Implement of reactivity perturbation on JMCT

In reactor design and safety analysis, the statistical error of Monte Carlo code often covers the actual reactivity difference between two systems with only a little difference. Generally, perturbation method is used to solve this problem. In this work, reactivity perturbation is implemented into JMCT code. High order perturbations and fission source perturbation are considered in order to get a high precision. Models with global perturbation and local perturbation are tested. The systems with fast and thermal neutron spectrum are also tested. Some of the above models have positive reactivity perturbations, while others have negative perturbations.