United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 90-78: Previously Unidentified Release Path from Boiling Water Reactor Control Rod Hydraulic Units

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

                              December 18, 1990


Information Notice No. 90-78:  PREVIOUSLY UNIDENTIFIED RELEASE PATH FROM 
                                   BOILING WATER REACTOR CONTROL ROD 
                                   HYDRAULIC UNITS


Addressees: 

All holders of operating licenses or construction permits for boiling water 
reactors (BWRs).

Purpose:

This information notice is intended to alert addressees to potential 
problems pertaining to a previously unidentified release path from the 
control rod drive hydraulic systems in boiling water reactors that may lead 
to design basis accident radiation doses significantly exceeding the values 
specified in the Final Safety Analysis Report.  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 June 1989, a design review for unmonitored release paths at Washington 
Nuclear Power Plant Unit 2 resulted in the discovery of a previously 
unidentified radiation release path in the control rod drive hydraulic 
system.  This path is postulated to result from the following sequence of 
conditions.  The two control rod drive pumps are shut down following a 
design basis accident.  There is a break outside of the reactor building 
(secondary containment) in the non-seismically qualified piping or tankage 
to which the control rod hydraulic system is connected (see Figure 1).  
Reactor coolant leaks past the double seals in any of the 185 control rod 
drives and the valves in their associated hydraulic control units.  The 
leakage flows back through one or more of the four headers connecting each 
of the 185 hydraulic control units to the common control rod drive (CRD) 
pump header.  The leakage then flows through the CRD pump header and the 
control rod drive pumps to the break or the condensate storage tank located 
outside of the reactor building. 

Much of the pathway to the condensate storage tank lies outside of the 
reactor building and includes piping that is not seismically qualified.  In 
addition, this piping passes close to the air intakes for the control room 
ventilation system.  Consequently, a failure of the supply piping for the 
drive pumps 



9012120148 
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during an accident would not only greatly increase the potential external 
release, but could increase the radiation dose to the control room 
operators.  General Electric-Nuclear Energy performed calculations for the 
newly postulated release path for a design basis accident with concurrent 
failure of the non-seismically qualified supply piping for the control rod 
drive system at Washington Nuclear Power Plant Unit 2.  The calculations 
incorporated the source terms specified in Regulatory Guide 1.3 and assumed 
that 10% of the iodine would escape as a gas from the liquid release.  These 
calculations produced a 30-day thyroid dose for control room operators of 
121 rem per gpm of leakage, a 30-day thyroid dose at the outer boundary of 
the low-population zone of 86 rem per gpm of leakage, and a 2-hour thyroid 
dose at the exclusion zone boundary of 36 rem per gpm of leakage.  A generic 
communication discussing this concern was sent by General Electric-Nuclear 
Energy to each BWR utility in July 1989.

Discussion:

For the control rod hydraulic systems at General Electric boiling water 
reactors, the inboard isolation for the primary containment is provided by 
the double seals in the control rod drives, and the outboard isolation for 
the primary containment is provided by valves within the hydraulic control 
units.  However, past leak tests of the rod drive seals that were performed 
by General Electric produced a maximum of 5 gpm per drive.  Leakage from the 
hydraulic control units can also be significant.

As shown in Figure 1, four paths lead from each of the 185 hydraulic control 
units to the common CRD pump header.  Three of these paths, the accumulator 
charging header, the drive header and the cooling header include check 
valves to prevent the return of water from the hydraulic units.  In 
addition, water escaping through the accumulator charging header must leak 
through the insert side scram valve, and water escaping through the drive 
header must leak through one of the directional control valves.  However, 
the check valve in the exhaust header is oriented so as to permit the flow 
of water back to the CRD pump header.  The exhaust water then flows via the 
CRD pump header back to the reactor vessel (or, as at Washington Nuclear 
Power Plant Unit 2, to the reactor water cleanup system) along with the 
excess pump flow.  Therefore, only one normally closed valve prevents water 
that is leaking out of each of the 185 control rod drives from returning 
through the associated exhaust header to the CRD pump header.  During 
startup testing at Limerick Unit 1, and at Susquehanna Unit 1, the total 
leakage from all of the hydraulic control units combined was measured at 5 
gpm and 11 gpm, respectively.  Both of these reactors include additional 
check valves at the discharge of the control rod drive pump (area A in 
Figure 1).  A partial audit by the NRC staff indicates that many of the 
newer BWR plants have check valves installed in the discharge pipe of the 
control rod drive pumps.  However, this audit also showed that other BWR 
plants, mostly the earlier ones, did not have such check valves.  The 
control rod drive system for BWR/6 plants is designed with a testable check 
valve and a motor-operated isolation valve.  Therefore, this pathway is 
applicable to pre-BWR/6 plants only.

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                                                       IN 90-78
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Combining the General Electric dose calculations for the postulated path 
with the leak rates measured from the hydraulic units at either Limerick or 
Susquehanna produces dose rates significantly in excess of the values in the 
Final Safety Analysis Report.  Independent calculations by the NRC staff 
produced offsite dose values that were comparable to the General Electric 
results.  Radiation release by this path is not possible as long as the 
control rod drive pumps are kept running.  However, continued operation of 
these pumps following an accident cannot be assured, particularly if the 
non-seismically qualified suction piping were to fail.

This problem was resolved at Washington Nuclear Power Plant Unit 2 by the 
installation of two check valves in series in the common discharge pipe from 
the control rod drive pumps (at area A in Figure 1) to prevent backflow out 
of the reactor building (secondary containment).  The Washington Nuclear 
Power Plant Unit 2 installation includes provisions for leak testing the 
valves and a leak rate criterion of 0.01 gpm was established for these 
valves.  

This information notice requires no specific action or written response.  If 
you have any questions about the information in this notice, please contact 
one of the technical contacts listed below or the appropriate NRR project 
manager.




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


Technical Contacts:  Charles R. Nichols, NRR
                     (301) 492-0854

                     Donald C. Kirkpatrick, NRR
                     (301) 492-1849

Attachments:
1.  Figure 1.  BWR Control Rod Drive System
2.  List of Recently Issued NRC Information Notices 
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