RIS 2011-12 guidance indicated that the licensees voltage calculations should provide the basis for proper operation of the plant safety-related (SR) electrical distribution system, when supplied from the
offsite circuits via the transmission network. These calculations should demonstrate that the voltage requirements (both starting and running voltages) of all plant SR systems and components are satisfied based on operation of the transmission system (including the bounding transmission system single contingency in terms of voltage drop). In addition, during accident conditions, the nuclear unit generator trip and associated transmission system voltage drop should be factored into the accident case voltage calculations since unit trip occurs as a result of the accident. Licensee calculations should also demonstrate that all SR systems and components will function as designed with proper starting and running voltages during all plant conditions and the
DVRs will not actuate (separating the transmission network supply). The team noted that Constellation had not performed calculations to demonstrate that the offsite electric power system had sufficient capacity and capability to assure the voltage requirements for all SR systems and components initiated during the onset of a design basis accident were met. Specifically, Constellation assumed that following the accident signal and resulting
turbine trip, that the main generator breaker stays connected as a synchronous condenser providing voltage support for an additional 60 seconds after the
reactor trip. After this time delay, relays then open the breaker in the 115 kilovolt (kV)
switchyard which results in a
115kV transmission system voltage drop to 108.9kV. This voltage level becomes the source of power to the 480V safety-related buses after the unit generator breaker opens. At this time, however, all of the accident initiated SR loads have already started and sequenced onto the buses as running loads. The team noted that due to the voltage drop at 60 seconds, the SR buses drop to approximately 382V or 79 percent of rated voltage, which is below the degraded voltage relay setting of 420V. Constellation credits operation of the #7 transformer and/or #6 transformer/767 regulator automatic
LTC to restore the bus voltages to above the
DVR reset within the design time delay of a nominal 68-125 seconds. The team determined that Constellation had not performed dynamic motor starting calculations to evaluate the terminal voltages to SR equipment while connected to the onsite main generator or offsite power during accident initiated load sequencing to ensure and demonstrate that the voltage requirements of all plant SR systems and components are satisfied. These calculations had not been performed, in part, because Constellation credits a dynamic
emergency diesel generator (
EDG) voltage analysis as bounding the voltage drops that would occur if on the main generator or offsite power. As a result, there are no calculations that show the voltage profile of the 480V SR buses, the terminal voltage of SR motors during motor starting and sequencing, or whether the
DVR setpoints are exceeded or reset during the load sequencing event. The team determined that based on the assumption of reliance on the generator to boost voltage, the effect of the
EDG being a bounding calculation may be technically accurate. However, the design assumption of the generator providing this voltage boost may be inconsistent with the
RIS 2011-12 guidance for what is assumed in showing that offsite power, independently can provide the source of power to ensure engineered safety feature (
ESF) loads have the proper voltage supply during sequencing. Degraded Voltage Relay Scheme Constellation uses an inverse-time delay (TD) ITE 27 undervoltage relay for the degraded voltage relays. The time delay as specified in TS
SR 3.3.4.2 is
68-125 seconds at 420V. The team noted that the
DVR time delay exceeds the TD for Ginna assumed in the
UFSAR accident analyses, which is a nominal 10 seconds and is the time required for the
EDGs to start and be ready to accept load. The team also determined that Constellation had not performed an analysis to verify that all SR equipment would not become unavailable due to protective device actuation for a degraded voltage within the bandwidth of the
DVR and loss-of-voltage (LOV) setpoints for the existing time delay prior to re-sequencing onto the
EDGs. The team reviewed the March 26, 1981, Amendment 38 to the operating license No.
DPR-18, which docketed that the original TD setting of 8.5 seconds to above 1000 seconds, ensured that all Class 1E motors will start successfully and be loaded onto the
EDG within the time assumed in the
FSAR accident analysis. The team could not determine what the basis for this conclusion was because the TD approved was an inverse time characteristic which appeared to have a range outside of existing accident analyses assumptions for the timing of core re-flood during the limiting design basis accident. In response to the teams concerns, Constellation entered the issue into their corrective action program as CR-2013-004767 for further review recognizing that while the TD is consistent with their approved licensing bases, it is not consistent with current NRC guidance. The team will coordinate with the NRCs Office of Nuclear Reactor Regulation to review the adequacy of Constellations approved licensing and design bases to evaluate whether this issue constitutes a violation and to ensure Constellation is meeting all NRC regulations and requirements. Pending resolution of this issue and determination of any potential enforcement actions, this item is an Unresolved Item.
