05000263/LER-2014-007
| Monticello Nuclear Generating Plant | |
| Event date: | 04-17-2014 |
|---|---|
| Report date: | 06-12-2014 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 2632014007R00 - NRC Website | |
Reported lessons learned are incorporated into the licensing process and fed back to industry.
Send comments regarding burden estimate to the FOIA, Privacy and Information Collections Branch (T-5 F53), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by Internet e-mail to Infocollects.Resource@nrc.gov, and to the Desk Officer, Office of Information and Regulatory Affairs, NEOB-10202, (3150-0104), Office of Management and Budget, Washington, DC 20503. If a means used to Impose an information collection does not display a currently valid OMB control number, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection,
EVENT DESCRIPTION
On April 17, 2014, Monticello Nuclear Generating Plant (MNGP) personnel discovered a previously unrecognized failure to take appropriate actions of Technical Specification (TS) Limiting Condition for Operation (LCO) 3.4.9. On seven occasions during the last three years the Reactor Pressure Vessel (RPV) [PRV] pressure dropped below 0 psig. Computer points that measure the RPV pressure do not go below 0 psig. As a result, data for condenser vacuum was examined to determine when the RPV pressure may have been less than 0 psig. The following dates were identified where a vacuum was drawn on the reactor vessel during startup activities: May 22, 2011, June 25, 2011, October 27, 2011, December 8, 2011, July 16, 2013, and February 5, 2014. During these events the vacuum drawn on the vessel was approximated to be -3 psig based on the vacuum observed in the condenser. One instance on June, 3, 2013, was also identified as drawing a vacuum on the vessel during preparation for the class 1 pressure test; vacuum was approximately -17.15 psig.
At the time of discovery, the plant was operating in Mode 1 at approximately 88% rated thermal power.
There were no other structures, systems, or components inoperable that contributed to the event.
EVENT ANALYSIS
This condition is being submitted in accordance with 10 CFR 50.73(a)(2)(i)(B) Operation or Condition Prohibited by Technical Specifications.
Operation of the RPV is required to be within the limits of the Pressure-Temperature (P-T) Curves in the Pressure Temperature Limits Report (PTLR) at all times as stated in TS 3.4.9. In Mode 1, 2, or 3, TS action 3.4.9.A requires restoration of parameter(s) to within limits within 30 minutes and determine if the Reactor Coolant System (RCS) is acceptable for continued operation within 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />. If the requirements of the LCO are not met in modes other than 1, 2, or 3, the required action is to initiate action to restore parameter(s) to within limits immediately and determine if RCS is acceptable for operation prior to entering Mode 2 or 3.
The P-T curves for Monticello used during plant startup bound reactor pressure conditions from 0 psig to 1300 psig. A vacuum drawn on the RPV during startup was not considered during development of the curves and is not reflected on the P-T curves. Monticello may draw a slight vacuum on the vessel during startup when the Main Steam Line Isolation Valves [ISV] are open to the Condenser [SG] and the Mechanical Vacuum Pump [SH] is in service. The vacuum is initiated on the RPV in Mode 4 and continues through the shift into Mode 2 and is terminated once the plant startup begins.
SAFETY SIGNIFICANCE
The structural integrity of the reactor vessel and the RCS remains acceptable at all conditions including a slight vacuum generated during start-up operations. The acceptability of continued operation of the RPV under a vacuum has been validated by evaluation. Operation of the RPV under a vacuum does not impact the limits described in the PTLR and 10CFR50 Appendix G. The integrity of the RCS is also confirmed by the leak tight nature of the RPV and the demonstration of margin to collapse.
The 10CFR50 Appendix G requirements describe the minimum temperature operation of the vessel at core not critical and core critical conditions. These requirements have no lower limit for vessel pressure and make no distinction between operation at 0 psig and vacuum on the RPV. Monticello maintained the minimum temperature during all plant start-up evolutions where a vacuum was drawn on the RPV and was within operational limits to prevent failure of the RPV from brittle fracture.
CAUSE
The cause of the failure to enter the appropriate technical specification actions was that station personnel did not recognize a vacuum was drawn on the RPV and the implications for compliance with P-T curves.
CORRECTIVE ACTION
The PTLR curves will be updated to recognize that the RPV may be operated at a vacuum.
PREVIOUS SIMILAR EVENTS
There were no previous similar Licensee Events Reports identified for the MNGP in the past three years relative to RPV P-T curve limitations.