Abstracts Category : Other

Development and applications of multi-physics capabilities in a continuous energy Monte Carlo neutron transport code

by Ville Valtavirta

The accurate modeling of nuclear reactors is essential to the safe and economic operation of current and future reactor types. Due to the physical feedback effects between the neutron distribution in the reactor core and the fuel and coolant material temperatures and densities, all of the three fields need to be solved simultaneously in order to obtain a solution for the behavior of the nuclear reactor. This coupled problem has traditionally been solved using a two-stage approach for the neutron transport. In this approach, the neutron interaction properties of different parts of the reactor core are first averaged using accurate neutron transport methods in a series of small scale simulations. These averaged quantities are then used in a simplified neutron transport model to obtain the full-core solution in a reasonable time allowing for multiple iterations between the solvers of the different physical fields. In recent years, advances in methodology as well as in computational power have made it possible to apply the accurate neutron transport methods directly to the full core problem, enabling modeling of the important feedback effects in more detail than has been possible with the two-stage approach. Moving to direct modeling of the coupled problem with the accurate neutron transport methods initially developed for the lattice calculations in the first part of the two-stage approach requires various changes to the neutron transport methods. In this thesis, capabilities required to solve the neutron transport part of the coupled problem are developed and implemented in the continuous energy Monte Carlo neutron transport code Serpent 2. Coupled calculation schemes were developed and implemented both for internally and externally coupled calculations for three different simulation types: Steady state calculations, burnup calculations and time-dependent transient calculations.The new coupled calculation capabilities were applied to the effective fuel temperature approximation, in which the complex fuel temperature distribution in a fuel rod or a fuel assembly is replaced with a single effective temperature for the neutron transport calculations. The new capabilities made it possible to estimate the effects of this approximation by providing an accurate reference solution using realistic temperature distributions provided by an internally or externally coupled fuel temperature solver.; TiivistelmNeutronien kyttytymisen ja reaktorisydmen materiaalien lmptilan ja tiheyden vlill olevat fysikaaliset takaisinkytkennt tekevt reaktorimallinnuksesta kytketyn ongelman: reaktorisydmen tehojakauman ratkaisemiseksi tytyy samanaikaisesti ratkaista mys jhdytteen ja polttoaineen lmptila- ja tiheysjakaumat. Menetelmilt vaaditun lyhyen laskenta-ajan vuoksi neutronien kyttytyminen ratkaistaan tyypillisesti kytten kaksivaiheista laskentaketjua. Ketjun ensimmisess osassa reaktorisydmen eri alueille lasketaan tarkoilla menetelmill joukko neutronien vuorovaikutuksia kuvaavia keskiarvoistettuja parametreja.Advisors/Committee Members: Leppnen, Jaakko, Adj. Prof., VTT Technical Research Centre of Finland Ltd, Finland (advisor).