The team identified an unresolved issue pertaining to the susceptibility of a design basis event concurrent with a degraded 4Kv vital bus. Specifically,
PSEGs UFSAR section 8.3, Onsite Power System, states the following: The Offsite Power System in combination with the onsite distribution system has been shown by analysis and test to possess sufficient capacity and capability to automatically start and subsequently operate all safety loads within their voltage ratings for anticipated
transients and accidents. The worst sustained undervoltage condition in the onsite distribution system was found to occur with a severely degraded 500 Kv offsite system simultaneous with a concurrent loss-of-coolant (
LOCA) on Unit 2 and unit trip on Unit 1 (or vice versa). The team was concerned that a postulated sustained undervoltage condition may result in the degraded voltage relays (second level undervoltage protection) not having sufficient voltage to reset following the start of the postulated accident initiated loads. If this were to occur, the 4Kv vital bus voltages on all three buses could remain below the degraded voltage setpoint of 94.6% but higher than the loss-of-voltage setpoint of 70% for the duration of the degraded voltage time delay of 13 seconds nominal. Subsequently, the 4Kv vital buses would automatically disconnect from offsite power and transfer to the
emergency diesel generators as designed. However, during the 13 seconds of degraded voltage, the safety related loads may have inadequate voltages to start or run and therefore go into a stall condition that could trip their associated protective relays or thermal
overload relays (
TOLs). The team noted that the safety related
MOV thermal
overload relays are not bypassed on an accident condition for the Salem Units. After disconnection from offsite power and transfer to the
EDG, any loads that tripped on their protective devices would reset and restart on the
EDG with the exception of safety related
MOVs or motors that have
TOLs. For those loads, they would become unavailable and not restart on the
EDG. The
TOLs can only be reset from the affected motor control centers in order to restart the motors or
MOVs. It was not clear if this scenario is within Salems licensing basis.
PSEG was researching licensing documentation in response to the question. Regardless of their licensing bases,
PSEG conservatively determined to take an action relative to the potential for degraded offsite power conditions. At the time of the inspection they issued a standing order to clarify that in lieu of entering
TS 3.8.1.1 action d, when both trains of offsite power are declared
inoperable due to degraded voltage on the grid, they will enter
TS 3.0.3. This issue was unresolved pending determination of
PSEGs licensing bases.
PSEG entered the issue into their corrective action program as Notifications 20379249, 20379246, and 20379520. (
URI 05000272;311/2008007-04, Vital Bus Degraded Voltage Licensing Bases
