Time dependent leakage model to identify defective fuel assemblies in VVER-type nuclear reactors

Dr. Imre Szalóki, Gábor Radócz, Dr. Tamás Pintér, Péter Jakab Rozmanitz, Ilona Varjúné Baracska, Dr. Anita Gerényi

29th Symposium of AER on VVER Reactor Physics and Reactor Safety (2019, Energoland, Mochovce NPP, Slovakia)
core surveillance and monitoring


Time dependent leakage model
to identify defective fuel assemblies in VVER-type nuclear reactors

Dr. Imre Szalóki1, Gábor Radócz1, Dr. Tamás Pintér2,
Péter Jakab Rozmanitz3, Ilona Varjúné Baracska3, Dr. Anita Gerényi1

1 Institute of Nuclear Techniques, Budapest University of Technology and Economics, Budapest, Hungary
2 Paks II. Nuclear Power Plant Ltd., Hungary
3 MVM Paks Nuclear Power Plant Ltd. (PA Zrt.), Hungary

In order to minimize the radioactive contamination in the coolant loops and in the reactor core of nuclear reactors it needs to be available an effective procedure and spectroscopic hardware systems for perception of leakage fuel. Leaking state of the fuel assemblies are indicated by the level of specific radioactivity’s of the fission products, the transuranium radioisotopes and the time-dependent behaviour of these radioactivity’s in the primary coolant. The characteristic parameters of the time-dependent leakage process can be calculated from the specific activities and the ratios of the radioactivity of iodine isotopes such as 131I, 132I, 134I, 134I, 135I, while the ratio of radioactivity of the isotopes 134Cs, 137Cs can be used to predict the burnup level. Several leakage models have been developed to evaluate the integrity state of fuel assemblies over the last two to three decades. Most of these models are based on calculations from activity measurement at equilibrium state of the reactor. Due to the stationary model is only applicable to equilibrium state, it cannot provide a meaningful description of leakage processes in non-stationary (dynamic) situations, for example spiking and time dependent leakage phenomenon. In the recent years, in an R+D project, we have developed a new calculation model based on the operating conditions of Paks Nuclear Power Plant reactors and on radioactivity data of leaked fission products. These activity data were originated from an automated gamma spectrometric measuring system operating an 8-hour time resolution.
The core of the leakage-model consists of a system of differential equations that describes most of the processes of the formation of fission products and activation events both in the fuel and in the primary coolant. Moreover, the dynamic model considers the surface contamination of the fuel assemblies, radioactive decay processes in the coolant and in the fuel, describes the spiking events as well. The activities of isotopes in the coolant depend on the technological parameters of the reactors such as intensity and efficiency of the purification devices, power of the reactor, those are considered in the model. Recently, by solution of this dynamic model combining with the stationary leakage calculations, the criterion factor and variation of the number of leaking fuel assemblies are determined over time in the reactor campaigns. Reaction rates of neutron-induced activation processes in fuel elements and in the primary coolant were determined by burnup calculation using the MCNP6 program.
This work has been partly carried out in the frame of projects supported by MVM Nuclear Power Plant Paks, Hungarian Atomic Energy Authority and VKSZ-14-1-2015-0021 Hungarian project supported by the National Research, Development and Innovation Fund.

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