Information Notice No. 84-21: Inadequate Shutdown Margin
SSINS No.: 6835
NUCLEAR REGULATORY COMMISSION
OFFICE OF INSPECTION AND ENFORCEMENT
WASHINGTON, D.C. 20555
March 28, 1984
Information Notice No. 84-21: INADEQUATE SHUTDOWN MARGIN
All nuclear research reactor facilities holding an Operating License (OL).
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).
March 28, 1984
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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
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
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
P. T. Burnett, Region II
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