Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
NUCLEAR REGULATORY COMMISSION
OFFICE FOR INSPECTION AND ENFORCEMENT
WASHINGTON, D.C. 20555
November 22, 1977
IE Bulletin No. 77-06
POTENTIAL PROBLEMS WITH CONTAINMENT ELECTRICAL PENETRATION ASSEMBLIES
Description of Circumstances:
On October 3, 1977 Northeast Nuclear Energy Company reported to the NRC
Region 1 Office that two control valves installed inside containment at
Millstone Unit No. 2 demonstrated abnormal operational characteristics. The
licensee reported that an unexpected closure of a letdown flow stop valve
occurred. While investigating this problem, the normally closed safety
injection recirculation return line drain valve was found to be in the open
position. Investigation of these events revealed the cause for failure to be
electrical shorts between conductors within a containment low voltage
penetration assembly.
The licensee subsequently determined that the wiring for both of the valves
shared the same low voltage module in an electrical penetration. Electrical
tests by the licensee revealed that 15 of the 85 conductors in the suspect
connector module exhibited decreased insulation resistance between
conductors. Based on this finding, it is believed that an electrical path
between adjacent circuits in the connector module was established. This
resulted in spurious operation of the valves. Similar resistance checks
performed on the remaining low voltage modules within the affected
penetration assembly revealed 17 additional conductors with reduced
insulation resistances. All conductors with resistances less than 20 megohms
were disconnected and their circuits were reconnected through spare
conductors.
Examination of the three remaining low voltage penetration assemblies,
identified 7 additional conductors with resistances of less than 20 megohms.
Each of these circuits were also reconnected through a spare conductor.
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IE Bulletin No. 77-06 November 22, 1977
Investigation showed that the reduced insulation resistance was probably
caused by moisture accumulation within the penetration assembly together
with small fissures in the epoxy seals surrounding each conductor in the
module. The licensee believes that moisture penetrating these cracks reduced
the insulation resistance between adjacent conductors. To prevent further
degradation from moisture buildup within the penetration assemblies, the
licensee re-established a dry nitrogen pressure of 24 PSIG in the
penetrations.
Subsequently the licensee reported that a second event of a similar nature
occurred on October 14, 1977. In this instance the sample isolation valve
for the pressurizer surge line failed to close on command. Investigation
into this event indicated that electrical shorts between conductors due to a
moisture accumulation problem was the probable cause for valve misoperation.
The shorted wires were disconnected and the valve was de-energized in the
closed position.
In discussions on the issue with the licensee and the electrical penetration
vendor, General Electric Company, NRC staff determined that maintenance of
nitrogen pressure is essential to the integrity of both high and low voltage
penetration assemblies. The General Electric Company specifies in its
penetration assembly maintenance and operation manual that a 15 PSIG dry
nitrogen pressure should be maintained on low voltage units while 30 PSIG
should be maintained on high voltage units.
Action To Be Taken By Licensees Of All Power Reactor Facilities With An
Operating License:
Containment Electrical Penetrations - For safety related systems
1.0 Do you have containment electrical penetrations that are of the G. E.
Series 100, or are otherwise similar in that they depend upon an epoxy
sealant and a dry nitrogen pressure environment to ensure that the
electrical and pressure characteristics are maintained so as to ensure
the functional capability as required by the plant's safety analysis
report; namely, (1) to ensure adequate functioning of electrical
safety-related equipment and (2) to ensure containment leak tightness?
1.1 Have you experienced any electrical failures with this type of
penetration?
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IE Bulletin No. 77-06 November 22, 1977
2.0 For those penetrations referenced in Item 1 above, have you maintained
the manufacturer's prescribed nitrogen pressure at all times?
2.1 If you have operated the penetrations 1 without maintaining a nitrogen
pressure was any degradation of insulation resistance or anomolous
component operation detected?
2.2 If no measurements were taken during periods when nitrogen pressure was
not maintained, how were you assured that the insulation resistance was
not degrading or degraded?
2.3 How do you determine that circuit insulation resistances values are
satisfactorily maintained?
3.0 Is there a need, as determined by either the vendor or yourself, to
maintain penetrations pressurized during a LOCA?
3.1 What measures have you taken to ensure that penetrations of this type
will perform their design function under LOCA conditions? (design
reviews, analyses or tests)
3.2 Are the measures that provide this assurance adequate to satisfy the
Commission's regulations (GDC 4, Appendix A to Part 50; QA Criteria,
Appendix B to Part 50)
4.0 Provide your response to Items 1.0 through 3.2 above in writing within
10 days. In addition, provide an oral response by 4:00 p.m.(Local Time)
November 25, 1977. Responses should be submitted to the Director of the
appropriate NRC Regional Office. A copy of written responses should be
forwarded to the U. S. Nuclear Regulatory Commission, Office of
Inspection and Enforcement, Division of Reactor Operations Inspection,
Washington, D. C. 20555.
Approved by GAO, B180225 (R0072); clearance expires 7-31-80. Approval was
given under a blanket clearance specifically for identified generic
problems.
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