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
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. Decay30 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
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