ML20174A116

VBWR Request for Additional Information Enclosure

ML20174A116
Person / Time
Site:	Vallecitos Nuclear Center, Vallecitos
Issue date:	07/15/2020
From:	Office of Nuclear Material Safety and Safeguards
To:	GE Hitachi Nuclear Energy
Parrott J
Shared Package
ML20174A114	List: ML20174A115 ML20174A116
References
EPID L-2017-LLA-0052
Download: ML20174A116 (7)

Text

Enclosure REQUEST FOR ADDITIONAL INFORMATION GE HITACHI REQUEST FOR ALTERNATE DECOMMISSIONING SCHEDULES FOR THE SHUTDOWN REACTORS AT THE VALLECITOS NUCLEAR CENTER GE HITACHI VALLECITOS BOILING WATER REACTOR DOCKET NO. 50-18 Regulatory Requirement Paragraph (c) in 10 CFR 20.1406, Minimization of contamination, requires each licensee, to the extent practical, to conduct operations to minimize the introduction of residual radioactivity into its site, including the subsurface, in accordance with the existing radiation protection requirements in 10 CFR Part 20, Standards for Protection Against Radiation, Subpart B, Radiation Protection Programs, and the radiological criteria for license termination in 10 CFR Part 20, Subpart E, Radiological Criteria for License Termination. GE Hitachi Nuclear Energys (GEH) compliance with 10 CFR 20.1406(c) is required to ensure that its requested exemption will not present an undue risk to the public health and safety, as required by the exemption criteria in paragraph (a)(1) of 10 CFR 50.12, Specific exemptions.

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Background===

As described in the U.S. Nuclear Regulatory Commission (NRC) staff regulatory guidance position 5 in NRC Regulatory Guide 4.22, Decommissioning Planning During Operations, in 10 CFR 20.1406(c), the NRC requires all licensees to minimize the introduction of radiological contamination into the site environment. To do so, licensees should implement procedures and practices that minimize the occurrence of leaks and spills from piping, tanks, and storage containers. Licensees should also have procedures that will: (1) identify the plant systems and radioactive materials storage containers with the potential for leaks and spills; (2) identify, to the extent practical, degraded equipment and containers before release and spills occur; and (3) detect leaks and spills throughout the facility as soon as they occur or soon thereafter.

These principles apply to all of the NRC licensed facilities at the Vallecitos Nuclear Center (VNC), including the Vallecitos Boiling Water Reactor (VBWR), to minimize the introduction of residual radioactivity into the site, including the subsurface.

By letter dated January 18, 2018 (ADAMS Accession No. ML17312B359), the NRC issued a request for additional information (RAI) to GEH to provide, among other things, an engineering structural analysis of the three shutdown reactor facilities at the VNC that includes an analysis of GEHs ability to maintain integrity of the building for purposes of ensuring confinement of the residual radioactivity. The request also stated that the analysis should include an assessment of the shutdown reactors that demonstrates GEHs ability to maintain structural integrity and confinement of residual radioactivity during and after a seismic event that could affect the facilities over the time of extended decommissioning. This request was related to maintaining the structural integrity of the shutdown reactors to address the requirements of 10 CFR 20.1406(c).

In its response to this RAI, by letters dated March 13, 2019 (ADAMS Accession No. ML19087A221) and November 15, 2019 (ADAMS Accession No. ML19319B845), GEH

2 described the residual radioactivity in the VBWR and provided three non-publicly available reports prepared by Structural Integrity Associates (SI) for GEH:

VNC EVESR, VBWR & GETR Facility and Material Degradation Evaluations (referred to as the VNC Degradation Evaluations in this RAI), beginning on page 15 of the document at ADAMS Accession No. ML19087A222.

Maintenance and Surveillance Plan: Vallecitos Nuclear Center Shutdown Reactors (referred to as the Maintenance and Surveillance Plan in this RAI), ADAMS Accession No. ML19319B974.

