05000483/LER-2006-009

LER-2006-009, Callaway Plant Unit 1

Callaway Plant Unit 1
Event date:	11-30-2006
Report date:	01-26-2007
4832006009R00 - NRC Website

A. REPORTABLE EVENT CLASSIFICATION

OTHER: Voluntary Report - any event or condition that does not meet the criteria for required reporting, if the licensee believes that the event or condition might be of safety significance or of generic interest or concern.

Prior to October 2006, the NRC's regulatory position on the application of technical specification requirements to redundant actuation equipment was not clearly established for Callaway Plant or the industry. As such, this voluntary report is submitted to address potential generic interest or concern.

B. PLANT OPERATING CONDITIONS PRIOR TO THE EVENT

Mode 1, 100 Percent Reactor Power

START OF THE EVENT AND THAT CONTRIBUTED TO THE EVENT

No structures, systems or components were Inoperable at the start of the event which contributed to the event.

Background

The Callaway Plant design includes one Main Steam Isolation Valve (MSIV) on each of four main steam lines. Each MSIV has two actuator trains energized from separate safety-related sources, each of which is capable of independently closing the MSIV. In the event of a failure of both trains of actuation for an MSIV (e.g., a loss of power to both trains) the valve will fail "as is". Final Safety Analysis Report (FSAR) Section 10.3.1.1, "Safety Design Bases", states that component redundancy is provided, consistent with General Design Criteria 34, so that MSIV safety functions can be completed assuming a single active component failure coincident with the loss of off-site power. FSAR Section 10.3.3, "Safety Evaluation", states that no single failure will compromise the MSIV system safety functions.

Requirements for the MSIVs are specified in Callaway Technical Specification (T/S) 3.7.2, "Main Steam Isolation Valves". Until recently, however, the MSIV actuator trains themselves were not explicitly addressed by MSIV T/S 3.7.2. MSIV actuator train inoperability was addressed by application of a Technical Specification Interpretation (TSI) based on the following premises:

(1) Due to redundancy in actuator design (i.e., recognizing that only one MSIV actuator train is needed for the associated MSIV to close on demand), an MSIV was considered to be operable with only one actuator train operable.

(continued) (2) As a prudent measure, both actuators were routinely tested for each MSIV during the performance of applicable surveillances on the MSIVs. Wording in the T/S 3.7.2.1 Surveillance Requirement Bases suggested that this testing should be done; however, it was not interpreted that the surveillance had to be satisfied via both actuator trains for each performance. The surveillance could be satisfied with either actuator train.

Following conversion to Improved T/S in April 2000 (for conformance to the new Standard Technical Specifications in NUREG-1431), the TSI was incorporated into a plant procedure that addressed operability issues. The content of the procedure remained unchanged from that of the TSI.

In December 2004 the content of the original TSI was relocated from the plant procedure to Chapter 16 of the FSAR with implementation of FSAR Change Notice 03-048. This resulted in the imposition of an FSAR controlled allowed outage time (AOT) for an inoperable MSIV actuator train. The AOT was imposed as a restriction that was considered to be above and beyond the T/S requirements for the MSIVs. The AOT specified for a single inoperable train of actuation was 64 hours7.407407e-4 days <br />0.0178 hours <br />1.058201e-4 weeks <br />2.4352e-5 months <br />, which was based on an allowed outage time that had evolved from operating experience under the TSI. The AOT was also supported by both engineering evaluation and probabilistic risk assessment.

Description On 12/29/2004, Callaway Plant experienced a failure of the active four-way valve component of a single actuator train on the 'C' MSIV, ABHVO020. This event resulted in the first usage of the FSAR Chapter 16 requirements specifying a 64-hour AOT for a single MSIV actuator train. During this time the MSIV itself was still considered Operable. The actuator train was restored within approximately 46 hours5.324074e-4 days <br />0.0128 hours <br />7.60582e-5 weeks <br />1.7503e-5 months <br />.

A corrective action document was initiated on 1/4/2005 in response to a review of FSAR Change Notice 03-048. The review focused on FSAR Chapter 16 provisions associated with inoperable MSIV actuator trains and the FSAR Chapter 15 accident analyses. A discrepancy was identified between the T/S 3.7.2 Bases (Applicable Safety Analyses); FSAR Chapter 16 provisions for MSIV actuator trains; and the FSAR Chapter 6.2, "Containment Systems", discussions on MSIV modeling in the containment analysis.

