05000335/LER-2012-008
| Docket Number Sequential Revmonth Day Year Year Month Day Year St. Lucie Unit 2 05000389Number No. | |
| Event date: | 08-23-2012 |
|---|---|
| Report date: | 10-22-2012 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition |
| 3352012008R00 - NRC Website | |
FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3)
Description of the Event
was defueled in a refueling outage. During implementation of a design modification to the U2 control room air conditioning system (CRACS)[EIIS:VI:ACU], FPL discovered an original design error (legacy issue) with the control circuitry for the CRACS swing train unit, HVA-ACC-3C, that was applicable to both St. Lucie Units 1 and 2.
The condition involves the ability of the CRACS swing unit to automatically start after a loss of offsite power (LOOP) coincident with a design basis accident. The CRACS swing train unit control circuitry does not contain EDG sequencing relays or latching relays that are installed on the HVA/ACC-3A and 3B units. The EDG sequencing and latching relays ensure that the unit will restart on LOOP as long as the unit was in operation prior to the LOOP. The swing CRACS unit, HVA/ACC-3C, is not equipped with these relays and will not restart on a LOOP even if it was in operation prior to the LOOP. The single failure criteria is no longer met with the swing CRACS unit, HVA/ACC-3C, in operation as a failure of either the HVA/ACC-3A or 3B CRACS unit results in no operating CRACS units post-LOOP.
This discovery had no immediate effect on Unit 1, the operating unit, as both the HVA/ACC-3A and 3B CRACS units were in operation at the time of discovery.
Compensatory measures were immediately implemented to require entering the appropriate Technical Specification (TS) action statement for an inoperable CRACS unit should the swing CRACS unit, HVA/ACC-3C, be in operation. At the time of discovery, St. Lucie Unit 2 was defueled and the limiting condition for operation (LCO) for the CRACS TS was not applicable.
Cause of the Event
The cause of this event was an original design error (legacy issue) that existed from the original construction of both of the St. Lucie Units.
The immediate corrective action included issuance of a prompt operability determination (POD) for St. Lucie Unit 1 that implemented compensatory measures to ensure that the swing CRACS unit, HVA-ACC-3C, could not be credited for meeting the TS LCO requirements.
Design modifications were developed and implemented on St. Lucie Unit 1 and are planned for St. Lucie Unit 2 swing CRACS units to ensure that the swing unit will start automatically post-LOOP if the unit was running prior to the event. These modifications eliminate the legacy single failure vulnerability issue. Additionally, the Unit 2 HVA/ACC-3A and 3B CRACS units will have automatic start capability to ensure that they will start post-LOOP or post-ESFAS if aligned in the AUTO position.
Procedure changes were developed and implemented on St. Lucie Unit 1 and are planned for St. Lucie Unit 2 to enhance CRACS operational instructions. Unit 1 requires two CRACS units to be in operation at all times. Unit 2 will allow a single CRACS unit to be in operation with the second unit aligned for automatic start. Both Unit's procedures require that the swing unit, HVA/ACC-3C, has to be in operation if it is being credited for meeting the TS LCO requirements.
Analysis of the Event
This condition is reportable under 10 CFR 50.73(a)(2)(ii)(B) as any event or condition that resulted in the nuclear power plant being in an unanalyzed condition that significantly degraded plant safety because of the failure to meet the single FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3) failure criteria. For either St. Lucie Unit, single failure would not be met when the swing CRACS unit, HVA-ACC-3C, was in operation. In addition, Unit 2 failed to meet single failure because the standby (e.g., not operating) HVA/ACC-3A or 3B unit on Unit 2 would not have restarted on a LOOP because the latching relay logic was not made up.
This condition is also reportable under 10 CFR 50.73(a)(2)(i)(B) as operation prohibited by the plant's Technical Specifications, as both St. Lucie Units have operated in the past three years crediting the swing CRACS unit, HVA-ACC-3C, for longer than the allowed outage time for an inoperable CRACS unit.
Each of the St. Lucie CRACS systems is similar, and is provided with three 50 percent capacity units and is sized for control room personnel habitability and equipment cooling purposes. On St. Lucie Unit 1, two units are normally in operation. On St.
Lucie Unit 2, one unit is normally in operation with the other unit(s) in standby available for manual alignment should a failure occur. Upon a LOOP, the air conditioner units are automatically loaded on the emergency diesel generator sets.
