05000483/LER-2003-003
| Callaway Plant Unit 1 | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(C), Loss of Safety Function - Release of Radioactive Material 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident |
| 4832003003R00 - NRC Website | |
I. DESCRIPTION OF THE REPORTABLE EVENT
A. REPORTABLE EVENT CLASSIFICATION
This event is being reported under 10CFR50.73(a)(2)(v)(C) and 10CFR50.73(a)(2)(v)(D), an event or condition that could have prevented the fulfillment of a safety function to control the release of radioactive material or mitigate the consequences of an accident.
B. PLANT OPERATING CONDITIONS PRIOR TO THE EVENT
Mode 1 at 100 percent power.
C. STATUS OF STRUCTURES, SYSTEMS OR COMPONENTS THAT WERE INOPERABLE AT THE START
OF THE EVENT AND THAT CONTRIBUTED TO THE EVENT
Not applicable.
D. NARRATIVE SUMMARY OF THE EVENT, INCLUDING DATES AND APPROXIMATE TIMES
On March 13, 2003 while operating at 100 % power, Callaway Plant determined a problem existed in the current safety analysis for a steam generator (S/G) tube rupture accident accompanied by an overfill condition of the ruptured S/G. This problem was discovered while reviewing future plant modification packages. The Callaway Plant screening process for modification development identified that these modifications could potentially have an adverse impact on the SGTR overfill analysis. During the re-analysis of the SGTR overfill sequence, all input values were re-validated. Preliminary analysis results for the current plant configuration with the re-validated inputs indicated that post-accident doses would involve a more than minimal increase from the radiological consequences currently presented in the Callaway FSAR. The input parameters having the dominant adverse impact on the analysis results were operator action times credited in the analysis. Since the SGTR overfill case was not explicitly addressed in the FSAR, credited operator action times were not maintained valid.
Current Technical Specifications (T/S) allow a reactor coolant system Dose Equivalent Iodine (DEI) value of 1.0 microcurie per gram. To assure compliance with FSAR analysis limits, plant procedures have been changed to administratively reduce the steady state DEI limit to 0.3 microcurie per gram, a value that has not been exceeded in the last three years. Current steady state DEI concentration in the reactor coolant system is 0.001769 microcurie per gram. The new lower DEI limit will ensure that if an overfill condition were to occur during a S/G tube rupture, post accident radiological consequences would not exceed the limits contained in the FSAR and the Standard Review Plan.
A formal root cause evaluation team was assembled to determine why the postulated plant conditions, including operator response times explicitly modeled in the analysis, had not been maintained current. As a result of this review, it was determined that in 1986 Callaway submitted an analysis for a SGTR with a stuck-open auxiliary feedwater (AFW) flow control valve (FCV). This analysis concluded that overfill was precluded during a design bases SGTR overfill sequence. In response, the NRC requested Union Electric (present day AmerenUE) submit a S/G tube rupture analysis which included a resultant overfill condition in the ruptured S/G. In 1987, Callaway submitted via letter ULNRC 1518, the analysis for a SGTR with a failed AFW flow control valve that did include overfill. ULNRC 1518 also included information regarding the impact of plant uprating, 15 percent tube plugging and Vantage 5 fuel transition on the overfill analyses. Plant uprating, Vantage 5 fuel transition and 15 percent tube plugging were other licensing initiatives being pursued by Union Electric during the same time frame as the SGTR analysis effort. Union Electric calculations performed in 1987 provide the bases for the analyses results presented in ULNRC 1518.
4 The intended scope of these calculations was as follows:
- SGTR with Overfill Thermal Hydraulic analysis
- Radiological Consequences Calculation for SGTR with overfill
- Design Bases calculation to address the effects of uprating, Vantage 5 fuel, and 15 percent tube plugging on the overfill analyses. This calculation demonstrates margin prior to overfill.
In 1988, the NRC requested additional information regarding operator response times for S/G tube rupture events.
