Resolution of Generic Safety Issues: Issue 75: Generic Implications of ATWS Events at the Salem Nuclear Plant (Rev. 1) ( NUREG-0933, Main Report with Supplements 1–35 )
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DESCRIPTION
Historical Background
On two occasions (February 22 and 25, 1983), Salem Unit 1 failed to scram automatically due to failure of both reactor trip breakers to open on receipt of an actuation signal. In both cases, the unit was successfully tripped by manual action.444 The failure of the breakers was attributed to excessive wear from improper maintenance of the undervoltage (UV) relays which receive the trip signal from the protection system and result in the breakers opening mechanically.
Three separate NRC actions were initiated to address this problem. One was plant-specific and was addressed before the restart of Salem 1; this was completed in the "Salem Restart Evaluation."454 The second action was an investigation into the Salem events and the circumstances leading to them; this was reported in NUREG-0977.447 The third action was the formation of an NRC task force to study the overall generic implications of this event; the results of this study were reported in NUREG-1000445 (Volumes 1 and 2). In addition, a number of issues raised448, 449, 451 by the staff were closely related to the design and testing of the reactor protection system (RPS).
Safety Significance
Failure to scram (also commonly referred to as ATWS) could rupture the RCS or distort ECCS valves such that a core-melt would result (see USI A-9, "ATWS").
Possible Solution
As a result of its work, the Salem Generic Implications Task Force published its findings in NUREG-1000,445 Volume 1, and later outlined proposed actions for licensees, applicants, and the NRC staff in SECY-83-248.521 NRC issued the required actions for licensees and applicants in Generic Letter 83-28520 on July 8, 1983. Both the required actions of Generic Letter 83-28520 and the internal staff actions were published in August 1983 as NUREG-1000,445 Volume 2 (See Table 3.75-1).
AEOD also reviewed the Salem event and published AEOD/P301522 which focused on two issues: (1) the adequacy of NRC's reporting requirements; and (2) the potential for trends and patterns analyses to predict such events before they occur. Based on the conclusions of this report, a number of actions were planned by AEOD (see Item B.11 of Table 3.75-1). In addition, the report endorsed a number of the other planned actions. NRR reviewed the final report522 and endorsed the AEOD planned actions.523 NRR also concluded523 that the generic letter520 addressed the safety issues which were of mutual concern. With respect to the related issues, the staff reviewed452 the design of the reset for a valid protection system signal451 and concluded that the existing
| (A) | Licensee Actions [50.54(f) Letter] | Priority* | MPA No. |
|---|---|---|---|
| TABLE 3.75-1445 | |||
| 1.Post-Trip Review | NOTE 3(a) | B-76, B-85 | |
| 2.Equipment Classification and Vendor Interface | NOTE 3(a) | B-77, B-86 | |
| 3.Post-Maintenance Testing | NOTE 3(a) | B-78, B-79,B-87, B-88 | |
| 4.Reactor Trip System Reliability | NOTE 3(a) | B-80, B-81,B-82, B-89,B-90, B-91,B-92, B-93 | |
| (B) | Staff Actions | ||
| 1.NRC Oversight of Licensee Management Performance | LI | NA | |
| 2.NRC Role in Reactor Restart Decisions | LI | NA | |
| 3.NRC Regulation of Licensee Maintenance Activities | HFPP | NA | |
| 4.Quality Assurance (Operations) | NOTE 3(b) | NA | |
| 5.Vendor Inspection | LI | NA | |
| 6.Review of IE Bulletin Requirements | LI | NA | |
| 7.Other Safety-Related UV Trip Attachments | NOTE 3(a) | NA | |
| 8.Review of Human Factors Issues | HFPP | NA | |
| 9. Amendment to ATWS Rule | A-9 | NA | |
| 10. Preservation of Evidence | LI | NA | |
| 11. Collection and Analysis of Operating Experience | LI | NA | |
| 12.General Operating Criteria | LI | NA |
* For a description of the terms used for priority, see Table II in the Introduction.
design was acceptable. The two other issues which deal with the testing of the RPS448,449 were considered to be within the scope of the Salem Task Force review.450 Specifically, the requirements regarding the RPS reliability improvements445 and the post-maintenance testing adequately addressed these issues.
PRIORITY DETERMINATION
This issue was broken down into 16 distinct parts: 4 licensee actions and 12 staff actions (See Table 3.75-1). The following is a summary of these 16 actions.
(A) Licensee Actions(4)
(1) Post-Trip Review
This action considered of two parts:
(a) Program Description and Procedure: Licensees and applicants were required to describe their programs for ensuring that unscheduled reactor shutdowns were analyzed and a determination made as to when their plants could be restarted safely.445 MPA B-76 was established by NRR/DL for implementation purposes.
