The MCNP6 computer program has been successfully extended to simulate reactor dynamics problems with moving parts of the geometries. Different from the dynamic method developed in other Monte Carlo codes, a movement scheme has been developed to account for the geometrical parts motion during the particle random walk. The MCNP6 computer program has been used to simulate two transient experiments of the Giacint critical assembly. The MCNP6 calculated total neutron flux was compared with that from the Serpent simulation. An excellent agreement was obtained between the results of the two Monte Caro computer programs. The MCNP6 calculated total neutron flux was also compared with the two measured transients. The MCNP6 results predicted a faster transient than the experimental data. The MCNP6 transient simulation was improved with an adjusted geometrical model which shifts the fuel rods slightly to match the measured reactivity worth due to the drained water. (c) 2021 Elsevier Ltd. All rights reserved.

This paper simulates seven transient experiments with geometrical movements performed at the GIACINT facility of Belarus. These experiments include three slow control rods insertion in more than 25 s, one fast control rod insertion in less than 1 s, and three water moderator draining transients. In the fast control rod insertion experiment, the control rods are first pushed by a mechanical spring and then let fall by gravity in the assembly. In this experiment, the control rods speed varies because of the initial spring force, the gravitational acceleration, and the water buoyancy force. In all other transient experiments, the geometry change occurs with a constant speed. The Serpent computer program was utilized to simulate these transient experiments with geometrical changes by performing two separate computations. One time-independent computation is performed in criticality mode and the other time-dependent computation is performed in dynamic-source mode. The first computation writes the neutron population and delayed neutron precursors for starting the transient simulation. The second computation divides the transient time into several time bins, reads the two files from the first computation, and updates them at the end of each time bin. The Serpent simulation results obtained for this set of experiments are in good agreement with the time dependent experimental measurements. In addition, Serpent and MCNP simulations results show an excellent agreement for the water moderator transient. Published by Elsevier Ltd.

UZrCN fuel is a high density, high temperature fuel that has potential for application in different types of reactors, comprising research reactors, including of conversion HEU on LEU fuel. Criticality experiments on U-H2O lattices with LEU UZrCN fuel rods were performed using "Giacint" critical facility of the Joint Institute for Power and Nuclear Research - Sosny of the National Academy of Science of Belarus. Five configurations of critical assemblies were studied. The fuel rod pitch is 18, 32 or 47 mm with triangular grid. The criticality conditions were obtained by adjusting the water moderator height in the studied lattices. The fuel was designed and produced by the Scientific Research Institute Scientific Industrial Association "LUCH" under the Russian Research Reactor Fuel Return Program. The fuel material UZrCN has U-235 enrichment of 19.75%. The uranium metal density is 10.8 g/cm3. The total fuel rod length is 620 mm, and the active fuel length is 500 mm. The outer fuel rod diameter is 12 mm, and the fuel material diameter is 10.7 mm. The clad material is stainless steel or niobium. The results of experiments on critical assemblies have been analyzed by creating detailed calculation models and performing simulations for the experiments. This paper presents the obtained experimental and analytical results. The "Giacint" and "Crystal" critical facilities will be used to determine neutron-physical characteristics of critical and subcritical assemblies on the fast neutron, modeling physical features of cores of advanced reactor and accelerator driven systems, cooled gas, and liquidmetal coolants. The critical and subcritical assemblies represent uniform hexagonal lattices of fuel assemblies (39 mm pitch), each of which consists of 7 fuel rods and have no cladding. Three types of fuel assemblies with different matrix material (air, aluminum and lead) were investigated. The side radial reflector is Be (internal layer) and stainless steel (external layer). The description of the design and the composition of the critical and subcritical assemblies with UZrCN fuel, the results of calculation are presented.

Benchmark criticality experiments for hexagonal lattice of fuel and absorbing rods in water were performed using “Giacint” critical facility of the Joint Institute for Power and Nuclear Research – Sosny of the National Academy of Science of Belarus. The fuel rods pitch is 21 mm and the absorber rods pitch is 63 mm with a triangular grid. The fuel composition – UO2 with 21% uranium-235 enrichment. The absorber composition – B4C. The moderator and reflector H20. The criticality conditions obtained by adjusting the water moderator height in the studied lattices. The results of experiments on critical assembly have been analyzed by creating detailed calculation models and performing simulations for the experiments. The analyses used the MCNP-4C and MCU-PD computer programs. In addition, the impact of using different nuclear data files are examined.

At critical facility "Giacint" are carried out experimental researches on the physics of nuclear reactors, criticalities of fissionable materials and nuclear safety, including work on replacement HEU on LEU nuclear fuel. New nuclear fuel will be created for research purposes at the critical facility "Giacint" on development of perspective cores for small size power and test reactors. The fuel consists of uranium-zirconium carbon nitride U0,9Zr0,1C0,5N0,5 with 19.75% enrichment by U-235. Density of uranium more than 10,5 g/cm. The clad material is stainless steel or niobium. The fuel assembly for critical facility "Giacint" consists from seven fuel rods and does not have the clad. Fuel rods and fuel assemblies were tested for conformity to requirements of the technical specifications. For carrying out at critical facility "Giacint" of benchmark experiments with use of these fuel rods were developed uranium-water critical assemblies and critical assemblies without moderator and made their equipment. In the paper will be described named rods and fuel assemblies, geometry and composition specified critical assemblies and presented the program of experiments on them.