A review of the accident analysis Calculation of Record M-YY-43, "Containment Pressure!Temperature Response to a Main Steam Line Break (MSLB)", identified a scenario that requires both MSIV actuation trains for accident mitigation. This case assumes the loss of off-site power and the failure of one instrument logic train as the single failure. These assumptions result in the unavailability of an emergency diesel generator, one containment heat removal train, and one MSIV actuator train. Bounded containment pressure and temperature results for the scenario were dependent upon revised assumptions regarding the minimum heat removal capability of the remaining operable containment coolers.

In review of these issues, the Nuclear Regulatory Commission (NRC) raised questions regarding the basis of the Callaway FSAR Chapter 16 provisions for inoperable MSIV actuator trains. The NRC subsequently took the position that MSIV actuator trains are "attendant" equipment for the MSIVs (based on the T/S definition of Operability) and; therefore, both MSIV actuator trains are required for MSIV Operability. As such, Regional NRC staff took the position that the T/S requirements for MSIV inoperability should have been imposed for events involving MSIV actuator inoperability. This issue was identified as an Unresolved Item (URI), and resolution was sought through NRC Office of Nuclear Reactor Regulation (NRR) involvement. Both branches of the NRC and the Licensee agreed that the issue was not a significant safety concern.

(continued) While awaiting NRC resolution, Callaway Plant continued to address the concern under the corrective action program. Callaway Plant staff subsequently determined from the issues raised by this concern that the T/S was inadequate with respect to requirements for MSIV actuator trains. A license amendment request (LAR) was therefore initiated to clarify the effect of actuator train inoperability on the MSIVs and the required treatment of actuator train inoperability.

In May 2005, AmerenUE submitted the LAR to revise the T/S to include actuator trains in the scope of the MSIV Limiting Condition for Operation (LCO) under T/S 3.7.2. The amendment request sought to explicitly address the actuator trains and to establish specific conditions and Completion Times for the actuator trains in addition to the MSIVs themselves. This was consistent with the position that the actuator trains should have a reasonable allowed outage time prior to declaring the affected MSIV inoperable.

During the NRC review process for the license amendment, administrative controls were established at Callaway Plant to provide conditions and actions for plant Operators to follow in the event an MSIV actuator train became inoperable. Based on interactions with the NRC, these controls were ultimately made to match the controls specified in the proposed LAR.

On June 16, 2006, the NRC approved and issued License Amendment 172. The T/S revisions incorporated by the amendment were unchanged from those requested under the LAR. Under the amended T/S, declaring an MSIV actuator train inoperable does not require concurrently declaring the associated MSIV inoperable while the Required Actions for the inoperable actuator train are in effect.

In parallel with the processing of Amendment 172, NRC resolution of the URI continued. The internal NRC evaluation was ongoing between January 2005 and October 2006 and culminated in the issuance of an internal letter on October 19, 2006. NRC resolution of the historical treatment of MSIV actuator train inoperability included several meetings during the evaluation period. The meetings included an internal NRC meeting in January 2006 and several subsequent meetings between the NRC, Licensees, and the Industry with participation from the Nuclear Energy Institute and the Technical Specifications Task Force. The final meeting was conducted in October 2006 and was concluded without reaching consensus on the final resolution. Subsequently, final NRC resolution was provided internally via the October 19, 2006, letter entitled, "Operability Determination for Callaway Plant Technical Specifications Requirements When One main Steam Isolation Valve Actuator Train is Removed from Service (ML061730396)". Callaway Plant was officially informed of the resolution in an NRC inspection exit meeting on November 30, 2006, when Callaway Plant was notified of a potential noncited violation based on the position documented in the resolution letter.

