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Geometry models for Monte Carlo transport codes have been using standard constructive solid geometry (CSG). The standard approach is using analytical equations for defining surfaces from which spatial cells are constructed. However, this approach can be quite time consuming and possibly error prone for complex models. Monte Carlo transport codes are continuously developed, one of the paths is using CAD-based mesh geometry. MCNP6 features unstructured meshes (UM) created with Abaqus/CAE as geometry description. Attila4MC package for creation of UM geometry from CAD model can be used for MCNP6 models. VVER-1000 fuel assembly model in UM geometry was created for TVSA-T.mod.2 fuel type. Basic validation of the model was performed, initially for criticality calculations. In the future, the model will be used for criticality safety analyses, preparation of boundary conditions for diffusion codes and radiation shielding analyzes of spent fuel transport and storage facilities.

The problem with higher nuclear fuel enrichment is its high initial reactivity. It has a negative effect on the peaking factor, which is one of the license conditions. The second major problem is the ability to control the reactivity of the reactor, and thereby maintaining the multiplication factor in the core equal to 1. Long-term control of the reactivity in PWR reactors is typically conducted by the concentrated boric acid (H3BO3) in the coolant; its highest possible concentration is determined by the requirement to maintain a negative reactivity coefficient. Another option are burnable absorbers. This work deals with usage of hafnium as an advanced type of burnable absorber. Based on the model of computing code UWB1 for the study of burnable absorbers, a new cladding of nuclear fuel is designed with a thin protective layer made of hafnium. This cladding is used as a burnable absorber that helps reducing excess of fuel reactivity and prolongs the life of the fuel assemblies, which increases economic coefficient of the use of nuclear power plants. This cladding would also work as a protective layer increasing endurance and safety of nuclear power plants. Today zirconium alloys are exclusively used for this purpose. The main disadvantages of zirconium alloys include rapid high temperature oxidation of zirconium — a highly exothermic reaction between zirconium and water steam at temperatures above 800 °C. During this reaction hydrogen and inconsiderable amount of heat are released. Hydrogen excess, released heat, and damaged cover of fuel may deepen the severity and consequences of possible accidents. Another disadvantage of zirconium alloys is their gradual oxidation under standard operating conditions and ZrH formation, which leads to cladding embrittlement.

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