05000416/LER-2001-004
| Grand Gulf Nuclear Station, Unit 1 | |
| Event date: | |
|---|---|
| Report date: | |
| 4162001004R01 - NRC Website | |
A. Reportable Occurrence Operating License (OL) NPF-29 Condition 2.0 (1) authorizes the operation of the facility at reactor core thermal power (CTP) levels not to exceed 3833 megawatts thermal (MWt). As a result of a non- conservative main steam moisture carryover fraction in the reactor CTP calculation, GGNS may have unknowingly operated at a reactor core power level slightly in excess of the licensed CTP limit.
Additionally, following further review, it was found that an older version of steam table data was used to calculate the saturated steam enthalpy. This also resulted in possible operation at a reactor core power level slightly in excess of the licensed CTP limit. Such operation was in violation of OL Condition 2.0 (1) and is being reported pursuant to OL Condition 2.F which requires a 24 hour2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> NRC notification for violating the GGNS Operating License followed by a written follow-up report in accordance with the requirements described in 10CFR50.73(b), (c) and (e). Although, this event is not reportable per 10CFR50.73 criteria, it is being reported in accordance with OL Condition 2.F.
This condition does not represent a safety system functional failure.
B. Initial Conditions At the time of discovery of the condition, the plant was in OPERATIONAL CONDITION 1 with reactor water level at approximately 36 inches and reactor water temperature at approximately 532 degrees F. The plant was operating at approximately 100 percent power.
C. Description of Occurrence On October 3, 2001, Grand Gulf evaluated the potential for a calculational error similar to that reported recently at other BWR plants. General Electric recently provided information related to the GE methodology used to calculate CTP. A review of the GE information and GGNS calorimetric calculations identified the error at Grand Gulf.
GGNS currently uses a value of 0.1 percent for the moisture carryover fraction in the CTP calculation which is performed by the process computer. BWRs with steam dryers similar to the GGNS design reported moisture carryover fractions of less than 0.003 percent. This value is consistent with the GGNS carryover fraction measured during initial startup testing at near rated power conditions.
Although GE recommends a value of 0.0 percent be used, changing the carryover fraction from 0.1 to 0.0 percent results in an increase in calculated CTP of approximately 0.082 percent (3.1MWt). This indicates that, due to the non-conservative calculation, GGNS unknowingly slightly exceeded the maximum allowed value for CTP in it's past operating history.
FACILITY NAME (1) DOCKET (2) LER NUMBER (6) Attachment 2 to GNRO-2001/00093 C. Description of Occurrence (continued) At the request of GGNS, GE reviewed the bases for the saturated steam correlation used in the GGNS plant calorimetric heat balance and concluded that the correlation was based on version 2 of the 1936 Keenan and Keyes data instead of more recent 1969 data. Consequently, the saturated steam enthalpy calculation in the GGNS process computer's calorimetric heat balance applies steam table data which is superseded with more conservative values. A further comparison between the 1969 Keenan and Keyes data and the 1967 ASME data indicated that the 1967 ASME data is even more conservative. Based on a steam flow of 16.5 Mlb/hr and a 1.1 Btu/lb difference between the enthalpy correlations at 1040 psia, the application of the more conservative 1967 ASME steam tables increases the calculated core thermal power by approximately 5.5 MW at full power conditions. This non-conservatism, discovered on December 4, 2001, combined with the moisture carryover error, produces a total Core Thermal Power increase of 8.6 MWt or 0.224 percent.
D. Apparent Cause A moisture carryover assumption of 0.1 percent percent in addition to the older steam table data have been applied in the GGNS calorimetric heat balance since receiving a full power license. Like other BWRs, GGNS applied the generic moisture carryover assumptions provided by General Electric.
E. Corrective Actions
Corrective Actions:
1. In both cases, reactor core thermal power was administratively limited to account for the non- conservatism in the calculated CTP.
2. A preliminary review was performed of the other inputs that are not monitored and concluded that these parameters appear to be conservative for application in the heat balance calculation.
3. Applicable programs/procedures were revised to apply a 0.0 percent moisture carryover value and an updated correlation for saturated steam in the plant calorimetric heat balance.
FACILITY NAME (1) PAGE (3) LER NUMBER (6) DOCKET (2) Attachment 2 to GNRO-2001/00093 F. Safety Assessment The impact of steam carryover fraction on the process computer heat balance calculations was provided by GE Nuclear Energy in a report dated September 2001. The GE report stated that the error in calculated CTP is an order of magnitude less than the precision of the MCPR safety limit evaluation process. Additionally, the error in calculated CTP is significantly less than the precision of the process computer core thermal power estimate. Therefore, the use of the carryover fraction of 0.1 percent, although non-conservative, was of minimal safety consequence and the health and safety of the general public were not compromised. Use of the older steam table data was not addressed in the GE Nuclear Energy report. However, the same general conclusions can be drawn regarding the resultant minor error.
G. Additional Information
As a result of these events, CR-GGN-2001-1645 and CR-GGN-2001-1899 were issued.
A previous similar event occurred on December 4, 1995. GGNS exceeded maximum power level specified in Operating License Condition 2.0 (1) due to a non-conservative error in the CTP calculation and reported the condition via LER 95-013-00.
Energy Industry Identification System (EIIS) codes, if applicable, are identified in the text within brackets [ ].