Two methods for the representation of moving control rods are defined. These methods have been used in recent analogue and digital computer studies on finite difference models of axial transients. The errors involved in their use are evaluated for a typically severe case of flux distortion by comparison with a one-group diffusion theory approach.nIt is shown that with either method, maximum error in rod penetration does not exceed about 0*06 of an interval for a 16 interval model or 0.17 of an interval for an 8 interval model. However, changes in reactivity are estimated much more accurately by the "quadratic" method than by the "linear" method.
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