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The modified power method has been applied to reduce the inter-cycle correlation of the Monte Carlo simulation in this study. Previously it has been developed and implemented in the Monte Carlo simulation to accelerate the source convergence. The convergence of the original Monte Carlo simulation determined by the decay rate of the 1st mode in the source, which will finally converge to the 0th mode. The 1st mode is also the most important origin of the inter-cycle correlation of the sources of different cycles. The modified power method can give the first several eigenmode solutions at the same time, so it is efficient for both source convergence acceleration and inter-cycle correlation reduction by subtracting the higher modes from the source. The 1D slab problem is modeled in this study to demonstrate the performance of the modified power method. Introduction The Monte Carlo (MC) method has been widely used in neutron transport simulation of the criticality analysis of the nuclear power systems. To solve the criticality eigenvalue and the corresponding eigenvector (the fission source), the power method is exclusively adopted, for with the cycle/generation concepts are introduced. The convergence rate, i.e., the number of iterations/cycles required for the eigenvector/source convergence, is determined by the dominance ratio k1/k0, where k0 and k1 are the 0th and 1st eigenvalues. This means the convergence may be very slow if the dominance ratio of the system is close to 1.0. Many source convergence acceleration techniques have been developed to solve the high dominance ratio problems, such as the coarse mesh finite difference method, which adopts the solution of a lower order operator (diffusion) to accelerate the transport solution. In our previous works, the modified power method (MPM) has been adopted to accelerate the source convergence, which can be thought as a kind of coarse mesh rebalance technique. It can also generate multiple eigen-solutions, and the convergence acceleration of the fission source, i.e., the fundamental/0th eigenmode, can be achieved by the subtraction of the other modes from the fundamental/0th mode. On the other hand, the inter-cycle correlation is also related to the higher modes. For the high dominance ratio problems, there is strong inter-cycle correlation, as the biases introduced in previous cycle will decay very slow, while the decay rate is also determined by the eigenvalue ratios, especially the dominance ratio k1/k0. Therefore, the modified power method should be also efficient for reducing the inter-cycle correlation. The theory of the modified power method will be briefly reviewed in the next section, followed by the numerical tests about the 1D slab problems and the conclusion. The Modified Power Method In the original MC simulation, one particle only carries one weight to represent the source that will finally converge to the fundamental mode. For the MPM, we introduced a weight vector for one particle, each element of the vector corresponding to one independent source, which will finally

Monte Carlo Inter-Cycle Correlation Reduction with the Application of the Modified Power Method