United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 88-47: Slower-Than-Expected Rod-Drop Times

                                  UNITED STATES
                          NUCLEAR REGULATORY COMMISSION
                             WASHINGTON, D.C.  20555

                                  July 14, 1988

Information Notice No. 88-47:  SLOWER-THAN-EXPECTED ROD-DROP TIMES


All holders of operating licenses or construction permits for 
pressurized-water reactors. 


This information notice is being provided to alert addressees to a potential 
problem regarding slower-than-expected rod drop times.  It is expected that 
recipients will review the information for applicability 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.

Description of Circumstances:

In a letter dated May 5, 1988, the licensee of Arkansas Nuclear One Unit 2 
(ANO-2) informed the NRC that they were unable to meet the Technical Specifi-
cation regarding maximum drop time for control rods.  The licensee discovered 
the problem as a result of a change in the methodology used for measuring rod-
drop times.  The test was part of the startup physics testing after refueling.
The new test methodology requires the interruption of power to the rod drive 
mechanisms for all rods simultaneously by means of the reactor trip breakers.  
The previous methodology called for the interruption of power to each 
individual drive mechanism.  Simultaneous interruption of power to all drive 
mechanisms leads to a slower dissipation of stored energy in the holding 
coils, hence increasing the measured rod-drop time.  The new methodology is a 
more realistic representation of the safety function.  The licensee found it 
necessary to review all of the accident analyses in the Final Safety Analysis 
Report to ensure continued conformance with the licensing basis.


The rod-drop test at ANO-2 showed that the maximum drop time was 3.2 seconds, 
although the Technical Specification required less than 3.0 seconds.  The 
staff found that the actions taken by the licensee were acceptable for dealing 
with the changes to the safety margins.  The licensee subsequently submitted a 
request to amend the Technical Specification on an emergency basis.  Because 
problems similar to that at ANO-2 may occur at other plants, the staff would 
like to inform all licensees of this experience so that they may take 

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                                                                 July 14, 1988
                                                                 Page 2 of 2

action to avoid requesting amendments on an emergency basis in regard to this 
matter.  The staff intends to incorporate the lessons learned from the ANO-2 
experience in future reviews of accident analyses results, such as reload 

No specific action or written response is required by this information notice.
If you have any questions about this matter, please contact the technical 
contact listed below or the Regional Administrator of the appropriate regional 

                              Charles E. Rossi, Director
                              Division of Operational Events Assessment
                              Office of Nuclear Reactor Regulation

Technical Contact:  N. Prasad Kadambi, NRR
                    (301) 492-1153

Attachment:  List of Recently Issued NRC Information Notices
.                                                            Attachment
                                                            IN 88-47 
                                                            July 14, 1988 
                                                            Page 1 of 1

                             LIST OF RECENTLY ISSUED
                            NRC INFORMATION NOTICES 
Information                                  Date of 
Notice No._____Subject_______________________Issuance_______Issued to________

88-46          Licensee Report of            7/8/88         All holders of OLs
               Defective Refurbished                        or CPs for nuclear
               Circuit Breakers                             power reactors. 

88-45          Problems In Protective        7/7/88         All holders of OLs
               Relay and Circuit                            or CPs for nuclear
               Breaker Coordination                         power reactors. 

88-44          Mechanical Binding of         6/24/88        All holders of OLs
               Spring Release Device                        or CPs for nuclear
               in Westinghouse Type                         power reactors. 
               DS-416 Circuit Breakers 

88-43          Solenoid Valve Problems       6/23/88        All holders of OLs
                                                            or CPs for nuclear
                                                            power reactors. 

88-42          Circuit Breaker Failures      6/23/88        All holders of OLs
               Due to Loose Charging                        or CPs for nuclear
               Spring Motor Mounting Bolts                  power reactors. 

88-41          Physical Protection           6/22/88        All holders of OLs
               Weaknesses Identified                        or CPs for nuclear
               Through Regulatory Ef-                       power reactors. 
               fectiveness Reviews (RERs) 

88-40          Examiners' Handbook for       6/22/88        All holders of OLs
               Developing Operator                          or CPs for nuclear
               Licensing Examinations                       power reactors. 

88-39          LaSalle Unit 2 Loss of        6/15/88        All holders of OLs
               Recirculation Pumps With                     or CPs for BWRs. 
               Power Oscillation Event 

88-38          Failure of Undervoltage       6/15/88        All holders of OLs
               Trip Attachment on General                   or CPs for nuclear
               Electric Circuit Breakers                    power reactors. 

88-37          Flow Blockage of Cooling      6/14/88        All holders of OLs
               Water to Safety System                       or CPs for nuclear
               Components                                   power reactors. 
OL = Operating License
CP = Construction Permit 
Page Last Reviewed/Updated Friday, May 22, 2015