Tier 1 Seismic Evaluation of the Vallecitos Boiling Water Reactor Shutdown Facility (referred to as the VBWR Seismic Evaluation in this RAI), ADAMS Accession Nos.

ML19319B946 and ML19319B968.

The VNC Degradation Evaluations identified the VBWR structure as having different levels of extensive concrete cracking due to reinforcement corrosion and/or mechanical stress, likely indicative of degradation of the structural capacity of the reinforced concrete sections; corrosion of the structural steel inside the building; the presence of an extensive amount of efflorescence that could indicate internal structural weakness; and evidence of groundwater intrusion and accumulation at the basement suggesting through-wall corrosion penetration of the VBWR steel housing, estimated by a corrosion rate calculation to have occurred over approximately 48 years.

The Maintenance and Surveillance Plan is the GEH plan to monitor and manage the condition of the shutdown facilities, including the VBWR, to minimize introduction of residual radioactivity into the site.

The VBWR Seismic Evaluation focused on the reactor vessel supports and basement concrete structures and included an analysis of the structures ability to maintain integrity and stability (without collapse) of the reactor vessel and containment building in a seismic event for purposes of ensuring confinement of residual radioactivity.

The staff also notes that in Section 4.1 of GEHs November 15, 2019 RAI response, GEH Third Reply to NRC Request for Information Needed for GEH Exemption Request for Alternate Decommissioning Schedules (GEH Third Reply) (ADAMS Accession No. ML19319B848), GEH states: The approach used [for the VBWR structural integrity evaluation] was to compute demand-to-capacity (D/C) ratios for the components under dead load and seismic loading, using intact nominal dimensions and conservative assumptions where information is missing or unclear.

Issues and Request Based on the NRC staff review of the VNC Degradation Evaluations and the Maintenance and Surveillance Plan related to maintaining structural integrity of the VBWR containment structure, the staff identified the following issues that require further information and clarification. GEHs responses will enable the staff to make a safety determination on whether the structural integrity of the degraded VBWR containment will be maintained under a seismic event and structural degradations will be adequately managed during the extended decommissioning period in a manner that minimizes the introduction of residual radioactivity into the site, including the subsurface.

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1. Tier 1 Seismic Evaluation of the Vallecitos Boiling Water Reactor Shutdown Facility and GEH Third Reply a) Section 3.2, Assumptions, modeling assumption item 2 of the VBWR Seismic Evaluation states that effectively rigid support from soil is applied on the compression side of the structure and contact with soil is represented as rigid. GEH did not include information to support modeling soil stiffness as rigid (i.e., fixed boundary conditions below grade as opposed to fixed base at foundation level) and this modeling assumption may adversely affect the results and/or conclusions of the structural evaluation.

Request: Provide the technical rationale to justify that modeling soil stiffness as rigid (i.e., fixed boundary conditions for all below grade portions as opposed to fixed base at foundation level when soil-structure interaction is not considered) in the VBWR containment structural analysis model is appropriate and will not adversely affect the results and/or conclusions of the structural evaluation. If GEH determines that the modeling assumption is not appropriate or in error, then provide an evaluation of the results and/or conclusions of the VBWR containment structural evaluation using an appropriate and justified modeling assumption.

b) Section 4.1, Drawing Review, of the VBWR Seismic Evaluation states that the metal containment is a 0.93 steel shell that surrounds the entire structure. Section 3.2, Assumptions, contains several modeling assumptions made to conservatively account for missing data or lack of supporting documents. Section 3.2, item 11 states that housing (i.e., the steel shell) is assumed uniform in thickness and no material loss through corrosion was modeled. However, Section 3.8, Corrosion of Exterior BWR Steel of the VNC Degradation Evaluations states that drawings indicate a 9.5 mm (3/8) steel shell (housing) completely surrounding the concrete below grade, and the exposed shell region above grade was measured to be 15/16 (0.93). It also states that the exterior steel housing has become compromised based on the presence of infiltrated water and large amounts of efflorescence in the VBWR basement. It is unclear if the structural model reflected this significant difference in nominal thicknesses for the steel shell below grade and above grade, and evidence of through-wall corrosion of housing below grade, or if it assumed the steel shell (housing) to be 0.93 for the entire steel containment. This may significantly affect the results and conclusions of the structural evaluation.

