A new finite difference approach to single-phase Stefafl problems is presented by using fixed-time step and variable space interval method .In fixed and equal lime steps, by taking the positions of moving interfaces obtained from computation as the grid points, and the difference between them as the space steps, meshes can be generated in computational domains gradually and automatically, the temperature is calculated at these grid points and the positions of moving interfaces of the next lime step are obtained.

A code is developed to compute the α eigenvalue by the multigroup discrete ordinate method.It can overcome the difficulty of α eigenvalue calculation below criticality to a considerable extent.A processing technique is also devised to apply to the system with a cavity at the center.The calculation results are believable and the algorithm is reliable.