ENS 46519
ENS Event | |
|---|---|
18:44 Jan 3, 2011 | |
| Title | Single Point of Failure Vulnerability Discovered Which Could Potentially Affect Accident Response |
| Event Description | On Monday, January 3, 2011, at 1344 EST, it was discovered by engineering that a single point of vulnerability exists at Susquehanna Steam Electric Station affecting both Unit 1 and Unit 2.
A potential single HVAC control component has been discovered whose failure could result in a spurious Steam Leak Detection (SLD) isolation causing a loss of generation, loss of the normal heat sink (main condenser) and a loss of HPCI and RCIC. The SLD delta temperature (delta T) instrumentation is dependent on proper operation of the Unit 1 and Unit 2 RB [Reactor Building] HVAC heater temperature controller during cold weather operation. The Unit 1 and Unit 2 RB HVAC heaters are controlled by a single temperature controller which sends a signal to multiple step controllers. Failure of the temperature controller could cause the heaters to turn off. This would cause a significant decrease in RB HVAC supply temperature which results in a significant increase in measured SLD delta T during cold winter months. This could cause an isolation of the MSIVs, HPCI, RCIC and RWCU within a short period of time. This is reportable pursuant to 10CFR50.72(b)(3)(v) for a condition that at discovery could have prevented fulfillment of a safety function needed to mitigate the consequences of an accident and 10CFR50.72(b)(3)(ii) for and event that resulted in the nuclear power plant being in an unanalyzed condition. The licensee notified the NRC Resident Inspector.
On January 3, 2011, SSES reported the discovery of a single point vulnerability that affected both Unit 1 and Unit 2 (EN # 46519). The vulnerability involved failure of a temperature controller that had the potential to result in a Steam Leak Detection (SLD) isolation causing a loss of generation, loss of the normal heat sink (main condenser), and a loss of HPCI and RCIC. The condition was reported pursuant to 10CFR50.72(b)(3)(v) as a condition that at discovery could have prevented fulfillment of a safety function needed to mitigate the consequences of an accident and pursuant to 10CFR50.72(b)(3)(ii) as an event that resulted in the nuclear power plant being in an unanalyzed condition. SSES has further evaluated the condition and determined that the condition did not meet reporting criterion 10CFR50.72(b)(3)(v) for a condition that at discovery could have prevented fulfillment of a safety function needed to mitigate the consequences of an accident. The level of judgment in reporting under this criterion Is a reasonable expectation of preventing fulfillment of a safety function. Alternately stated, the condition is reportable if there was reasonable doubt that the safety function would have been fulfilled if the system had been called upon to perform it. Technical evaluation of the condition concluded the following: there is reasonable assurance (high degree of confidence) that the HPCI, RCIC, Main Steam Isolation and RWCU systems will remain operable. The identified condition does not adversely affect the operability of the affected systems. The identified condition increases the probability that a failure of the Reactor Building HVAC temperature controller TC-17589 or TC-27589 could cause the MSIV's, HPCI and RCIC to isolate during cold weather operation. However, this increase in probability is very small and is not sufficient to erode the confidence in the reasonable expectation of operability. For a system isolation to occur, the heater controller had to fail and concurrently the outside air temperature had to be below approximately 10?F. The Reactor Building HVAC temperature controllers are highly reliable and the outside air temperatures required for this event occur infrequently (there have only been 11 instances of temperatures at Susquehanna dropping below 10 deg F in the past two years). Since both infrequent conditions have to occur concurrently, it is unlikely that this postulated failure would occur. As a result, reporting pursuant to 10CFR50.72(b)(3)(v) is retracted; however, the condition remains reportable pursuant to 10CFR50.72(b)b)(3)(ii) as an event that resulted in the nuclear power plant being in an unanalyzed condition. The NRC Resident Inspector has been notified. Notified R1DO (T.Dimitriadis)
On January 3, 2011, SSES reported the discovery of a single point vulnerability that affected both Unit 1 and Unit 2 (EN #46519). The vulnerability involved failure of a temperature controller that had the potential to result in a Steam Leak Detection (SLD) isolation causing a loss of generation, loss of the normal heat sink (main condenser), and a loss of HPCI and RCIC. The condition was reported pursuant to 10CFR50.72(b)(3)(v) as a condition that at discovery could have prevented fulfillment of a safety function needed to mitigate the consequences of an accident and pursuant to 10CFR50.72(b)(3)(ii) as an event that resulted in the nuclear power plant being in an unanalyzed condition. On February 28, 2011, SSES retracted the 10CFR50.72(b)(3)(v) portion of the report on the basis that there was reasonable assurance that the HPCI, RCIC, Main Steam Isolation and RWCU systems would remain operable. The reasonable assurance was based on the very small probability of system isolation that required failure of the highly reliable heater controller had to fail concurrent with outside air temperature below approximately 10?F. This notification is intended to revise the basis for retraction of the 10CFR50.72(b)(3)(v) portion of the report. The revised basis is that the conditions required to prevent fulfillment of a safety function did not exist at the time of discovery. The licensee has notified the NRC Resident Inspector. Notified R1DO (Rogge). |
| Where | |
|---|---|
Susquehanna  Pennsylvania (NRC Region 1) | |
| Reporting | |
| 10 CFR 50.72(b)(3)(ii)(B), Unanalyzed Condition 10 CFR 50.72(b)(3)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident | |
| Time - Person (Reporting Time:+2.42 h0.101 days <br />0.0144 weeks <br />0.00331 months <br />) | |
| Opened: | Gordon Robinson 21:09 Jan 3, 2011 |
| NRC Officer: | Donald Norwood |
| Last Updated: | Jun 7, 2011 |
| 46519 - NRC Website | |
Unit 1 | |
|---|---|
| Reactor critical | Critical |
| Scram | No |
| Before | Power Operation (98 %) |
| After | Power Operation (98 %) |
Unit 2 | |
|---|---|
| Reactor critical | Critical |
| Scram | No |
| Before | Power Operation (94 %) |
| After | Power Operation (94 %) |