United States Nuclear Regulatory Commission - Protecting People and the Environment

Credit for Control Rods Without Scram Capability in the Calculation of the Shutdown Margin

                                  UNITED STATES
                          NUCLEAR REGULATORY COMMISSION
                             WASHINGTON D.C.  20555

                                January 26, 1989

                                   CAPABILITY IN THE CALCULATION OF THE 
                                   SHUTDOWN MARGIN


All holders of operating licenses or construction permits for test and 
research reactors.


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.


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.


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.  

.                                                            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

89-03          Potential Electrical          1/11/89        All fuel cycle and
               Equipment Problems                           major nuclear 

89-02          Criminal Prosecution of       1/9/89         All holders of a
               Licensee's Former President                  U.S. NRC specific
               for Intentional Safety                       license.

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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