United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 96-10 - IN 96-10: Potential Blockage by Debris of Safety System Piping which is not used During Normal Operation or Tested During Surveillances

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

                               February 13, 1996


NRC INFORMATION NOTICE 96-10:  POTENTIAL BLOCKAGE BY DEBRIS OF SAFETY SYSTEM
                               PIPING WHICH IS NOT USED DURING NORMAL
                               OPERATION OR TESTED DURING SURVEILLANCES


Addressees

All holders of operating licenses or construction permits for nuclear power
reactors.

Purpose

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to the potential for the blockage, by debris, of
safety system piping in boiling-water reactors (BWRs) or pressurized-water
reactors (PWRs).  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.

The source of the information contained in this information notice was the
incident reporting system (IRS) of the Organization for Economic Cooperation
and Development Nuclear Energy Agency.  The Spanish regulatory staff
authorized public dissemination of this information.

Description of Circumstances

During refueling outages at PWRs in Spain, containment sumps are usually
inspected visually to ensure that the sump, screens, and suction inlet pipes
in the emergency core cooling system (ECCS) are free of debris, and that the
sumps screens are not corroding.

In November 1993, the licensee of a PWR in Spain inspected its containment
sumps during outage surveillance.  The licensee did not consider it unusual
for the sumps to contain water because there is some leakage to the sump
during functional testing of a three-way valve which connects the ECCS borated
tanks with the containment sumps.  However, in this case, the individual
inspecting the sump believed that the water was unusually dirty, so he had it
siphoned off.  Once the water had been removed, he could see that three of the
four sumps had debris in the bottom below the suction pipe for the ECCS.  A
closer examination of the ECCS suction lines revealed that two of the four 


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ECCS lines taking suction from the sumps were partially blocked by debris. 
One of the two partially blocked lines had almost half the flow area of the
pipe blocked off; the other was blocked less.  It is believed that the debris 
had been there since the plant was commissioned, and that this event demon-
strated a significant failure of the surveillance program.  Figure 1 shows a
cross-sectional view of the sump and indicates where the debris was found.

Upon further review, the Spanish regulators noted that there are no technical
specification requirements to inspect the piping from the containment sumps to
the ECCS to ensure that it is clear of any obstruction by debris.  It was also
noted that many sections of piping on safety systems in both PWRs and BWRs are
only called upon to function during accident conditions, and are not used
during normal operation or tested during functional surveillance tests. 
Confidence in the operability of these pipes was based on the fact that they
had not been used since they were tested before plant startup.

In response to the event, Spanish regulators issued a generic letter which
asked their licensees to take four actions.  First, they were requested to
establish administrative controls, including a locking device, on the sump
hatch and on any other removable sump parts (PWRs only).  Second, they were
requested to inspect the containment sump and the piping between the sump and
the first ECCS valve which separates the part of the system that is
functionally tested from the part that is not (PWRs only).  Third, they were
requested to inspect the sump every refueling outage.  If more than 1 kilogram
of debris was found in the sump, licensees were then requested to reinspect
the piping between the sump and the functionally tested portion of the ECCS
(PWRs only).  And fourth, licensees were requested to identify piping segments
of safety systems through which flow does not occur, either during normal
operation or during surveillance testing, and to analyze whether there are
reasonable guarantees that no obstruction exists (BWRs and PWRs).  Examples of
the piping segments that would require this analysis are crossover piping from
the essential service cooling system to the auxiliary feedwater system (PWRs),
relief valve discharge piping (PWRs), low-pressure coolant injection suction
piping from the condensate storage tank (BWRs), and crossover connections
between low-pressure coolant injection trains (BWRs).

Discussion

The licensee in this case concluded that the safety significance of this event
was small because the partial blockage of the lines would not prevent the ECCS
from providing sufficient core cooling.  However, it was also noted that some
of the debris could have been entrained in the water flow and could have
detrimental effects on other parts of the system (e.g., pump and valve
components and heat exchangers).  The licensee initial corrective actions
(prior to issuance of the generic letter) included cleaning up the sumps and .                                                            IN 96-10
                                                            February 13, 1996
                                                            Page 3 of 4


suction lines, locking sump hatches, and establishing administrative controls. 
These corrective actions address the root cause of the presence of debris in
the sump and suction line.

It has been noted in previous NRC generic communications that debris could
prevent such safety systems as the ECCS from functioning reliably throughout a
design-basis accident.  These previous NRC communications have dealt with the
potential for debris to clog the ECCS strainers (operational debris, loss-of-
coolant-accident-generated debris, or a combination of both).  The debris
described in this information notice is another example of the potential
fordebris to impair the operation of safety systems.  Although the licensee in
this case has determined that the debris found in its sump was insufficient to
prevent the ECCS from performing its safety function, such debris could damage
individual components of the safety system (e.g., pumps), degrade system
performance, or (in combination with the debris generated during a loss-of-
coolant accident) could accelerate a loss of net positive suction head for the
ECCS pumps. 

Related Generic Communications

.     NRC Bulletin 93-02 and Supplement 1:  "Debris Plugging of Emergency Core
      Cooling Suction Strainers," dated May, 11, 1993, and February 18, 1994.

.     NRC Information Notice 92-85:  "Potential Failures of Emergency Core
      Cooling Systems Caused by Foreign Material Blockage," dated 
      December 23, 1992.

.     NRC Information Notice 92-71:  "Partial Plugging of Suppression Pool
      Strainers at a Foreign BWR," dated September 30, 1992.

.     NRC Information Notice 89-77:  "Debris in Containment Emergency Sumps
      and Incorrect Screen Configurations," dated November 21, 1989.

.     NRC Information Notice 88-87:  "Pump Wear and Foreign Objects in Plant
      Piping Systems," dated November 16, 1988.

.     NRC INFORMATION NOTICE 88-28:  "Potential for Loss of Post LOCA
      Recirculation Capability Due to Insulation Debris Blockage," dated 
      May 19, 1988.

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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.


                                          signed by

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

Technical contact:  Eric J. Benner, NRR
                    (301) 415-1171
                    Internet:ejb1@nrc.gov

                    Robert B. Elliott, NRR
                    (301) 415-1397
                    Internet:rbe@nrc.gov

Attachments:
1.  Figure 1:  Scheme of Containment Sump

(See File IN96017.WP1 for Figure 1 in Word Perfect Graphics)
Page Last Reviewed/Updated Thursday, November 21, 2013