Resolution of Generic Safety Issues: Issue 161: Use of Non-Safety-Related Power Supplies in Safety-Related Circuits (Rev. 1) ( NUREG-0933, Main Report with Supplements 1–34 )
This issue was identified1481 by NRR to address the concerns raised during the licensing of Nine Mile Point Unit 2.
On February 7, 1985, Niagara Mohawk submitted to the NRC a report on "Non-Class 1E Devices Connected to Class 1E Power Supplies" which stated that "the non-Class 1E devices to be analyzed were identified by a study of Elementary Diagrams and Elementary Diagram Device Lists for all safety systems in the GE scope of supply for NMP2." These devices were asserted to be acceptable by the licensee and the vendor (GE) based on an FMEA; however, this FMEA was not accepted by the staff. Ultimately, of the 239 identified components, 35 were isolated with qualified isolation devices and 76 were upgraded to Class 1E by the licensee in order to meet the regulatory requirements imposed by the staff.
The fundamental concern in this issue was whether the staff's actions in requiring the isolation or replacement of 111 out of 239 components during the licensing review was a change in regulatory position and, if so, whether the position should be backfit to all licensed facilities. The position taken by the staff in 1985 during the OL review of Nine Mile Point 2 was necessary to meet the regulations in effect at that time. It was GE's contention that the design approach at Nine Mile Point 2 was similar to that used at other plants and was accepted by the NRC during the licensing review of BWR plants.
The determination of whether a component or system is an associated circuit is based on the application of IEEE 384-1977 and IEEE 279-1971. Any non-Class 1E component or system which interconnects with a Class 1E component or system is an associated circuit, unless it is adequately isolated. IEEE 279-1971, which was incorporated by reference into 10 CFR 50.55a(h), does not identify associated circuits but only states that "the transmission of signals from protection system [Class 1E] equipment for control system [non-Class 1E] use shall be through isolation devices which shall be classified as part of the protection system." This restriction was relaxed in IEEE 384-1977 to define an associated circuit as any circuit comprised of non-Class 1E components that is not isolated from a Class 1E circuit by an isolation device. This configuration is acceptable if the associated circuit is "analyzed or tested to demonstrate that Class 1E circuits are not degraded below an acceptable level." IEEE 384-1977 was adopted in Regulatory Guide 1.75, Revision 3, in September 1978.
Prior to the publication of IEEE 384, the classification of an associated circuit did not exist and any non-Class 1E component should have been isolated from a Class 1E component or system by a qualified isolation device. Therefore, it was unlikely that the staff accepted a practice in the past which they found unacceptable in 1985, since the requirements in 1985 were less strict than the requirements before 1977. GE was unable to document previous acceptance by the staff of the design practice under contention. In addition, the criteria used by the staff in 1985 to determine whether a circuit is an associated circuit and whether the associated circuit should be isolated would also be applied currently to OL applicants.
The components at Nine Mile Point 2 were analyzed by GE and resulted in 128 of the 239 identified components being accepted as associated circuits without modification; however, the remaining 111 components were upgraded to Class 1E or were isolated. It appears reasonable to assume that the determination made by NRR regarding the need for components to be Class 1E is correct. Thus, this issue was determined to be a matter of compliance with existing regulations1482 and was DROPPED from further consideration as a new and separate issue. In an RES evaluation,1564 it was concluded that consideration of a 20-year license renewal period did not change the priority of the issue.