The final NRC position stated that for the Callaway Plant MSIVs, with one of the redundant actuator trains out of service, the MSIV should have been declared inoperable under T/S 3.7.2 (prior to implementation of License Amendment 172). Because T/S Surveillance Requirement 3.7.2.2 applies to both actuator trains of the MSIV, the surveillance requirement could not be met with an actuator train out of service. Therefore, T/S LCO 3.7.2 would not be met with an inoperable actuator train because Surveillance Requirement 3.7.2.2 could not be met. Both actuator trains must in fact be tested to demonstrate that the MSIV is operable as designed and described in the FSAR. The Callaway licensing basis requires the MSIV actuators to function to effect closure of the associated MSIV on an Engineered Safety Feature Actuation Signal for steam line isolation. Therefore, the MSIV actuators perform a required support function for the safety-related MSIVs. Since the actuators were not included in the MSIV T/S LCO (prior to Amendment 172), the MSIV actuators are non-T/S support functions required by the T/S definition of Operability.

E. METHOD OF DISCOVERY OF EACH COMPONENT, SYSTEM FAILURE, OR PROCEDURAL ERROR

On 12/29/2004, Callaway Plant experienced a failure of the active four-way valve component of a single actuator train on the 'C' MSIV, ABHVO020. This event resulted in the first usage of the FSAR Chapter 16 requirements specifying a 64-hour AOT for a single inoperable MSIV actuator train. During the actuator train AOT, the MSIV itself was still considered Operable. Following this event a discrepancy was identified during a review of FSAR Change Notice 03-048, which added MSIV actuator train inoperability guidance to FSAR Chapter 16.7.12. The discrepancy was between T/S Bases descriptions of MSIV modeling in the containment analysis and descriptions in FSAR Chapter 6.2 of the same modeling. A review of the Callaway calculation of record for the containment pressure/temperature response to a Main Steam Line Break showed that the FSAR discussion was correct and that the T/S Bases discussion was inadequate.

Following significant and lengthy Licensee and NRC evaluation, the NRC issued a final interpretation in the form of an internal letter. The letter provided the basis for the proper application of T/S requirements to the MSIVs for a single train of actuation being out of service. This NRC position resulted in the issuance of a potential violation for inadequate T/S compliance, which was discussed with the Licensee during an inspection exit meeting on 11/30/2006.

II.� EVENT DRIVEN INFORMATION

A. SAFETY SYSTEMS THAT RESPONDED

No automatic actuations occurred and no safety systems were required to respond to this event.

B. DURATION OF SAFETY SYSTEM INOPERABILITY

With regard to the determinations set forth in the final NRC position, there have been instances in the three years prior to implementation of License Amendment 172 on June 16, 2006, when the applicable T/S Action statements were not entered and associated Completion Times were not met. The applicable T/S Actions and Completion Times were as follows:

3.7.2.A� With one MSIV inoperable in MODE 1, restore MSIV to OPERABLE status within 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br />.

3.7.2.B� With Required Action and associated Completion Time of condition A not met, be in MODE 2 within 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br />.

3.7.2.0 � With one or more MSIVs inoperable in MODE 2 or 3, close MSIV within 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> and verify MSIV is closed once per 7 days.

3.7.2.D� With Required Action and associated Completion Time of condition C not met, be in MODE 3 within 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br />, AND be in MODE 4 within 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br />.

(continued) Research of MSIV inoperability and MSIV actuator train unavailability for the three years prior to License Amendment 172 was performed to determine actual out of service times and to establish if Callaway Plant failed to declare the associated MSIV inoperable in accordance with T/S requirements in effect at the time. The review determined that there were eighteen instances where an MSIV actuator train was out of service and the associated MSIV was not declared inoperable. Six instances exceeded the MSIV AOT and resulted in a failure to enter MODE 2 within the required 6-hour Completion Time. In three of those instances the MSIV was not closed within 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> of being inoperable in MODE 2 or 3 and the 6-hour time allowed to be in MODE 3 was exceeded. In one case the requirement to be in MODE 4 within 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> was not met. The six cases are as follows:

Begin Date/Time� End Date/Time� Total Duration� MSIV 8-Hr Completion Time Exceeded Hr:Min:Sec� Hr:Min:Sec Sep 7, 2003 / 5:02� Sep 8, 2003 / 11:06� 30:04:02� 22:04:02� , Nov 16,2003 / 4:41� Nov 17,2003 / 10:42� 30:01:02� 22:01:02 Dec 29,2004 / 7:55� Dec 31,2004 / 6:12� 46:17:00� 38:17:00 Jan 10,2005 / 10:28� Jan 11,2005 / 8:17� 21:48:08� 13:48:08 Mar 31,2005 / 22:05� Apr 1,2005 / 16:02� 17:57:30� 9:57:30 Nov 16,2005 / 8:05� Nov 16,2005 / 20:41� 12:36:01� 4:36:01 None of the six instances exceeded the 72-hour allowed outage time for an actuation train under the current Technical Specifications.

