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UNITED STATESNUCLEAR REGULATORY COMMISSIONOFFICE OF NUCLEAR REACTOR REGULATIONWASHINGTON, D. C. 20555-0001

April 23, 1999

- Addressees:
- Purpose:
- Description of Circumstances:
- Discussion:

#### Addressees:

All holders of operating licenses for nuclear power reactors, except those who have permanently ceased operations andhave certified that fuel has been permanently removed from the reactor.

#### Purpose:

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice supplement to inform addressees ofrecent experience related to the requirements of 10 CFR 50.46 regarding reporting of changes or errors in emergency corecooling system (ECCS) evaluation models. The material in this supplement focuses on small-break loss-of-coolant accident(LOCA) analyses and on reporting by individual licensees. The material discussed in the original information notice issuedin April 1997, focused principally on large-break LOCA analysis and on calculational activities by fuel vendors at theirfacilities. It is expected that recipients will review the information for applicability to their facilities and consider actions asappropriate to avoid similar problems. No specific action or written response is required by this notice.

#### Description of Circumstances:

Deficiencies have been identified in the review and treatment of LOCA analyses in that reports required by 10 CFR50.46(a)(3) were not submitted when required. When the situation was noted, a licensee commented that his utility andperhaps other utilities had difficulty in understanding and applying the reporting requirements of 10 CFR 50.46(a)(3).

The Duquesne Light Company (DLC) submitted its annual 10 CFR 50.46 report for Beaver Valley Power Station, Units 1and 2 on September 26, 1996. When, as part of its followup to Information Notice 97-15, the NRC staff reviewed thesubmittal, it appeared that large-break loss-of-coolant accident (LBLOCA) analyses had been appropriately treated but thatthe results of the small break loss-of-coolant accident (SBLOCA) analyses had not been reviewed and treated in accordancewith the regulations. At least 190� F of adjustments to peak cladding temperature (PCT) had been made since the lastSBLOCA analyses. However, the DLC annual report lacked sufficient detail for the staff to determine if the errors,changes, and other factors leading to this amount of adjustment had been reported under 10 CFR 50.46(a)(3)(i) and whetherthe licensee had taken appropriate action as defined in 10 CFR 50.46 (a)(3)(ii). The staff, therefore, initiated followupactivities with the licensee that eventually led to a request for additional information and several conference calls.

For changes in PCT with absolute values totaling more than 50� F, 10 CFR 50.46(a)(3)(ii) requires that, within 30 days, thelicensee report the existence of the deviations to the NRC and propose a schedule and method to deal with the situation.Further, the licensee must either reanalyze with an approved model for the present configuration to show compliance with10 CFR 50.46 requirements or take other appropriate action.

DLC submitted another 10 CFR 50.46 report on March 21, 1997. In this report, the sum of the absolute values of additionalchanges and errors in PCT made in the SBLOCA analyses since the previous report exceeded 50� F. However, the changesand errors had not been reported within 30 days, nor was a schedule proposed for addressing the significance of thedeviation as required by 10 CFR 50.46(a)(3)(ii). The staff sent a request for additional information dated May 19, 1997, tothe licensee to follow up on this. In a letter of October 10, 1997, DLC stated its intention to reanalyze both the large-breakand small-break LOCA calculations and proposed a schedule date of June 1999 for completion. The NRC staff found theproposed schedule and resolution acceptable.

In its letter, DLC also suggested that the NRC disseminate more broadly detailed information on reporting of informationto satisfy 10 CFR 50.46(a)(3) requirements. This supplement to the information notice is in part a response to thatsuggestion.

The staff's experience with DLC led it to examine in detail the annual submittals of other licensees. At least three othershad made adjustments to their LOCA analyses of record. These adjustments resulted in PCT changes of more than 50� Fwhich exceeded the significance criterion of 10 CFR 50.46(a)(3). Although these licensees submitted the requisite reports,they failed to propose a schedule either to reanalyze with an approved model or to justify other appropriate actions.

#### Discussion:

The ECCS acceptance criteria rule requires in 10 CFR 50.46(a)(3)(i) that licensees

> ...estimate the effect of any change to or error in an acceptable evaluation model or in the application of an acceptableevaluation model to determine if the change or error is significant. For this purpose, a significant change or error isone which results in a calculated PCT different by more than 50 degrees F from the temperature calculated for thelimiting transient using the last acceptable model, or is a cumulation of changes and errors such that the sum of theabsolute magnitudes of the respective temperature changes is greater than 50 degrees F.  

The next section of the rule, 10 CFR 50.46(a)(3)(ii), requires that

> ...for each change to or error discovered in an acceptable evaluation model or in the application of such a model thataffects the temperature calculation, the licensee shall report the nature of change or error and its estimated effect onthe limiting ECCS analysis to the Commission at least annually as specified in 10 CFR 50.4. If the change or error issignificant, the licensee shall provide this report within 30 days and include with the report a proposed schedule forproviding a reanalysis or taking other action as may be needed to show compliance with 10 CFR 50.46 requirements.  

The preceding two quotations define what must be done when errors or changes to an approved evaluation model arefound. The scope of the evaluation modeling required is discussed in 10 CFR 50.46(a)(1)(i) which states

> ECCS cooling performance must be calculated for a number of postulated LOCAs of different sizes, location, andother properties sufficient to provide assurance that the most severe postulated loss of coolant accidents arecalculated.  

Several assessments covering the entire spectrum of break sizes are generally necessary to determine whether the 10 CFR50.46(b)(1) PCT criterion of 2200� F is violated for the most limiting conditions. To meet this requirement, licensees oftenemploy separate approved evaluation methodologies for large-break and for small-break LOCAs. These separatemethodologies are all part of the "acceptable evaluation model." Changes and errors must logically be tracked separatelyfor each evaluation methodology and the effect and "significance" of them assessed independently for each methodology.

When changes and errors arise in the evaluation model for either the LBLOCA or SBLOCA, the licensee reports theirexistence, nature, and estimated effect at least annually. If the absolute sum of these changes and errors in PCT for eitherLBLOCA or SBLOCA is greater than 50� F; i.e., is "significant", then the licensee must report within 30 days and, in thatreport, propose a schedule for reanalysis or for such other action to comply with 10 CFR 50.46 requirements as isappropriate.

This information notice supplement requires no specific action or written response. If you have any questions about theinformation in this notice, please contact one of the technical contacts listed below or the appropriate Office of NuclearReactor Regulation (NRR) project manager.

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