Request: Provide the technical rationale with supporting evidence to justify that modeling the thickness of below-grade steel shell (housing) completely surrounding the concrete below grade as a uniform nominal 15/16 in the VBWR containment structural analysis model is appropriate and will not adversely affect the results and conclusions of the structural evaluation; the response should factor implications of the information noted above that (i) drawings indicate that the below grade steel housing nominal thickness is apparently 3/8 thick and (ii) there is evidence of further material loss due to corrosion. If GEH determines that the modeling assumption is not appropriate or in error, then provide an evaluation of the results and conclusions of the VBWR containment structural evaluation using an appropriate and justified modeling assumption.

c) Among the modeling assumptions in Section 3.2, Assumptions, of the VBWR Seismic Evaluation, made to conservatively account for missing data or lack of supporting documents, item 16 states that the temporary columns shown in the foundation plan

4 were assumed to be retained after backfilling and are credited in the analysis as restraints for torsional displacement. However, given the temporary configuration of the columns for original construction support, information regarding the condition and potential degradation, connection, and confirmation of existence of these temporary columns and their connections to the structure were not provided to demonstrate that these columns can perform the function for which it is credited in the VBWR containment seismic evaluation.

Request: Provide the technical basis with supporting evidence to justify that the temporary columns exist in the assumed configuration and are capable of performing the structural function for which they are credited in the VBWR structural integrity evaluation through the extended decommissioning period. Alternatively, provide an evaluation of the results and conclusions of the VBWR containment structural evaluation without crediting the temporary columns for structural function.

d) Section 5.1.6, Main Floor, of the VBWR Seismic Evaluation states in part that stresses in the main floor are not excessively high but were over the limit when adding the dynamic soil pressure. Section 5.2, D/C Ratio Results Discussion, also states that no failure of the floor diaphragm is predicted as a result of the Tier 1 evaluation. However, Section 5.1.6 further states that the floor was analyzed as intact and was not checked with cracking introduced at or near the reactor pit. The NRC staff notes that Section 2.5.1, Ground Level Main Floor, Elevation 539 - 6, of the walkdown described in the VNC Degradation Evaluations, identified numerous cracks on this operating floor, including 6.5 mm cracking, described in the report as fairly large; thus, it is not clear how the floor could be considered acceptable if the analysis did not evaluate these degradations.

Request: Provide an evaluation of the impact of the existing cracking, projected through the extended decommissioning period, on the structural capacity and integrity of the VBWR main (operating) floor. Alternatively, provide the technical rationale and justification for why the structural integrity of the main floor is considered acceptable by GEH if the structural evaluation did not consider existing degradations such as significant cracking projected through the extended decommissioning period.

Additionally, include the controlling Demand to Capacity (D/C) ratios with locations for the main floor.

e) Chapter 2, Technical Approach of the VBWR Seismic Evaluation states, The VBWR model makes use of nominal dimensions and elastic material properties. However, the capacities, where appropriate, include reductions in member size and/or strength to approximate the as-found moderately degraded condition in a conservative manner.

Further, Section 6.1, Scope of the Tier 1 Evaluation, of Chapter 6, Conclusions, of the report states, Final D/C ratios include adjustments for concrete cracking and corrosion of steel, to represent conditions observed during our site walkdowns. Also, Section 6.6, Summary, of the report states, Computed capacities include projected degradation of the structure, based on inspection observations.

