Information Notice 2004-14, Use of Less Then Optimal Bounding Assumptions in Criticality Safety Analysis at Fuel Cycle Facilities
ML041760122 | |
Person / Time | |
---|---|
Issue date: | 07/19/2004 |
From: | Beckner W, Pierson R Office of Nuclear Material Safety and Safeguards, NRC/NRR/DIPM |
To: | |
References | |
IN-04-014 | |
Download: ML041760122 (6) | |
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
OFFICE OF NUCLEAR MATERIAL SAFETY AND SAFEGUARDS
WASHINGTON, D.C. 20555 July 19, 2004 NRC INFORMATION NOTICE 2004-14: USE OF LESS THAN OPTIMAL BOUNDING
ASSUMPTIONS IN CRITICALITY SAFETY
ANALYSIS AT FUEL CYCLE FACILITIES
Addressees
All licensees authorized to possess a critical mass of special nuclear material.
Purpose
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice (IN) to alert
addressees to a safety concern arising from the use of less than optimal bounding assumptions
in criticality safety analysis at fuel cycle facilities. 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 IN are not new NRC requirements;
therefore, no specific action nor written response is required.
Description of Circumstances
Under 10 CFR Parts 70 and 76, certain licensees processing, storing, or handling critical
masses of fissile material are required to analyze all accident scenarios leading to criticality and
provide reliable controls to assure that inadvertent criticality events are highly unlikely. Typical
criticality analysis identifies credible accident sequences leading to criticality; identifies
reasonable bounding assumptions related to the processes, equipment, or material analyzed;
and establishes limits or boundaries of processes, equipment, or material within which
bounding assumptions are applicable. Criticality may be deemed not credible when inherent
features of the process, equipment, or material in a specific accident sequence leading to
criticality can be shown to constrain the reactivity of fissile material within subcritical limits. The
safety concern arises when accident scenarios leading to criticality are deemed not credible, based on bounding assumptions that are less than optimal for the system involved.
Recently, a licensee reported an event, to NRC, concerning operation of an incinerator outside
of the approved safety basis. The licensee had performed a criticality safety evaluation of an
incinerator approximately 8 years previously and concluded that criticality was not credible
outside of the primary combustion chamber. Licensee nuclear criticality safety (NCS) analysis
focused on accumulation of sufficient mass in the incinerator system to support criticality.
Based on mass limits on the input waste stream, licensee NCS engineers determined that most
mass resulting from incineration would accumulate in the primary combustion chamber and that
ash resulting from incineration would never exceed a concentration of 21.6 weight percent
uranium (wt% U) which is always subcritical in infinite media at the optimal moderator ratio.
Based on this conclusion, criticality safety limits and controls were developed and implemented
only for the primary combustion chamber, which was a small fraction of the incinerator system.
Licensee NCS engineers believed that very limited amounts of ash would carry over from the
incinerator primary combustion chamber to the remainder of the incinerator system and that
mass controls on the primary combustion chamber would limit uranium concentration in the ash
to less than 21.6 wt% throughout the incinerator system. This led the licensees NCS engineers
to conclude that criticality outside the primary combustion chamber was not credible.
On March 5, 2004, the licensee reported an event concerning the accumulation of significant
quantities of ash outside the primary combustion chamber at concentrations in excess of
21.6 wt% U. The licensees investigation revealed that ash deposits at various locations in the
incinerator routinely exceeded the 21.6 wt% uranium concentration assumed to be bounding for
ash and that the mass of ash deposited also exceeded expectations.
Discussion:
In the described event, the chosen uranium concentration was arbitrary and did not bound the
subject fissile system. 21.6 wt% U is not a natural limit on U concentration in incinerator ash
and is less than optimal because higher uranium concentrations produce a more reactive fissile
system. The value results from an infinite media calculation where optimum moderation
conditions are established with ash replaced by a uranium dioxide and water mixture. The
limiting concentration is related to the critical point, in this case keff = 0.98.1 To complete
analysis of the incinerator, the licensee looked at data from selected parts of the incinerator
system and concluded that 21.6 wt% U far exceeded uranium concentrations typically expected
in the system. However, U concentration frequently exceeded 21.6 wt% in the primary and
secondary combustion chambers and flue. The licensee failure to recognize that the actual U
content of the ash was related, in part, to weak documentation of bounding assumptions and
poor definition of incinerator system boundaries to which the bounding assumptions applied.
The accumulation of material outside the primary combustion chamber at the U concentrations
seen, along with the availability of water in the incinerator off-gas quench system, results in the
conclusion that criticality was actually credible in the incinerator secondary combustion
chamber.
