IAEA Technical Meeting on Flexible (Non-Baseload) Operation Approaches for Nuclear Power Plants Regulatory Challenges and Fuel Performance Paul Clifford United States of America
Agenda 1. Regulatory Challenges Impacts on Design and Licensing Bases Impacts on Core Reload Design 2. Fuel Performance PCI/SCC Protection 2
Feasibility Study Prior to undertaking flexible operation, a comprehensive review of NPP design and licensing bases are necessary. o Technical Specification LCOs and SRs o Safety Analysis o PRA o Operating Procedures and Training Capabilities to support flexible operation depend on many parameters and will vary for each plant design. 3
Licensing Basis While the operating license encompasses all modes of operation, from refueling to full power, a presumption of base-load operation may be inherent in the plant s licensing basis. Plant modifications and changes to design and licensing bases documents are likely for existing fleet and may also be needed for new reactor designs. 4
Bases Documents Plant Technical Specifications (TS) and Updated Final Safety Analysis Report (UFSAR) capture the design and licensing bases. o Limiting conditions of operation (LCO) based upon the initial conditions assumed in the safety analysis. o Reactor protection systems (RPS) and engineered safety feature (ESF) system functions, setpoints, and capabilities o Surveillance requirements (SR) designed to ensure the protective capabilities of these systems o Instrumentation uncertainties, drift, calibration, and response time addressed 5
Impact on Safety Analysis UFSAR safety analysis may be limited in scope to HZP and HFP transients and accidents initiated from nominal, steady-state conditions. o Predicated on base-load operation o Flexible operation challenges underlying assumption Transients and accidents initiated from intermediate power levels may produce worse results and need to be evaluated. Transients and accidents initiated from off-nominal conditions likely to yield worse results and need to be evaluated. 6
Impact on Safety Analysis Flexible operation may challenge the calibration and validation of analytical models used in the safety analysis. Plant system models are often calibrated to plant operating conditions. For example, PWR primary-to-secondary heat transfer coefficient may be adjusted to achieve operating SG pressure. These system models would need to be calibrated to intermediate power level operating conditions. Fuel rod thermal-mechanical models would need to be revised and validated against data from power ramp testing. 7
Reload Safety Analysis To minimize reload design cost, many NPP have opted for cycle-independent reload safety analysis. The reload checklist process involves comparing cycle-specific core physics (based on planned operation) and reload parameters against bounding parameters which form the bases of the safety analysis analysis-of-record. o This approach reduces reload design cost; however, decreases operating margin and fuel management flexibility Flexible operation requires operating margin (e.g., wide band on allowable AXPD and CEA insertion). 8
Reload Safety Analysis To support flexible operation, NPP may need to migrate to cycle-specific reload safety analysis based on as-built core loading and as-burned reload depletions. o This approach improves operating margin and fuel management flexibility; however, increases reload design cost Planned power maneuvering (e.g., daily LF) could be built into core reload depletion and safety analysis. Unplanned power maneuvering may alter power and burnup profiles, change core physics parameters, and necessitate re-analysis. 9
Core Power Distribution Detailed 3D power/flux profiles are generated periodically to measure or confirm power peaking factors used in the monitoring of important safety margins including maximum linear heat generating rate (LHGR) and DNB/CHF limits. o Coupling coefficients are generated based upon predicted power distribution and used to generate 3D power/flux profiles of the core using incore instrument signals Flexible operations creates two concerns related to power distribution and peaking factors: 1. Accuracy of coupling coefficients and resulting power profile and peaking factors 2. Periodicity for generating 3D power/flux profile during and following flexible operation 10
Core Power Distribution The second item becomes even more important for reactor designs without fixed incore detectors. NPP operators may need to consider the potential impacts of flexible operation on predicted core physics parameters and safety margins. An accurate and rapid indication of core power distribution is necessary to protect against violating fuel maneuvering and local power peaking limits during plant maneuvering, especially when flexible operation is achieved via control rod motion. 11
Excore Detector Decalibration NPPs rely on fast-responding, safety-grade excore detectors to measure core power and axial power distribution, as well as to initiate a timely reactor trip signal if power exceeds a prescribed setpoint. Excore detectors are periodically calibrated to other measurements of reactor power. Power measurement uncertainties usually increase at lower power. Any change in the relationship between interior power distribution and excore neutron flux would effectively decalibrate the excore detectors. Periodicity of excore calibration during and following flexible operation will need to be addressed, especially when flexible operation is achieved via control rod motion. 12
