PRIMARY SYSTEM HYDRAULIC CHARACTERISTICS AFTER MODIFICATION OF REACTOR COOLANT PUMPS` IMPELLER WHEELS AT BOHUNICE NPP
22nd Symposium of AER on VVER Reactor Physics and Reactor Safety (2012, Průhonice, Czech Republic)
CORE MONITORING, SURVEILLANCE AND TESTING
Abstract
A coolant flow through the reactor is usually determined after annual outages at Slovak
NPP in two distinct ways. First method is determination on the basis of the secondary system
parameters – measurement of thermal balances. The value achieved by this method is used as
the input parameter in the Table of allowed reactor operation modes. The second method
draws from the primary system parameters – measurement of primary system hydraulic
characteristics.
Flow nozzles used for the measurement of feed water flow behind high pressure heaters
were replaced at both Bohunice Units during outages in 2008. The feed water flow behind
high pressure heaters is one of the main parameters used for the determination of coolant flow
through the reactor by the first method. Compared to the measurement executed during
previous fuel cycles, the calculated coolant flow through the reactor decreased considerably
after the change of flow nozzles. The imaginary change of coolant flow through the reactor at
Unit 3 was -1,6 %; and at Unit 4 -2,6 %. This change was not proved by the parallel
measurement of primary system hydraulic characteristics. Later it was found out that the
original flow nozzles used for 25 years were substantially deposited (original inner diameter
of the nozzles was reduced by about 0,6?0,9 mm). Therefore feed water flow measurement
was untrustworthy within the recent years.
On the findings stated above, Bohunice NPP has decided to increase coolant flow
through the reactor by changing the reactor coolant pumps impeller wheels. Modification of
impellers wheels is planned within years 2012 to 2014. The 4 impeller wheels at Unit 4 and 2
at Unit 3 were replaced during the outage in 2012. General contractor for Bohunice NPP is
VUJE, Inc. The supplier of impeller wheels is the Russian company CKBM St. Petersburg
through the Czech company SIGMA (as a subcontractors of VUJE, Inc.). The Division for
Preparation of NPP Operation is responsible for preparation, execution and evaluation of