Information Notice 2005-12, Excessively Large Criticality Safety Limits Fail to Provide Double Contingency at Fuel Cycle Facility

Excessively Large Criticality Safety Limits Fail to Provide Double Contingency at Fuel Cycle Facility

ML051080466
Person / Time
Issue date:	05/17/2005
From:	Hiland P, Pierson R NRC/NMSS/FCSS, NRC/NRR/DIPM/IROB
To:
References
IN-05-012
Download: ML051080466 (4)

UNITED STATES

NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR REACTOR REGULATION

OFFICE OF NUCLEAR MATERIAL SAFETY AND SAFEGUARDS

WASHINGTON, DC 20555 May 17, 2005 NRC INFORMATION NOTICE 2005-12: EXCESSIVELY LARGE CRITICALITY SAFETY

LIMITS FAIL TO PROVIDE DOUBLE

CONTINGENCY AT FUEL CYCLE FACILITY

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 inform

addressees of a concern related to criticality safety at fuel fabrication and other facilities

processing, storing, or handling critical masses of fissile material. It is expected that licensees

will review this information 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

During a recent plant-wide review of criticality safety analysis at a fuel cycle licensee, a licensee

criticality safety engineer questioned the utility of a control scheme that relied on mass and

moderator controls for double contingency in a large uranium dioxide powder hopper.

Experience and process knowledge caused the engineer to believe that the controls were too

high and that exceeding one limit could lead to a critical configuration under credible

circumstances, without requiring the other limit to be exceeded.

Licensee investigation revealed that the system had previously relied on a level limit and that

during a revision of the safety basis, the controls had been changed to support new mass and

moderator limits. The licensee determined that the new mass and moderator limits had been

derived from the previous level limit (i.e., by determining how much mass was present at the

maximum allowed level and then determining how much moderator was required to make the

system critical in that configuration). There was no accompanying analysis to demonstrate that

the new values were actually limiting. The licensee demonstrated that, in fact, increasing the

amount of mass could make the system critical without exceeding the moderation limit.

DISCUSSION

Under 10 CFR Parts 70 and 76, certain licensees processing, storing, or handling critical

masses of fissile material are required to analyze accident scenarios leading to criticality, establish subcritical limits, and provide reliable controls to provide an acceptable level of

assurance that the subcritical limits are not exceeded. Dual, independent controls provide the

basis for double contingency protection against inadvertent criticality. Fuel cycle licensees

generally implement criticality safety controls on important process parameters that, when

upset, may create a critical system.

NRC has several concerns about this issue. The first is that the practice of deconstructing an

existing criticality safety limit was apparently the licensees common practice, which many

licensee criticality safety engineers used for years, resulting in a number of questionable control

arrangements in the technical safety basis. This practice is associated with an expansion of an

established limit. The second concern is the failure to perform needed analysis to assure that

newly developed criticality safety limits would not lead to a critical system by their individual

failure and would ensure double contingency. No explicit modeling was performed for this

analysis, and such modeling is not normally performed for limits not based on computer analysis

due to conservatism normally employed. The final concern is the failure of licensee technical

reviewers and management to detect the excessively high limits during routine review and

approval. Review of these new limits did not include thorough examination and questioning of

existing technical analysis and assumptions.

Failure to establish and maintain fully independent nuclear criticality safety controls exposes

fuel cycle licensees to the possibility of failure of double contingency. Licensees should

consider actions, as appropriate, to identify and mitigate this vulnerability. Appropriate actions

may include: (1) identification of double contingency arrangements based on tabulated values, hand calculations, or other related approximations, such as one-dimensional computer analysis, that may result in the questionable limits being identified, (2) analysis to establish the adequacy

of the controls intended to support the limits identified in item (1), and (3) incorporation of

appropriate requirements into routine review and approval protocol to assure implementation of

suitably robust double contingency arrangements when new criticality safety limits are derived

from previous limits.

CONTACT

This IN requires no specific action nor written response. Questions about this matter may be

referred to the technical contact listed below.

/RA/ /RA/

Patrick L. Hiland, 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

Dennis Morey, NMSS

301-415-6107 e-mail: dcm@nrc.gov

Attachment: List of Recently Issued NMSS Generic Communications

Note: NRC generic communications may be found on the NRC public Web site, http://www.nrc.gov, under Electronic Reading Room/Document Collections.

CONTACT

This IN requires no specific action nor written response. Questions about this matter may be

referred to the technical contact listed below.

/RA/ /RA/

Patrick L. Hiland, 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

Dennis Morey, NMSS

301-415-6107 e-mail: dcm@nrc.gov

Attachment: List of Recently Issued NMSS Generic Communications

Note: NRC generic communications may be found on the NRC public Web site, http://www.nrc.gov, under Electronic Reading Room/Document Collections.

ADAMS ACCESSION #: ML051080466 OFC FCSS/TSG Tech Editor FCSS/TSC FCSS/TSG NRR

NAME DMorey:dw* EKraus* RCesaro* MGalloway* JWermiel

DATE 04/ /05 04/29/05 05/03/05 05/03/05 05/10/05 OFC NRR NRR FCSS

NAME CRoque-Cruz PHiland RPierson

DATE 05/13/05 05/13/05 05/ 17 /05 OFFICIAL RECORD COPY

Attachment 1 IN 2005 -12 Recently Issued NMSS Generic Communications

Date GC No. Subject

Addressees

04/18/05 RIS-05-06 Reporting Requirements for All material licensees possessing

Gauges Damaged at portable gauges, regulated under

Temporary Job Sites 10 CFR Part 30.

04/14/05 RIS-05-04 Guidance on the Protection All holders of operating licenses

of Unattended Openings or construction permits for nuclear

that Intersect a Security power reactors, research and test

Boundary or Area reactors, decommissioning

reactors with fuel on site, Category 1 fuel cycle facilities, critical mass facilities, uranium

conversion facility, independent

spent fuel storage installations, gaseous diffusion plants, and

certain other material licensees.

02/28/05 RIS-05-003 10 CFR Part 40 All persons possessing aircraft

Exemptions for Uranium counterweights containing

Contained in Aircraft uranium under the exemption in

Counterweights - Storage 10 CFR 40.13(c)(5).

and Repair

05/17/05 IN-05-013 Potential Non-conservative All licensees using the Keno-V.a

Error in Modeling criticality code module in

Geometric Regions in the Standardized Computer Analyses

Keno-v.a Criticality Code for Licensing Evaluation (SCALE)

software developed by Oak Ridge

National Laboratory (ORNL)

05/17/05 IN-05-012 Excessively Large Criticality All licensees authorized to

Safety Limits Fail to Provide possess a critical mass of special

Double Contingency at Fuel nuclear material.

Cycle Facility

04/07/05 IN-05-010 Changes to 10 CFR Part 71 All 10 CFR Part 71 licensees and

Packages certificate holders.

IN-05-007 Results of HEMYC All holders of operating licenses

04/01/05 Electrical Raceway Fire for nuclear power reactors, except

Barrier System Full Scale those who have permanently

Fire Testing ceased operations and have

certified that fuel has been

permanently removed from the

reactor vessel, and fuel facilities

licensees.

03/10/05 IN-05-005 Improving Material Control All licensees authorized to

and Accountability Interface possess a critical mass of special

with Criticality Safety nuclear material.

Activities at Fuel Cycle

Facilities

Note: NRC generic communications may be found on the NRC public website, http://www.nrc.gov, under Electronic Reading Room/Document Collections.