Less than optimal bounding assumptions for criticality safety are most often seen in ventilation
and off-gas systems, waste-processing systems, and incinerators. Licensees should consider
actions, as appropriate, to mitigate this vulnerability. These actions could include reviewing all
accident sequences where less than optimal bounding assumptions were used to establish
NCS controls or determine that criticality is not credible. Actions could also include verifying
1 keff is the effective neutron multiplication factor for the system under consideration.
The licensee had defined the critical point in its license as keff =0.98. that bounding assumptions are actually bounding, for example, by reviewing available material
composition data for evidence that attributes such as isotope concentration are actually within
expected values.
This IN requires no specific action nor written response. If you have any questions about the
information in this notice, please contact the technical contact listed below.
/RA/ /RA/
William D. Beckner, Chief Robert C. Pierson, Director
Reactor Operations Branch Division of Fuel Cycle Safety
Division of Inspection Program Management and Safeguards
Office of Nuclear Reactor Regulation Office of Nuclear Material Safety
and Safeguards
Technical Contact:
Dennis Morey, NMSS
301-415-6107 E-mail: dcm@nrc.gov
Attachments:
1. List of Recently Issued NMSS Information Notices
2. List of Recently Issued NRC Information Notices that bounding assumptions are actually bounding, for example, by reviewing available material
composition data for evidence that attributes such as isotope concentration are actually within
expected values.
This IN requires no specific action nor written response. If you have any questions about the
information in this notice, please contact the technical contact listed below.
/RA/ /RA/
William D. Beckner, Chief Robert C. Pierson, Director
Reactor Operations Branch Division of Fuel Cycle Safety
Division of Inspection Program Management and Safeguards
Office of Nuclear Reactor Regulation Office of Nuclear Material Safety
and Safeguards
Technical Contact:
Dennis Morey, NMSS
301-415-6107 E-mail: dcm@nrc.gov
Attachments:
1. List of Recently Issued NMSS Information Notices
2. List of Recently Issued NRC Information Notices
ML041760122 OFC TSG TSG Tech ED TSG OI
NAME DMorey:dw RCesaro Ekraus: by fax MGalloway MKFahey
DATE 6/ 24 /04 6/ 29 /04 6/ 22 /04 6/ 30 /04 7/ 01 /04 OFC NRR FCSS
NAME WBeckner RPierson
DATE 7/ 02 /04 7/ 19 /04 C = COVER E = COVER & ENCLOSURE N = N0 COPY
OFFICIAL RECORD COPY
Attachment 1 LIST OF RECENTLY ISSUED
NMSS INFORMATION NOTICES
_____________________________________________________________________________________
Information Date of
Notice No. Subject Issuance Issued to
_____________________________________________________________________________________
2004-13 Registration, Use, and Quality 06/30/2004 All materials and
Assurance Requirements for decommissioning reactor
NRC-Certified Transportation licensees.
Packages
2004-03 Radiation Exposures to 02/24/2004 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 Use Commission (NRC) medical-use
licensees and NRC master
materials license medical-use
permittees.
2003-22 Heightened Awareness for 12/09/2003 All medical licensees and NRC
Patients Containing Detectable Master Materials Licens
Amounts of Radiation from medical use permittees.
Medical Administrations
2003-21 High-Dose-Rate-Remote- 11/24/2003 All medical licensees.
Afterloader Equipment Failure
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
Attachment 2 LIST OF RECENTLY ISSUED
NRC INFORMATION NOTICES
_____________________________________________________________________________________
Information Date of
Notice No. Subject Issuance Issued to
_____________________________________________________________________________________
2004-13 Registration, Use, and Quality 06/30/2004 All materials and
Assurance Requirements for decommissioning reactor
NRC-Certified Transportation licensees.
Packages
2004-12 Spent Fuel Rod Accountability 06/25/2004 All holders of operating licenses
for nuclear power reactors, research and test reactors, decommissioned sites storing
spent fuel in a pool, and wet
spent fuel storage sites.
2004-11 Cracking in Pressurizer Safety 05/06/2004 All holders of operating licenses or
and Relief Nozzles and in construction permits for nuclear
Surge Line Nozzle power reactors, except those that
have permanently ceased
operations and have certified that
fuel has been permanently
removed from the reactor.
2004-10 Loose Parts in Steam 05/04/2004 All holders of operating licenses
Generators for pressurized-water reactors
(PWRs), except those who have
permanently ceased operations
and have certified that fuel has
been permanently removed from
the reactor.
2004-09 Corrosion of Steel 04/27/2004 All holders of operating licenses
Containment and Containment for nuclear power reactors except
Liner those who have permanently
ceased operation and have
certified that fuel has been
permanently removed from the
reactor vessel.
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