Further, key walkdown observation No. 1 in Section 4.1, VBWR Structural Integrity Assessment, of GEH Third Reply (from Section 4.1, Key Conclusions from Site Visit, of the VNC Degradation Evaluations) states, The VBWR containment building has cracks with width 0.1 mm (hairline) to 4 mm (large) due to corrosion of the reinforcing bars and cracking of the operating floor 6 mm (very large) and on the concrete

5 containment walls, 0.3 mm to 1.5 mm (larger than hairline cracks). Further, key walkdown observation No. 6 in Section 4.1 of GEH Third Reply (from Section 4.1 of the VNC Degradation Evaluations) states:

Extensive cracking of concrete was observed in VBWR that could have been produced by mechanical stress or corrosion of the embedded reinforcing steel (rebar). Several correlations are observed between the reinforcing steel layout/rebar pattern and the locations of cracking, and it appears corrosion of the rebar is responsible for cracking directly along the concrete surface over the rebar observed in most locations. This corrosion likely indicates significant degradation of the structural capacity of the reinforced concrete sections. To account for the observed degradation, the load carrying capacity of the concrete was reduced when evaluating the stress results of the analysis.

The NRC staff further notes that Section 4.1 of GEH Third Reply states:

Computed load carrying capacities of the components were reduced to account for projected degradation of the structure based on inspection observations.

Nominal sizes and dimensions were used in the analysis to compute demands.

In order to address the potential structural strength loss due to concrete cracking and corrosion of steel, the following adjustments were made to the capacities when calculating D/C ratios:

Concrete strength reduced from 3000 psi [used in structural analysis] to 2000 psi Corrosion factor 0.9 on rebar area Corrosion factor 0.9 on bolt area The NRC staff notes that the reduced concrete strength and minimal corrosion factors on rebar and bolt areas do not appear to be representative of the key walkdown observations related to concrete and reinforcing steel degradations noted above.

Further, there is no information provided of the rationale or evidence used in determining that these nominal reductions are representative of degradations observed during walkdowns and projected degradations through the extended decommissioning period as asserted in the statements above.

Request: With reference to (1) the corrosion degradation issues (rebar, bolt) and associated assumed corrosion reduction factors, and (2) the assumed reduced compressive strength of 2000 psi identified above, provide the technical rationale with supporting evidence used in determining that the corrosion reduction factors and reduced concrete compressive strength, to account for reduction of strength due to degradations of the affected reinforced concrete and bolt components, are representative or conservative of the observed degradation projected through the extended decommissioning period. Also, include the controlling D/C ratios and locations for the degraded reinforced concrete containment components below the main floor to substantiate the statements asserted in the first paragraph of this issue (e).

6 f) Key walkdown observation No. 8 in Section 4.1 of Third GEH Reply states that a calculation of the corrosion rate of steel suggests that through-wall penetration of the VBWR 3/8 or 9.5 mm thick steel housing, with an estimated corrosion rate of 0.2 mm/year, could have occurred in approximately 48 years and could have been responsible for the presence of water and significant efflorescence in the VBWR basement. The significance of this conclusion is that water could continue to penetrate the VBWR.

Further, key walkdown observation No. 9 in Section 4.1 of Third GEH Reply states:

The extensive amount of efflorescence in the VBWR is considered a concern since it could indicate internal structural weakness. It is believed that the massive amounts of efflorescence identified in the VBWR is due to the hydrostatic pressure from exterior water that penetrated the exterior steel housing. The load carrying capacity of concrete was reduced when evaluating the stress results of the analysis to account for the observed degradation.

Monitoring of the groundwater infiltration and condition of the structure is discussed in Section 5.1.

