X.M2 (NUREG-2191 R0)
X.M2 NEUTRON FLUENCE MONITORING
Program Description
This aging management program (AMP) provides a means to ensure the validity of the neutron fluence analysis and related neutron fluence-based, time-limited aging analyses (TLAAs). In so doing, this AMP also provides an acceptable basis for managing aging effects attributable to neutron fluence in accordance with requirements in Title 10 of the Code of Federal Regulations (10 CFR) 54.21(c)(1)(iii). This program monitors neutron fluence for reactor pressure vessel (RPV) components and reactor vessel internal (RVI) components and is used in conjunction with the Generic Aging Lessons Learned for Subsequent License Renewal (GALL-SLR) Report AMP XI.M31, “Reactor Vessel Material Surveillance.” Neutron fluence is a time-dependent input parameter for evaluating the loss of fracture toughness due to neutron irradiation embrittlement. Accurate neutron fluence values are also necessary to identify the RPV beltline region, for which neutron fluence is projected to exceed 1 × 1017 n/cm2 (E > 1 MeV) during the subsequent period of extended operation.
Neutron fluence is an input to a number of RPV irradiation embrittlement analyses that are required by specific regulations in 10 CFR Part 50. These analyses are TLAAs for subsequent license renewal applications (SLRAs) and are the topic of the acceptance criteria and review procedures in Standard Review Plan for Review of Subsequent License Renewal Applications for Nuclear Power Plants (SRP-SLR) Section 4.2, “Reactor Vessel Neutron Embrittlement Analyses.” The neutron irradiation embrittlement TLAAs that are within the scope of this AMP include, but are not limited to: (a) neutron fluence, (b) pressurized thermal shock analyses for pressurized water reactors, as required by 10 CFR 50.61 or alternatively, if applicable, for the current licensing basis (CLB)] by 10 CFR 50.61a; (c) RPV upper-shelf energy analyses, as required by Section IV.A.1 of 10 CFR Part 50, Appendix G, and (d) pressure-temperature (P-T) limit analyses that are required by Section IV.A.2 of10 CFR Part 50, Appendix G and controlled by plant technical specifications (TS) update and reporting requirements (i.e., the 10 CFR 50.90 license amendment process for updates of P-T limit curves located in the TS limiting conditions of operation, or TS administrative control section requirements for updates of P-T limit curves that have been relocated into a pressure-temperature limits report).
The calculations of neutron fluence also factor into other analyses or technical report methodologies that assess irradiation-related aging effects. Examples include, but are not limited to: (a) determination of the RPV beltline as defined in Regulatory Issue Summary 2014-11, “Information On Licensing Applications For Fracture Toughness Requirements For Ferritic Reactor Coolant Pressure Boundary Components,” (b) evaluation of the susceptibility of RVI components to neutron radiation damage mechanisms, including irradiation embrittlement (IE), irradiation-assisted stress corrosion cracking (IASCC), irradiation-enhanced stress relaxation or creep (IESRC) and void swelling or neutron induced component distortion; and (c) evaluating the dosimetry data obtained from an RPV surveillance program.
Guidance on acceptable methods and assumptions for determining reactor vessel neutron fluence is described in the U.S. Nuclear Regulatory Commission (US NRC) Regulatory Guide (RG) 1.190, “Calculational and Dosimetry Methods for Determining Pressure Vessel Neutron Fluence.” The methods developed and approved using the guidance contained in RG 1.190 are specifically intended for determining neutron fluence in the region of the RPV close to the active fuel region of the core and are not intended to apply to vessel regions significantly above and below the active fuel region of the core, nor to RVI components. Therefore, the use of RG 1.190-adherent methods to estimate neutron fluence for the RPV regions significantly above and below the active fuel region of the core and RVI components may require additional justification, even if those methods were approved by the US NRC for RPV neutron fluence calculations. This program monitors in-vessel or ex-vessel dosimetry capsules and evaluates the dosimetry data, as needed. Such dosimetry capsules may be needed when the reactor surveillance program has exhausted the available capsules for in-vessel exposure.
Evaluation and Technical Basis
- 1. Scope of Program: The scope of the program includes RPV and RVI components that are subject to a neutron embrittlement TLAA or other analysis involving time-dependent neutron irradiation. The program monitors neutron fluence throughout the subsequent period of extended operation for determining the susceptibility of the components to IE, IASCC, IESRC, and void swelling or distortion. The use of this program also continues to ensure the adequacy of the neutron fluence estimates by: (a) monitoring plant and core operating conditions relative to the assumptions used in the neutron fluence calculations, and (b) continuously updating the qualification database associated with the neutron fluence method as new calculational and measurement data become available for benchmarking. This program is used in conjunction with GALL-SLR Report AMP XI.M31, “Reactor Vessel Material Surveillance.”