(b) Data and Information Capability: Licensees and applicants were required to have, or have planned, the capability to record, recall, and display data and information to permit diagnosis of the causes of unscheduled reactor shutdowns, prior to restart, and for ascertaining the proper functioning of safety-related equipment.445 MPA B-85 was established by NRR/DL for implementation purposes.
(2) Equipment Classification and Vendor Interface
This action consisted of two parts:
(a) Reactor Trip System Components: Licensees and applicants were required to confirm that all components whose functioning is required to trip their reactors are identified as safety-related on documents, procedures, and information-handling systems used in their plants to control safety-related activities, including maintenance, work orders, and parts replacement. In addition, this action called for licensees and applicants to establish, implement, and maintain a continuing program to ensure that vendor information is complete, current, and controlled throughout the life the plants and appropriately referenced or incorporated in plant instructions and procedures.445 MPA B-77 was established by NRR/DL for implementation purposes.
(b) Programs for All Safety-Related Components: Licensees and applicants were required to submit a description of their programs for safety-related equipment classification and vendor interface.445 MPA B-86 was established by NRR/DL for implementation purposes. The requirements of this action were later clarified by Generic Letter 90-03.1303
(3) Post-Maintenance Testing
This action consisted of two parts:
(a) Reactor Trip System Components: Licensees and applicants were required to: (i) submit the results of their review of test and maintenance procedures and TS to assure that post-maintenance operability testing of safety-related components in the reactor trip system (RTS) was required to be conducted, and that the testing demonstrated that the equipment was capable of performing its safety functions before being returned to service; (ii) submit the results of their check of vendor and engineering recommendations to ensure that any appropriate test guidance was included in the test and maintenance procedures or the TS, where required; and (iii) identify, if applicable, any post-maintenance test requirements in existing TS which can be demonstrated to degrade rather than enhance safety.445 MPAs B-78, B-79, B-87, and B-88, were established by NRR/DL for implementation purposes.
(b) All Other Safety-Related Components: Licensees and applicants were required to: (i) submit a report documenting the extension of test and maintenance procedures and TS review to assure that post-maintenance operability testing of all safety-related equipment was required to be conducted and that the testing demonstrated that the equipment was capable of performing its safety functions before being returned to service; (ii) same as (3)(a)(ii) above; and (iii) identify, if applicable, any post-maintenance test requirements in existing TS which were perceived to degrade rather than enhance safety.445 MPAs B-78, B-79, B-87, and B-88 were established by NRR/DL for implementation purposes.
(4) RTS Reliability
This action consisted of five parts:
(a) Vendor-Related Modifications: This action required the review of all vendor-recommended reactor trip breaker modifications for verification that each modification had been implemented. In those cases where a modification was not implemented, a written evaluation of the technical reasons was required.445 MPA B-80 was established by NRR/DL for implementation purposes.
(b) Preventative Maintenance and Surveillance Program for Reactor TripBreakers: Licensees and applicants were required to describe their preventative maintenance and surveillance programs to ensure reliable reactor trip breaker operation.445 MPAs B-81 and B-89 were established by NRR/DL for implementation purposes.
(c) Automatic Actuation of Shunt Trip Attachment for W and B&W Plants: This action required W and B&W reactors to be modified by providing automatic RTS actuation of the breaker shunt trip attachments; these attachments were to be considered safety-related (Class 1E).445 MPAs B-82 and B-90 were established by NRR/DL for implementation purposes.
(d) Improvements in Maintenance and Test Procedures for B&W Plants: Licensees and applicants with B&W reactors were required to apply safety-related maintenance and test procedures to the diverse reactor trip feature provided by interrupting power to control rods through the silicon controlled rectifiers.445 MPA B-91 was established by NRR/DL for implementation purposes.
(e) System Functional Testing: This action required all plants to perform on-line functional testing of the RTS, including independent testing of the diverse trip features.445 MPAs B-92 and B-93 were established by NRR/DL for implementation purposes.
(B) Staff Actions(12)
(1) NRC Oversight of Licensee Management Performance
This action consisted of two parts:
(a) Systematic Assessment of Licensee Performance(SALP): The purpose of this action was to improve the inputs to SALP reports by AEOD, OIE, and NRR by broadening the data base, ensuring NRR management involvement, and ensuring that the plant project managers and technical reviewers are cognizant of the importance of management issues on a continuing basis.445 This action was classified as a Licensing Issue.
