Information Notice No. 88-76: Recent Discovery of a Phenomenon Not Previously Considered in the Design of Secondary Containment Pressure Control

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

                               September 19, 1988


Information Notice No. 88-76:  RECENT DISCOVERY OF A PHENOMENON NOT 
                                   PREVIOUSLY CONSIDERED IN THE DESIGN OF 
                                   SECONDARY CONTAINMENT PRESSURE CONTROL 


Addressees: 

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

Purpose: 

This information notice is being provided to alert addressees to a recent 
discovery of a phenomenon not previously considered in the design of the 
secondary containment pressure control system, which could cause the secondary
containment pressure to rise above allowable values.  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 do not constitute NRC 
requirements; therefore, no specific action or written response is required. 

Description of Circumstances: 

In Licensee Event Report (LER) 87-065-00, dated November 10, 1987, Niagara 
Mohawk Power Corporation, licensee for Nine Mile Point, Unit 2 (NMP 2), noti-
fied the NRC that the NMP 2 secondary containment had not been maintained at 
the required subatmospheric pressure at higher building elevations because of 
a phenomenon not considered in the design of the secondary containment 
pressure control system. 

At NMP 2, the instruments that measure the differential pressure (delta P) 
between the interior of the secondary containment and the atmosphere sense 
pressure at an elevation of approximately 265 feet (near the bottom of the 
building).  The delta P at the upper portion of the building (at an elevation 
of 435 feet) is obtained by taking into account the interior and exterior 
static pressure gradients between the elevations.  The design of the system 
did not take into account the temperature-induced difference in the pressure 
gradients inside and outside the secondary containment.  Whenever the outside 
temperature is lower than the temperature maintained in the secondary contain-
ment, the vertical pressure decrease at the higher elevation outside the 
secondary containment is greater than the pressure decrease inside the secon-
dary containment because of the higher density of the colder air. 




8809130350
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                                                            IN 88-76
                                                            September 19, 1988
                                                            Page 2 of 3


The calculated values for reactor building delta P as a function of outside 
air temperature at an elevation of 435 feet demonstrate this effect (Table 1).
For outside temperatures lower than reactor building temperatures, pressure 
differentials between the inside and outside of the building at upper 
elevations were calculated to be less negative than the allowable value of 
-0.25 inch water gauge (WG).  The installed instrumentation was insufficient 
to accurately determine reactor building differential pressure at higher 
elevations.

Discussion: 

In a postulated accident, the secondary containment structure, which is nor-
mally maintained at a pressure lower than atmospheric, and supporting systems 
would collect and process radioactive material that may leak from the primary 
containment.  Whenever an outward positive pressure exists across the 
secondary containment boundary, the leakage prevention function of the 
secondary containment is assumed to be negated and all primary containment 
leakage is assumed to be released directly into the environment.  Under these 
circumstances, the offsite dose limits stated in 10 CFR Part 100 for fission 
product releases from postulated accidents could be exceeded. 

As stated in Branch Technical Position (BTP) CSB 6-3 (NUREG-0800), a 
"positive" pressure in this regard is defined as any pressure greater than 
-0.25 inch WG, to conservatively account for wind loads and the uncertainty in
pressure measurements.  In addition to these factors, the problems at NMP 2 
show that the effect of delta P gradients caused by low outside air 
temperatures can be an important factor in the design of the secondary 
containment pressure control.  Because its density is higher, cold air exerts 
more force per increment of elevation than warm air.  Thus, while maintaining 
the -0.25 inch WG differential pressure in lower portions of the secondary 
containment, the delta P decreases at higher elevations and becomes "positive"
as demonstrated in Table 1.  It should also be noted that this effect 
increases as humidity increases in the reactor building. 

Subsequent to discovery of this phenomenon, the licensee took several cor-
rective actions.  The setpoint on the delta P pressure transmitters was reset 
from 0.33 inch to 0.76 inch vacuum WG.  The licensee's analysis indicated that
this would assure a delta P of at least 0.25 inch vacuum WG at upper 
elevations for a temperature differential of 85xF between reactor building 
interior and exterior.  A modification was also initiated to relocate the 
delta P elements to the roof of the reactor building.  After completion of 
this modification the delta P setpoint would be reset to the original value of
0.33 inch vacuum WG.  With implementation of this modification, a minimum 
delta P of 0.25 inch vacuum WG would be established in the reactor building.  
A considerably larger delta P in the lower elevations of the reactor building 
would occur on days with low outside temperature. 

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                                                            IN 88-76
                                                            September 19, 1988
                                                            Page 3 of 3


No specific action or written response is required by this information notice.
If you have any questions about this matter, please contact the technical 
contact listed below or the Regional Administrator of the appropriate NRC 
regional office. 




                              Charles E. Rossi, Director
                              Division of Operational Events Assessment
                              Office of Nuclear Reactor Regulation


Technical Contact:  Jack Kudrick, NRR
                    (301) 492-0871


Attachments:  
1.  Table 1 - Effect of Outside Temperature on Reactor
      Building Delta P
2.  List of Recently Issued NRC Information Notices
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                                                            Attachment 1
                                                            IN 88-76
                                                            September 19, 1988
                                                            Page 1 of 1


                                     TABLE 1


            EFFECT OF OUTSIDE TEMPERATURE ON REACTOR BUILDING DELTA P


              OUTSIDE TEMPERATURE           REACTOR BUILDING DELTA P*
                     (xF)                   (inches water gauge [WG])

                    85                                 -0.25
                    60                                 -0.17
                    40                                 -0.10
                    20                                 -0.03
                     0                                 +0.06
                   -20                                 +0.15








*Reactor building delta P at elevation 435 feet with -0.25 inch (water gauge)
 measured at elevation 265 feet; building temperature at 85xF, 0% humidity. 
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