EPRI | Nuclear Sector Roadmaps August 2015

IN USE: ENSURING REACTOR PRESSURE VESSEL INTEGRITY THROUGH EIGHTY YEARS OF OPERATION

ISSUE STATEMENT

The ability to monitor and demonstrate the structural integ-rity of the reactor pressure vessel (RPV) through 80 years of operation is essential to ensure continued operation of the nuclear fleet. For an RPV subjected to years of neutron radi-ation, adequate demonstration of integrity will become increasingly difficult when applying existing analytical tools and correlations, many of which were established using tech-nology and analytical capabilities dating from the 1970s.

Revisions to analytical tools, material property databases and embrittlement correlations are necessary to accurately predict the service life of the RPV. Technical advances are needed to:• Ensure that necessary research data is available to identify

and assess fluence- and flux-based damage mechanisms at the levels anticipated through 80 years of operation.

• Establish a damage mechanism-based definition of the region of the reactor vessel that must be evaluated for maintenance of structural integrity. Improving the under-standing of these damage mechanisms could help inform regulations.

• Ensure that design materials, end-of-license fluence, and flux are considered to optimize design and fabrication practices for advanced light water reactors.

Ongoing research and regulatory interactions are needed to ensure that the appropriate analytical tools and correlations are developed to analyze and model vessel integrity for safe and efficient operation through 80 years.

DRIVERS

The primary driver for reactor pressure vessel integrity is to ensure that plants can safely and efficiently operate through 80 years without significant operational constraints or miti-gation of RPV embrittlement. This encompasses operating restrictions associated with assuring RPV integrity during start-up and shutdown activities (e.g., avoiding prolonged startup/shutdown times because of constrained pressure-temperature operating envelop), and the assessment and evaluation of neutron irradiation embrittlement, thermal embrittlement, and other damage mechanisms that may compromise the ability to meet reactor vessel fracture tough-ness requirements (e.g., meeting the LTOP setpoint, LTOP limitations, pressure-temperature limitations PTS screening criteria and upper shelf energy toughness requirements through 80 years).

RESULTS IMPLEMENTATION

Successful management of reactor vessel integrity issues will protect the only non-replaceable component in the nuclear steam supply system. This research will ensure that regula-tions reflect known technical constraints, but also that plants avoid operational constraints that degrade plant perfor-mance. The end product of this R&D will be EPRI reports that support the bases for ASME Code and NRC regula-tions affecting RPV asset management.

As data gathering and modeling proceed, the needs of the advanced light water reactor fleet will be considered to ensure that long-term plans envelope advanced designs. Moreover, where program results may affect design or fabrication prac-tices, this guidance will be made available to vendors via the EPRI Advanced Light Water Reactor Utility Requirements Document.

PROJECT PLAN

The project plan is comprised of the following elements:

Operational Support Through 80 Years• PWR utilities implemented EPRI’s coordinated reactor

vessel surveillance program in 2012. From now to 2025, this program will generate additional high-fluence sur-veillance data and irradiated material samples needed to support embrittlement correlation databases and damage mechanism assessments at fluences representative through 80 years of operation.

• EPRI and the PWR Owners Group (PWROG) will con-duct R&D to determine whether the Code and regulatory requirements for RPV integrity could put plants at safety and economic risk in coming decades. This will be accom-plished through technical support of ASME Code activi-ties and collaboration with NRC Research in several areas: RPV surveillance issues, ASME Section XI Appen-dix G operating limit issues (e.g., pressure-temperature (P-T) and LTOP); RPV low upper shelf energy (USE) and equivalent margins analysis (EMA) issues; development of fracture toughness testing methods; and RPV “extended beltline” Issues.

• EPRI and the PWROG will evaluate the operational impacts related to embrittlement correlations and envi-ronmental damage projections for materials and compo-nents in the “extended beltline.”

Materials Degradation and Aging August 2015

• EPRI and the PWROG will address the impact of uncer-tainties that may exist for plants that must use alternative procedures to determine initial RTNDT and initial Upper Shelf Energy for vessels fabricated to ASME Code versions prior to the Summer 1972 Addenda of ASME III 1971.

• BWR vessel embrittlement issues will be evaluated, including development of a program for BWRs to meet 10 CFR 50 Appendix H surveillance requirements through 80 years of operation.

• EPRI will conduct R&D to assess the generic impact of the Doel-3 RPV NDE experience (i.e., potential hydro-gen flaking in core shell forgings and issues regarding potential nonhomogeneity of forging materials). As required, the technical bases for changes to the ASME Boiler & Pressure Vessel Code will be generated to ensure that plants have the necessary Code tools and guidance to evaluate a large number of quasi-laminar flaws.

• EPRI and the PWROG will develop the technical bases and data to support broader adoption of Master Curve technology as an approved alternative for RPV integrity assessment, both in the ASME Code (Section XI Appen-dices A, G and K) and in regulations. This activity will also support resolution of issues relating to the initial frac-ture toughness of RPV materials.

