Solution of the 6. Dynamic AER Benchmark Using the Coupled Code DYN3D/ATHLET

The 6th dynamic benchmark is a logical continuation of the work to validate systematically coupled neutron kinetics / thermohydraulics code systems for the estimation of the transient behaviour of VVER type nuclear power plant which was started in the 5th dynamic benchmark. This benchmark concerns a double ended break of one main steam line (asymmetrical MSLB) in a VVER-440 plant. The core is at the end of first cycle in full power conditions. The asymmetric leak causes a different depressurization of all steam generators. New features in comparison to the 5th dynamic benchmark were included: asymmetric operation of the feed water system, consideration of incomplete coolant mixing in the reactor vessel, and the definition of a fixed isothermal recriticality temperature for normalising the nuclear data.

The solution of this benchmark was obtained by means of coupled code system DYN3D/ATHLET. There, the whole core including neutron kinetics and thermohydraulics processes was calculated by three dimensional neutron kinetics core model DYN3D. The thermal hydraulics behaviour of primary and secondary circuit was modelled by the system code ATHLET.
For the purpose of realisation of the fixed isothermal recriticality temperature Trecrit = 210°C the cross section data of absorber were adapted.

During the transient calculation of MSLB scenario, the reactor scram was initiated with the assumption that two of the most reactive control rods in the sector of highest overcooling are stuck at their fully withdrawn position. Accordingly, the power is decreased due to reactor scram. Thereafter, the reactivity feedback caused by the decrease of coolant temperature entailed the increase of the reactor power. Based on an assumed delayed start of the high pressure safety injection, the recriticality of the shut-off reactor is achieved. The value of total power in the second maximum is about 80 MW.

In this paper the behaviour of the main thermohydraulic and neutron kinetic parameters is presented.