Analysis Associated with Uncontrolled Dilution of Boric Acid Concentration in the Reactor VVER-1000/320

28th Symposium of AER on VVER Reactor Physics and Reactor Safety (2018, Olomouc, Czechia)
[5] Reactor dynamics and safety analysis


Jan Hádek, Radim Meca (ÚJV Řež)


The paper contains a description of conservative analysis of initiating event associated with
uncontrolled dilution of boric acid concentration in the reactor VVER-1000/320 of Temelín
NPP with TVSA-T fuel assemblies at hot full power and four coolant loops in operation. This
event is included in the group of beyond design basis accidents. The failure of operator
intervention to isolate dilution routes at intervals shorter than 30 minutes is assumed. The aim
of analysis is to determine also the time interval which is necessary for interventions leading to
the deterrence of fuel damage. Since the response of the whole NPP system influences
the course of safety important parameters of the reactor core, the calculations were made by
an externally coupled version of the 3D reactor dynamic code DYN3D and the thermohydraulic
system code ATHLET. The two-group neutronic cross section library of homogenized cross
sections parameterized for different thermal-hydraulic states prepared for TVSA-T fuel
assemblies is used. It is shown that, in addition to exceeding the DNBR limit of more than 90
minutes from the start of the transient, the remaining safety acceptance criteria will not be
violated until the end of the calculation.

This work was funded by the Project between ČEZ, a. s. – Temelín NPP and ÚJV Řež, a. s.
(Contract No. 14SMP258).