Positive Difference Schemes in Neutron Spectral Codes

In the paper a new finite difference scheme assuring positivity of solutions in spectral codes based upon the Greuling-Goertzel approximation is developed. The method can be adopted in any code of this type without changing the execution time and storage requirements. The method has been applied in the Algol version of the BIGG code and found advantageous in the case of comparatively wide lethargy intervals when the standard central difference formulas could lead to instabilities.