Information Notice 2004-14, Use of Less Then Optimal Bounding Assumptions In Criticality Safety Analysis at Fuel Cycle Facilities

Use of Less Then Optimal Bounding Assumptions In Criticality Safety Analysis at Fuel Cycle Facilities

ML041760122
Person / Time
Issue date:	07/19/2004
Revision:	0
From:	Beckner W D, Pierson R C Office of Nuclear Material Safety and Safeguards, NRC/NRR/DIPM
To:
References
IN-04-014
Download: ML041760122 (6)

UNITED STATES NUCLEAR REGULATORY COMMISSIONOFFICE OF NUCLEAR REACTOR REGULATIONOFFICE OF NUCLEAR MATERIAL SAFETY AND SAFEGUARDSWASHINGTON, D.C. 20555July 19, 2004NRC INFORMATION NOTICE 2004-14:USE OF LESS THAN OPTIMAL BOUNDINGASSUMPTIONS 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 alertaddressees to a safety concern arising from the use of less than optimal bounding assumptions in criticality safety analysis at fuel cycle facilitie It is expected that recipients will review the information for applicability to their facilities and consider actions, as appropriate, to avoid similar problem 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 criticalmasses 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 unlikel 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 applicabl 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 limit 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 outsideof the approved safety basi 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 chambe Licensee nuclear criticality safety (NCS) analysis focused on accumulation of sufficient mass in the incinerator system to support criticalit ML041760122 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. Based on mass limits on the input waste stream, licensee NCS engineers determined that mostmass 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 rati 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 theincinerator 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 syste This led the licensee's 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 significantquantities of ash outside the primary combustion chamber at concentrations in excess of 21.6 wt% The licensee's 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 thesubject fissile syste .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 syste The value results from an infinite media calculation where optimum moderation conditions are established with ash replaced by a uranium dioxide and water mixtur The limiting concentration is related to the critical point, in this case keff = 0.9 To completeanalysis 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 syste However, U concentration frequently exceeded 21.6 wt% in the primary and secondary combustion chambers and flu 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 applie 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 ventilationand off-gas systems, waste-processing systems, and incinerator Licensees should consider actions, as appropriate, to mitigate this vulnerabilit 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 credibl Actions could also include verifying 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 respons If you have any questions about theinformation in this notice, please contact the technical contact listed below./RA//RA/William D. Beckner, ChiefRobert C. Pierson, DirectorReactor Operations BranchDivision of Fuel Cycle Safety Division of Inspection Program Management and Safeguards Office of Nuclear Reactor RegulationOffice of Nuclear Material Safety and Safeguards

Technical Contact:

Dennis Morey, NMSS301-415-6107 E-mail: dcm@nrc.gov

Attachments:

List of Recently Issued NMSS Information Notices 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 respons If you have any questions about theinformation in this notice, please contact the technical contact listed below./RA//RA/William D. Beckner, ChiefRobert C. Pierson, DirectorReactor Operations BranchDivision of Fuel Cycle Safety Division of Inspection Program Management and Safeguards Office of Nuclear Reactor RegulationOffice of Nuclear Material Safety and Safeguards

Technical Contact:

Dennis Morey, NMSS301-415-6107 E-mail: dcm@nrc.gov

Attachments:

List of Recently Issued NMSS Information Notices List of Recently Issued NRC Information NoticesML041760122OFCTSGTSGTech EDTSGOINAMEDMorey:dwRCesaroEkraus: by faxMGallowayMKFaheyDATE6/ 24 /046/ 29 /046/ 22 /046/ 30 /047/ 01 /04OFCNRRFCSSNAMEWBecknerRPiersonDATE7/ 02 /047/ 19 /04C = COVERE = COVER & ENCLOSUREN = N0 COPYOFFICIAL RECORD COPY

______________________________________________________________________________________OL = Operating License CP = Construction PermitAttachment 1 LIST OF RECENTLY ISSUEDNMSS INFORMATION NOTICES_____________________________________________________________________________________InformationDate of Notice N SubjectIssuanceIssued to

_____________________________________________________________________________________2004-13Registration, Use, and QualityAssurance Requirements for NRC-Certified Transportation Packages06/30/2004All materials and decommissioning reactor licensees.2004-03Radiation Exposures toMembers of the Public in Excess of Regulatory Limits Caused by Failures to Perform Appropriate Radiation Surveys During Well-logging Operations02/24/2004All well-logging licensees.2004-02Strontium-90 Eye ApplicatorsNew Calibration Values and Use02/05/2004All U.S. Nuclear RegulatoryCommission (NRC) medical-use licensees and NRC master materials license medical-use permittees.2003-22Heightened Awareness forPatients Containing Detectable Amounts of Radiation from Medical Administrations12/09/2003All medical licensees and NRCMaster Materials Licens medical use permittees.2003-21High-Dose-Rate-Remote-Afterloader Equipment Failure11/24/2003All medical licensees.Note:NRC generic communications may be received in electronic format shortly after they are issued bysubscribing to the NRC listserver as follows:To subscribe send an e-mail to <listproc@nrc.gov >, no subject, and the following command in themessage portion:subscribe gc-nrr firstname lastname

______________________________________________________________________________________OL = Operating License CP = Construction PermitAttachment 2 LIST OF RECENTLY ISSUEDNRC INFORMATION NOTICES_____________________________________________________________________________________

InformationDate of Notice N SubjectIssuanceIssued to

_____________________________________________________________________________________2004-13Registration, Use, and QualityAssurance Requirements for NRC-Certified Transportation Packages06/30/2004All materials anddecommissioning reactor licensees.2004-12Spent Fuel Rod Accountability06/25/2004All holders of operating licensesfor nuclear power reactors, research and test reactors, decommissioned sites storing spent fuel in a pool, and wet spent fuel storage sites.2004-11Cracking in Pressurizer Safetyand Relief Nozzles and in Surge Line Nozzle05/06/2004All holders of operating licenses orconstruction permits for nuclear power reactors, except those that have permanently ceased operations and have certified that fuel has been permanently removed from the reactor.2004-10Loose Parts in SteamGenerators05/04/2004All holders of operating licensesfor pressurized-water reactors (PWRs), except those who have permanently ceased operations and have certified that fuel has been permanently removed from the reactor.2004-09Corrosion of SteelContainment and Containment Liner04/27/2004All holders of operating licensesfor nuclear power reactors except 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 bysubscribing to the NRC listserver as follows:To subscribe send an e-mail to <listproc@nrc.gov >, no subject, and the following command in themessage portion:subscribe gc-nrr firstname lastname