transient calculations of coolant mixing in vver-440/213 rpv

TRANSIENT CALCULATIONS OF COOLANT MIXING
IN VVER-440/213 RPV
Béla KISS, Ildikó BOROS, Dr. Attila ASZÓDI
Budapest University of Technology and Economics
Institute of Nuclear Techniques, Hungary
kiss@reak.bme.hu, boris@reak.bme.hu, aszodi@reak.bme.hu
ABSTRACT
The Budapest University of Technology and Economics, Institute of Nuclear Techniques (BME
NTI) has been working since 2001 on the three-dimensional CFD model of the reactor pressure
vessel of the VVER-440 type reactors. During this time period – due to the development of the
available computational capacity – a very complex and detailed model of the RPV has been
developed.
The new model can be applied for steady state calculation during normal operational condition
and for different transient analyses as well. The main results of the sensitivity study in steady
state were presented on the 18 th AER Symposium. Three different transient calculations were
performed with this new model. One of these was the participation in a benchmark exercise on
the start-up of the 6 th main coolant pump, which is aimed to compare the capabilities of mixing
models of 1D system codes with the results of CFD simulation. Two another transient
calculations were made to investigate the mixing of the injected emergency coolant in different
parts of the RPV.
INTRODUCTION
A detailed RPV model of VVER-440/213 type reactor was developed in BME NTI in the last
year. This model contains the main structural elements as inlet and outlet nozzles, guide baffles
of hydro-accumulators, alignment drifts, perforated plates, brake- and guide tube chamber and
simplified core. For the meshing and simulations ANSYS softwares (ICEM-11.0 and CFX-11.0)
were used. With the new vessel model a series of parameter studies were performed considering
turbulence models, discretisation schemes, and modeling methods. In steady state the main
results were presented last year on AER Symposium. The model is suitable for different transient
calculations as well. In this paper the results of three transient simulations performed with this
model are presented.

START UP OF THE 6 th MAIN COOLANT PUMP
The purpose of the suggested benchmark was the comparison of calculation results with different
codes on the effects of coolant mixing in the VVER-440/213 reactor vessel. The task of this
benchmark is to investigate the start up of the 6th main coolant pump. A computation was carried
out with the help of ATHLET/BIPR-VVER code in Kurchatov Institute for this transient and was
repeated with ANSYS CFX 11.0 at our Institute.
For the transient calculation a special vessel model has been used, which contained the geometry
only up to the core outlet (Fig 1). The pressure lost in the core and the effect of the core on the
coolant flow was considered with a porous region model of the core. The SST turbulence model
was selected for the calculation.
Fig. 1: Model for the transient calculation
The examined situation was the start up of the 6 th loop while the other 5 loops are in normal
operation. The coolant mass flow of the 6 th loop reaches the nominal rate (~1400 kg/s) during 20
seconds (Fig 2).
Fig. 2: Mass flow and temperature boundary conditions

The results show that the temperature gradient is steep in the vicinity of the inlet nozzle at the
beginning of the transient. It is an important result from the point of view of thermal stresses.
The temperature distribution becomes homogenous at the end of the transient (Fig 3).
Fig. 3: Temperature distribution on the wall of RPV
ECS INJECTION INTO 2 nd , 3 rd AND 5 th COLD LOOPS
Cold water was injected into the 2 nd , 3 rd and 5 th loops from the Emergency Cooling System (ECS)
during the next transient. The injection pipes are connected perpendicular to the cold leg loops
from above. The boundary conditions of the simulation were calculated by APROS.
The injection starts in 10 th seconds. The mass flow of the injected coolant (max. 13,5 kg/s) is
significantly less compared to the mass flow of the loops (min. 1510 kg/s). The temperature of
the ECCS coolant is 32 o C (Fig 4).

Mass flow and temperature of the coolant in the 6 loops
Mass flow and temperature of the injected water
Fig. 4: Mass flow and temperature boundary conditions calculated by an APROS model
The temperature of the RPV decreases during the transient, but the effect of the injected colder
water is not considerable. The colder water does not have enough time for the mixing inside the
loop, the mixing happens after entering into the pressure vessel (Fig 5).

Fig. 5: Tempreature distribution on the wall of RPV and pipe
ECS INJECTION ABOVE THE CORE
The assumed initial conditions of the next transient: the reactor is in hot shut down condition, i.e.
the main coolant pumps are stopped. There is natural circulation in two loops (2 nd and 6 th loops
are opened), the coolant mass flow rate in these two loops is 40 kg/s. The temperature of the
coolant is 140 o C, the pressure is 0,5 MPa. The ECCS injects water into the 4 th loop and through
the hydro-accumulator nozzles. The mass flow of the injected water is 50 kg/s, the temperature is
35 o C.
The duration of the transient was 60 s. In the calculation buoyancy was taken into account, and
because of the large coolant temperature differences temperature dependent material properties
were used.
For this simulation only the core and the structural elements above the core were modelled in
details. The goal was to simulate the mixing in the upper plenum so this is the reason why the
lower plenum was neglected for this case.
Fig. 6: The model of the core and the upper plenum
The injected water cools down the coolant in the 4 th loop and enters into the vessel in the 23 rd
second. The colder water flows around the perforated reactor pit and passes through the

perforations into the upper plenum. The coolant through the hydro-accumulators flows directly
onto the top of the core and flows down through the fuel assemblies.
Fig. 7: Tempreature distribution on the wall
The colder water of the ECCS cools down the fuel assemblies gradually during the transient.
Fig. 8: Tempreature distribution at the bottom of the core

The streamlines show the flow path of the coolant through HA in the RPV. The water goes down
directly into the core and does not mix with the warmer water in the upper plenum.
Fig. 9: Streamlines in the pressure vessel
SUMMARY
In this paper the results of three transient simulations were presented, performed in the Budapest
University of Technology and Economics, Institute of Nuclear Techniques (BME NTI). Due to
the development of the available computational capability, it is already achievable to include the
direct model of all internals in the CFD model of the pressure vessel. It opens the door to perform
detailed simulations for different transient cases.
Three different transient events were computed with the model. The performed simulations
showed that the model is applicable for the computation of different cases.
The first transient was the start up of the 6 th main coolant pump. The temperature distribution is
very inhomogeneous on the wall of the RPV at the beginning of the transient.
The second transient was the injection from the ECS into 2 nd , 3 rd and 5 th cold loops. The mass
flow of the cold water was negligible compared to the mass flow of the loops so it did not have
large influence.
The third ones was the injection from ECS into the 4 th loop and through the nozzles of HA above
the core.. The colder water fills the 4 th loop and flows into the RPV. The coolant from HA flows
down into the core and starts to cool down the fuel assemblies.