United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 95-03, Supplement 1: Loss of Reactor Coolant Inventory and Potential Loss of Emergency Mitigation Functions While in a Shutdown Condition

                                UNITED STATES 
                         NUCLEAR REGULATORY COMMISSION
                     OFFICE OF NUCLEAR REACTOR REGULATION
                          WASHINGTON, D.C. 20555-0001

                                 March 25, 1996


NRC INFORMATION NOTICE 95-03, SUPPLEMENT 1:  LOSS OF REACTOR COOLANT INVENTORY 
                                             AND POTENTIAL LOSS OF EMERGENCY 
                                             MITIGATION FUNCTIONS WHILE IN A   
                                             SHUTDOWN CONDITION


Addressees

All holders of operating licenses or construction permits for PWR power
plants.

Purpose

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice supplement to alert addresses to insights from additional analysis
related to reactor coolant system drain-down events with the potential for
adversely impacting accident-mitigation capability.  It is expected that
recipients will review the information in this supplement in conjunction with
Information Notice 95-03 for applicability to their facilities and consider
actions, as appropriate, to avoid similar problems.  However, suggestions
contained in this information notice supplement are not NRC requirements;
therefore, no specific action or written response is required.

Background

Information Notice 95-03, "Loss of Reactor Coolant Inventory and Potential
Loss of Emergency Mitigation Functions While in a Shutdown Condition," issued
on January 18, 1995, discusses the Wolf Creek drain-down event of    
September 17, 1994.  In that event, operators were attempting to reborate
residual heat removal train B while at the same time, maintenance personnel
were repacking a residual heat removal train A to train B crossover isolation
valve.  Train B is reborated by recirculating water through a loop that
contains the residual heat removal system piping, the refueling water storage
tank, a containment spray pump, a manual refueling water storage tank
isolation valve, and a residual heat removal system crossover line.  

When the storage tank isolation valve was opened for the reboration process
and the train A to train B crossover isolation valve was opened for stroke
testing, a drain-down path was inadvertently created from the reactor coolant
system to the refueling water storage tank.  As a result, an unintentional
reactor coolant system flow path was created allowing approximately 
35,000 liters (9200 gallons) of reactor coolant to transfer to the refueling
water storage tank.  If the drain-down had not been promptly terminated, the
operability of the emergency core cooling system would have been compromised. 
Also, reactor coolant system water flashing to steam in the piping or in the
refueling water storage tank would likely have created conditions conducive to
water hammer.  

9602050208.                                                            IN 95-03, Supp. 1
                                                            March 25, 1996
                                                            Page 2 of 3


This information notice supplement discusses additional insights that have
been gained from analyses of the Wolf Creek event.  

Discussion

Licensee and staff analyses of the event assumed failure to isolate the 
drain-down path.  If this were to occur, within 5 minutes, reactor vessel
water level could drain to the bottom of the hot leg and as a consequence the
operating residual heat removal pump would lose suction, cavitate, and fail. 
Continued boil-off would result in uncovering of the core in less than one
hour.  If the event were to occur at the beginning of mode 4, at a higher
reactor coolant temperature and pressure, the transient could evolved even 
faster.  

Failure to quickly isolate the flow path could result in the refueling water
storage tank suction header filling with steam which would continually
discharge into the refueling water storage tank.  This steam could lead to
water hammer events with the potential for mechanical damage to pump
components, piping, and tank structural components.  Even if the drain-down
path is isolated, if sufficient steam ingress has occurred, a potential
continues to exists for water hammer and mechanical damage to components.  In
addition, there exists the potential for containment bypass through the
refueling water storage tank. 

Considering the size of the header and the geometrical arrangement of that
pipe (i.e., no U or inverted U arrangements) it is estimated that, after
drain-down isolation, water from the refueling water storage tank will flow
back down into the header.  However, adequate suction may or may not be
available to all emergency core cooling system pumps because of steam or air
pockets in the intakes of individual pumps.  Therefore, even with isolation,
the risk of pump damage could remain for some time.  

Plants could be susceptible to events of this nature while operating the
shutdown cooling system when the reactor coolant system is at more than 121 �C
(250 �F).  These conditions exist shortly after the plant is shut down for
refueling, maintenance, or a forced outage.  

A special NRC report dated March 1995 and titled "Reactor Coolant System
Blowdown at Wolf Creek on September 17, 1994" (AEOD/S95-01), identified 
19 loss-of-coolant events that have occurred at reactors during shutdown.  Of
these 19 events, only 2 have taken place at temperatures and pressures
sufficient to result in voiding refueling water storage tank piping.  Con-
sidering PWR operating experience, the staff estimated the initiating event
frequency may be equal to or greater than 1E-3 per reactor year.  The
initiating event frequency and the heavy dependence upon short term operator
action, highlight the importance of careful planning, accuracy of adminis-
trative procedures, and disciplined adherence to those procedures.    
.                                                            IN 95-03, Supp. 1
                                                            March 25, 1996
                                                            Page 3 of 3


This information notice supplement requires no specific action or written
response.  If you have any questions about the information in this supplement,
please contact the technical contact listed below or the Office of Nuclear
Reactor Regulation project manager.          


                                          signed by

                                    Dennis M. Crutchfield, Director
                                    Division of Reactor Program Management
                                    Office of Nuclear Reactor Regulation

Technical contact:  Lambros Lois, NRR
                    (301) 415-3233
                    internet:lxl1@nrc.gov
Page Last Reviewed/Updated Monday, November 18, 2013