Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
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
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IN 90-78
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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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