United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 84-21: Inadequate Shutdown Margin

                                                 SSINS No.: 6835           
                                                 IN 84-21

                                 UNITED STATES
                         NUCLEAR REGULATORY COMMISSION
                     OFFICE OF INSPECTION AND ENFORCEMENT
                            WASHINGTON, D.C. 20555

                                March 28, 1984

Information Notice No. 84-21: INADEQUATE SHUTDOWN MARGIN

Addressees:

All nuclear research reactor facilities holding an Operating License (OL).

Purpose:

This information notice is provided as a notification of a potentially 
significant problem pertaining to the lack of an adequate shutdown margin at
a  nuclear research reactor facility. It is expected that recipients will
review  the information for applicability to their facilities and consider
actions, if  appropriate, to preclude a similar problem occurring at their
facilities.  Suggestions contained in this information notice do not
constitute NRC  requirements and, therefore, no specific action or written
response is  required.

Description of Circumstances:

On May 31, 1983, a report from a university operating a research reactor gave 
notice that the facility had been operated with a shutdown margin less 
negative than the 0.4% dk/k required by Technical Specifications, for the 
reactor core in a xenon-free condition with the highest worth rod stuck out. 
On May 30, 1983, the shutdown margin was estimated to be -0.35% dk/k. The 
control rod worth curves used in making this determination had been obtained 
by measurement on December 20, 1982. The core configuration, at that time, 
consisted of 24 fuel assemblies (a 5 x 5 array with one corner fuel assembly 
removed) of which 4 were partial assemblies containing control rods. The core 
was reflected on three sides by graphite and on one side by water. One 
graphite reflector assembly was displaced by a pneumatic conveyor facility. 
There were no other experiments or experimental facilities within 3 inches
of  the core.

Over the ensuing months, experiments were added to the reflector immediately 
adjacent to the core displacing either graphite or water. The negative 
reactivity effects of the experiments and fuel burnup were compensated from 
time to time by the addition of partial or complete fuel assemblies to one
row  of the grid plate. By May 25, 1983, core size had been increased to 27
fuel  assemblies (22 full fuel assemblies, 4 partial rodded assemblies, and
1  partial unrodded assembly).


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                                                 IN 84-21
                                                 March 28, 1984            
                                                 Page 2 of 2

Remeasurements of control rod worths conducted in June 1983, for the 27 
assembly core revealed the worths to be significantly reduced for two of the 
three control rods. Remeasurement of the reactivity worth of experiments 
produced values less negative than those used in the May 30, 1983, estimation 
of the shutdown margin.

From these measurements, taken in June 1983, it was concluded that the 
Technical Specification for shutdown margin had been seriously violated
during  May. On May 25, 1983, the reactor would have been supercritical with
a  reactivity between +0.20 and +O.26% dk/k in the xenon-free condition with
the  highest worth rod stuck out.

The incorrect control of the shutdown margin for the research reactor stemmed 
from two causes:

1. The practice of recalibrating control rods only when the location of the 
control rods was changed, rather than in response to a change in core 
configuration.

2. The practice of using reactivity values for experiments and additional
fuel  assemblies in configurations that are different from those in which the
values  were obtained.

To prevent similar occurrences, operators of research reactors are advised
to  review their practices and controls in this area of reactivity
accountability.

No written response to this information notice is required. If you have any 
questions about this matter, please contact the Regional Administrator of the 
appropriate NRC Regional Office, or this office.


Edward L. Jordan, Director
Division of Emergency Preparedness 
and Engineering Response
Office of Inspection and Enforcement

Technical Contact: R. M. Young, IE
                   (301) 492-7275

                   P. T. Burnett, Region II
                   (404) 242-2600

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