Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
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.
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IN 88-76
September 19, 1988
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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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