Monte Carlo model for neutron production by the interactions of low energy deuterons in solid targets : doktorska disertacija

english

doktorska dela

2012

Univerza v Mariboru, Fakulteta za gradbeništvo, prometno inženirstvo in arhitekturo

The construction of the nuclear fusion plant 'ITER' has started in 2009 at Cadarache, France. The ITER machine represents a milestone in the civil use ofthe nuclear fusion energy. The physics of ITER is based on the fusion reaction between deuteron and triton nuclei (D-T). The deuteron-deuteron reaction (D-D) is also interesting and is foreseen for the next generation of fusion reactors. The experimental activities carried out in the context of the ITER neutronics involve intense fields of neutrons produced with a linear accelerator for deuterons, a target containing tritium or deuterium and auxiliary structures, such as the detector system, cooling system, room walls,etc. The experimental sources of uncertainty cannot be eliminated, and what is requested for an experiment to be of benchmark quality is the accurateassessment of any uncertainty that is present in the measurements. Theuncertainties due the neutron source term are often referred by the experimentalists as 'resolution' uncertainties. The precise evaluation of the individual sources of uncertainty cannot be achieved with experimental techniques alone. The problem of modelling deuteron interactions in solid tritiated or deuterated targets represents an interesting case concerning the choice and use of physics models and nuclear data. It requires accurate treatment of elastic scattering processes (requiring physics models), inelastic electronic collisions (usually provided in the form of tabulated data) and fusion nuclear reactions data (for which evaluated nuclear data are available and should be used). A synopsis is included to make the reader acquainted with ion transport models and nuclear cross sections for deuteron and neutron interactions. The doctoral thesis is aimed at proving the hypothesis that a computational model can be developed, which would include the features observable in a fusion neutronics experiment and that their implementation would be based on solid physics ground and numerically stable. The software and databases generally available in nuclear engineering are often inadequate for solving problems of coupled ion/neutron transport because of the intrinsic limitations in physics (e.g. energy cut-offs, model limitations) or in the geometric details or in the nuclear data usage. It is necessary to model the deuteron interactions inside the target and get information on the deuteron energy and direction while slowing down. This issue is a problem of ion transport in matter. For the computer simulation of the slowing down and scattering of ions in materials, the Monte Carlo methods have a number of distinct advantages over deterministic formulations based on transport theory. For instance, the first allow more rigorous treatment of elastic scattering. Many codes for tackling this problem are available. Among these, the SRIM code (2008 version) and the MCNPX code (version 2.6) are well-established. Both codes rely on physical models for the deuteron transport in matter. The MCNPX models are designed for 200 MeV and higher deuteron energies and their application is not reliable below. In the MCNPX code, the deuteron transport and the nuclear reactions on deuteron interactionoccur above some energy threshold in the MeV energy region, so neutrons cannot be generated with MCNPX at the low deuteron energies of the neutron generators under discussion. On the other hand, SRIM has seen a thorough validation of the ion transport module till low ion energies because of its wide range of applications, but the SRIM code cannot be used to producethe source neutrons because it does not include nuclear reactions at all. These drawbacks can be overcome by modelling the nuclear reaction that yields the neutrons and introducing approximations in the ion transport (e.g. neglecting angular straggling) or even in the angular effects of the target and the nuclear reaction (e.g. resorting to independent evaluations of neutronangular emission) or by tailoring a code for the specific problem.

07.08.2023