All holders of operating licenses or construction permits for nuclear power reactors.
The U.S. Nuclear Regulatory Commission (NRC) is issuing this generic letter to: (1) notify addressees about problems with testing of safety-related logic circuits, (2) request that all addressees implement the actions described herein, and (3) require that all addressees submit a written response to this generic letter regarding implementation of the requested actions.
The NRC staff had previously issued the following information notices (Ins) regarding problems with testing of safety-related logic circuits: IN 88-83, "Inadequate Testing of Relay Contacts in Safety-Related Logic Circuits," dated October 19, 1988; IN 91-13, "Inadequate Testing of Emergency Diesel Generators (EDGs)," dated March 4, 1991; IN 92-40, "Inadequate Testing of Emergency Bus Undervoltage Logic Circuitry," dated May 27, 1992; IN 93-15, "Failure to Verify the Continuity of Shunt Trip Attachment Contacts in Manual Safety Injection and Reactor Trip Switches," dated February 18, 1993; and IN 93-38, "Inadequate Testing of Engineered Safety Features Actuation Systems," dated May 24, 1993. Despite these notices, recent events have occurred similar to those described in the INs which indicate that licensees have not taken sufficient action to correct previously identified problems in logic circuit surveillance testing. On March 7, 1995, NRC issued IN 95-15, "Inadequate Logic Testing of Safety-Related Circuits," which informed licensees about these recent events at Cooper Nuclear Station, Fermi 2, Waterford 3, Grand Gulf Nuclear Station, and Arkansas Nuclear One, Unit 1 and Unit 2.
Description of Circumstances
The NRC has documented a significant number of instances involving problems with logic testing of safety-related circuits in the information notices described above. These information notices discuss events at various pressurized water and boiling water reactors. The examples of problems with logic testing cover a wide range of systems including safety injection system actuation, containment spray system actuation, residual heat removal system actuation, diesel generator load sequencing, and reactor protection system actuation. In most cases, the affected logic circuits functioned properly when testing in accordance with technical specification (TS) requirements was performed. The NRC has taken enforcement action in many of these cases since they resulted in TS violations. The details of these instances are included in the information notices cited above. An example of the details associated with this issue at Fermi Station is repeated here.
On July 15, 1994, during a routine review of surveillance procedures required by the Fermi Unit 2 TS, the licensee (Detroit Edison Company) discovered that neither the procedures used for testing the load shedding of the 4160 volt Residual Heat Removal (RHR) pumps nor the related instrumentation and control (I&C) logic functional test procedure provided for the full testing of the RHR pump start logic. The conductors which connect the I&C and electrical portions of the circuit were not tested. Also, the test procedures did not include verification that the switchgear breaker would not close with an undervoltage signal present at the bus.
After investigating further, the licensee discovered additional deficiencies in the undervoltage functional test surveillance procedures including the logic functional test surveillance procedures for the three other engineered safety buses. Also, the surveillance test overlap did not include sufficient overlap of the logic circuit to cover the degraded voltage trip input to the non-interruptible air supply system isolation logic, the degraded voltage trip input to the bus feeder breaker position, and the alternate automatic closure circuits for the EDG output breakers. The licensee further determined that the 480 volt load shed logic had not been fully tested.
On September 9, 1994, the licensee identified additional surveillance deficiencies and expanded the investigation of its surveillance procedures for EDGs and I&C overlap testing. During this investigation, the licensee determined that (1) multiple pathways for starting an EDG through the emergency core cooling system (ECCS) logic were not being tested separately, (2) emergency equipment cooling water (EECW) actuation from the load sequencer was not being differentiated from EECW actuation on reactor building closed cooling water low pressure, and (3) test acceptance criteria permitted performance outside of the TS limits.
In October and November 1994, the licensee identified several other test deficiencies in its surveillance procedures. These deficiencies were related to the core spray system, RHR system, reactor protection system, safety relief valves, alternate rod insertion and main steam isolation valve leakage control system logic, remote shutdown panel, primary containment manual isolation valves, and alternate shutdown panel transfer switches.
To address the above deficiencies, the licensee has taken the following corrective actions: (1) reviewed deficient procedures and performed required surveillance to establish operability, (2) reviewed similar procedures to identify other deficiencies, (3) is creating electrical overlap drawings, and (4) is training authors and technical reviewers of procedures to be fully aware of logic surveillance requirements. The NRC staff issued a notice of violation to Detroit Edison Company concerning the above issue (NRC Inspection Report No. 50-341/94-12).
A number of NRC regulations document the requirements to test safety-related systems to ensure that they will function as designed when called upon. For example, Title 10 of the Code of Federal Regulations (10 CFR), Section 50.36, "Technical Specifications," paragraph (c)(3) states that, "surveillance requirements are requirements relating to test, calibration or inspection to assure that the necessary quality of systems and components is maintained, that facility operation will be within the safety limits, and that the limiting conditions of operation will be met." Surveillance requirements to assure continued operability of safety-related logic circuits have been included in the plant-specific technical specifications for all operating nuclear power plants.
Other documents that provide a basis for these requirements include:
- 10 CFR 50.55a, "Codes and Standards," paragraph (h) which includes reference to Institute of Electrical and Electronic Engineers (IEEE) Standard 279, "Criteria for Protection Systems for Nuclear Power Generating Stations"
- Appendix A to 10 CFR 50, General Design Criterion (GDC) 21, "Protection System for Reliability and Testability"
- Appendix A to 10 CFR 50, General Design Criterion (GDC) 18, "Inspection and Testing of Electric Power Systems"
- Appendix B to 10 CFR 50, Criterion XI, "Test Control"
- Regulatory Guide (RG) 1.118, "Periodic Testing of Electric Power and Protection Systems"
- RG 1.32, "Criteria for Safety-Related Electric Power Systems for Nuclear Power Plants"
The NRC staff finds that the failure to adequately test safety-related actuation logic circuitry is safety significant in that inoperable essential electric components required for automatic actuation of post-accident mitigation systems may be undetected for extended periods. This is particularly true for the reactor protection system, whose unavailability is shown in probabilistic risk assessments to be a dominant contributor to potential core damage scenarios. Undetected reactor protection system availability/reliability degradation is also a potentially significant contributor to overall risk. Unavailability of those circuits associated with automatic emergency core cooling system (ECCS) actuation, especially in a loss-of-offsite-power situation, is a lesser contributor to overall risk but is important in ensuring post-accident recovery in accordance with licensing bases. Failure to automatically actuate safety systems also places the additional burden on the operators of having to manually actuate required functions and thus increases the chance for operator error.
The NRC staff notes that even in cases where surveillance testing of the logic circuits has not been complete, it is likely that only very small portions of the circuit have been omitted from the test. Further, the NRC staff is not aware of instances of specifically identified surveillance inadequacies that resulted in the unavailability of the safety system when called on during an event. Nevertheless, as indicated above, the NRC staff finds that compliance with the plant-specific technical specifications is essential in order to maintain the validity of the assumptions in the licensing basis accident analyses. On the basis of the recent events, previously issued INs, complexity of the logic, and contribution to the core damage frequency, the NRC staff has further determined that licensees should review their surveillance procedures for the reactor protection system, EDG load shedding and sequencing, and actuation logic for the engineered safety features systems to ensure that complete testing is being performed as required by the technical specifications.
The NRC staff requests that all addressees take the following actions: