United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 96-31: Cross-Tied Safety Injection Accumulators

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

                                        May 22, 1996


NRC INFORMATION NOTICE 96-31:  CROSS-TIED SAFETY INJECTION ACCUMULATORS


Addressees

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

Purpose

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to the potential for operation in an unanalyzed
condition with safety injection (SI) accumulators cross-tied.  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 in this information notice are not NRC requirements;
therefore, no specific action or written response is required.

Description of Circumstances

On March 8, 1996, the licensee for Indian Point Nuclear Generating Unit No. 3
(IP3) reported that the plant may have operated outside its design basis
because the safety injection accumulators had periodically been cross-tied for
short periods of time.  The IP3 technical specifications require the cross-
connection of the safety injection accumulators once every shift to perform a
channel check when an accumulator second pressure or level instrument channel
is inoperable.  The licensee had also cross-tied safety injection accumulators
together to sluice water or nitrogen from one accumulator to another to ensure
adequate water or nitrogen.  An evaluation by the licensee engineering staff
(confirmed by Westinghouse) shows that the plant may not be protected if
accumulators are cross-tied during some loss-of-coolant accidents because
nitrogen pressure is postulated to bleed off through the faulted loop to the
containment.  

Since the IP3 licensee report, several other licensees have reported that
their plant procedures also allow cross-connection of safety injection
accumulators, in some cases, all of the accumulators in order to equalize
pressure.  No other licensee has reported a requirement to perform this
operation.  The IP3 licensee has submitted an application to amend its
technical specification to remove the cross-connection requirement.  Other
licensees have taken administrative action to prohibit cross-connection of the
accumulators.  

Discussion

The safety injections accumulators are pressure vessels filled with borated
water and pressurized with nitrogen gas.  The accumulators are isolated from 

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                                                                     May 22, 1996
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the reactor coolant system cold legs by two check valves in series.  Should
reactor coolant system pressure fall below the accumulator pressure, the check
valves would open and borated water would be injected into the reactor coolant
system.  The accumulators function as passive engineered safety features and
perform a critical function in mitigating a loss-of-coolant accident.  As
stated in the IP3 final safety analysis report, the design capacity of the
accumulators is based on the assumption that flow from one of the accumulators
will spill onto the containment floor through the ruptured loop.  The flow
from the three remaining accumulators will provide water to reflood the core.

If two or more safety injection accumulators are cross-connected during a
postulated large-break loss-of-coolant accident, the nitrogen gas pressure of
the cross-connected accumulators on non-faulted loops will decrease because of
gas escaping through cross-connected lines to the accumulator in the broken
loop, through the ruptured pipe, and into the containment.  Licensee
calculations showed that the pressure of cross-connected accumulators on non-
faulted loops would decrease below the value assumed in the safety analysis
report.  The IP3 licensee, with confirmation from Westinghouse, infers that
the peak cladding temperature would exceed 1204 �C [2200 �F] using the design-
basis model, but calculations were not performed because fewer than three
accumulators injecting were not considered in their licensing-basis analyses.

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:  Frank Orr, NRR              
                     (301) 415-1815              
                     Internet:fro@nrc.gov

                     John Tappert, NRR
                     (301)415-1167
                     Internet:  jrt@nrc.gov

Attachment:  List of Recently Issued NRC Information Notices

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