United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 87-56: Improper Hydraulic Control Unit Installation at BWR Plants

                                                        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
                                                            Page 2 of 3


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
                                                            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 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
Page Last Reviewed/Updated Tuesday, November 12, 2013