Information Notice 2001-15, Non-Conservative Error Minimum Critical Power Ratio Limits
| ML012710347 | |
| Person / Time | |
|---|---|
| Issue date: | 10/29/2001 |
| From: | Imbro E Operational Experience and Non-Power Reactors Branch |
| To: | |
| Hodge, CV, NRR/DRIP/REXB, (415-1861) | |
| References | |
| TAC MB02454 IN-01-015 | |
| Download: ML012710347 (5) | |
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555-0001 October 29, 2001 NRC INFORMATION NOTICE 2001-15: NON-CONSERVATIVE ERRORS IN MINIMUM
CRITICAL POWER RATIO LIMITS
Addressees
All holders of operating licenses or construction permits for boiling water reactors (BWRs).
Purpose
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice to inform the
addressees that NRC recently received reports of nonconservative errors affecting the
minimum critical power ratio (MCPR) operating and safety limits. It is expected that recipients
will review the information for applicability to their facilities and consider actions, as appropriate, to avoid similar problems. However, suggestions contained in this information notice are not
NRC requirements; therefore, no specific action or written response is required.
Background
Thermal limits on nuclear power plant operation are established to ensure that fuel cladding
integrity is not lost because of overheating. Fuel cladding integrity provides one of the
protections of public health and safety against uncontrolled release of radioactivity into the
environment. One of these limits, the MCPR safety limit, is defined as the smallest allowed
ratio of the critical bundle power (defined as that required to produce a critical stage of boiling
called transition boiling somewhere in the bundle) to the actual bundle power. A higher value of
this ratio may be used as an operating limit based on the single failure (operator error or
equipment malfunction causing a transient) design basis requirement.
Description of Circumstances
In June 2001, the fuel vendor General Electric reported that licensees that implemented stability
detect and suppress trip systems at their plants may be making nonconservative errors in their
licensing calculations for reloads, resulting in inadequate MCPR safety limit protection (Part 21 Report 2001-23-0).
Optional stability solutions requiring these calculations are defined as Options I-D, II, and III in
the vendors document NEDO-32465-A, Reactor Stability Detect and Suppress Solutions
Licensing Basis Methodology for Reload Applications, August 1996. This document gives two
generic so-called Delta CPR/Initial CPR Vs. Oscillation Magnitude (DIVOM) curves, one for
core-wide mode oscillations and one for regional mode oscillations. The curves relate
normalized critical power ratio to hot bundle oscillation magnitude.
In Option I-D, the generic core-wide curve is used to confirm that the flow-biased average
power range monitor (APRM) flux trip provides adequate MCPR safety limit protection for a
core-wide mode oscillation initiating on the rated flow control line.
Option II is not specifically addressed in the vendors document, but the vendor states that the
generic regional mode curve has been used at Nine Mile Point Unit 1 to confirm that the APRM
trip gives adequate MCPR protection for a regional mode oscillation initiating on the rated flow
control line.
In Option III, the generic regional mode oscillation curve is used to determine the setpoints for
the implemented stability detect and suppress trip system used to provide adequate MCPR
protection. Plants using this option sometimes call these systems oscillation power range
monitors.
In recent evaluations, the vendor identified a nonconservative error in the high peak bundle
power-to-flow ratios in the generic regional DIVOM curve and in the high core-average power- to-flow ratios in the generic core-wide DIVOM curve. The generic regional DIVOM curve
overpredicts the Option III system trip setpoint, and the generic core-wide DIVOM curve
overpredicts the flow-biased APRM flux trip setpoint.
The vendor stated that it had informed all affected nuclear power plants, including General
Electric boiling water reactors (BWRs), and the industry BWR owners group. The vendor
described compensatory actions taken by the Hatch licensee for both Units 1 and 2. The Perry, Nine Mile Point Unit 2, and Fermi Unit 2 licensees notified the NRC of their compensatory
actions (Event Notifications 38099, 38106, and 38119 respectively).
The vendor provided updated generic DIVOM curves and a corresponding figure of merit to
determine curve applicability in August 2001. The vendor expected to review applicable studies
and, if necessary, develop new methodologies and obtain licensing approval for them by the
end of 2002.
In March 2001, the fuel vendor Framatome ANP Richland reported having used an
inappropriate reference temperature in establishing MCPR operating limits for several BWRs.
The result was to overpredict the thermal conductivity of the fuel. The affected plants were
Dresden Units 2 and 3, Quad Cities Units 1 and 2, LaSalle Units 1 and 2, and Susquehanna
Units 1 and 2. The largest correction was to increase the previous MCPR limit by 0.02 for
power less than 60-percent rated power (Part 21 Report 2001-14-0).
The vendor stated that it had demonstrated continued compliance with the overpressurization
criteria of the American Society of Mechanical Engineers (ASME) code and had installed the
appropriate reference temperature in the computer code used for the MCPR analysis. In October 2000, Siemens Power, the predecessor of Framatome ANP Richland, reported a
deviation in the pump model of the COTRANSA2 computer code, resulting in improper
calculation of the flow resistance during a pump seizure event. The affected nuclear power
plants were Dresden Units 2 and 3 and Susquehanna Units 1 and 2 (Part 21 Report 2000-23-
0).
In October 2000, the Hope Creek licensee was doing surveillance testing of a recirculation
pump motor-generator set. The licensee reported finding that the scoop tube mechanical and
electrical stop overspeed setpoints were nonconservatively high (less than or equal to 109 percent for the mechanical stop and less than or equal to 107 percent for the electrical stop). If
the analyzed reactor recirculation pump runaway transient occurs, the flow-dependent MCPR
may be exceeded (Event Notification 37418).
