05000220/LER-2011-002

LER-2011-002, Changes and Errors in the Methodology Used by GE Hitachi to Demonstrate Compliance with 10 CFR 50.46 Acceptance Criteria

Nine Mile Point Unit 1
Event date:	05-06-2011
Report date:	07-01-2011
2202011002R00 - NRC Website

A. PRE-EVENT PLANT CONDITIONS:

Prior to this event, Nine Mile Point Unit 1 (NMP1) was operating at 0 percent power.

B. EVENT:

On May 3, 2011, Nine Mile Point Nuclear Station, LLC (NMPNS) was informed by its fuel vendor, GE Hitachi (GEH), of a change in its Emergency Core Cooling System (ECCS) evaluation model that would affect the calculation of peak cladding temperature (PCT) and maximum local oxidation (MLO) at NMP1. This change was required to address three individual errors and a model change identified by GEH for the NMP1 calculation of record. On May 6, 2011, based on the information provided by GEH, NMPNS determined that correction of one of the identified errors, GEH Notification Letter 2011-01, Impact of CORCL Bundle Power Correction — Part-length Rods, resulted in an increase in the calculated PCT and MLO above the 10 CFR 50.46 acceptance criteria of 2200 degrees F and 17 percent, respectively. Also, a change in the ECCS evaluation model, GEH Notification Letter 2011-05, Impact of Update in CORCL Code Version, resulted in an increase in the calculated MLO above the 10 CFR 50.46 acceptance criteria of 17 percent. A description of the four individual notifications regarding the ECCS evaluation model, as detailed by the vendor, is as follows:

GEH Notification Letter 2011-01, Impact of CORCL Bundle Power Correction — Part-length Rods:

An option in the CORCL code distributes power in a manner considering part-length rods in the bundle. This modeling technique has been found to be non-conservative in that it slightly under predicts the total power generated in the hot bundle. As such, the PCT and cladding oxidation results in calculations using this option would be non-conservative.

This error impacted the NMP1 GE11 fuel with a PCT effect of +60 degrees F and an oxidation effect of +4 percent.

GEH Notification Letter 2011-03, Impact of Updated Formulation for Gamma Heat Deposition to Channel Wall for 9x9 and 10x10 Fuel Bundles:

In the input formulation for the SAFER model, input coefficients are used to direct the deposition of gamma and neutron radiation energy produced by fuel fissions and decay heat, determining whether it would heat the fuel rod, cladding, channel, or control rod structural materials. The contribution of heat from gamma ray absorption by the channel was found to have been minimized. The method had been simplified such that initially all the energy was assumed to be deposited in the fuel rods prior to the Loss-of-Coolant- Accident (LOCA) and then adjusted such that the correct heat deposition was applied after the scram. This modeling was determined to be potentially non-conservative.

This error impacted the NMP1 GE11 fuel with a conservative PCT effect of -25 degrees F and an oxidation effect of -11.5 percent.

GEH Notification Letter 2011-04, Impact of Droplet Flow Distribution Array Alignment to Rod Groupings Error:

Programmed enhancements to the CORCL code allowed for an increased number of rod groupings to be defined so as to more accurately represent bundle configuration in the ECCS-LOCA analysis. It was noted that an array in the model, which describes distribution of droplets and film cooling from core spray across the several groupings of rods and the channel, was not populated with corresponding additional elements. This had the effect of denying the channel and peripheral groupings of this core spray distribution, preferentially distributing liquid film and droplets with cooling effect to represented rod groupings. This condition was determined to be potentially non-conservative for calculating temperatures in those groupings where the PCT may occur.

This error impacted the NMP1 GE11 fuel with a conservative PCT effect of -30 degrees F and an oxidation effect of -8.5 percent.

GEH Notification Letter 2011-05, Impact of Update in CORCL Code Version:

The CORCL code has been updated to Version CORCLO7E3. The update provided added functionality of the code with respect to power distribution, increased the number of rod groups that can be modeled, included PRIME-based properties on fuel, corrected film cooling that was credited and provided other updates by way of code maintenance.

This model change impacted the NMP1 GE11 fuel with a PCT effect of +35 degrees F and an oxidation effect of +11.5 percent.

