Tesis

The Codes and Models Section of the Nuclear Fuels Management has been developing a fuel code called DIONISIO, whose purpose is to simulate the behavior of a typical fuel rod of a nuclear power reactor. The code currently has more than 70 interconnected models that address most of the phenomena that occur under irradiation under normal reactor operating conditions. Some of the phenomena that the code predicts are the thermal and mechanical behavior, the release of fission gases, the swelling and densification of the tablets, the thermohydraulic conditions of the channel surrounding the bar, the growth of the oxide layer, among others. Likewise, the code has a module to simulate the behavior of plate-type fuels, a design that is commonly used in research reactors.

Likewise, and through another doctoral work completed in 2021, the code's prediction range has been extended to accident scenarios, particularly those caused by loss of coolant, called LOCA by its acronym in English (Loss Of Coolant Accident). With this objective, several models were developed to describe the behavior of the cooling fluid in the power reactors, as well as those that take place in the fuel rod cladding, typically Zircaloy alloy, such as its oxidation and the consequent growth of different phases (ZrO2, alpha- Zr, beta- Zr), at high temperatures, and the capture and release of hydrogen from the dissociation of water molecules from the coolant. Likewise, models were incorporated to describe the macroscopic mechanical deformation of the cladding under highly demanding conditions, like the ballooning and the possible catastrophic rupture (burst).

The development of this complex module involved, first, the separate development and evaluation of models to describe the different physical processes and, subsequently, their incorporation into the code as new particular subroutines, respecting the architecture, mode of execution and versatility of the rest of the code. The new accident module allows analyzing and quantifies the behavior of the refrigerant fluid, whether in single or double phase, evaluating at every moment the system pressure, the refrigerant flow and the vapor fraction in the different axial positions of the channel. It is noted that, in an accident context, the rapid variation experienced by the conditions to which the fuel rod is subjected gives rise to greater complexity in the calculation, since certain simplifying assumptions that are used in normal operating conditions must be abandoned.

In close relation to these aforementioned developments, and as a result of different current factors in nuclear technology, it is proposed to extend the operation of the code to new fuels, those called ATF (Advance Technological Fuels). These fuels are built with novel materials or materials that include a substantial improvement from the point of view of specifications related to both safety and sustainability.

ATF materials are part of future nuclear reactors, those called Generation III+ and Generation IV, which seek, in addition to increasing energy efficiency, to promote the sustainability of nuclear energy, always improving its safety. In this sense, one of the first definitions of ATF materials arises, any material that increase the safety range of the reactors that use them, either because their properties allow working in less demanding conditions, or because given a hypothetical accident, these materials provide better responses to extreme requests.

Due to this future scenario, developing an ATF materials library for a fuel code like DIONISIO is a necessary and fundamental step from a technological point of view, since although Argentina does not currently have ATF fuel developments, it is expected that in a next future will have them, if we want to remain be one of the few nations that produce nuclear technology on the planet.