The inspectors identified an unresolved item related to the acceptability of the 230 kV preferred offsite power system to meet design basis requirements. Additional NRC review is needed to determine if the preferred offsite system has sufficient capacity and capability to supply the engineered safety features buses for all required accidents and
transients. On April 10, 2009, the inspectors identified that the plant electrical design analysis may not be adequate to demonstrate that the 230 kV preferred offsite power system had sufficient capacity and capability to meet station loads following an accident on one unit and concurrent
safe shutdown on the remaining unit or for a concurrent
safe shutdowns on both units. The Diablo Canyon offsite power sources include a normal supply from the 230 kV distribution system and a delayed supply from the 500 kV distribution system. The normal supply is required to immediately power the engineered safety feature systems following a station accident or a
reactor trip. The delayed supply backs up the normal supply and can be aligned to power the engineered safety feature systems in about 30 minutes. NRC Safety Evaluation Report, Safety Evaluation by the Directorate of Licensing U.S. Atomic Energy Commission in the Matter of Pacific Gas And Electric Company Diablo Canyon Nuclear Power Station, Units 1 and 2 San Luis Obispo County,
California Docket Nos. 50-275 And 50-323, and Final Safety Analysis Report Update, Section 8.1, Offsite Power Systems, established
IEEE Standard 308-1971, Class
IE Electrical Systems, as part of the preferred offsite power system design basis.
IEEE Standard 308-1971, Section 8.1.1, Multi-Unit Station Considerations, stated: Capacity. A multi-unit station may share preferred power supply capacity between units. In such a case, as a minimum the total preferred capability must be sufficient to operate the engineering safety features for a design basis accident on one unit and those systems required for concurrent
safe shutdown on the remaining units. The type of accident and shutdown and the unit assumed to have the accident, shall be those which give the largest total preferred capability requirements. Pacific Gas and Electric used Design Calculation
357AA-DC, Units 1 and 2 Load Flow, Short Circuit and Motor Starting Analysis, September 24, 2007, to ensure that the preferred offsite power system was capable of meeting design basis electrical load requirements. The inspectors identified that Calculation
357AA-DC did not include load flow cases representing the largest total capability for an accident on one unit and concurrent
safe shutdown of the other unit or concurrent
safe shutdown of both units. Calculation
357AA-DC modeled the limiting load flow cases as an accident (or unit trip) on one unit while assuming a previous shutdown had occurred on the other unit. The load flow modeling was based on the assumption that plant operators would perform an orderly shutdown entailing the manual transfer of electrical loads to the 230 kV system at a time of low electrical demand from the accident or tripped unit. On June 26, 2009, the licensee completed a preliminary re-evaluation of preferred offsite power supply load flow assuming an accident on one unit and a concurrent
safe shutdown on the remaining unit, and for an assumed concurrent
safe shutdown on both units. The licensee concluded the voltage at the 4160 Class 1E vital buses would be less than adequate to support operation of the engineering safety features under design conditions. The licensee also analyzed the plant response based on actual available 230 kV
switchyard voltages between November 2008, and June 26, 2009. For these cases, the licensee concluded that 4160 Class 1E vital bus voltages would have intermittently dropped below the minimum voltage required for operability of the engineering safety features. The inspectors concluded that actual 230 kV system voltage recovered prior to exceeding the 72-hour action time for
Technical Specification 3.8.1,
AC Sources - Operating, for any single occurrence. Pacific Gas and Electric had previously identified that the 230 kV offsite power source had insufficient voltage (reported as Licensee Event Report 1-95-007, 230 kV System May Not Be Able to Meet its Design Requirements for all Conditions Due to Personal Error). The corrective actions included increasing the capability of the startup transformers and installation of large capacitor banks at the plant
switchyard and Mesa Substation. When sizing the replacement transformers, the licensee assumed that the preferred offsite power system only needed to have the capacity and capability for an accident or trip on one of the two units. In a licensing position paper for the 230 kV system loading requirements (Letter File 227961, from the Director Licensing to Director, Electrical
I&C Engineering September 27, 1995), the licensee added the word orderly to the safety shutdown requirements specified in
IEEE Standard 308-1971. The licensee did not perform a
10 CFR 50.59 evaluation of this change nor seek prior NRC approval. As a result, the licensees previous corrective actions were insufficient to restore the preferred offsite power system to compliance with the design basis. This issue is unresolved pending NRC review of the 230 kV preferred offsite power system design basis requirements. Unresolved Item: 05000275;323/2009003-01
