Bulletin 91-01: Reporting Loss of Criticality Safety Controls
OMB No: 3150-0009
NRCB 91-01
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR MATERIAL SAFETY AND SAFEGUARDS
WASHINGTON, D.C. 20555
October 18, 1991
NRC BULLETIN 91-01: REPORTING LOSS OF CRITICALITY SAFETY CONTROLS
Addressees
All fuel cycle and uranium fuel research and development licensees.
Purpose
This bulletin requests that addressees inform the Commission of their
criteria and procedures that assure the prompt evaluation and reporting of
the degradation of any controlled parameters used to prevent nuclear
criticality to licensee management and the immediate reporting to the
Commission of any significant degradation of such controls as required by 10
CFR 20.403(a). A written response to this bulletin is required.
Background
On October 3, 1990, all licensees who possess more than a critical mass
quantity of special nuclear material were informed of the need for
management attention to the establishment and maintenance of their nuclear
criticality safety program (Information Notice No. 90-63, attached). That
Notice referred to previous Information Notice No. 89-24, dated March 6,
1989, also on the subject of criticality safety. The Commission needs
assurance that proper attention is being addressed to these important
criticality concerns. Also, licensees must assure that significant
degradation of any controls used to prevent criticality is promptly reported
to management, and as required by 10 CFR 20.403(a)(1), to the Commission, to
assure that appropriate actions are taken to prevent further system
degradation.
Description of Circumstances
In May 1991, a process upset occurred in the solvent extraction portion of a
uranium recycle unit of a fuel manufacturing operation. The process upset
caused the accumulation of enriched uranium in favorable geometry tanks in a
waste processing area. When these tanks filled, their uranium contents were
transferred, in some instances without sample measurement, to an unfavorable
geometry tank waste accumulation tank, and then to a second, unfavorable
geometry waste treatment tank. Although the upset was observed by operators
late on an evening shift, the process was not shut down until around 5:30
the next morning, when measurements indicated high uranium concentrations in
the favorable geometry tanks. A high concentration of uranium was measured
in the waste treatment tank at 7:00 a.m.
9110180125
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October 18, 1991
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Licensee management was made aware of the incident later that morning, and a
technical evaluation/recovery team was established. The NRC was notified of
the incident around 3:45 p.m. that day. A criticality incident did not
occur, but the margin of safety was reduced. The licensee removed the
excess uranium from the waste treatment tank over a period of several days
using centrifuge techniques.
The licensee's investigation team concluded that there were several areas of
operational control which were significant contributors to the incident,
including: (1) failure to always follow procedures or inadequate
procedures; (2) insufficient supervision and/or technical support of
operations; (3) lack of adequate overchecks/audits on conformance with
criticality safety control requirements; and (4) inadequate records systems.
The Commission's investigation is reported in NUREG-1450.
Discussion of Safety Significance
Because of the above event and knowledge of similar circumstances at other
licensed activities, the Commission is concerned that there may be
insufficient attention by licensees to the need for internal reporting and
prompt evaluation of failures of controlled parameters related to
criticality safety. The Commission is also concerned that licensees may not
have procedures in place to assure compliance with the requirements under 10
CFR 20.403 to report immediately to the Commission any significant failure
of criticality safety controls. As discussed in the appendix to this
bulletin, several controls may be used to maintain a controlled parameter
for preventing a criticality excursion. If substantial control over a
controlled parameter is lost, the event should be reported to the
Commission.
Following are specific examples of events related to criticality control
that should be reported to the Commission immediately:
1. Complete loss of a controlled parameter.
2. Substantial degradation of a controlled parameter.
3. Failure of a controlled parameter previously identified by the
Commission or the licensee's criticality safety specialists as
requiring reporting upon failure.
4. Determination that a criticality safety analysis was deficient in
evaluating actual plant conditions and necessary controlled parameters
were not established.
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October 18, 1991
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5. An unusual event or condition for which the severity and remedy are not
readily determined.
Reports of such events must be made to the NRC Operation Center, which is
staffed 24 hours per day, and to the appropriate Regional Administrator.
Some types of events, though not warranting reporting to the Commission,
nevertheless merit attention within the licensee's own organization,
particularly by the criticality safety specialists. The Commission intends
that all events involving degradation of criticality controls be reported
for evaluation within the licensee's organization. Since some events which
can occur in process systems and their relative importance to criticality
safety cannot be determined before the event, criteria are needed in order
to make the judgment as to whether the event should be reported to the
Commission as required by 10 CFR 20.403(a). Each recipient of this
bulletin, therefore, should assure that as to the extent of degradation,
licensees are encouraged to report to the Commission. If initial
indications of an event do not seem to warrant Commission notification, but
further developments prove it to be more serious, the Commission should be
immediately notified at that point.
Addressees are also reminded that all necessary corrective actions must be
taken promptly, regardless of any reporting action. Reports to the
Commission do not require that corrective actions be completed prior to
reporting.
Requested Action
Addressees are requested to evaluate their criticality safety criteria and
procedures, modify them as appropriate to assure that events involving
degradation of controls will be promptly evaluated and reported to licensee
management and NRC as appropriate, and provide a description of their
criteria and procedures to NRC. In completing this evaluation, licensees
should include the following:
1. Based on your current analyses of criticality safety, and any further
criticality analyses that may be necessary for this determination,
identify and examine each individual controlled parameter whose failure
could contribute to a decrease in criticality control. For each
individual controlled parameter, determine whether or not degradation
of the system of controls would constitute a significant loss of
effectiveness. Loss of a single controlled parameter should be
considered to have occurred upon total failure or substantial
degradation of the control.
