Information Notice 2004-04, Fuel Damage During Cleaning at a Foreign Pressurized Water Reactor
| ML040540778 | |
| Person / Time | |
|---|---|
| Issue date: | 02/24/2004 |
| From: | Beckner W NRC/NRR/DIPM/IROB |
| To: | |
| Dozier J, NRR/IROB 415-1014 | |
| References | |
| IN-04-004 | |
| Download: ML040540778 (8) | |
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, DC 20555-0001 February 24, 2004 NRC INFORMATION NOTICE 2004-04: FUEL DAMAGE DURING CLEANING AT A
FOREIGN PRESSURIZED WATER REACTOR
Addressees
All holders of operating licenses for light-water 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 to inform
addressees about a recent fuel damage event that occurred during chemical cleaning of the fuel
at a foreign pressurized-water reactor (PWR). The fuel cleaning system involved in the event
was not of domestic (U.S.) design or manufacture; however, the fuel and processes used
at the affected PWR are similar to those that may be used in domestic light-water reactors.
This event involved a release of radioactive material to the environment and was publicly
reported in the news media. The occupational dose to workers was well within regulatory limits, and the estimated dose to members of the public was a small fraction of regulatory limits
and less than 1 days exposure to natural background radiation. The event was classified as
Level 3 on the International Nuclear Event Scale, based on substantial damage to irradiated fuel.
The NRC expects recipients to review the information in this notice for applicability to their facilities
and consider actions, as appropriate, to avoid similar problems. However, suggestions contained
in this information notice do not constitute NRC requirements and, therefore, do not require
any specific action or written response.
Description of Circumstances
The event in question occurred at a foreign PWR that was undergoing chemical cleaning
of the fuel because the reactor had experienced corrosion product deposition that affected
core thermal performance. This corrosion product deposition was attributable to a variety of factors, including earlier steam generator chemical decontamination. The plants management elected
to conduct the chemical cleaning, which was originally scheduled to take place during an
upcoming refueling outage to improve core thermal performance during the subsequent
operating cycle.
The facility had received licensing approval for the fuel cleaning system, based in part on
previous successful cleaning of seven-assembly batches of fuel that had decayed for more than
a year. The operator installed the fuel cleaning system in the cask transfer area adjacent to the
spent fuel pool. The fuel cleaning system consisted of a large tank designed to hold 30 fuel
assemblies, other tanks to hold and collect the cleaning solution, pumps to circulate the cooling
water and cleaning solutions, filters to trap removed corrosion products, and valves and hoses
to control and direct the flow of the cleaning solution. The tank containing the fuel assemblies was
insulated to facilitate the maintenance of temperatures to support the cleaning process. Openings in
the top of the tank consisted only of a small line to vent noncondensible gases and a covered
access opening to permit movement of the fuel assemblies. (The latter opening was closed
during the cleaning process.) The cooling flow entered the bottom of the tank, flowed up
through the fuel assemblies, flowed down around the shroud surrounding each assembly, and
exited the tank through a plenum at the bottom. However, bypass flow paths reduced the
effectiveness of the available cooling flow. The flow during the cleaning process was high, but
the standby cooling pump used after completion of the cleaning cycle produced much lower
flow.
Following reactor shutdown on March 29, 2003, the reactor operator began fuel cleaning operations
with fuel removed from the reactor vessel. The operators successfully cleaned three batches
of recently irradiated fuel without incident. However, unlike the earlier batches, the operators
did not remove the tank access cover shortly after the completion of the cleaning process
for the fourth batch on April 10, 2003, because the crane was being used for another task.
The 30 fuel assemblies in the tank represented about 10 percent of the core and had an estimated
decay heat rate of 240 to 270 kW. Analyses later demonstrated that the single cooling pump
in operation after the completion of the cleaning process provided insufficient flow to effectively
cool the fuel assemblies with the access cover in place because bypass flow paths and the
development of a negative thermal head precluded upward flow through the fuel assemblies.
The decay heat began producing steam within the cleaning tank. Because the vent line
was inadequate for the rate of steam generation, the steam displaced the water surrounding
the fuel assemblies within several hours. In a steam environment, the fuel temperature increased
rapidly to an estimated 1,200 Kelvin. This temperature increase caused a minor release
of noble gases from the fuel assemblies, which was detected by plant instruments. In response
to this release, plant operators decided to remove the cleaning tank access cover. The absence
of instrumentation within the cleaning tank precluded the assessment of conditions within
the cleaning tank prior to removal of its access cover. When the access cover seal was broken
early in the morning of April 11, 2003, water entered the tank and contacted the hot fuel assemblies.
