Development of advanced leakage models for lattice calculations based on the method of characteristics.

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The long-term objectives of this research program are to develop and implement in transport based lattice codes new or improved numerical simulation techniques to make them more precise and efficient tool to analyse neutrons behaviour in nuclear reactor. This implies: developing new computational models to treat more complex systems; programming robust method to accelerate the solution algorithm for large neutron transport problems; and developing computation schemes benefiting from recent advances in computer technology. Most of the development we proposed will be implemented in the code DRAGON we developed at Polytechnique Montreal. This freely distributed software is used in Canada and abroad for the safety analysis of CANDUs as well as in Laboratories and Universities throughout the world for advanced reactor studies.***Lattice codes are mainly used to provide spatially homogenized and energy condensed cross sections for finite core calculation software. For currently operating reactors, this implies performing 2D or 3D neutron transport simulations over fully reflected fuel assemblies. However, nuclear reactors are finite which means neutron will escape from the lattices. This is where leakage models are useful because they provide a mean to simulate this effect in a critical lattice in addition to producing a more representative flux distribution to use for fuel burnup and lattice homogenization. Currently, the most advanced heterogeneous leakage method implemented in lattice codes is the simplified heterogeneous B1 model but its use is limited to the collision probability (CP) method in fully reflected symmetric 2D geometries (axially infinite cell).***The short-term goal of this research program is to take advantage of the fact that the method of characteristics (MoC) programmed in DRAGON can solve the transport equation for high order angular flux modes to develop the heterogeneous BN leakage model. As a first step we will implement, my students and I, an advanced B1 heterogeneous leakage method in DRAGON using the MoC in fully reflected non-symmetric 2D geometries. For the second step, work will proceed on generalizing the BN method to 3D geometries with mixed boundary conditions (void axially and white reflection radially). Thirdly, we will develop a 3D cyclic tracking procedure so that mirror like reflection boundary conditions can be treated in the 3D transport solution. This will increase substantially the performance of DRAGON for 3D simulations by removing the computational penalties resulting to the presence of small surfaces at external boundaries. Finally we will propose and implement an advanced acceleration techniques for 3-D full core Monte Carlo simulations. This will be based on an iterative algorithm to solve the fine mesh MoC equation with incoming angular flux boundary conditions generated by a coarse mesh Monte Carlo simulation.**