Á Tóta, A. Keresztúri, A. Molnár, I. Panka, , Hungarian Academy of Sciences, Hungaria

23rd Symposium of AER on VVER Reactor Physics and Reactor Safety (2013, Štrbské Pleso, Slovakia)
Reactor dynamics and safety analysis


The fulfillment of the safety analysis acceptance criteria and the normal operation safety
related limitations are usually evaluated by using separate hot channel or/and hot assembly
thermal hydraulic calculations. In accordance with the present structure of the VVER-440
fuel assemblies, the subchannel thermal hydraulic calculation of the recent KARATE-440
code system exploits the presence of the assembly shroud by applying zero mixing
between the assemblies. However, the shroud-less assemblies of the expected new NPP
units in Hungary or even those of some recently foreseen VVER-440 are raising several
questions; like what is the role and the influence of the mixing from or/and to the
neighboring assemblies, and in this new situation which is the smallest possible calculation
domain in the vicinity of the hot channel. Focusing on ATWS problems, the paper
evaluates the thermal hydraulic influence of the environment of the hottest subchannel by
supplementing the calculation domain step by step with the closest pins and subchannels of
the neighboring assemblies. The pin power distributions are also varied. The conservatism
of the closed assembly approximation for the shroud-less case, neglecting the hydraulic
mixing effect between the assemblies, is also analyzed.

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