The NRC request covered operator response times for two SGTR cases:
- SGTR with a failed AFW flow control valve (overfill) The operator response time values measured during the failed ASD cases were classified as valid. The overfill simulator cases were, at the time, deemed not to represent valid data for operator response times. Response times for the failed ASD cases obtained from these exercises were used in calculations and submitted to the NRC in ULNRC 2145.
In 1990, the NRC issued an SER, which stated that Callaway could successfully mitigate a design-bases SGTR with overfill and maintain radiological consequences within the limits of the Standard Review Plan and 10CFR100. The SER further stated that the NRC staff did not concur with Union Electric's conclusion regarding no steam generator overfill following a design-bases SGTR. Callaway did not accept the NRC's conclusion and documented this in CN 90-68. Based upon this disagreement, Callaway only incorporated the SGTR with a stuck open auxiliary steam dump case into the FSAR. The 1987 calculations that demonstrated overfill were viewed as being "beyond-design-bases" or "beyond-licensing-bases" and were therefore not treated as analyses which were required to be maintained current.
The error was identified in 2003 while reviewing plant modifications involving the Main Feedwater Isolation valves and Auxiliary Feedwater check valves. Westinghouse was contracted to perform a quantitative analysis the SGTR overfill sequence. As a part of the re-analysis effort, all analysis input values were re-validated. The Westinghouse preliminary results indicated that the current plant configuration and procedures would result in radiological consequences exceeding the limits of the Standard Review Plan. The input parameter having the dominant adverse effect was the "as-found" values for operator response times. As a result of the new calculated doses, on 3/13/03, Callaway issued an Operations Night Order and administratively reduced the Dose Equivalent Iodine (DEI) limit to 0.3 micro-curies per gram instead of the T/S limit of 1.0 micro-curies per gram. � The administrative limit for DEI-131 has been incorporated into plant procedures. Additionally, Callaway's Emergency Operating Procedure (EOP) E-3 is being revised to enhance operator response times and these new times are being used in a new Westinghouse analysis that will demonstrate compliance with accident dose limits.
E. METHOD OF DISCOVERY OF EACH COMPONENT, SYSTEM FAILURE, OR PROCEDURAL ERROR
The problem with the SGTR accident analysis was discovered while reviewing two plant modification packages for the Main Feedwater Isolation Valve (MFIV) actuators and for AFW check valves.
H. � EVENT DRIVEN INFORMATION
A. SAFETY SYSTEMS THAT RESPONDED
Not applicable for this event.
B. DURATION OF SAFETY SYSTEM INOPERABILITY
Not applicable for this event.
C. SAFETY CONSEQUENCES AND IMPLICATIONS OF THE EVENT.
Based on actual DEI values over the last three years, calculated doses following a SGTR overfill event would not have exceeded any regulatory limits or the accident consequences stated in the FSAR for SGTR.
An evaluation determined that the event described in this LER is of very low risk significance.
III. CAUSE OF THE EVENT
The cause of the event is due to Callaway failing approved by the NRC.
to explicitly incorporate the SGTR with overfill accident analysis
IV. CORRECTIVE ACTIONS
There were two immediate actions taken upon discovery of this event. 1) An administrative DEI limit of 0.3 micro- curies per gram Iodine was imposed. 2) All dose assessment coordinators were informed that in the event of a SGTR with water release, the software currently used for dose assessment must be performed using field monitoring data.
Procedures have been revised to reflect this guidance.
Additional actions being pursued include:
- Revising and training on Emergency Operating Procedure E-3
- Reanalysis of the SGTR accident using revalidated operator response times
- Acquiring NRC approval of new dose calculation methodology
V. PREVIOUS SIMILAR EVENTS
No similar events were identified where NRC SER conclusions failed to be appropriately incorporated into Callaway's FSAR.
VI. ADDITIONAL INFORMATION
The system and component codes listed below respectively.
are from the IEEE Standard 805-1984 and IEEE Standard 803A-1984 System: � Not applicable for this event Component: � Not applicable for this event.