05000282/LER-2012-001
| Prairie Island Nuclear Generating Plant | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident 10 CFR 50.73(a)(2)(vii), Common Cause Inoperability |
| 2822012001R00 - NRC Website | |
Energy industry identification system (EMS) codes are identified in the text within brackets [xx].
EVENT DESCRIPTION
On October 27, 2011, an error was identified in the Pre-Operational (Pre-Op) test records for the Unit 1 diesel generators (EDGs) [EK], D1 and D2. The records calculated the net brake horsepower with the equation BHPn=(KWa X Efficiency)/0.746. The correct equation would have the efficiency term in the denominator, BHPn=(KWa)/(Efficiency X 0.746). This error, along with the subsequent discovery of an additional error regarding fuel oil energy content assumptions, had downstream effects on the values calculated elsewhere, including the Fuel Oil (FO) Storage calculation (ENG ME-020). As a result, the required volume of stored diesel fuel oil to operate D1 or D2 and a diesel driven Cooling Water [BI] pump [P] for 14 days in the event of a postulated maximum probable flood increased. A past operability review using the corrected requirement was performed for the period from November 3, 2008, to December 21, 2011. During 2010 there were multiple times where the fuel oil volume maintained on site was below the revised requirement (2202 hours0.0255 days <br />0.612 hours <br />0.00364 weeks <br />8.37861e-4 months <br /> total with a longest continuous timeframe of 738 hours0.00854 days <br />0.205 hours <br />0.00122 weeks <br />2.80809e-4 months <br />).
EVENT ANALYSIS
Four Design Class I FO storage tanks [TK] supply fuel oil to D1 and D2. Each tank is equipped with a submersible transfer pump [P] to pump fuel oil from the tank to the nominal capacity 500 gallon day tank of either EDG. For the two diesel-driven cooling water pumps, fuel oil supply is from two Design Class 1 FO storage tanks. Each tank is provided with a submersible transfer pump to transfer fuel oil to either diesel-driven cooling water pump nominal capacity 500 gallon day tank.
Procedures, piping and valving are provided to transfer fuel oil from any one fuel oil storage tank to any other FO storage tank by using the proper valve lineup. The submersible pump associated with each FO storage tank has the capability to pump fuel oil to the transfer house where the oil is then routed to the desired fuel oil storage tank.
The Unit 1 design minimum storage capacity of diesel fuel oil is based on one EDG (D1 or D2), plus one diesel-driven cooling water pump for 14 days. To meet this requirement Unit l's Technical Specification (TS) Limiting Condition for Operation (LCO) 3.7.8 Condition D requires a minimum diesel-driven pump stored fuel oil supply of 19,500 gallons. TS 3.8.3 Condition A requires a minimum EDG fuel oil supply of 42,000 gallons for a combined minimum total diesel fuel oil storage requirement of 61,500 gallons. A revised total diesel fuel oil storage requirement, which used the corrected diesel generator brake horsepower and corrected for additional calculation errors resulted in a minimum requirement of 67,370.76 gallons. This value was used in assessing past operability from November 3, 2008, to December 21, 2011. This past operability assessment determined that for a single period of up to 738 continuous hours and 2202 total hours the revised requirements were not met, rendering D1 and D2 inoperable.
On February 15, 2012, with Unit 1 in Mode 1 at 100% power, a reportability review was completed It determined that this event was reportable under 10 CFR 50.73(a)(2)(i)(B) as an operation or condition prohibited by TS. In addition, the event is reportable under 10 CFR 50.73(a)(2)(vii) as a common-cause inoperability of independent trains or channels and 10 CFR 50.73(a)(2)(v)(D), an event or condition that could have prevented fulfillment of a safety function needed to mitigate the consequences of an accident.
CAUSE
The causal evaluation determined that calculation errors that resulted in non-conservative requirements for TS LCOs 3.7.8 and 3.8.3 were caused by a lack of technical rigor in the design process.
SAFETY SIGNIFICANCE
The minimum available fuel oil during the periods of inoperability was approximately 64,630 gallons without crediting day tank volumes, compared to the required 67,370.76 gallons. Although not available to be credited during a plant screenhouse internal flooding event at the time, two safety related diesel-driven cooling water pump storage tanks were also available. The stored diesel fuel in either tank, in addition to the diesel fuel credited in the four Design Class I fuel oil storage tanks, would have been sufficient to meet the revised requirements. As a result, there were no radiological, environmental or industrial impacts associated with this event. Therefore, this event did not affect the health and safety of the public.
CORRECTIVE ACTIONS
A major revision to ENG-ME-020 will be completed, which requires all design inputs to be verified.
Compensatory measures were updated to ensure a minimum 14-day supply of diesel fuel oil until a previously submitted Technical Specification amendment to revise LCOs 3.7.8 and 3.8.3 is approved.
PREVIOUS SIMILAR EVENTS
No similar licensee event reports at the Prairie Island Nuclear Generating Plant involving calculation errors leading to non-conservative Technical Specifications were identified in the past three years.