The NMP1 ECCS evaluation model was revised prior to startup from the Spring 2011 refueling outage to address the potential errors and the model change (i.e., prior to formal GEH notification). The NMP1 Maximum Average Planar Linear Heat Generation Rate (MAPLHGR) limits were adjusted through a plant monitoring system (i.e., 3D Monicore) update to maintain the existing PCT and MLO margins. The updated MAPLHGR limits have also been included in the NMP1 Core Operating Limits Report (COLR) for the current operating cycle (Cycle 20) which commenced following the 2011 outage. As such, NMP1 is in full-compliance with 10 CFR 50.46 requirements. In addition, based on a 3 year historical review of the maximum daily Maximum Average Planar Ratios (MAPRAT), it was determined that NMP1 had not operated in an unanalyzed condition that significantly degraded plant safety.

THE EVENT:

None.

D. DATES AND APPROXIMATE TIMES OF MAJOR OCCURRENCES:

RFO21 - The NMP1 evaluation model was revised prior to startup from the Spring 2011 refueling outage to address the potential errors and the model change.

MAPLHGR limits were adjusted through a plant monitoring system (i.e., 3D Monicore) update to maintain the existing PCT and MLO margins.

5/6/2011 On May 6, 2011, based on the information provided by GEH, NMPNS determined that the identified effect of one of the errors resulted in an increase in the calculated PCT and MLO above the 10 CFR 50.46 acceptance criteria. Also, the identified effect of the model change resulted in an increase in the calculated MLO above the 10 CFR 50.46 acceptance criteria. Condition Report 2011- 004603 was generated. Further review of 10 CFR 50.46(a)(3)(ii) determined that an 8-hour notification report to the NRC was required under 10 CFR 50.72(b)(3)(ii)(B). The notification was completed as required.

6/6/2011 In accordance with 10 CFR 50.46(a)(3)(ii), NMPNS provided a 30-day notification letter to the NRC describing a significant change to and errors in the NMP1 ECCS evaluation model.

E. OTHER SYSTEMS OR SECONDARY FUNCTIONS AFFECTED:

No systems or secondary functions were affected by this condition. These Notifications did not affect Nine Mile Point Unit 2.

F. METHOD OF DISCOVERY:

NMPNS was informed by its fuel vendor GEH of a change in its ECCS evaluation model that could affect NMP1. This change was required to address three individual errors and a model change identified by GEH for the NMP1 calculation of record.

G. MAJOR OPERATOR ACTION:

None required.

H. SAFETY SYSTEM RESPONSES:

None required.

II. CAUSE OF EVENT:

The apparent causes of the subject errors and model change are as follows:

Notification Letter 2011-01:

A modeling technique in the CORCL code has been found to be non-conservative, under predicting the total power generated in the hot bundle.

Notification Letter 2011-03:

SAFER code input coefficients used to direct the deposition of gamma and neutron radiation energy produced by fuel fissions and decay heat, was concluded to be potentially non-conservative.

Notification Letter 2011-04:

The CORCL code modeling of rod groupings is potentially non-conservative for calculating temperatures in the rod groups where the PCT might occur.

Notification Letter 2011-05:

The CORCL code has been updated for the purpose of addressing acknowledged errors and to provide added functionality of the code with respect to power distribution, increasing the number of rod groups that can be modeled, including PRIME-based properties on fuel, correcting film cooling credited, and providing other updates by way of code maintenance.

The causal statements above are based upon the actual errors and changes within the ECCS/LOCA model and do not address the human performance aspects of how these errors occurred. As such, GEH is performing a root cause analysis for the above errors.

Condition Report CR-2011-004603 applies to this LER

III. ANALYSIS OF THE EVENT:

As reported by GEH in 10 CFR 50.46 Notification Letters 2011-01, 2011-03, 2011-04 and 2011-05, the effect of each error and model change is listed in Table 1:

Table 1 Notification Letter Bundle Type

PCT

Effect (F) Oxidation Effect (%) 2011-01 GE-11 60 4.0 2011-03 GE-11 -25 -11.5 2011-04 GE-11 -30 -8.5 2011-05 GE-11 35 11.5 The NMP1 LOCA analysis of record results in a PCT of 2150 degrees F relative to the 2200 degrees F criterion of 10 CFR 50.46(b)(1) and a MLO of criterion of 10 CFR 50.46(b)(2). Since the errors and model change listed above were issued from GEH at the same time, their total impact, as well as individual effects, is considered with respect to PCT and MLO to assess reporting requirements and safety significance:

Total Impact:

With respect to the total impact of the three errors and model change, GEH has provided a 3 percent setdown of the GE11 MAPLHGR values for NMP1 Cycle 20 to address the effects on PCT and MLO listed in Table 1. Although the effect of error Notifications 2011-03 and 2011-04 do not meet the threshold for reportability under 10 CFR 50.46, they contribute to the total impact addressed with the 3 percent MAPLHGR setdown. The 3 percent adjustment was implemented at NMP1 prior to startup from the Spring 2011 refueling outage, which commenced Cycle 20.

