Bulletin 91-01: Reporting Loss of Criticality Safety Controls

                                                       OMB No:  3150-0009 
                                                       NRCB 91-01 

                                UNITED STATES
                          WASHINGTON, D.C.  20555 

                              October 18, 1991 



All fuel cycle and uranium fuel research and development licensees. 


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. 


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 

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. 


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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 

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 

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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                                                            October 18, 1991
                                                            Page 4 of 5 

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 

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 

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 

                                                         Attachment 2 
                                                         NRCB 91-01 
                                                         October 18, 1991 
                                                         Page 1 of 1 



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.


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 
                                   - Volume and density measurements 

�    Concentration                 - Dual sampling and dual analysis 
                                   - On-line monitoring 

�    Automatic Engineered          - Temperature
     Systems                       - Pressure 
                                   - Moisture 



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