Failure of all CRACS units to start post-LOOP would result in the slow heat up of the control room envelope (CRE). Eventually, personnel habitability issues would drive the operators to manually start the available CRACS units before the temperature excursion could adversely affect equipment within the control room.
The St. Lucie Unit 1 control room emergency air cleanup system (CREACS) and St. Lucie Unit 2 control room emergency cleanup system (CRECS) [EIIS:VI:FLT] are designed to provide air cleaning for the CRE atmosphere so that airborne radiological doses, experienced by control room personnel following a design basis accident (DBA) do not exceed limits imposed by Generic Design Criterion (GDC) 19.
Upon receipt of a containment isolation signal (CIS) or a high radiation signal from either of the outside air intakes, both of the CREACS/CRECS fans are automatically started, the outside air intakes are isolated, the kitchen and toilet exhaust ducts are isolated and the system recirculates the control room air through the HEPA filters and charcoal adsorbers for removal of radioactive particles and iodine (St.
Lucie Unit 1 has only one HEPA filter and charcoal adsorber shared between the CREACS fans, St. Lucie Unit 2 has independent HEPA filter and charcoal adsorber trains for each CRECS fan).
Although the CREACS/CRECS fans provide the radiological cleanup, the CRACS fans provide the required mixing of the CRE. The CREACS/CRECS fans draw air from the CRACS return duct, filter the air and discharge the filtered air back into the CRACS return duct. Without the CRACS fans operating the CREACS/CRECS fans will essentially recirculate the air inside the CRACS duct and filtration of the CRE will not occur.
Although the operators have detailed procedural requirements for operation of CREACS/CRECS post-accident, there was no specific procedural guidance to ensure that CRACS units are in operation for dose mitigation purposes.
An extent of condition and cause assessment was completed for all safety-related swing components and only the CRACS units were subject to this legacy design issue.
Analysis of Safety Significance This condition has no effect on core damage or large early release probabilities because no fission product barriers or mitigation equipment necessary to preserve these barriers are affected. Thus, this condition had no adverse effect to the FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3) health and safety of the public at large. Failure of CRACS post-LOOP is a GDC-19 control room habitability issue.
The current design of the CRACS circuit for HVA/ACC-3C will not meet the requirements for a LOOP coincident with the DBA and a single failure when utilizing the HVA/ACC-3C unit. Thus, the HVA/ACC-3C unit cannot be utilized for this purpose. Additionally, St. Lucie Unit 2 normally operates with only one CRACS fan in service with one additional fan in standby. In this alignment, only the running CRACS fan will restart following LOOP, and prior to implementation of the design modification to remove this vulnerability, this operational alignment would not meet the requirements for a LOOP coincident with the DBA and a single failure.
Although manual action can be taken to start the CRACS units to provide cooling of the CRE, there is not sufficient time for manual action during a DBA to ensure that the CRACS fan(s) are operating to provide mixing of the CRE air to support the CREACS/CRECS filtration function assumed for the GDC 19 control room dose analysis of record. However, it is reasonable to assume that the operators would initiate the CRACS fans based on personnel comfort as the control room heats up without any air conditioning. Based on this assumption, and the inherent conservatisms within the control room GDC 19 analysis of record, any postulated dose to the operators would not exceed the GDC 19 requirements of 5 rem TEDE by any appreciable amount.
Corrective Actions
The corrective actions listed have been entered into the site CAP. Any changes to the actions below will be processed in accordance with the CAP.
1. The control circuitry for the St. Lucie Unit 1 swing CRACS unit, HVA/ACC-3C, was modified to ensure that the swing CRACS unit will start post-LOOP if it was operating prior to the event.
2. The control circuitry for the St. Lucie Unit 2 swing CRACS unit, HVA/ACC-3C, will be modified in a similar manner as that completed on Unit 1 during the SL2-20 refueling outage. Additionally, the Unit 2 HVA/ACC-3A and 3B CRACS units will have automatic start capability to ensure that they will start post-LOOP or post- ESFAS if aligned in the AUTO position or if operating prior to the event.
3. St. Lucie Unit 1 procedures were changed to require two CRACS units to be in operation at all times.
4. St. Lucie Unit 2 procedures will be changed during the SL2-20 refueling outage to allow one CRACS unit to be operating with a second unit aligned for automatic start.
Similar Events None
Failed Components
None