An unresolved item (
URI) was identified when an
operability determination dated January 22, 2008, was required to ensure that latent fabrication and calculation errors did not create unacceptable reductions in net positive suction head requirements for pumps in
emergency core cooling systems. This new design information was associated with the already installed containment recirculation
sump strainer modification. The associated vendor calculation, TD 6002 05, for clean strainer head loss omitted the head loss component associated with the orifices located in the strainer support plate. The size of the orifice beneath each strainer tube was not large enough to prevent head loss in excess of the net positive suction head required per the design conditions defined in the purchase specification supplied to the strainer vendor. The additional head loss due to the calculation correction was 2.28 feet. This resulted in required net positive suction head being less than available. Wolf Creek performed an
operability determination review to demonstrate that the head loss margin could be recovered. The
operability determination on January 22, 2008, addressed the smaller support plate orifice holes by using additional margin gained by separating the head loss of the
RHR and
containment spray piping systems to demonstrate lower losses and additional water inventory in containment prior to
containment spray swapover to the
sump. Wolf Creek is replacing the strainer support plate with larger orifices to regain head loss margin in Refueling Outage 16. However, additional concerns were provided to the licensee by the vendor on April 1, 2008, addressing nonconservative temperature correction through the orifices. Subsequent to this, the licensee will need to perform additional analyses to determine if negative margin existed during the last cycle and if the new orifice holes will provide positive margin. At the completion of the inspection period, there were still unresolved questions about the assumptions and results associated with the calculations used for regaining net positive suction head margin. These concerns require additional inspection and, when completed, the inspection results will require significance determination. This issue is considered unresolved pending additional NRC review of Wolf Creek
operability determination calculations:
URI 05000483/2008002-03, Containment
Sump Net Positive Suction Head Losses
