Information Notice No. 96-58: RCP Seal Replacement with Pump on Backseat
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555-0001
October 30, 1996
NRC INFORMATION NOTICE 96-58: RCP SEAL REPLACEMENT WITH PUMP ON BACKSEAT
All holders of operating licenses or construction permits for pressurized-
water reactors (PWRs).
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to a potential problem during replacement of
reactor coolant pump (RCP) seal packages. 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 are not NRC requirements; therefore, no
specific action or written response is required.
Description of Circumstances
On March 1, 1996, Sequoyah Unit 1 had a forced outage (Mode 5) to replace an
RCP seal package that had been leaking. The licensee elected to perform this
maintenance activity with the reactor coolant system (RCS) depressurized and
the RCS level within the normal pressurizer (PZR) level band. With RCS level
in the PZR, the RCS maintenance boundary for the seal replacement activity was
provided by the weight of the pump impeller/shaft assembly (approximately 6000
pounds) which had been placed on its backseat. To achieve backseating, the
RCP motor coupling bolts were loosened; this lowered the pump on its backseat
above the RCP thermal barrier housing.
Preceding the outage, site Engineering had determined that an upward pressure
of approximately 15 psig would lift the pump assembly off its backseat
resulting in RCS leakage. This information was never given to the operators.
In addition, the operators were not given specific guidance regarding the
maximum PZR level or a PZR level operating range which should be maintained to
ensure that the static pressure of the RCS was less than the pressure required
to lift the impeller/shaft assembly off its backseat. Operators questioned
the lack of guidance and were subsequently given specific information
regarding acceptable PZR level.
9610250137. IN 96-58
October 30, 1996
Page 2 of 2
The Sequoyah RCPs (Westinghouse model A193S) were designed with a backseat
that provides an RCS pressure boundary to permit replacement of the seal
package. The weight of the impeller/shaft assembly provides pressure boundary
control for this maintenance activity. However, excessive RCS static head
(high pressurizer level) could lift the assembly off its backseat, resulting
in RCS leakage into the maintenance area.
The staff recognizes that by replacing the RCP seal with level in the
pressurizer the licensee avoids risks associated with reduced inventory or
midloop operation. However, problems associated with this operation also
exist. Therefore, licensees may wish to consider (1) determining a maximum
pressurizer level to preclude unseating the pump, (2) providing operators with
control data that can be used with available instrumentation (e.g.,
pressurizer level limits) to ensure that RCS pressure does not cause the
impeller/shaft assembly to lift off its seat, and (3) providing operators with
cautions to prevent unwarranted safety injection into the RCS which could
increase the probability of lifting the pump internals.
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 Office of
Nuclear Reactor Regulation (NRR) project manager.
signed by D.B. Matthews for
Thomas T. Martin, Director
Division of Reactor Program Management
Office of Nuclear Reactor Regulation
Technical contacts: Robert D. Starkey, RII
William F. Burton, NRR
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