Introduction

In this chapter the idea of the CSFM technique is extended to calculate multiple perturbations in the eigenvalue of the Boltzmann transport equation with a single Monte Carlo simulation. The actual Monte Carlo simulation is done in an artificial reference system different from the unperturbed and all perturbed systems. The simulations of the unperturbed and all perturbed systems are correlated to the simulation of the reference system. This allows the determination of multiple $\Delta$Ks with a single Monte Carlo simulation, yielding a significant reduction in computational effort.