Information Notice No. 96-27: Potential Clogging of High Pressure Safety Injection Throttle Valves During Recirculation

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

                                  May 1, 1996



All holders of operating licenses or construction permits for pressurized
water reactors (PWRs).


The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to potential clogging of high pressure safety
injection (HPSI) throttle valves during the recirculation phase of a design-
basis loss-of-coolant accident because of restrictive flowpaths in the valves. 
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

Description of Circumstances

On February 20, 1996, Northeast Utilities (the licensee) shut down Millstone
Unit 2 because of the discovery of a potential common-mode failure mechanism
affecting the HPSI system.  

During a review of an Institute of Nuclear Power Operations (INPO) Operational
Experience report concerning potential clogging of emergency core cooling
system (ECCS) throttle valves at the Diablo Canyon Nuclear Power Plant, the
licensee determined that eight throttle valves in the HPSI injection lines in
Unit 2 were susceptible to the failure mechanism identified in the INPO
report.  The failure mechanism described in the INPO report was the potential
for clogging of ECCS throttle valves because of entrained debris that could
pass through the containment sump strainers but not be able to pass through
the throttle valves.  The licensee based its susceptibility determination on
the fact that the openings in the containment sump strainers were 0.47 cm
[0.187 inch] and the minimum dimension within the valve flow-path was 0.32 cm
[0.125 inch].  The throttle valves cannot be remotely actuated and are
inaccessible during a design-basis accident.


Because the containment sump strainer openings are 0.47 cm [0.187 inch] and
the most restrictive dimension through which flow must pass within the
throttle valve is 0.32 cm [0.125 inch], the potential exists for particles 

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of sizes between these dimensions to pass through the containment sump
strainers, clog the throttle valves, and cause a partial or complete loss of 
post-accident HPSI flow during the recirculation phase of an accident.  

At Millstone Unit 2, the normal lineup for the recirculation phase is such
that all recirculation flow passes through the HPSI system.  The licensee has
adopted this arrangement because of potential structural and vibrational
loading of the LPSI system during recirculation operation.

The licensee is evaluating this condition for a more thorough determination of

Similar Events

The safety assessment performed by Pacific Gas and Electric for the Diablo
Canyon plant indicated that the LPSI system, which does not have as
restrictive a flowpath, can provide both cold-leg and hot-leg flow during the
recirculation phase of a design-basis accident.  Therefore, core cooling would
not be compromised even if no flow were able to pass through the HPSI system. 
Also, Pacific Gas and Electric has indicated that, because of the small
difference in size between the containment sump strainers and the minimum
throttle valve dimension, debris that could pass through the sump strainers
that could be larger than the throttle valve minimum dimension would likely be
fragmented as it passed through the LPSI and HPSI pumps, and that high
differential pressure at the valve would likely force the debris through the

The safety assessment performed by Northeast Utilities for another unit,
Millstone Unit 3, indicated that the ECCS pumps in the recirculation flow path
will pulverize any material that passes through the sump screens, preventing
clogging of the throttle valves.  Also, the licensee stated that the flow
velocity at the sump strainer will be approximately  0.05 m/sec [0.15 ft/sec]
during a large-break loss-of-coolant accident.  The licensee has stated it
expects that at this flow velocity debris would settle out and not be
transported through the sump strainers.  In addition, the licensee stated that
even if two of the eight throttle valves become clogged, the system would
still perform its safety function of providing long-term cooling to the
reactor core.

On the basis of its review of the information provided by the licensee of
Millstone Unit 3 and as discussed above, the NRC staff concluded that fragile
debris entering the sump screens is likely to be fragmented by the charging
pumps and the HPSI pumps so that there is reasonable assurance that this
debris is unlikely to clog the throttle valves at Millstone Unit 3.  As this
conclusion is based on specific design features at Millstone Unit 3, it may
not be applicable to other plants.

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                                                            May 1, 1996
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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 or the appropriate Office of
Nuclear Reactor Regulation (NRR) project manager.

                                          signed by

                                    Brian K. Grimes, Acting Director
                                    Division of Reactor Program Management
                                    Office of Nuclear Reactor Regulation

Technical contacts:  Eric J. Benner, NRR
                     (301) 415-1171

                     Chu-Yu Liang, NRR
                     (301) 415-2878

Page Last Reviewed/Updated Thursday, March 25, 2021