United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 96-05: Partial Bypass of Shutdown Cooling Flow from the Reactor Vessel

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

                            January 18, 1996


NRC INFORMATION NOTICE 96-05:  PARTIAL BYPASS OF SHUTDOWN COOLING FLOW FROM
                             THE REACTOR VESSEL


Addressees

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

Purpose

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to potential problems associated with monitoring
reactor coolant temperature while operating in the shutdown cooling mode of
the residual heat removal system.  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 are not NRC requirements; therefore, no specific
action or written response is required.

Description of Circumstances

On July 8-9, 1995, partial bypass of shutdown cooling flow from the reactor
vessel occurred at the Hope Creek Nuclear Station.  With residual heat removal
pump B running, the recirculation pump discharge valves for both A and B
recirculation loops were stroked to prevent thermal binding (Figure 1, which
for clarity shows only recirculation and residual heat removal loops B). 
However, the A discharge valve still became thermally bound.  To prevent the B
discharge valve from thermally binding, the licensee partially opened the B
discharge valve.  Because recirculation pump discharge valve B was open, some
of the shutdown cooling flow bypassed the reactor vessel and went through
recirculation loop B.  This reduced shutdown cooling flow through the core and
increased core temperature until boiling occurred and the reactor vessel
started to pressurize.  Plant operators were unaware of the heatup because the
bypass flow caused indicated water temperature, measured at the inlet to the
residual heat removal heat exchanger, to be less than the unmeasured reactor
vessel exit water temperature at the recirculation outlet nozzle.  The
temperature of water exiting the reactor vessel is normally lower than the
core exit temperature at the top of the active fuel due to internal bypass
flows.  The reduced shutdown cooling flow exacerbated this condition.  Two
transitions from cold to hot shutdown occurred without being recognized by the
operating crew.  The event was terminated when the licensee closed
recirculation pump discharge valve B and restarted residual heat removal
pump B.


9601110058.                                                        IN 96-05
                                                        January 18, 1996
                                                        Page 2 of 3


Discussion

To prevent thermal binding during plant cooldowns, Hope Creek operating
procedures specified that the recirculation pump discharge valves be
alternately opened and closed.  When the recirculation pump discharge valve is
stroked open and then closed in a short time, the effect on shutdown cooling
is negligible.  During this event, however, plant operators deviated from the
operating procedure and left the recirculation pump discharge valve partially
open.  As a result, part of the shutdown cooling flow was diverted away from
the reactor vessel, significantly reducing core flow.  After this event, the
licensee decided not to stroke the recirculation pump discharge valves and
removed this step from the operating procedures.

The reactor vessel level during shutdown operation is important in maintaining
cold shutdown conditions.  Hope Creek operating procedures specified that
reactor vessel water level be maintained about 4.6 m [15 ft] above the top of
the active fuel, a level corresponding to the water spillover elevation in the
steam separators.  The significant flow reduction through the core allowed the
coolant in the core region to heat up and the reactor vessel to pressurize. 
The licensee later determined that reactor vessel level should be maintained
above the elevation of the steam separator spillover point to ensure adequate
core cooling during shutdown conditions.  Maintaining this level would allow
natural circulation within the reactor vessel if the shutdown cooling system
was not available.  The licensee revised the operating procedures to specify
that reactor vessel water level be maintained above the bottom of the
predryers on the steam separators, about 6.1 m [20 ft] above the top of the
active fuel, during cold shutdown operation.  

Valid core outlet temperature information was not available during most of
this event.  During the event, the residual heat removal system pumps were
stopped some of the time for surveillance testing of the residual heat removal
system valves.  During such times, measurements were not available to indicate
the temperature of water exiting from the reactor vessel, normally taken as
the temperature of water at the residual heat exchanger.  At other times, the
bypass flow through the open recirculation pump discharge valve caused the
measured water temperature to be lower than the temperature of water exiting
the reactor vessel.  Finally, significant bypass flow via inactive jet pumps
inside the reactor vessel meant that the temperatue of water exiting the
reactor vessel was not representative of the temperature of water exiting the
core.  (The last is true even during normal shutdown operation unless all jet
pumps are active.)  Other temperature indications, such as the reactor vessel
metal and reactor water cleanup system inlet temperature, do not represent
water temperature exiting the core.  As a consequence, water in the core
apparently reached saturation temperature faster than anticipated when the
residual heat removal system pumps were stopped because the core was initially
hotter than shown by available instrumentation.  At times when a residual heat
removal pump was running, the jet pump bypass flows led to a temperature
indication lower than the core exit temperature.  Both situations led to
unanticipated and initially unrecognized pressurization.
.                                                        IN 96-05
                                                        January 18, 1996
                                                        Page 3 of 3


The licensee revised operating procedures to provide additional guidance for
monitoring reactor vessel pressure and vessel metal temperature when the
shutdown cooling system is not in service.  This guidance includes considering
pressurization of the reactor vessel as a transition to the hot shutdown
condition.

Related Generic Communication

Information Notice 93-45, "Degradation of Shutdown Cooling System
Performance," June 16, 1993

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


                                 signed by

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

Technical contacts:  Robert Summers, Region I
                    (609) 935-5373
                    Internet:rjs@nrc.gov

                    Scott Morris, Region I
                    (609) 935-5373
                    Internet:sam1@nrc.gov

                    C. Vernon Hodge, NRR
                    (301) 415-1861
                    Internet:cvh@nrc.gov

Attachments:  
1.  Figure 1.  Hope Creek Shutdown Cooling System with Open              
     Recirculation Pump Discharge Valve
2.  List of Recently Issued NRC Information Notices

(See File IN96005.WP1 for Figure 1 in Word Perfect Graphics)
Page Last Reviewed/Updated Thursday, November 21, 2013