Credit for Control Rods Without Scram Capability in the Calculation of the Shutdown Margin
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON D.C. 20555
January 26, 1989
Information Notice No. 89-09: CREDIT FOR CONTROL RODS WITHOUT SCRAM
CAPABILITY IN THE CALCULATION OF THE
SHUTDOWN MARGIN
Addressees:
All holders of operating licenses or construction permits for test and
research reactors.
Purpose:
This information notice is being provided to alert addressees to potential
problems from allowing credit to be taken for the reactivity worth of control
rod(s) that do not have scram capability in the calculation of the shutdown
margin. It is expected that recipients will review the information for ap-
plicability to their facilities and consider actions, as appropriate, to
avoid similar problems. However, suggestions contained in this information
notice do not constitute NRC requirements; therefore, no specific action or
written response is required.
Background:
An NRC inspection at a non-power reactor facility identified a situation in
which a shutdown margin less than that required by the licensee's technical
specification was possible. The reactor involved has two types of control
rods: shim rods, which are magnetically coupled to their drive motors and
which are decoupled and rapidly inserted into the reactor core upon receipt
of a scram signal, and a regulating rod, which is fixed to its drive motor
and drives into the core upon receipt of a scram signal. The regulating
rod was used by the licensee, in addition to the shim rods, to demonstrate
compliance with the technical specification for the shutdown margin.
Discussion:
The shutdown margin is a measure of the reactivity necessary to provide confi-
dence that a reactor can be made subcritical by a predetermined amount by
means of the control and safety systems, starting from any permissible
operating con-dition. In the calculation of the shutdown margin, it is assumed
that the most reactive control device (i.e., the highest worth rod) is in its
most reactive condition and that the reactor will remain subcritical without
further operator action. The shutdown margin should also be essentially
instantaneously avail-able (within the technical specification limit on rod
drop time) upon receipt of a reactor scram signal. If the shutdown-initiating
event is a loss of facility electrical power, the non-scramming control rod(s)
which are fixed to their drive motors would not drive into the reactor core.
8901190445
. IN 89-09
January 26, 1989
Page 2 of 2
Therefore, if the reactivity worth of the non-scramming control rods(s)
is required to be instantaneously available to meet the shutdown margin
technical specification requirement, it may not be possible to make the
reactor subcritical in a controlled, safe manner.
Subsequent calculations have shown that the licensee did not need to include
the regulating rod in the calculations to meet the technical specification
requirement for shutdown margin. However, to ensure that the non-scrammable
control rod is not used in the calculation of the shutdown margin (the
technical specification would have allowed this), an amendment to the facility
operating license was requested by the licensee and approved by the NRC. The
technical specification states that the shutdown margin is based on scrammable
control rods only.
No specific action or written response is required by this information notice.
If you have any questions about this matter, please contact one of the
technical contacts listed below or the Regional Administrator of the
appropriate regional office.
Charles E. Rossi, Director
Division of Operational Events Assessment
Office of Nuclear Reactor Regulation
Technical Contacts: John Thompson, NRR
(301) 492-1175
Al Adams, NRR
(301) 492-1121
Attachment: List of Recently Issued NRC Information Notices
. Attachment
IN 89-09
January 26, 1989
Page 1 of 1
LIST OF RECENTLY ISSUED
NRC INFORMATION NOTICES
_____________________________________________________________________________
Information Date of
Notice No._____Subject_______________________Issuance_______Issued to________
89-08 Pump Damage Caused by 1/26/89 All holders of OLs
Low-Flow Operation or CPs for nuclear
power reactors.
89-07 Failures of Small-Diameter 1/25/89 All holders of OLs
Tubing in Control Air, Fuel or CPs for nuclear
Oil, and Lube Oil Systems power reactors.
Which Render Emergency Diesel
Generators Inoperable
89-06 Bent Anchor Bolts in 1/24/89 All holders of OLs
Boiling Water Reactor or CPs for BWRs
Torus Supports with Mark I steel
torus shells.
89-05 Use of Deadly Force by 1/19/89 All holders of OLs
Guards Protecting Nuclear for nuclear power
Power Reactors Against reactors.
Radiological Sabotage
89-04 Potential Problems from 1/17/89 All holders of OLs
the Use of Space Heaters or CPs for nuclear
power reactors and
test and research
reactors.
89-03 Potential Electrical 1/11/89 All fuel cycle and
Equipment Problems major nuclear
materials
licensees.
89-02 Criminal Prosecution of 1/9/89 All holders of a
Licensee's Former President U.S. NRC specific
for Intentional Safety license.
Violations
88-23, Potential for Gas Binding 1/5/89 All holders of OLs
Supp. 1 of High-Pressure Safety or CPs for PWRs.
Injection Pumps During a
Loss-of-Coolant Accident
89-01 Valve Body Erosion 1/4/89 All holders of OLs
or CPs for nuclear
power reactors.
_____________________________________________________________________________
OL = Operating License
CP = Construction Permit
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