Application de la theorie des perturbations generalisees et des algorithmes stochastiques afin d'ameliorer les reflecteurs des reacteurs CANDU6

The CANDU6 reactor has many differences from other power reactors among which it has horizontal fuel bundles that can be refueled on power and uses natural uranium (0.7% of uranium 235) as fuel. Another important characteristic of CANDU6 reactors is the heavy water moderator and reflector. Heavy water surrounding the reactor core reflects back some of the neutrons escaping from the core and saves neutrons that are then available for the fission process. Many parameters are important to define the combination of materials to be used in the reflector e.g. materials compatibility, economy, strength, shielding, etc... The ideal reflector has a high probability of scattering neutrons and a low absorption probability. Heavy water being very expensive, the use of a large heavy water reflector presents a large cost. The purpose of this work is to perturb the CANDU reflector in order to obtain a more uniform flux distribution throughout the reactor core while lowering the reflector cost.;Generalized Perturbation theory (GPT) is widely used in reactor physics to evaluate variation of the multiplication factor in a critical reactor. One area where GPT methods have been particularly successful is in Light Water Reactors (LWR) fuel management analysis. A similar approach has been proposed to optimize fuel management in a CANDU reactor at equilibrium refueling. A new application of GPT to evaluate the variation of zonal power as a result of the reflector perturbation is proposed in this project. This approach allows us to define modification sites in the reflector of a CANDU6 reactor.;The Taboo search and the genetic algorithm had been used to obtain the new reflector configuration. An objective function which tries to flatten the power distribution among the 14 standard control zones is used.;It is important to note that we have reached our objective and the reflector has improved the flux distribution. This reflector has also decreased the amount of heavy water which automatically implies a very important decrease of the financial cost of building a new CANDU6 reactor.;We have defined different metal mixtures and introduced them into the reflector. These mixtures contain concentrations of Fe56, Cr52, Ni58 and Mn55 atoms and are added at different positions of the reflector with various concentrations. The idea is to design a new heterogeneous reflector still containing mostly heavy water but with interstitial metal components. This would reduce the cost of installation of new plant and renovation of existing plants.