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 U

235/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). 

ML060880311
IN 2006-10

Page 2 of 3

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 U

235/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 U

235/gallon) would bound all known

U

235 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.
IN 2006-10

Page 3 of 3

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
Enclosure

IN 2006-10

Recently Issued NMSS Generic Communications

Date GC No. Subject Addressees

01/26/06 RIS-02-15,

Rev. 1

NRC Approval of Commercial Data

Encryption Products For the

Electronic Transmission Of

Safeguards Information

All authorized recipients and holders of

sensitive unclassified safeguards

information (SGI).

01/24/06 RIS-06-01 Expiration Date for NRC-Approved

Spent Fuel Transportation Routes

The U.S. Nuclear Regulatory Commission

(NRC) licensees who transport, or deliver

to a carrier for transport, irradiated

reactor fuel (spent nuclear fuel (SNF)).

01/13/06 RIS-05-27,

Rev. 1

NRC Regulatory Issue Summary

2005-27, Rev. 1, NRC Timeliness

Goals, Prioritization of Incoming

License Applications and

Voluntary Submittal of Schedule

for Future Actions for NRC Review

All 10 CFR Parts 71 and 72 licensees

and certificate holders.

03/21/06 IN-02-23,

Supl. 1

Unauthorized Administration of

Byproduct Material for Medical Use

All medical licensees.

01/19/06 IN-06-02 Use of Galvanized Supports and

Cable Trays with Meggitt Si 2400

Stainless- Steel-jacketed Electrical

Cables

All holders of operating licenses for

nuclear reactors except those who have

permanently ceased operations and have

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.