Data Modeling• EPRI will support NRC development of an updated RPV

database, the Radiation Embrittlement Archive Project (REAP), a web-based tool available to the public for research on new embrittlement trend curves and surveil-lance data analysis.

• EPRI and the PWROG will assess appropriate stress intensity, flaw distribution and fluence attenuation mod-els for use in assessing materials/components of the “extended beltline.”

• EPRI will work with its international partners (e.g., Mate-rials Aging Institute) to develop advanced modeling tools and techniques (e.g., multiscale) and to assess susceptibil-ity of high-temperature RPV components to thermal aging embrittlement.

• EPRI will support efforts to assess and update the Indus-try-standard embrittlement trend correlation (ASTM E900). This will include examination of high fluence sur-veillance specimens using advanced microscopy tech-niques to identify new embrittlement mechanisms (if any) at high fluences.

• EPRI will develop a PWR Supplemental Surveillance Program, consisting of two surveillance capsules contain-ing selected, previously-irradiated and tested, high-value PWR surveillance specimens. These capsules will be irra-diated in two PWR host plants for ten years and will

provide significantly more high fluence surveillance data than a typical plant capsule. The PSSP capsules will pro-vide data to inform or validate future generation ETCs, supporting RPV operations through 80 years.

• EPRI will collaborate with other research organizations to support post-irradiation testing of RPV steels irradiated in the DOE/UCSB ATR-2 capsule for the purpose of increasing the mechanistic understanding of high-fluence embrittlement mechanisms that may impact second license renewal decisions.

• EPRI will develop an embrittlement trend correlation for prediction of Charpy upper shelf energy decrease result-ing from neutron embrittlement.

• EPRI will seek a suitable PWR nozzle material and con-duct through-wall material property testing in order to increase knowledge of embrittlement in the extended beltline, including Albedo effects.

Regulatory and Communications• Since NRC is responsible for drafting and issuing regula-

tions dealing with irradiation damage mechanisms, EPRI and the PWROG will regularly interface with the NRC Research and Regulation branches.

• EPRI will continue to integrate PWROG, BWRVIP and MRP activities relative to RPV integrity and regularly communicate their activities with the NRC.

• EPRI will continue to participate in and support ASME Section III, ASME Section XI, and ASTM E-10 activities related to reactor vessel integrity.

RISKS

There are both economic and operational risks to the indus-try if this research is not conducted. Overly conservative RPV material damage mechanism models could lead to operational constraints and significant costs for analysis, inspection, and mitigation.

Project risks are associated with the potential identification of new and/or more significant material damage than expected. For example, the data from the coordinated and supplemental PWR surveillance capsule programs may confirm additional embrittlement at fluences beyond 8 x 1019 n/cm2 for PWRs and flux effects (increased damage at lower flux) for BWRs limiting the ability to reach 80 years without aggressive mitigative management of the vessel. The embrittlement effects on nozzles, coupled with high stress intensity factors in areas of structural discontinuities, may make the nozzles controlling for pressure-temperature limits and require additional mitigative actions.

EPRI | Nuclear Sector Roadmaps August 2015

RECORD OF REVISION

This record of revision will provide a high level summary of the major changes in the document and identify the Road-map Owner.

revision description of change

0 Original Issue: August 2011 Roadmap Owner: Timothy Hardin

1 Revision Issued: August 2012 Roadmap Owner: Timothy Hardin

Changes: Added the development of a PWR Supplemental Surveillance Program (PSSP) capsule as an element of the Program Plan under “Data Modelling”; added an element to assess and update ASTM E900 ETC. Revised the flowchart to reflect these additions; updated overall schedules; and corrected the program interactions indicated by some connectors.

2 Revision Issued: August 2013 Roadmap Owner: Timothy Hardin

Changes: Minor editorial changes. Clarified the research areas for collaborative research with NRC. Clarified description of the PSSP, which will consist of two capsules, not a single capsule. Added project to perform materials testing on a decommissioned RPV nozzle (Zion). Added Research and Development efforts to address issues raised by the Doel-3 RPV forging issue (potential hydrogen flaking).

3 Revision Issued: August 2015 Roadmap Owner: Timothy Hardin

Changes: Added projects to resolve issues regarding initial fracture toughness of RPV materials and use of Master Curve technology for RPV integrity assessment. Added project to develop an embrittlement trend correlation for prediction of upper shelf energy decrease. Deleted project for testing of Zion RPV nozzle because DOE LWRS is not obtaining the nozzle. Added project to support testing of PWR surveillance materials in the DOE’s ATR2 capsule experiment. As requested by the Integration Committee, added a statement of future intent to obtain and test PWR nozzle material to investigate Albedo effects, when a suitable opportunity is available.

Materials Degradation and Aging August 2015