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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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