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
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

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 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 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 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 outsideof 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. 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 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 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 system. 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% U. 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 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 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 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 ventilationand 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 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 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: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 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:1. List of Recently Issued NMSS Information Notices

2. 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 No. 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 No. 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