Information Notice 2014-01, Fuel Safety Limit Calculations Inputs Were Inconsistent with NRC-Approved Fuel Design
| ML13325A966 | |
| Person / Time | |
|---|---|
| Issue date: | 02/21/2014 |
| From: | Michael Cheok, Kokajko L Division of Construction Inspection and Operational Programs, Division of Policy and Rulemaking |
| To: | |
| Mensah T, 301-415-3610 | |
| References | |
| TAC MF3105 IN-14-001 | |
| Download: ML13325A966 (5) | |
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
OFFICE OF NEW REACTORS
WASHINGTON, DC 20555-0001
February 21, 2014
NRC INFORMATION NOTICE 2014-01:
FUEL SAFETY LIMIT CALCULATION INPUTS
WERE INCONSISTENT WITH NRC-APPROVED
CORRELATION LIMIT VALUES
ADDRESSEES
All holders of an operating license or construction permit for a nuclear power reactor under
Title 10 of the Code of Federal Regulations (10 CFR) Part 50, Domestic Licensing of
Production and Utilization Facilities, except those that have permanently ceased operations
and have certified that fuel has been permanently removed from the reactor vessel.
All holders of and applicants for a power reactor early site permit, combined license, standard
design approval, or manufacturing license under 10 CFR Part 52, Licenses, Certifications, and
Approvals for Nuclear Power Reactors. All applicants for a standard design certification, including such applicants after initial issuance of a design certification rule.
PURPOSE
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice (IN) to inform
addressees of instances in which inputs into fuel safety limit calculations used critical heat flux
(CHF) correlation limit values that were different from those previously approved by the NRC
staff. The NRC expects that recipients will review the information for applicability to their
facilities and consider actions, as appropriate, to avoid similar problems. Suggestions contained
in this IN are not NRC requirements; therefore, no specific action or written response is
required.
BACKGROUND
General Design Criterion (GDC) 10, Reactor Design, in Appendix A, General Design Criteria
for Nuclear Power Plants, of 10 CFR Part 50 states that the reactor core shall be designed with
appropriate margin to assure that the specified acceptable fuel design limits are not exceeded
during any condition of normal operation, including the effects of anticipated operational
occurrences (AOOs). One of the specified acceptable fuel design limits for pressurized-water
reactors (PWR) is the departure from nucleate boiling ratio (DNBR) safety limit. To ensure that
the DNBR safety limit is not exceeded, a CHF correlation is developed which predicts the heat
flux at which the departure from nucleate boiling (DNB) occurs. The NRC staff reviews and
approves each CHF correlation for its application to a specific fuel type(s) to ensure
conservatism. Additionally, the NRC staff reviews the associated CHF correlation limit, often referred to in safety evaluations as the DNBR limit, which captures the uncertainty of the
correlation.
The CHF correlation limit is typically obtained by using measured CHF values at various
locations in the correlations application domain and dividing those values by the correlations
prediction of CHF at the same locations. The resulting ratios of measured-to-predicted data are
then used to quantify the correlations uncertainty. This quantification is usually performed by
calculating the 95th percentile at the 95th confidence level of the measured-to-predicted
distribution, generally referred to as the 95/95 statistic. Usually, the 95/95 statistic can be
calculated from the mean () and standard deviation () of the measured-to-predicted data as
well as Owens k-value, which is solely a function of the degrees of freedom. The equation for
the 95/95 statistic for a normal distribution is given as follows:
95 95
1
The CHF correlation limit is often the 95/95 statistic; however, it may be necessary to bias the
CHF correlation by choosing a correlation limit slightly higher than the 95/95 statistic. By using
a CHF correlation limit slightly higher, the CHF correlations predictions would be made more
conservative. For example, the 95/95 statistic may be 1.113, whereas the NRC-approved CHF
correlation limit may be 1.13.
The uncertainty of the CHF correlation can then be combined with system and operational
uncertainties using an NRC-approved statistical methodology to calculate the DNBR safety limit.
By demonstrating that this DNBR safety limit is not exceeded during any condition of normal
operation, including the effects of AOOs, there is assurance that the DNB-based specified
acceptable fuel design limit is not exceeded.
DESCRIPTION OF CIRCUMSTANCES
While performing a recent review of a statistical combination of uncertainties, the NRC staff
became aware of a possible inconsistency in calculating the DNBR safety limit. When the CHF
correlation uncertainty was combined with other uncertainties in order to generate the
statistically-based DNBR safety limit, statistical parameter inputs based on the calculated 95/95 statistic from the CHF correlation were used as opposed to statistical parameter inputs based
on the NRC-approved CHF correlation limit, as defined in the safety evaluation. In the observed
example, the methodology used to calculate the DNBR safety limit used the mean, standard
deviation, and Owens k-value for the measured-to-predicted data. While these parameters
were associated with the 95/95 statistic from the CHF correlation, they did not capture any
upward bias that was factored into the NRC-approved CHF correlation limit. In the case of the
observed example, additional conservatism associated with the upward bias was absent in the
statistical methodology application that was used to generate the statistically-based DNBR
safety limit.
DISCUSSION
Licensees rely on their safety analysis to demonstrate that the specified acceptable fuel design
limits are not exceeded during any conditions of normal operation, including the effects of
AOOs. The NRC staff has recently discovered that the DNBR safety limit generated from
statistical methodologies may not properly account for a conservative bias that may be included
in the NRC-approved CHF correlation limit as defined in the safety evaluation.
The correction of this inconsistency may increase the statistically-based DNBR safety limit. The
magnitude of the increase is dependent on the difference between the CHF correlations 95/95 statistic and the NRC-approved CHF correlation limit. While this difference in limits is typically
small, and often zero, the NRC staff estimates that a worst case increase in a DNBR safety
limit could be on the order of 1 to 2 percent. The NRC staff anticipates any increase in DNBR
safety limit would be minimal and would not impact plant operation.
CONTACT
This IN requires no specific action or written response. Please direct any questions about this
matter to the technical contact listed below or the appropriate Office of Nuclear Reactor
Regulation (NRR) project manager.
/RA/
/RA/
Michael C. Cheok, Director
Lawrence E. Kokajko, Director
Division of Construction Inspection
Division of Policy and Rulemaking
and Operational Program
Office of Nuclear Reactor Regulation (NRR)
Office of New Reactors (NRO)
Technical Contact:
301-415-1532
E-mail: Joshua.Kaizer@nrc.gov
Note: NRC generic communications may be found on the NRC public Web site, http://www.nrc.gov, under NRC Library.
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