Information Notice No. 89-73: Potential Overpressurization of Low Pressure Systems

                                UNITED STATES
                           WASHINGTON, D.C.  20555

                              November 1, 1989

                                   PRESSURE SYSTEMS 


All holders of operating licenses or construction permits for nuclear power 


This information notice is intended to alert addressees of the potential for
overpressurization of low pressure systems during testing of valves 
connecting these low pressure systems to higher pressure systems.  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: 

On September 5, 1989, the McGuire Unit 2 reactor was being returned to 
service following a refueling outage.  The reactor coolant system was at 
140 F and 325 psig (Mode 5).  The B-train of the residual heat removal 
system (RHRS) was in service.  Plant personnel were performing a required 
stroke test procedure on the A-train containment spray pump suction valve 
(NS18A).  The function of valve NS18A is to open during the recirculation 
phase of a loss-of-coolant accident (LOCA) to permit the A-train containment 
spray pump to take suction from the containment sump.  The plant has two 
containment sump suction lines, each of which serves a containment spray 
pump and an RHR pump for each of the two independent engineered safety 
feature trains.  The NS18A valve is interlocked so that it will not open 
unless the refueling water storage tank is isolated (by closing valve NS20A) 
from the containment spray system and the containment sump isolation valve 
is opened.  The containment sump was kept isolated for the duration of the 
test by bypassing the valve interlock in accordance with procedures.  Also 
in accordance with procedure, valve NS20A was closed to isolate the 
refueling water storage tank.  See attachment 1. 

The procedure for stroke testing the NS18A valve had undergone the normal 
reviews and approvals by utility personnel.  The plant staff did not realize 
that opening the NS18A valve would pressurize the containment spray system 
with reactor system pressure from the operating RHR loop nor did the test 
procedure contain an adequate caution.  The suction piping of the A and B 
RHR trains is normally cross-connected and was cross-connected at the time 
the NS18A valve was cycled.  The containment spray system is designed for 
220 psig, whereas the RHR system is 


                                                            IN 89-73 
                                                            November 1, 1989
                                                            Page 2 of 2 

designed for 450 psig.  When the NS18A valve was stroked open, the 
containment spray system was pressurized thus causing the relief valve 
upstream of the containment spray pump to open and discharge to the 
pressurizer relief tank.  The capacity of the relief valve was not 
sufficient to prevent overpressurization of the containment spray system, 
which caused failure of the containment spray heat exchanger bottom flange 
gasket.  The containment spray discharge header was isolated so that 
containment spray did not take place.  Operators quickly responded to the 
loss of reactor coolant by closing the NS18A valve.  An estimated 2200 
gallons of reactor coolant was lost through the failed containment spray 
heat exchanger during the approximately 1.5 minutes the NS18A valve was 
open.  Licensee personnel inspected the containment spray system piping not 
including the heat exchanger room.  Therefore, the licensee did not detect 
the failed heat exchanger gasket.  After the NS18A valve was closed, the 
containment spray system was aligned to take suction from the refueling 
water storage tank by opening the NS20A valve in accordance with procedures.  
With the NS20A valve opened, water from the refueling water storage tank 
began to flow into the containment spray system and onto the floor of the 
auxiliary building through the failed bottom flange gasket of the spray heat 
exchanger.  The leak continued for about 1 hour before being detected and 
isolated.  The licensee estimates that a combined total of 10,000 to 15,000 
gallons of reactor coolant water and water from the refueling water storage 
tank leaked through the failed gasket. 

The licensee revised the surveillance testing procedures for the containment
spray suction valve to caution that stroke testing should not be performed 
with the associated RHR train pressurized above 100 psig.  Testing 
procedures for valves in other interconnecting lines were examined to ensure 
that they contained proper precautions.  

This information notice requires no specific action or written response.  If 
you have any questions about this matter, please contact one of the 
technical contacts listed below or the appropriate NRR project manager. 

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

Technical Contacts:  Caudle Julian, RII
                     (404) 242-5585

                     Sammy Diab, NRR
                     (301) 492-1075

                     Walton Jensen, NRR
                     (301) 492-1190

1.  Figure - McGuire Unit 2 RHR and Containment Spray Systems 
2.  List of Recently Issued NRC Information Notices


Page Last Reviewed/Updated Friday, May 22, 2015