Resolution of Generic Safety Issues: Item A-2: Asymmetric Blowdown Loads on Reactor Primary Coolant Systems (Rev. 2) ( NUREG-0933, Main Report with Supplements 1–34 )
On May 7, 1975, the NRC was informed by VEPCO that an asymmetric loading on the reactor vessel supports resulting from a postulated reactor coolant pipe rupture at a specific location (e.g., the vessel nozzle) had not been considered by W or S&W in the original design of the reactor vessel support systems for North Anna Units 1 and 2. This item was originally identified in NUREG-03712 and was later declared a USI in NUREG-0510.186
In a postulated event at the vessel nozzle, asymmetric LOCA loading could result from forces induced on the reactor internals by transient differential pressures across the core barrel and by forces on the vessel due to transient differential pressures in the reactor cavity. With the advent of more sophisticated computer codes and the accompanying more-detailed analytical models, it became apparent to W that such differential pressures, although of short duration, could place a significant load on the reactor vessel supports, thereby affecting their integrity. This issue was determined by the NRC to have generic implications for all PWRs.CONCLUSION
This USI was RESOLVED with the publication of NUREG-0609699 and affected all operating and future PWRs. For operating PWRs, MPA D-10 was established by DL/NRR for implementation purposes. Generic Letter 84-04700 was also issued by the staff.