Information Notice 2006-10, Use of Concentration Control for Criticality Safety
| ML060880311 | |
| Person / Time | |
|---|---|
| Issue date: | 04/23/2006 |
| From: | Pierson R NRC/NMSS/FCSS |
| To: | |
| References | |
| IN-06-010 | |
| Download: ML060880311 (5) | |
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR MATERIAL SAFETY AND SAFEGUARDS
WASHINGTON, D.C. 20555-0001 April 23, 2006 NRC INFORMATION NOTICE 2006-10: USE OF CONCENTRATION CONTROL FOR
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 inform
addressees of a concern about the use of concentration control for criticality safety as the
primary nuclear criticality safety (NCS) control for unsafe-geometry vessels. It is expected that
licensees will review this information and consider actions, as appropriate, to avoid similar
problems. Suggestions contained in this IN are not 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 identify accident scenarios leading to criticality and
develop, implement, and maintain reliable controls to ensure that inadvertent criticality is highly
unlikely. Typical criticality safety analyses identify credible accident sequences leading to
criticality; identify bounding assumptions related to the processes, equipment, or material being
analyzed; and establish limits or boundaries of processes, equipment, or material that comply
with corresponding bounding assumptions. 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.
During a recent review of criticality safety analyses at a fuel cycle licensee facility, NRC
inspectors noted routine sampling results showing concentrations near a licensee-proposed
bounding concentration value in an unsafe-geometry tank. The fuel cycle licensee relied solely
on concentration control to maintain safety in an unsafe geometry tank. The licensee asserted
that the NCS method for controlling concentration in the tank was by limiting the concentration
in the waste stream leading into the tank. The licensee stated that the waste stream solution
was uniform on entry to the tank, and that settling could not result in an unsafe concentration.
The analysis demonstrated that by regulating the waste stream concentration to 0.06 grams
uranium-235 (U235)/liter (0.227 grams U235/gallon), the overall concentration in the tank was
guaranteed to remain below the maximum-assumed concentration of 8 grams U235/liter
(30.28 grams U235/gallon).
However, the licensee performed chemical analysis on settled solids in the tank and determined
that the solids contained fissile material near 8 grams U235/liter. As part of routine sampling, the
licensee found a sample with a concentration of 7.74 grams U235/liter (29.30 grams U235/gallon).
The licensee sparged the tank, but only in instances where a sample was to be extracted from
the waste solution. The sparging was not credited with, nor used to maintain uniformity in, the
tank.
DISCUSSION
The effective use of concentration control requires a system in which concentration changes
are well-understood and controlled. NRC is concerned that, in this instance, the licensee
maintained the use of concentration control as the single parameter for assuring criticality
safety without adequately maintaining a uniform solution and without treating settling in the tank
as an upset condition. In this case, the U235 concentration limit was chosen from expected
concentrations in the tank as a result of limiting inlet waste stream concentrations. The
licensee determined that 8 grams U235/liter (30.28 grams U235/gallon) would bound all known
U235 concentrations in the unfavorable tanks. Without ensuring uniformity within the tanks, it is
credible for settling to occur in the solution. The idea of settling within the tank was not
considered as an upset condition in this case. Had possible accumulations of settled solids
been further evaluated, it may have been shown to be credible for fissile material
concentrations in settled solids to exceed the 8-gram (0.018-pound) limit.
An inappropriate use of concentration control was highlighted in an earlier notice (IN-2004-14),
on use of a limit on uranium concentration that was less than bounding for the process in which
it was applied. A licensee determined that mass controls would limit the uranium concentration
in the incinerator ash to less than 21.6 percent throughout the incinerator system. However, material control and accountability (MC&A) sampling data showed concentration levels above
21.6 percent uranium in some parts of the incinerator system. Although the IN focused on the
need to establish appropriate interactions between criticality safety and MC&A staff, it also
provides another case which exemplifies the need, when using concentration control, for
licensees to ensure that they adequately capture all credible bounding scenarios which could
potentially impact their system.
Licensee NCS staff should fully understand their systems and all changes that could upset
concentration control in the system. Staff should also ensure that all credible scenarios are
addressed, and that analyses governing the process bound all such scenarios. During future
inspections, NRC inspectors will review systems using this control to ensure that proper
controls are in place and that they are properly implemented.
CONTACT
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/
Robert C. Pierson, Director
Division of Fuel Cycle Safety
` and Safeguards
Office of Nuclear Material Safety
and Safeguards
Technical Contact:
Natreon Jordan, NMSS
301-415-7648 E-mail: njj@nrc.gov
Enclosure:
List of Recently Issued NMSS Generic Communications
CONTACT
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/
Robert C. Pierson, Director
Division of Fuel Cycle Safety
and Safeguards
Office of Nuclear Material Safety
and Safeguards
Technical Contact:
Natreon Jordan, NMSS
301-415-7648 E-mail: njj@nrc.gov
Enclosure:
List of Recently Issued NMSS Generic Communications
ADAMS ACCESSION #: ML060880311 OFC FCSS/TSG Tech ED IMNS FCSS/TSG FCSS
NAME NJordan Ekraus by fax AMcIntosh MGalloway RPierson
DATE 3/ 30 /06 4/ 09 /06 4/ 20 /06 4/ 20 /06 4/ 23 /06
Enclosure
Recently Issued NMSS Generic Communications
Date GC No. Subject
Addressees
01/26/06 RIS-02-15, NRC Approval of Commercial Data All authorized recipients and holders of
Rev. 1 Encryption Products For the sensitive unclassified safeguards
Electronic Transmission Of information (SGI).
Safeguards Information
01/24/06 RIS-06-01 Expiration Date for NRC-Approved The U.S. Nuclear Regulatory Commission
Spent Fuel Transportation Routes (NRC) licensees who transport, or deliver
to a carrier for transport, irradiated
reactor fuel (spent nuclear fuel (SNF)).
01/13/06 RIS-05-27, NRC Regulatory Issue Summary All 10 CFR Parts 71 and 72 licensees
Rev. 1 2005-27, Rev. 1, NRC Timeliness and certificate holders.
Goals, Prioritization of Incoming
License Applications and
Voluntary Submittal of Schedule
for Future Actions for NRC Review
03/21/06 IN-02-23, Unauthorized Administration of All medical licensees.
Supl. 1 Byproduct Material for Medical Use
01/19/06 IN-06-02 Use of Galvanized Supports and All holders of operating licenses for
Cable Trays with Meggitt Si 2400 nuclear reactors except those who have
Stainless- Steel-jacketed Electrical permanently ceased operations and have
Cables certified that fuel has been permanently
removed from the reactor vessel; and fuel
cycle licensees and certificate holders.
Note: NRC generic communications may be found on the NRC public website at
http://www.nrc.gov, under Electronic Reading Room/Document Collections.