Plant heat-up is a process which all operating systems such as primary coolant
circuit, pressurizer, primary and secondary sides of the steam generators and etc. are transferred
from a cold shutdown to a hot standby status. During plant heat-up, some thermohydraulic
limitations such as maximum and minimum allowable pressure and maximum rate of increase in
pressure and temperature which are recommended by plant commissioning program and NPP
safety related documents should be considered. Maximum allowable pressure prevents brittle
fracture in reactor vessel, Minimum allowable pressure in the inlet of the reactor coolant pumps
(RCPs) prevent pump cavitations and maximum allowable rate of increase in temperature and
pressure respectively prevent thermal and mechanical shocks. Thus, tuning pressure and
temperature increasing rates during plant heat-up is important from plant safety point of view.
The RELAP5 system code was used to model and analysis the behavior of WWER-1000 plants
during heat-up. In plant heat-up, at first the primary circuit pressure increases by injection of N2
gas into pressurizer in order to providing minimum required NPSH for operation of the RCPs.
After short time RCPs are turned on to operate which increase the primary coolant circuit
temperature through friction losses. At a time which specified by heat-up procedure the
pressurizer heaters are turning on to increase the primary circuit pressure. Heat transfer from
primary to secondary side in the steam generators causes increasing of the secondary side
temperature and pressure. Temperature and pressure of primary and secondary circuits increase
until plant reaches to hot standby condition. The results show that the thermohydraulic
parameters during plant heat-up are in an acceptable range and have a good agreement with
available data in technical documents.