Information Notice No. 97-09: Inadequate Main Steam Safety Valve (MSSV) Setpoints and Performance Issues Associated with Long MSSV Inlet Piping

                                 UNITED STATES
                         NUCLEAR REGULATORY COMMISSION
                          WASHINGTON, D.C. 20555-0001

                                March 12, 1997

                                 WITH LONG MSSV INLET PIPING


All holders of operating licenses or construction permits for nuclear power


The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert licensees to the recent staff findings related to improper
main steam safety valve (MSSV) setpoints and MSSV performance issues
associated with long inlet piping.  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  

The licensee for Millstone Unit 2, a Combustion Engineering pressurized-water
reactor (PWR), reviewed the calculations used to determine MSSV setpoints to
assure that peak main steamline pressure did not exceed the allowable
pressure.  The licensee determined that the calculations may be inadequate
because the dynamic pressure loss between the main steamline and the MSSVs was
not modeled.  This stretch of piping is not modeled in the licensee's design-
basis event transient analysis.  The consideration of the dynamic pressure
drop in the piping would reduce the relieving capacity of the MSSVs. 
Therefore, the omission resulted in an underprediction of the peak main
steamline pressure by a relatively significant amount and also resulted in the
potential for the calculated peak pressure to exceed 110 percent of the design
pressure for the main steam system.  In addition, significant dynamic pressure
drops resulting from long inlet piping could cause unstable MSSV performance.

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                                                              March 12, 1997
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The peak main steam pressure for PWRs generally occurs when a main steam
isolation valve closure or a turbine trip is postulated.  Typically, as much
as 100 percent of the full main steam flow can be relieved through the MSSVs
following one of these events.  These high flowrates can create significant
dynamic (i.e., frictional and acoustic) pressure drops.

At Millstone Unit 2, 8 individual code safety relief valves are attached to
each of the 2 main steamlines with approximately 6.1 m [20 feet] of 15.24-cm
[6-inch] piping.  The full-flow pressure drop through this 6.1 m [20 ft] of
piping could be as high as 689 kPa [100 psi].  As a result of not modeling the
stretch of piping for each MSSV, the actual discharge capacity of the MSSVs
was overpredicted and the peak main steamline pressure was underpredicted. 
The relatively long stretch (approximately 6.1 m [20 feet]) of relatively
small (15.24-cm [6-inch]) piping between the main steamlines and the MSSVs
contributed to the magnitude of the underprediction of the peak pressure;
however, the dynamic pressure loss from all stretches of piping should be
accounted for in the analysis.  For plants that do not have long stretches of
narrow piping or have a large manifold, the MSSVs will not have as large a
pressure drop, however, the calculations may still be affected significantly
depending on the actual piping configuration in the plant.

In addition, long MSSV inlet piping may also affect the stability of the
MSSVs.  If dynamic pressure drops are great enough, the valve disks may
chatter because (after the valves open) the valve inlet pressures will
immediately drop below the valve reseating pressures.  Because significant
excess system pressure has not been relieved, the valves reopen and the
chattering cycle would continue.

Depending on the specific plant, this analysis can be performed by the
architect engineer, the nuclear steam supply system vendor, the fuel vendor,
or the licensee.  For example, the fuel vendor may supply the transient
analysis; however, the system pressure losses that are inputs to the transient
analysis are frequently provided by the architect engineer.  As a result, it
is important that controls are in place such that data communication across
organizational interfaces include all pressure losses in the transient

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                                                              March 12, 1997
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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 M.M. Slosson

                                      Thomas T. Martin, Director
                                      Division of Reactor Program Management  
                                      Office of Nuclear Reactor Regulation

Technical contacts:  Christopher P. Jackson, NRR      Charles G. Hammer, NRR
                     (301) 415-2947                   (301) 415-2791
                     E-mail:             E-mail:

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

Page Last Reviewed/Updated Friday, May 22, 2015