In September 2000, the Grand Gulf licensee reported that a main generator partial load
rejection can actuate a control circuit that may not always activate a reactor scram or
recirculation pump downshift as assumed in the analysis. This condition could adversely affect
MCPR limits (Event Notification 37342).
In July 2000, the Fitzpatrick licensee reported finding the mechanical stops to limit speed of
both recirculation pumps were set at values exceeding those assumed in the calculated
flow-dependent MCPR in the core operating limit report. Assuming initial low flow, this error
could result in exceeding the MCPR safety limit under postulated runaway recirculation flow
In January 1999, the fuel vendor Asea Brown Boveri-Combustion Engineering (ABB-CE)
reported that BWR MCPR analyses using the BISON fast-transient analysis code could be
nonconservative because the code incorrectly models the reactor vessel lower plenum volume
(Part 21 Report 1999-07-0).
Also in January 1999, ABB-CE reported a potential nonconservative critical power ratio
correlation (XL-S96) for SVEA-96 BWR fuel. This correlation is based on a cosine axial power
shape, which could lead to nonconservative MCPR operating limits and nonconservatively
monitored critical power ratios for top-peaked axial power shapes. This error affected WNP-2 for Cycle 14 after 5,600 MWD/MTU fuel exposure (Part 21 Report 1999-08-0).
In November 1998, ABB-CE reported that a defect in modeling radial power in the BWR fast- transient analysis code BISON-SLAVE could lead to nonconservative MCPR operating limits.
The defect involves an input option that was incorrectly described in the code user manual and
not the program coding itself. The affected nuclear power plants were WNP-2 and
Susquehanna Unit 1 (Part 21 Report 1998-69-0).
In September 1998, the fuel vendor Siemens Power reported that BWR MCPR operating limits
were nonconservative because in determining gap heat transfer coefficients used by the
RODEX2 code for BWR transient analyses, fuel cladding was assumed to be cold-worked
instead of annealed. The affected nuclear power plants were LaSalle Unit 2 for Cycle 8 and
Quad Cities Unit 2 for Cycle 15 (Part 21 Report 1998-55-0). In April 1998, the fuel vendor Siemens Power reported that additive constants used by the core
monitoring system to determine MCPR for Atrium-9B fuel were nonconservative and more
uncertain than previously estimated. The affected nuclear power plants were Dresden Unit 3 and Quad Cities Unit 2 (Part 21 Report 1998-33-0).
Discussion
All of these 11 events involved BWRs. They may be categorized as follows:
- 6 thermal modeling errors
- 2 other modeling errors
- 2 incorrect settings
- 1 control circuit modeling error
Each category represents a shortcoming in quality control measures. Individually, the events
had little safety significance. Collectively, a significant number of similar events in a relatively
short time may indicate a trend of deteriorating quality control of the MCPR safety parameter.
This information notice 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.
/RA/
Eugene V. Imbro, Acting Chief
Operational Experience
and Non-Power Reactors Branch
Division of Regulatory Improvement Programs
Office of Nuclear Reactor Regulation
Contacts: V. Hodge, NRR Tai L. Huang, NRR
301-415-1861 301-415-2867 E-mail: cvh@nrc.gov E-mail: tlh1@nrc.gov
Attachment: List of Recently Issued Information Notices
ML012710347 Template #:NRR-052
- Publicly Available G Non-Publicly Available G Sensitive G Non-Sensitive
OFFICE REXB TECH EDITOR SRXB SRXB REXB C:REXB
NAME CVHodge* cvh for * THuang* RCaruso* JTappert EVImbro
DATE 09/19/2001 09/19/2001 09/24/2001 09/24/2001 / /2001 / /2001
Attachment LIST OF RECENTLY ISSUED
NRC INFORMATION NOTICES
_____________________________________________________________________________________
Information Date of
Notice No. Subject Issuance Issued to
______________________________________________________________________________________
2001-14 Problems with incorrectly- 10/03/01 All holders of operating licenses
Installed Swing-Check Valves for nuclear power reactors, except those who have ceased
operations and have certified that
fuel has been permanently
removed from the reactor vessel
2001-13 Inadequate Standby Liquid 08/10/01 All holders of operating licenses
Control System Relief Valve for boiling water reactors
Margin
2001-12 Hydrogen Fire at Nuclear 8/08/01 All holders of operating licenses
(ERRATA) Power Stations or construction permits for
nuclear power reactors except
those who have ceased
operations and have certified that
fuel has been permanently
removed from the reactor vessel
2001-12 Hydrogen Fire at Nuclear 7/13/01 All holders of operating licenses
Power Stations or construction permits for
nuclear power reactors except
those who have ceased
operations and have certified that
fuel has been permanently
removed from the reactor vessel
2001-11 Thefts of Portable Gauges 07/13/01 All portable gauge licensees
2001-10 Failure of Central Sprinkler 06/28/01 All holders of licenses for nuclear
Company Model GB Series power, research, and test
Fire Sprinkler Heads reactors and fuel cycle facilities
2001-09 Main Feedwater System 06/12/01 All holders of operating licenses
Degradation in Safety-Related for pressurized water nuclear
ASME Code Class 2 Piping power reactors, except those who
Inside the Containment of a have permanently ceased
Pressurized Water Reactor operations and have certified that
fuel has been permanently
removed from the reactor vessel
______________________________________________________________________________________
OL = Operating License
CP = Construction Permit