-6 N-5 Increased from N-4 N-3 Decay 4.2 w/o to 4.9 w/o. N-2 N-1 N N+1 • Average Peripheral Burnup Decreased from 36 GWD/MTU to 28 GWD/MTU. 30 Day after Flux Neutron Neutron Energy Counts Cannot be Driven by Burned Fuel Alone 5 Cycle N-X Core Load 3 x 3 Built SSA in Row 1 SSA Additional moved to Peripheral Row 3 Assemblies Source Range Detector Response (cps) Response Detector Range Source Added Core Load Time 6 Cycle N-1 Core Load 3 x 3 Built SSA in Row 1 Source Range Detector Response (cps) Response Detector Range Source SSA moved to Row 3 Core Load Time 7 Cycle N Core Load Peripheral Assemblies Loaded SSA in Row 1 Range Detector Response (cps) Response Detector Range SSA in Row 2 Additional SSA Source Assemblies inboard Loaded Core Load Time 8 What are the Alternatives? • Utilize Scalar-Timer? – Costly Addition to the Core Reload Time, Maybe Added Critical Path • Leave SSA on Periphery? – ICRR Becomes too Non-Linear During Heatup/Dilution • Maintain High Burned Fuel on Periphery – Unsatisfactory Increase in Fuel Costs • New Secondary Source Design – Untested and Subcritical Calculations to Test Design is Difficult 9 VERA as an Investigation Tool • Create Irradiation Histories of Individual Secondary Source Assemblies – 8 As-Operated Cycles – 1 Design Cycle – 2 Projected Cycles (Introduction of new SSA Design) • Key off of Sb-124 N.D. parameterized to a neutron strength • Use SHIFT to Determine SRD Response Function and Scaling Factor 10 Tentative Measured-to-Predicted Results (Benchmarking) Source Range Detector Response (cps) Response Detector Range Source Core Load Time 11 Tentative Measured-to-Predicted Results (Benchmarking Cont’d) Source Range Detector Response (cps) Response Detector Range Source Core Load Time 12 Tentative Measured-to-Predicted Results (Benchmarking Cont’d) N32 Source Range Detector Response (cps) Response Detector Range Source N32 Core Load Time 13 Preliminary Results • Benchmarking Shows Ability to Predict Response – Some Cycles Have High RMS Values (>5%). Mostly due to the Noisiness of the Measured Detector Response • Large Variation in Measured Results Between the Two SRDs – Reasons are Being Investigated. Presumed to be Driven by the Gradient of the SSAs, or Different Settings in the SRD Logic • Forward Projections May be Used to Map Loading Strategy for Next Cycle Results are Promising! 14 Areas for Improvement • ORIGEN Photonuclear Interactions Missing – VERA Assumes a Static n/Ci-Sb124 Value • Full Core Depletion Necessary Since Shuffling of SSAs is not Tracked Explicitly. – Extensive Computational Time Expended • Fission Spectrum is not Passed from MPACT to SHIFT – Assumes the Watt Spectrum • Potential Inaccuracies in Importance Mapping – Unknown Impact 15 www.casl.gov 16.