However, the NRC staff notes that exposure to moisture increases the corrosion rate of observed rebar corrosion and the staff could not find any specific actions related to monitoring of groundwater infiltration and condition of the structure claimed to be discussed in Section 5.1, Recommendations from Walk Down Reviews, of GEH Third Reply that GEH intends to take. Further, SI recommended to GEH in Section 6.5, Recommendations, of the VBWR Seismic Evaluation, strength and petrographic testing of the concrete to verify adequate compressive strength and composition. However, GEH decided not to implement SIs recommendations for core sampling and associated material property, chemical and petrographic tests (intended to provide deterministic data and verification regarding the actual condition and quality of the concrete in the basement wall section) by considering them as enhancements on the basis that the structural analysis, which in this case is based on unverified assumptions, showed that containment integrity is maintained. The NRC staff has concerns with conclusions of analysis based on assumptions that are unverified regarding reinforced concrete condition and performance of the degraded VBWR containment structure, especially in degraded critical areas near the base, with significant efflorescence potentially indicative of structural weakness (compressive strength, stiffness (elastic modulus)) where seismic demand is expected to be maximized.

Request: With regard to the ongoing reinforced concrete degradation issues (i.e.,

significant cracking, efflorescence, and rebar corrosion), provide the technical justification and rationale that the unverified assumptions identified above are representative or conservative values for use in the structural evaluation, or verify the assumed values through, for example, the use of core sampling tests (such as those recommended by GEH consultant SI) to provide objective assessment and verification of the compressive strength, elastic modulus and condition of the degraded concrete with corroding reinforcement, and that the strength and quality of the degraded concrete and reinforcement in the lower levels (main floor and below) of the VBWR containment, assumed in the structural evaluation, are adequate to maintain sufficient integrity as structural reinforced concrete through the extended decommissioning period. If core sample testing is done, provide the results including the technical rationale and justification used for the selection of the number and locations of core samples tested.

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2. Maintenance and Surveillance Plan: Vallecitos Nuclear Center Shutdown Reactors a) The issues identified with the GEH Maintenance and Surveillance Plans for the EVESR and GETR, as described in NRCs RAI dated March 16, 2020 (ADAMS Accession No. ML20071G413), also apply to the Maintenance and Surveillance Plan for the VBWR.

Request: Provide the information requested in the NRCs RAI dated March 16, 2020 (ADAMS Accession No. ML20071G413).

b) In its review of the VBWR Maintenance and Surveillance Plan related to structural integrity, the staff identified the following additional issue for the VBWR given the ongoing and continued infiltration of groundwater into the containment due to through-wall corrosion penetration of the steel housing (at unknown numbers of locations) noted as key walkdown observation No. 8 in Section 4.1 of GEH Third Reply. In Section 3.2 of the VBWR Seismic Evaluation, GEH stated that it is unlikely that water collecting in the basement would be able to flow back out into the environment, and that the water level in the basement would have to rise to a level higher than the exterior groundwater level to overcome the hydrostatic pressure. Based on the review of VBWR Maintenance and Surveillance Plan, it is not clear how the level of the collected water inside the containment will be monitored and managed with respect to the groundwater level variation at the site to ensure that introduction of residual radioactivity into the site is minimized.

Request: Describe how the level of the ingress water inside the VBWR containment will be monitored and managed with respect to the groundwater level variation at the site to assure introduction of residual radioactivity into the site is minimized through the extended decommissioning period.

3. Supporting Future Actions: Section 5.0, Maintenance and Surveillance of Facilities, of GEH Third Reply includes disposition of recommendations from walkdown reviews, maintenance and surveillance of the shutdown facilities, and special surveillance should a seismic event occur. These include selective future actions that GEH will perform in support of providing reasonable assurance that introduction of residual radioactivity to the site will be minimized during extended decommissioning period. GEH states it has entered these actions into the GEH Corrective Action System for tracking and there may be additional future actions as a result of response to this RAI. However, there is no implementation schedule provided for future actions that GEH will perform.

Request: Clearly identify all the specific future actions (including those, if any, that may result from the response to this RAI) that GEH will perform in support of structural integrity to provide reasonable assurance that introduction of residual radioactivity to the site, including subsurface, will be minimized during the extended decommissioning period of the shutdown facilities (including VBWR) and provide the implementation schedule for each action.