- Updated neutron fluence calculations, plant modifications, and RPV surveillance program data are used to identify component locations within the scope of this program, including the beltline region of the RPV. Applicable requirements in 10 CFR Part 50, and if appropriate, plant TS, related to calculating neutron fluence estimates and incorporating those calculations into neutron irradiation analyses for the RPVs and RVIs must be met.
- 2. Preventive Actions: This program is a condition monitoring program through calculation of neutron fluence values, and continuous monitoring of their validity; thus, there are no specific preventive actions. Because this program can be used to verify that the inputs and assumptions associated with neutron fluence in the irradiation embrittlement TLAAs (described in SRP-SLR Section 4.2) remain within their respective limits, this program can prevent those TLAAs from being outside of the acceptance criteria that are set as regulatory or design limits in the analyses. Since the program is used to determine that the inputs and assumptions associated with neutron fluence in irradiation embrittlement TLAAs will remain within their respective limits, this program does have some preventative aspects to it.
- 3. Parameters Monitored or Inspected: The program monitors component neutron fluence as determined by the neutron fluence analyses, and appropriate plant and core operating parameters that affect the calculated neutron fluence. The calculational methods, benchmarking, qualification, and surveillance data are monitored to maintain the adequacy of neutron fluence calculations. Neutron fluence levels in specific components are monitored to verify component locations within the scope of this program are identified.
- Neutron fluence is estimated using a computational method that incorporates the following major elements: (1) determination of the geometrical and material input data for the reactor core, vessel and internals, and cavity; (2) determination of the characteristics of the neutron flux emitting from the core; (3) transport of the neutrons from the core to the vessel, and into the cavity; and (4) qualification of the calculational procedure.
- Guidance on acceptable methods and assumptions for determining RPV neutron fluence is described in US NRC RG 1.190. The use of RG 1.190-adherent methods to estimate neutron fluence for the RPV beltline regions significantly above and below the active fuel region of the core, and RVI components may require additional justification, even if those methods were approved by the US NRC for RPV neutron fluence calculations.
- 4. Detection of Aging Effects: The program uses applicant-defined activities or methods to track the RPV and RVI component neutron fluence levels. The neutron fluence levels estimated in this program are used as input to the evaluation for determining applicable aging effects for RPV and RVI components, including evaluation of TLAAs as described in SRP-SLR Section 4.2.
- 5. Monitoring and Trending: Monitoring and trending of neutron fluence are needed to ensure the continued adequacy of various neutron fluence analyses as identified as TLAAs for the SLRA. When applied to RVI components and to components significantly above and below the active fuel region of the core, the program also assesses and justifies whether the current neutron fluence methodology for the CLB is acceptable for monitoring and projecting the neutron fluence values for these components during the subsequent period of extended operation, or else appropriately enhances (with justification) the program’s monitoring and trending element activities accordingly on an as-needed basis. Trending is performed to ensure that plant and core operating conditions remain consistent with the assumptions used in the neutron fluence analyses and that the analyses are updated as necessary.
- Neutron fluence estimates are typically determined using a combination of plant and core operating history data that address past plant operating conditions, and projections that are intended to address future operation. Although projections for future operation may conservatively over-estimate the core neutron flux to cover potential variations in plant and core operation and increases in neutron flux at any given time, there is no explicit requirement to do so. Therefore, projections for future plant and core operation should be periodically verified to ensure that any projections used in the neutron fluence calculations remain bounding with respect to actual plant operating conditions.
- This program monitors in-vessel or ex-vessel dosimetry capsules and evaluates the dosimetry data, as needed. Additional dosimetry capsules may be needed when the reactor surveillance program has exhausted the available capsules for in-vessel exposure.
- 6. Acceptance Criteria: There are no specified acceptance values for neutron fluence; the acceptance criteria relate to the different parameters that are evaluated using neutron fluence, as described in SRP-SLR Section 4.2.
- US NRC RG 1.190 provides guidance for acceptable methods to determine neutron fluence for the RPV beltline region. It should be noted, however, that applying RG 1.190-adherent methods to determine neutron fluence in locations other than those close to the active fuel region of the core may require additional justification regarding, for example, the level of detail used to represent the core neutron source, the methods to synthesize the three-dimensional flux field, and the order of angular quadrature used in the neutron transport calculations. The applicability of existing qualification data may also require additional justification.
