05000317/FIN-2011005-05
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Finding | |
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| Title | Single Failure vulnerability for Low Pressure Safety Injection Flow Control Valve CV-306 |
| Description | An unresolved item (URI) was identified because additional NRC review and evaluation is needed to assess whether a performance deficiency exists associated with a single failure vulnerability for flow control valve CV-306. The LPSI system flow control valve CV-306 is located between the LPSI pumps and the LPSI injection header. It is an air-operated valve (AOV), and is located on a single pipe that branches into four lines for emergency core cooling system (ECCS) injection into the RCS. A flow controller is used during shutdown conditions to throttle CV-306, which would result in sending a portion of the flow through the shutdown cooling heat exchangers before returning to the RCS. An inadvertent full closure of this valve would isolate all LPSI flow to the RCS. Constellation\\\'s normal configuration of CV-306 is key-locked open, which means a control room two position key-lock switch (Auto and Open) is placed in the Open position and the key is removed. This configuration electrically removes the signal from the flow controller to the valve. The incident on December 1, 2010, occurred when a technician bumped his hardhat on the l/P converter during an adjacent instrument calibration activity, and the valve moved from 100 percent open to 75 percent open. Constellation determined that bumping the l/P had caused calibration shift, which caused the valve to partially close. The inspectors noted that the key-lock switch isolates the circuit between the flow controller and the l/P converter, thus any failure of the l/P could reposition the valve. Upon discovery by the control room operators, TS LCO 3.0.3 was entered as it was conservatively concluded that the LPSI f10wpath was inoperable. Subsequently, Constellation performed an engineering analysis, which determined that 75 percent open would have provided sufficient ECCS flow to the RCS during a postulated accident. The inspectors noted that TS Surveillance Requirement (SR) 3.5.2.1 requires operators to verify the following valves are in the listed position with power to the valve operator removed. CV-306 is one of the three valves listed in the associated TS SR and its required position is open. The 12-hour frequency surveillance is performed in the control room (actually performed every six hours) by recording that the key-lock switch is in the Open position and the valve is open (red light illuminated). Regarding the three valves listed in TS SR 3.5.2.1, the associated TS Basis states the following: Misalignment of these valves could render both ECCS trains inoperable; Securing these valves in position by interrupting the control signal to the valve operator, ensures that the valves cannot be inadvertently misaligned; and A 12-hour frequency is considered reasonable in view of other administrative controls ensuring that a mispositioned valve is an unlikely possibility. However, as was observed on December 1, 2010, an inadvertent misposition of CV-306 actually occurred. Further, there is no specific alarm or annunciation that alerts the operators that the valve is not in the full open position. The inspectors noted that typically, the action that accompanies that statement with power to the valve operator removed involves removing the motive force to the valve operator. For example, in the case of a motor-operated valve, the associated breaker is typically opened, and for an AOV, air is isolated to the operator or the valve is locked in the required position. In Supplement 1 to the CCNPP Units 1 and 2 Safety Evaluation (May 1973), the NRC documented that this single locked-open feature and fail-open AOV is provided through a key-lock in the electric control circuit in the control room. It also stated that notwithstanding this feature, a single failure such as a broken valve stem could cause the valve (an active component) to fail in a closed position and block the only LPSI flow path to the reactor coolant system. The applicant committed to modify the design so that no single failure could cause the valve to close. The modification consisted of a plug (jacking screw) that was inserted through the bottom of the valve body and mechanically prevented closure of the valve. In the time period between the licensing of the two units (circa 1974- 1976), a question was raised regarding CCNPP\\\'s ability to prevent boron precipitation during hot leg recirculation. While Supplement 1 above indicated the need for a jackscrew to maintain CV-306 open (to satisfy single failure), the licensee subsequently communicated a need to close CV-306 for establishing hot leg recirculation (it was presented as one of the options in docketed correspondence). In Supplement 5 to the Unit 2 Safety Evaluation (August 1976), Section 7.5.3, the NRC documented that to satisfy the single failure criterion, the applicant has proposed to lock out power to the motor operator of LPSI discharge valve CV-306 in the open position. We (the NRC) will include this requirement in the TSs. The boron precipitation concern appears to be the reason for the difference between the words/assumptions in Unit 1/2 Supplement 1 vs. Unit 2 Supplement 5. The inspectors noted that a jackscrew was originally installed in Unit 1 and then subsequently removed via a 10 CFR 50.59 screen/analysis in 1976. It was never installed in Unit 2. During the onsite inspection, the inspectors identified that, although originally considered as an option, the CV-306 valve is currently not used in establishing hot leg recirculation in the emergency operating procedures. The inspectors determined that a single failure vulnerability remained with Constellation\\\'s existing implementation of TS SR 3.5.2.1, in that, a single failure of a component such as the l/P converter could render all of LPSI inoperable. Further, in response to this concern, Constellation completed a Failure Mode and Effects Analysis, which identified the existence of two possible failure modes that could result in an inadvertent partial or full closure of CV-306 (I/P mechanical agitation, and I/P high output failure). However, Constellation stated they believed that Branch Technical Position ICSB 18, Application of the Single Failure Criterion to Manually-Controlled Electrically Operated Valves, contained a provision that would permit their configuration as meeting TS SR 3.5.2.1. The inspectors were evaluating whether Constellation was in compliance with TS SR 3.5.2.1 and the licensing/design basis of the LPSI system. Constellation stated that based upon the historical written communications and the existing licensing basis documentation, that their CV-306 configuration satisfied TS SR 3.5.2.1 and the licensing bases. Constellation initiated CR-2011-011314 on November 14, 2011, to formally address the concerns for this issue. In the interim, the inspectors noted that Constellation had subsequently isolated air to the CV-306 valve on each unit, and has, therefore, eliminated any immediate safety or TS compliance concerns. This issue will be opened as an URI in order to review and evaluate Constellation\\\'s corrective actions and determine if a performance deficiency exists with respect to the single failure vulnerability for flow control valve CV-306. |
| Site: | Calvert Cliffs  |
|---|---|
| Report | IR 05000317/2011005 Section 4OA5 |
| Date counted | Dec 31, 2011 (2011Q4) |
| Type: | URI: |
| cornerstone | Mitigating Systems |
| Identified by: | NRC identified |
| Inspection Procedure: | |
| Inspectors (proximate) | K Mangan M Patel S Pindale W Sherbin S Kobylarz L Doerflein E Keighleyd Silks Pindale G Dentel P Presby S Kennedy D Kern R Rolph J Hawkins E Torres K Cronk M Jennerich |
| INPO aspect | |
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Finding - Calvert Cliffs - IR 05000317/2011005 | |||||||||||||||||||||||||||||||||||||||||||||||||||||
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Finding List (Calvert Cliffs) @ 2011Q4
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