Probabilistic Risk Assessment In addition to traditional, deterministic safety analysis, NPPs may utilize probabilistic risk assessment (PRA) to riskinform plant operations and maintenance. Flexible operations may increase the probability of equipment failures, plant transients, and reactor trips. A systematic review of inputs and assumptions to the PRA model and supporting sensitivity analyses will be necessary. 13
Licensed Plant Operators Regulatory requirements related to controlling reactor power and reactivity management vary from country to country. In the United States, regulations do not permit the manipulation of the controls of any facility by anyone who is not a licensed operator or senior operator (10CFR50.54(i)). These regulations may restrict a NPP s ability to support flexible operations (e.g., continuous FC). 14
NRC Approved Advanced Designs The NRC has issued approvals for several advanced reactor designs including the following: o Combustion Engineering System 80+ PWR o General Electric Advanced BWR o Westinghouse AP1000 PWR o General Electric Hitachi ESBWR Several other advanced reactor design reviews are in progress: o AREVA EPR o Mitsubishi APWR o KEPCO APR-1400 15
NRC Approved Advanced Designs The ability of these advanced designs to support flexible operations (e.g., LF and FC) was never demonstrated nor part of the NRC s review and approval. The licensing basis of these advanced reactor design certifications presumes base-load operation. Changes to the design and licensing bases of these advanced designs may be necessary to support flexible operation. 16
Fuel Performance 17
Fuel and Operating Cost Fuel management is optimized to reduce fuel costs while satisfying plant capacity goals and safety requirements. o Fuel management scoping studies show an increase in fuel cost associated with flexible operation Additional thermal and mechanical cycling and wear on plant components will promote higher operating and maintenance cost. 18
Fuel Design Requirements Flexible operations challenges fuel performance and may necessitate design changes. o Extends cycle length and residence time for fuel assemblies. o Component corrosion limits o PWR grid-to-rod fretting and BWR debris fretting o Increases number of thermal and mechanical cycles for assembly component fatigue analysis o Impacts existing fuel performance models (e.g., FGR, rod growth) o Increases cladding stress and promotes PCI (cladding failure) 19
PCI Cladding Failure Modes PCI is a broad phenomenon that includes several failure modes. Cladding stress combined with the aggressive chemical agents present in fission products (e.g., iodine) may lead to cladding failure via stress corrosion cracking (referred to as PCI or PCI/SCC). 20
PCI/SCC Threshold Iterative FRAPCON-3.4 calculations demonstrate that allowable ΔLHGR decreases dramatically after hard contact and continues to decrease with exposure. PCI/SCC threshold based on Studsvik R2 ramp testing 7 6 200 Mpa Lower Threshold 250 Mpa BE Threshold Step Load Increase (KW/ft) 5 4 3 2 1 0 0 10 20 30 40 50 60 70 Fuel Rod Nodal Burnup (GWd/MTU) 21
PCI/SCC Protection Protection against PCI/SCC cladding damage during flexible operation requires the following: 1. Specific knowledge of each fuel rod design s susceptibility to PCI/SCC 2. Qualified analytical models and fuel design limits 3. Established fuel pre-conditioning and maneuvering guidelines. 4. Accurate core power distribution measurements In-pile power ramp testing of irradiated fuel rod segments is necessary to qualify analytical models and establish PCI/SCC threshold and maneuvering guidelines. 22
PCI/SCC Resistant Fuel PCI/SCC Resistant Fuel Rod Design Features: o Barrier cladding consists of a thin layer of natural or low alloy zirconium on the inside diameter of the fuel rod. This design has proven effective in BWRs o CANLUB is a thin lubricating film of graphite between the natural uranium fuel pellet and zirconium-alloy sheath. This design feature has proven effective in CANDU reactors o Fuel additives are under development and show promising results 23
Fuel Rod Design Conclusion: Fuel conditioning and maneuvering guidelines have proven effective at preventing PCI/SCC cladding failure during flexible operations. o Model predictions prior to maneuvering inform plant operations (e.g., ramp rates, rod motion) o Core monitoring during maneuvering protects established fuel design margins 24
Summary This presentation provided an overview of relevant topics which are addressed in the IAEA report. Flexible operations introduce challenges to plant systems and components as well as plant operations. Overall, operating experience in Germany and France continues to demonstrate that flexible operation is achievable in a safe and reliable manner. o Both countries completed detailed feasibility studies, upgraded instrumentation and controls, completed necessary plant modifications, enhanced Operator training, and addressed regulatory issues prior to implementing flexible operations 25
Cooperation for safe and peaceful use of nuclear energy THANK YOU FOR YOUR ATTENTION 26