C. SAFETY CONSEQUENCES AND IMPLICATIONS OF THE EVENT.

The MSIVs are credited in the Containment analysis for mitigating a Main Steam Line Break (MSLB).

The Containment analysis is based on the blowdown of one steam generator. All four of Callaway's steam generators are connected to a common steam header. Closure of the MSIVs prevents the three intact steam generators from feeding the break flow. Additionally, closure of all four MSIVs isolates the common steam header from the break. FailLire of a single MSIV to close would still prevent the three intact steam generators from blowing down. However, failure of one MSIV would increase the amount of mass available from the common steam header that feeds the break.

The impact of an inoperable MSIV actuator train on the deterministic safety analysis was reviewed using the CONTEMPT computer code. The inoperable MSIV actuator train taken in addition to a postulated single failure would result in additional mass release from the common main steam header.

This mass would result in an incremental increase in peak post-accident containment pressure of approximately 1.5 psig. At the time of this event, the Callaway Plant FSAR reported a peak post- accident Containment pressure of 48.1 psig. The design pressure of the Containment building is 60 psig. Therefore, the increased pressure would not have caused the Containment Design Bases Limit (continued) for Fission Product Barrier (DBLFPB) to be exceeded or altered. This event did not result in jeopardizing the ability of the Callaway Containment to accommodate peak post-accident pressure as described in General Design Criterion 50.

In addition, the issue described in this LER was evaluated with the Callaway PRA model. The evaluation determined that the incremental conditional core damage probability (ICCDP) attributable to this issue is significantly below 1E-6; therefore, this event was of very low risk significance. Use of the PRA model to evaluate the event provides for a comprehensive, quantitative assessment of the potential safety consequences and implications of the event, including the consideration of alternative conditions beyond those analyzed in the FSAR.

III.� CAUSE(S) OF THE EVENT AND CORRECTIVE ACTION(S) Prior to documenting a formal resolution in the October 2006 letter, the NRC's regulatory position on this issue was not clearly established for Callaway Plant or the industry. Historically, Callaway consistently implemented the position that unavailability of a single, redundant MSIV actuator train does not render the associated MSIV inoperable; and that a reasonable allowed outage time is permitted to restore the unavailable actuator train prior to declaring the MSIV inoperable. In addition, the industry position appeared to be consistent with this interpretation.

The most probable causes for the difference in interpretation of the original Callaway Plant MSIV T/S and the associated corrective action to prevent recurrence are as follows:

(1)The original T/S did not explicitly address the MSIV actuator trains (the authors and reviewers of the original T/S and T/S Bases did not explicitly address MSIV actuator and MSIV operability); (2)The redundancy in the MSIV actuator design permits an MSIV to close on demand, with one actuator train unavailable; (3)Plant operational experience demonstrated the fact that the 8-hour allowed outage time for an inoperable MSIV was too restrictive for restoring an unavailable actuator train; and finally (4)Allowing a reasonable time to restore the unavailable actuator train, without declaring the MSIV itself inoperable, was not a safety concern.

Subsequent to identification of concerns with the adequacy of T/S guidance related to MSIVs and their associated actuator trains, AmerenUE submitted a license amendment request to revise the T/S to explicitly include the MSIV actuator trains within the scope of the T/S 3.7.2 LCO. The amendment request sought to add the actuator trains to the T/S and to establish specific conditions and completion times apart from the MSIVs themselves. This LAR was approved and changes issued under License Amendment 172 on June 16, 2006.

IV.� PREVIOUS SIMILAR EVENTS Internal and external operating experience (OE) was reviewed. However, no relevant operating experience was identified that would have affected Callaway's historical interpretation and practice associated with unavailable MSIV actuator trains and inoperability of the associated MSIV.

V.�ADDITIONAL INFORMATION The system and component codes listed below are from the IEEE Standard 805-1984 and IEEE Standard 803A-1983, respectively.

Main Steam Isolation Valves�System: SB Component: ISV Main Steam Iso Valve Actuator System: SB Component: FSV