Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
SSINS No.: 6835
IN 87-56
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
November 4, 1987
Information Notice No. 87-56: IMPROPER HYDRAULIC CONTROL UNIT
INSTALLATION AT BWR PLANTS
Addressees:
All holders of operating licenses or construction permits for boiling water
reactors (BWRs).
Purpose:
This information notice is being provided to alert addressees to a potential
problem that could affect the ability of the hydraulic control units (HCUs) to
control the positioning of the control rods in the event of an earthquake. In
addition, the potential for damage to the control rod drive (CRD) system
withdraw lines that exists under certain conditions could result in a small-
break loss-of-coolant accident in the HCU area. It is expected that
recipients will review this 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.
Background:
The CRD system controls the position of the control rods within the reactor
core either to change reactor core power or to rapidly shut down the reactor
(scram). The HCU is a major component of the CRD system that incorporates all
the hydraulic, electrical, and pneumatic equipment necessary to move one CRD
mechanism during normal or scram operations. This equipment, which includes
the accumulators, CRD insert lines, CRD withdraw lines, and scram valves, is
supported by the HCU frames.
If a sufficiently large number of HCU frame bolts are missing or loose, a Safe
Shutdown Earthquake (SSE) could result in damage affecting the ability of the
CRD system to control the positioning of the control rods. In addition,
damage to a CRD withdraw line could result in a small-break loss-of-coolant
accident in the area of the HCUs.
Description of Circumstances:
On May 4, 1986, at the Perry Nuclear Power Plant Unit 1, a hold-down bolt on a
CRD system HCU was found to be missing. Four other HCUs were observed with
hold-down bolt heads not in contact with the HCU frame. On May 15, further
investigation revealed that all of the HCUs had been installed with bolt
torque
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November 4, 1987
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values differing from the requirements of the seismic testing in the Environ-
mental Qualification Report. Also, the upper support for each of the 16
branch junction modules (BJMs) was erroneously attached to an HCU frame. (A
BJM is an enclosure containing rod position sensor cable terminations and
voltage regulating transformers for the Rod Control and Information System.
It is mounted to one of a group of HCUs which it serves.) During
installation, the BJMs were attached to the HCUs in accordance with a General
Electric Co. (GE) drawing, but the HCU seismic test configuration used for
equipment qualification did not include a BJM.
After the degraded condition of the HCUs at the Perry plant was discovered, GE
issued letters to all BWR owners regarding Potentially Reportable Condition
(PRC) 86-08, "Improper Hydraulic Control Unit Installation." The letters
informed the owners of the condition of the Perry HCU units. The letters
concluded that neither the missing or loose hold-down bolts nor the erroneous
BJM attachment observed at the BWR/6 constituted a safety problem during a
faulted event at that plant, nor would they at any other BWR because if the
remaining hold-down bolts were snug tight, they would enable the HCU to scram
the control rods. The GE letters did not address the possibility of damage to
a CRD withdraw line that is attached to the HCU. A ruptured CRD line would
constitute a small-break loss-of-coolant accident.
During an inspection at Pilgrim Nuclear Power Station on March 13, 1987 (in
response to GE Letter No. G-HK-6-326 dated October 22, 1986, regarding
improper HCU installation), Boston Edison Company (BECo) identified loose or
missing bolts at the top and base of 31 of 145 HCU frames. Also, BECo found
flat washers were not installed with the base bolts, which was contrary to the
installation drawing. The BECo engineering evaluation of the as-found condi-
tions concluded that design-basis operability of the HCUs could not be estab-
lished for loading conditions postulated for the SSE. BECo installed the
correct bolting to ensure the integrity of the HCU structure.
Discussion:
Although the GE letter, based on the condition of the HCUs at the Perry plant,
provides some assurance that a safety problem does not necessarily exist, the
experience at the Pilgrim plant demonstrates that some improperly installed
HCU configurations might not meet seismic design criteria. The GE letter
pointed out that the responsibility for implementing adequate HCU bolt
hold-down torque rests with the utility and GE recommended that HCU
installations be checked to ensure that:
1. The HCU mounting, whether with bolts or welding, is consistent with the
seismic qualification test configuration. The utility may wish to use
the actual HCU seismic qualification torque value to ensure adequate
tightness.
2. Where BJMs are incorporated, they are not attached directly to the HCU
frame above the floor, except in cases where the BJM has been considered
in the equipment qualification results.
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November 4, 1987
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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 regional
office.
Charles E. Rossi, Director
Division of Operational Events Assessment
Office of Nuclear Reactor Regulation
Technical Contact: Samuel MacKay, NRR
(301) 492-8394
Attachment: List of Recently Issued NRC Information Notices