Subsequent video examination of the cleaning tank revealed severe fuel damage that resulted
from the water quenching of the hot fuel assemblies and their surrounding shroud. The operator
estimated that this fuel damage event resulted in the release of a few hundred Tera Becquerels
(about 10,000 Curies) of nobel gases, a few tenths of a Tera Becquerel (about 10 Curies)
of radioiodine, and less than one-hundredth of a Tera Becquerel (about a quarter of a Curie)
of other particulate radionuclides (principally Cesium isotopes). Offsite environmental measurements
were consistent with these release estimates.
The failure of the fuel cladding and surrounding shroud resulted in a redistribution of fuel material, with much of it settling to the bottom of the cleaning tank. This distribution of fuel material was outside the configurations analyzed to verify a substantial margin to criticality. To ensure
an adequate margin to criticality, the operators substantially increased the dissolved boron
concentration in the spent fuel pool and adjacent cask transfer pit.
Discussion:
Provision of adequate cooling, maintenance of a margin to criticality, and maintenance of fuel
integrity for fission product retention are essential functions for the safe storage of irradiated
fuel. These functions are normally accomplished through passive design features incorporated
in the design of the fuel and the storage racks.
This event demonstrates the importance of maintaining adequate cooling of fuel after discharge
from the reactor vessel. In this event, the design features that provide adequate natural circulation
cooling were not maintained in the design of the cleaning system. Instead, the cleaning system
design relied on forced circulation cooling without adequate consideration of the reliability
and capability provided for this function. The damage to the integrity of the fuel, which resulted
from the inadequate cooling, threatened the maintenance of an adequate margin to criticality
and released a substantial quantity of radioactive material to the environment.
Related Generic Communications:
The following NRC generic communications describe related reactor operating experience:
(1) Information Notice 97-85, Effects of Crud Buildup and Boron Deposition on Power Distribution
and Shutdown Margin, December 11, 1997.
(2) Information Notice 97-14, Assessment of Spent Fuel Cooling, March 28, 1997.
(3) Generic Letter 88-17, Loss of Decay Heat Removal 10 CFR 50.54(f), October 17, 1988.
(4) Generic Letter 87-12, Loss of Residual Heat Removal (RHR) while the Reactor Coolant System
(RCS) is Partially Filled, July 9, 1987.
This information notice does not require any 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 project manager in the NRCs Office of Nuclear Reactor Regulation.
/RA/
William D. Beckner, Chief
Reactors Operations Branch
Division of Inspection Program Management
Office of Nuclear Reactor Regulation
Technical contacts: S.R. Jones, NRR Jerry Dozier, NRR
(301) 415-2712 (301) 415-1014 Email: srj@nrc.gov Email: jxd@nrc.gov
Attachment: List of Recently Issued NRC Information Notices
ML040540778 OFFICE RSE:RORP:DRIP Tech Editor RSE:SPLB:DSSA
NAME IJDozier PAGarrity* SRJones
DATE 02/09/2004 02/09/2004 02/09/2004 OFFICE BC:SPLB:DSSA SC:OES:IRIB:DIPM C:IROB:IROB:DIPM
NAME JNHannon TReis WDBeckner
DATE 02/10/2004 02/20/2004 02/24 /2004
Attachment 1 LIST OF RECENTLY ISSUED
NRC INFORMATION NOTICES
_____________________________________________________________________________________
Information Date of
Notice No. Subject Issuance Issued to
_____________________________________________________________________________________
2004-03 Radiation Exposures to Pending All well-logging licensees.
Members of the Public in
Excess of Regulatory Limits
Caused by Failures to Perform
Appropriate Radiation Surveys
During Well-logging
Operations
2004-02 Strontium-90 Eye Applicators 02/05/2004 All U.S. Nuclear Regulatory
New Calibration Values and Commission (NRC) medical-use
Use licensees and NRC master
materials license medical-use
Permittees.
2004-01 Auxiliary Feedwater Pump 01/21/2004 All holders of operating licenses
Recirculation Line Orifice or construction permits for
Fouling - Potential Common nuclear power reactors, except
Cause Failure those that have permanently
ceased operations and have
certified that fuel has been
permanently removed from the
reactor.
2002-26, Sup 2 Additional Failure of Steam 01/09/2004 All holders of an operating license
Dryer After A Recent Power or a construction permit for
Uprate nuclear power reactors, except
those that have permanently
ceased operations and have
certified that fuel has been
permanently removed from the
reactor.
Note: NRC generic communications may be received in electronic format shortly after they are
issued by subscribing to the NRC listserver as follows:
To subscribe send an e-mail to <listproc@nrc.gov >, no subject, and the following
command in the message portion:
subscribe gc-nrr firstname lastname
______________________________________________________________________________________
OL = Operating License
CP = Construction Permit