Dottorato in ENERGIA E AMBIENTE

Titolo della tesi: Three-dimensional neutronics code development for design and safety of fusion devices

The focus of the present work is the development and applications of three-dimensional codes devoted to the assessment of the gamma dose rate due to the neutron-induced activation reactions in fusion reactor applications. In particular, the thesis deals with the development of the Advanced D1S dynamic tool for the shutdown dose rate assessment and the implementation of a new methodology to evaluate the dose rate from the Activated Corrosion Products (ACPs) in the water cooling loops. The evaluation of the shutdown dose rate is fundamental for shielding design, materials’ requirements, licensing and planning of maintenance operations in high performances fusion devices. The shutdown dose rate calculations require the combined use of radiation transports and inventory codes. The Advanced Direct 1-Step (D1S), based on MCNP5 Monte Carlo and FISPACT inventory codes, is one of the most validated tools used in complex fusion tokamak machine with unique computation capabilities such as the mesh tally maps and continuous time evolution assessment of the dose rate in a single run. In the Direct 1-Step approach a single transport simulation of neutrons and decay gammas is performed: photons are treated as promptly emitted and weighted by correction factors that take into account the build-up and decay of the considered nuclides. Advanced D1S has been validated through measurements at JET tokamak and comparison with Rigorous 2-Step codes and it has been also extensively used for ITER and DEMO calculations. The Advanced D1S Dynamic is an improved version of Advanced D1S with new features aimed at extending applications, overcome present limitations and at improving its versatility. The new developments are mainly related to the treatment of multi-step reactions. Furthermore, a new Python library has been developed to optimize the MCNP-FISPACT interfaces and data analyses. The new developments have been described in detail and the applications of Advanced D1S dynamic to the shutdown dose rate assessment in DEMO Water Cooled Lithium Lead reactor and to the Divertor Tokamak Test (DTT) facility are presented, as well as implications to safety related issues. The experimental validation of the code has been performed by comparing the results of the calculations with shutdown dose rate measurements performed at the Joint European Tokamak (JET). A novel advanced methodology for the three-dimensional assessment of the gamma dose rate due to the ACPs in water cooling loops has been developed. The formation and transport of ACPs in a reactor water cooling loop is a complex process, involving many different mechanisms that react among each other. The ACPs assessment is performed through the PACTITER code. The resulting activity due to deposit and oxide in the different zones of the loop is used to generate the decay gamma sources from ACPs in the zones of interest in the cooling loop. A proper workflow has been implemented in order to interface the ACP output results of PACTITER with MCNP. The ACPsourceETL python script uses the ACTIGamma tool, developed in collaboration with CCFE, to provide the decay gamma energy distributions. The gamma energy line probabilities and the surfaces are used to implement the ACPs gamma surface sources with proper spatial distribution in MCNP format for the three-dimensional gamma transport simulation. The implemented methodology has been successfully applied to the ITER WCLL TBM water cooling system in order to assess the spatial distribution of the ACPs dose rate at shutdown in Port Interspace and Ancillary Equipment Unit area in Port Cell.