Individual Impacts:

As shown in Table 1, under error notification 2011-01 the PCT effect is 60 degrees F which is greater than the existing design margin of 50 degrees F, resulting in a PCT which exceeds the 10 percent and a MLO effect of 11.5 percent for Notification 2011-05. Both these effects are greater than the existing design margin of 0.5 percent resulting in a MLO above the 10 CFR 50.46(b)(2) criteria. Because these individual Notifications were non-conservative relative to the 2200 degrees F criterion of 10 CFR 50.46(b)(1) and the 17 percent criterion of 10 CFR 50.46(b)(2), these events are reportable in accordance with 10 CFR 50.46(a)(3)(ii) which states "...any change or error correction that results in a calculated ECCS performance that does not conform to the criteria set forth in paragraph (b) of this section is a reportable event as described in 50.55(e), 50.72, and 50.73." Also, because this event did not meet the specific reporting requirements of 10 CFR 50.73, the "OTHER" box has been checked on the LER cover sheet.

NMPNS has concluded that this event does not represent a condition that significantly degraded plant safety or posed a threat to the health and safety of plant personnel or the public. This conclusion is based on the following:

• NMP1 is in compliance with 10 CFR 50.46 for the current operating cycle, Cycle 20, and has been since startup from the Spring 2011 refueling outage. Adjusted MAPLHGR limits were implemented at NMP1 to maintain the calculated PCT and MLO within the 10 CFR 50.46(b) acceptance criteria and to restore design margins. Specifically, a 3 percent setdown was applied to the existing MAPLHGR values to address the errors and model change. These changes were made prior to the NMP1 startup through a plant monitoring system update (i.e., 3D Monicore). The updated MAPLHGR limits have also been included in the NMP1 COLR for the current operating cycle, which commenced following the 2011 refueling outage.

• NMPNS performed a review of the maximum daily MAPRAT at NMP1 that occurred during Cycle 19 and the second half of Cycle 18 (i.e., a historical review of approximately 3 years). The maximum MAPRAT were then compared against a 3 percent screening criterion as this was the applied setdown for the MAPLHGR values used to address the Notification errors and model change. No instances were found in which the 3 percent screening criterion was exceeded. As such, NMP1 was not operated in an unanalyzed condition that significantly degraded plant safety and would not have exceeded 10 CFR 50.46 limits if a Design Basis Accident (LOCA) would have occurred.

As the effects of the errors and model change have been corrected through implementation of adjusted MAPLGHR values for current operation and the historical review of plant operation has shown NMP1 did not operate in an unanalyzed condition, no other corrective actions are required to address the four 10 CFR 50.46 notifications.

Note that although GNF2 fuel was installed during the 2011 refueling outage, NMP1 did not operate with GNF2 fuel prior to the MAPLGHR adjustments and model corrections being made.

Accordingly, the impact on GNF2 fuel is not presented in this LER.

This event has no effect on plant performance indicators.

IV. CORRECTIVE ACTIONS:

A. ACTION TAKEN TO RETURN AFFECTED SYSTEMS TO PRE-EVENT NORMAL STATUS:

No systems were affected.

B. ACTION TAKEN OR PLANNED TO PREVENT RECURRENCE:

Actions which were taken or are planned to be taken include the following:

The NMP1 GE11 MAPLHGR limits were adjusted through a plant monitoring system (i.e., 3D Monicore) update to maintain the existing PCT margin of 50 degrees F and MLO margin of 0.5 percent. The updated MAPLHGR limits have also been included in the NMP1 COLR for the current operating cycle (Cycle 20) which commenced following the 2011 outage. As such, NMP1 is in full compliance with 10 CFR 50.46 PCT and MLO requirements.

A. FAILED COMPONENTS:

None.

B. PREVIOUS LERs ON SIMILAR EVENTS:

None.

C. THE ENERGY INDUSTRY IDENTIFICATION SYSTEM (ENS) COMPONENT FUNCTION

IN THIS LER:

COMPONENT IEEE 803 FUNCTION IEEE 805 SYSTEM

IDENTIFIER IDENTIFICATION

None N/A N/A

D. SPECIAL COMMENTS:

None