POST TEST CALCULATIONS OF A SEVERE ACCIDENT EXPERIMENT FOR VVER-440 REACTORS BY THE ATHLET CODE
22nd Symposium of AER on VVER Reactor Physics and Reactor Safety (2012, Průhonice, Czech Republic)
NEUTRON KINETICS AND REACTOR DYNAMICS METHODS
Severe accident ? if no mitigation action is taken ? leads to core melt. The Fukushima
accident has triggered urgent discussions about the need for such type of investigation. An
effective severe accident management strategy can be the external reactor pressure vessel
cooling for corium localization and stabilization.
For some time discussion was going on, whether the in-vessel retention can be applied for
the VVER-440 type reactors. It had to be demonstrated that the available space between the
reactor vessel and biological protection allows sufficient cooling to keep the melted core in
the vessel, without the reactor pressure vessel losing its integrity.
In order to demonstrate the feasibility of the concept an experimental facility was realized
in Hungary. The facility called Cooling Effectiveness on the Reactor External Surface
(CERES) is modeling the vessel external surface and the biological protection of Paks NPP.
The scaling ratio for reactor surface is 1:40 but the scale of the elevation is 1:1 to ensure
correct representation of the natural circulation processes. The heat flux is generated by
electric heaters. A large number of pressure, temperature, water level, void and flow
measurements allow tracking of the coolant behavior and surface temperature. In the
experiments, the area of the coolant channel can be adjusted in the critical section (where the
gap size is minimal).
A model of the CERES facility for the ATHLET TH system code was developed. The
results of the ATHLET calculation agree well with the measurements showing that the vessel
cooling can be insured for a long time in a VVER-440 reactor.