- 7. Corrective Actions: Results that do not meet the acceptance criteria are addressed in the applicant’s corrective action program under those specific portions of the quality assurance (QA) program that are used to meet Criterion XVI, “Corrective Action,” of 10 CFR Part 50, Appendix B. Appendix A of the GALL-SLR Report describes how an applicant may apply its 10 CFR Part 50, Appendix B, QA program to fulfill the corrective actions element of this AMP for both safety-related and nonsafety-related structures and components (SCs) within the scope of this program.
- The program provides for corrective actions by updating the analyses for the RPV components, or assessing the need for revising the augmented inspection bases for RVI components, if the neutron fluence assumptions in RPV analyses or augmented inspection bases for RVI components are projected to be exceeded during the subsequent period of extended operation. Acceptable corrective actions include revisions to the neutron fluence calculations to incorporate additional operating history data, as such data become available; use of improved modeling approaches to obtain more accurate neutron fluence estimates; and rescreening of RPV and RVI components when the estimated neutron fluence exceeds threshold values for specific aging mechanisms.
- When the fluence monitoring activities are used to confirm the validity of existing RPV neutron irradiation embrittlement analyses and result in the need for an update of an analysis that is required by a specific 10 CFR Part 50 regulation, the corrective actions to be taken follow those prescribed in the applicable regulation.
- 8. Confirmation Process: The confirmation process is addressed through those specific portions of the QA program that are used to meet Criterion XVI, “Corrective Action,” of 10 CFR Part 50, Appendix B. Appendix A of the GALL-SLR Report describes how an applicant may apply its 10 CFR Part 50, Appendix B, QA program to fulfill the confirmation process element of this AMP for both safety-related and nonsafety-related SCs within the scope of this program.
- 9. Administrative Controls: Administrative controls are addressed through the QA program that is used to meet the requirements of 10 CFR Part 50, Appendix B, associated with managing the effects of aging. Appendix A of the GALL-SLR Report describes how an applicant may apply its 10 CFR Part 50, Appendix B, QA program to fulfill the administrative controls element of this AMP for both safety-related and nonsafety-related SCs within the scope of this program.
- 10. Operating Experience: The program reviews industry and plant operating experience (OE) relevant to neutron fluence. Applicable OE affecting the neutron fluence estimate is to be considered in selecting the components for monitoring. RG 1.190 provides expectations for updating the qualification database for the neutron fluence methods via the operational experience gathered from RPV material surveillance program data. This operational experience is in accordance with the requirements of 10 CFR Part 50 Appendix H.
- The program is informed and enhanced when necessary through the systematic and ongoing review of both plant-specific and industry OE including research and development such that the effectiveness of the AMP is evaluated consistent with the discussion in Appendix B of the GALL-SLR Report.
References
10 CFR Part 50, Appendix B, "Quality Assurance Criteria for Nuclear Power Plants.” Washington, DC: U.S. Nuclear Regulatory Commission. 2016.
10 CFR Part 50, Appendix G, “Fracture Toughness Requirements.” Washington, DC: U.S. Nuclear Regulatory Commission. 2016.
10 CFR Part 50, Appendix H, “Reactor Vessel Material Surveillance Program Requirements.” Washington, DC: U.S. Nuclear Regulatory Commission. 2016.
10 CFR 50.55a, “Codes and Standards.” Washington, DC: U.S. Nuclear Regulatory Commission. 2016.
10 CFR 50.60, “Acceptance Criteria for Fracture Prevention Measures for Lightwater Nuclear Power Reactor for Normal Operation.” Washington, DC: U.S. Nuclear Regulatory Commission. 2016.
10 CFR 50.61, “Fracture Toughness Requirements for Protection Against Pressurized Thermal Shock Events.” Washington, DC: U.S. Nuclear Regulatory Commission. 2016.
10 CFR 50.61a, “Alternate Fracture Toughness Requirements for Protection Against Pressurized Thermal Shock Events.” Washington, DC: U.S. Nuclear Regulatory Commission. 2016.
US NRC. Regulatory Guide 1.190, “Calculational and Dosimetry Methods for Determining Pressure Vessel Neutron Fluence.” Agencywide Documents Access and Management System (ADAMS) Accession No. ML010890301. Washington, DC: U.S. Nuclear Regulatory Commission. March 2001.