(b) INPO Evaluation Program: This action called for the staff to improve its review of INPO evaluation findings, licensee-proposed corrective actions, and subsequent INPO findings.445 This action was classified as a Licensing Issue.
(2) NRC Role in Reactor Restart Decisions
The purpose of this action was to enhance NRC determination of the safety significance of operating events and their effects on continued operations or restart.445 This action was classified as a Licensing Issue.
(3) NRC Regulation of Licensee Maintenance Activities
The purpose of this action was to strengthen NRC's program for regulation of equipment maintenance in operating reactors. NRC Inspection Procedure 62703 was to be modified to provide for observation of RTS breaker preventive maintenance on a periodic basis. In addition, the staff was to continue its development of new regulatory initiatives for licensee maintenance activities.445 This action was determined to be covered in the HFPP.
(4) Quality Assurance (Operations)
This action called for the staff to develop Revision 3 to Regulatory Guide 1.33225 to include detailed procedures and more stringent criteria for operational QA programs.445 The appropriate revisions were made to the guide to address the Salem ATWS events; however, because of other deficiencies, publication of the guide was deferred pending industry issuance of ANS 3.2. Thus, this action was RESOLVED with no new requirements for operating plants. The impact of this action on future plants will be determined when Revision 3 to Regulatory Guide 1.33225 is issued.1300
(5) Vendor Inspection
The staff identified a need for greater assurance that the quality standards specified by nuclear equipment suppliers for components used in safety-related systems include reliability and design life margin, commensurate with their importance to safety.445 This action called for the NRC vendor inspection program to be revised to address these areas and, thus, was classified as a Licensing Issue.
(6) Review of IE Bulletin Requirements
The Salem breaker failures were closely related to problems previously identified and communicated to licensees in IE Bulletin Nos. 77-021304 and 79-091305 as well as IE Circular No. 81-12.1306 This action was classified as a Licensing Issue and called for the staff to ensure that bulletin responses are promptly reviewed and that, where appropriate, bulletin actions are promptly incorporated into TS, rules and regulations, SRP,11 codes and standards, or inspection requirements.445
(7) Other Safety-Related UV Trip Attachments
This action called for the staff to prepare a bulletin to require all licensees to identify, test as appropriate, and confirm maintenance procedures and safety classifications for breakers with UV trip attachments in safety-related systems other than the RTS.445 IE Bulletin No. 83-081299 was issued to address the staff's concern. Thus, this action was RESOLVED and requirements were issued.
(8) Review of Human Factors Issues
This action called for the staff to include a more balanced treatment of the implications of ATWS in its review of human factors issues.445 This action was classified as a Licensing Issue and was addressed by the staff in the HFPP.
(9) Amendment to ATWS Rule
The staff developed a proposed amendment to the ATWS rule (USI-A-9) to reflect certain implications of the Salem events. This proposal addressed the need to upgrade further the degree of diversity in the existing W RTS design.445 Thus, this action was covered in the resolution of USI A-9, "ATWS."
(10) Preservation of Evidence
This action called for the development of generic instructions to the Regions to ensure preservation of evidence subsequent to an incident and, thus, was classified as a Licensing Issue.445
(11) Collection and Analysis of Operating Experience
This action was classified as a Licensing Issue and called for the staff to develop an information notice to clarify the reporting requirements associated with component failures, such as reactor trip breaker failures.445
(12) General Operating Criteria
This action called for the staff to consider the development of a set of general operating criteria analogous to the general design criteria. It was believed that such criteria would codify good operating practices and would address the following areas: management philosophy and policies; management oversight and review responsibility; organization and communication; operating and emergency procedures; staffing, qualifications, and training; assurance of quality in operations; preventive and corrective maintenance; data collection, use, retention, and reporting; operating experience evaluation; replacement of equipment; planning and scheduling of safety-related work; and post-maintenance and periodic inspection and testing.445
An inter-office task force was established to investigate the need for, and benefits of, the NRC developing and codifying general operating criteria for nuclear power plants. A preliminary survey of regulations and guidance was performed by this task force. After staff review of this survey, it was recommended that general operating criteria not be promulgated and any deficiencies in regulations and guidance be addressed within the existing regulatory structure.1301 Thus, this Licensing Issue was resolved.
CONCLUSION
The four parts of this issue involving required actions for licensees and applicants were resolved with the issuance of Generic Letter 83-28520 and IE Bulletin Nos. 83-011297 and 83-04.1298 Sixteen MPAs were issued by the staff to track the implementation of the required actions. (See Table 3.75-1, Part A.) The remaining twelve parts of the issue were staff actions that have been addressed as stated above. (See Table 3.75-1, Part B.) Thus, this issue was RESOLVED and requirements were issued.
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