Information Notice 2005-12, 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
Technical Contact:
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
Technical Contact:
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
NAME
DMorey:dw*
EKraus*
RCesaro*
MGalloway*
JWermiel
DATE
04/ /05
04/29/05
05/03/05
05/03/05
05/10/05 OFC
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
Gauges Damaged at
Temporary Job Sites
All material licensees possessing
portable gauges, regulated under
04/14/05 RIS-05-04
Guidance on the Protection
of Unattended Openings
that Intersect a Security
Boundary or Area
All holders of operating licenses
or construction permits for nuclear
power reactors, research and test
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
Exemptions for Uranium
Contained in Aircraft
Counterweights - Storage
and Repair
All persons possessing aircraft
counterweights containing
uranium under the exemption in
05/17/05 IN-05-013 Potential Non-conservative
Error in Modeling
Geometric Regions in the
Keno-v.a Criticality Code
All licensees using the Keno-V.a
criticality code module in
Standardized Computer Analyses
for Licensing Evaluation (SCALE)
software developed by Oak Ridge
National Laboratory (ORNL)
05/17/05 IN-05-012
Excessively Large Criticality
Safety Limits Fail to Provide
Double Contingency at Fuel
Cycle Facility
All licensees authorized to
possess a critical mass of special
nuclear material.
04/07/05 IN-05-010
Changes to 10 CFR Part 71
Packages
All 10 CFR Part 71 licensees and
certificate holders.
04/01/05 IN-05-007
Results of HEMYC
Electrical Raceway Fire
Barrier System Full Scale
Fire Testing
All holders of operating licenses
for nuclear power reactors, except
those who have permanently
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
and Accountability Interface
with Criticality Safety
Activities at Fuel Cycle
Facilities
All licensees authorized to
possess a critical mass of special
nuclear material.
Note: NRC generic communications may be found on the NRC public website, http://www.nrc.gov, under Electronic Reading Room/Document Collections.