2. Any list of methods of control, such as indicated in the appendix,
should be considered from the point of view of importance to
maintaining its associated controlled parameter. The possibility of
combinations of loss of more than one control should be considered;
i.e., are some controls likely to fail simultaneously and what is the
level of significance of such an occurrence?
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NRCB 91-01
October 18, 1991
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3. Whenever an event occurs in which criticality safety controls do not
function entirely as expected, a management-established reporting
system should ensure that proper levels of licensee management will be
promptly informed. This reporting system will allow plant and safety
management to evaluate the significance of a criticality event
precursor and to take appropriate action. Significant loss of control
may dictate activation of the Emergency Plan during the evaluation and
recovery activities.
Reporting Requirements
Within 90 days of the receipt of this bulletin, pursuant to 10 CFR 70.22(d),
each recipient shall provide the Commission with a statement describing its
reporting criteria and management implementation procedures for evaluation
and reporting related to loss of criticality safety controls which meet the
requirements of 10 CFR 20.403(a). The statement should indicate the means
whereby responsible licensee management will be made aware of any relevant
failures, the criteria used by licensee management to determine the
importance of those failures to criticality safety, and the related
reporting levels. The statement should also indicate how the determination
will be made that a controlled parameter is sufficiently degraded such that
any Emergency Plan procedures will be activated and indicate how the
implementation of these procedures for reporting will be documented.
Implementing procedures and documentation will be reviewed during NRC
inspection.
The written reports required above shall be submitted to the U.S. Nuclear
Regulatory Commission, ATTN: Document Control Desk, Washington, D.C. 20555.
In addition, a copy shall be submitted to the appropriate Regional
Administrator. The reporting requirements for reports in response to this
bulletin are covered by OMB clearance number 3150-0009, which expires May
31, 1994. The estimated average number of burden hours is 80 person hours
per licensee response, including those needed to assess the new
recommendations, search data sources, gather and analyze the data, and
prepare the required letters. Send comments regarding this burden estimate
or any other aspect of this collection of information, including suggestions
for reducing this burden, to the Information and Records Management Branch,
Division of Information Support Services, Office of Information Resources
Management, U.S. Nuclear Regulatory Commission, Washington, DC 20555, and to
the Paperwork Reduction Project (3150-0009), Office of Information and
Regulatory Affairs, NEOB-3019, Office of Management and Budget, Washington,
DC 20503.
On November 19, 1991, the Commission will sponsor a workshop concerning
reporting of criticality safety events. The location for the workshop will
be announced as soon as arrangements can be made.
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October 18, 1991
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If you have questions about this matter, please contact one of the technical
contacts listed below.
Richard E. Cunningham, Director
Division of Industrial and
Medical Nuclear Safety
Office of Nuclear Material Safety
and Safeguards
Technical contacts: George H. Bidinger, NMSS
(301) 492-0683
Robert E. Wilson, NMSS
(301) 492-0126
A. Thomas Clark, NMSS
(301) 492-3424
Attachments:
1. Information Notice No. 90-63, "Management Attention to the Establishment
and Maintenance of a Nuclear Criticality Safety Program"
2. Appendix - Principals of Criticality Safety
3. List of Recently Issued NMSS Bulletins
4. List of Recently Issued NRC Bulletins
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Attachment 2
NRCB 91-01
October 18, 1991
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APPENDIX
PRINCIPALS OF CRITICALITY SAFETY
The basic tenet of nuclear criticality prevention is that at least two,
unlikely, independent, and concurrent changes in process conditions must
occur before a criticality accident is possible. This is the so-called
"double contingency" principle in ANSI/ANS-8.1-1983. Controls or systems of
controls are used to limit process variables in order to maintain safe
operating conditions. A list of these typical controls is presented below.
The analysis for criticality safety should identify the multiple scenarios
by which nuclear criticality can occur. Then controlled parameters and
their supporting methods of control must be established to prevent each
scenario. A defense-in-depth of two or more controlled parameters is
necessary to make criticality unlikely, thereby satisfying the double
contingency principle. If a controlled parameter is lost or is
substantially degraded, the special nuclear material could threaten to cause
exposures to radiation or release of radioactive materials as described in
10 CFR Part 20.403(a) and should be reported immediately to the Commission.
Several controls may be involved in maintaining a controlled parameter. For
example, a large tank used to receive process solution containing a fissile
material might use Raschig rings for one control parameter and concentration
limits for another. Both parameters may have several related controls to
assure that their loss is highly unlikely. The Raschig rings may be
inspected before and after installation, both as to boron content and volume
fill level. There may be several controls on concentration, such as initial
feed preparation, in-line monitors, and sampling and analysis. The product
of all the controls combined provide the "highly unlikely" character of the
failure of the controlled parameters.
STANDARD CONTROLLED PARAMETERS AND TYPICAL SUPPORTING CONTROLS
Controlled Parameters Typical Supporting Controls
� Favorable Geometry - Configuration control - Periodic
examination (Quality Assurance)
- Spacing
� Use of Poisons - Configuration control
- Neutron absorption measurement
- Periodic examination and analysis
- Concentration
� Mass - Batch size weighing, sampling and
analysis
- Volume and density measurements
� Concentration - Dual sampling and dual analysis
- On-line monitoring
� Automatic Engineered - Temperature
Systems - Pressure
- Moisture
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