In the spent fuel dissolving vessel, the resonant isotopes in fuel pellets is surrounded by fissile solution. Complicated resonance interaction between the solid fuel and fissile solution is induced. Conventional self-shielding methods based on the equivalence theory for PWR cell and assembly will not apply because the neutron flux spectrum in the fissile solution are quite different from those PWR cases where no resonant isotope is in the moderator. In the present paper, Sub-group method is applied to calculate the space-dependent self-shielded multi-group cross-section. Sub-group parameters such as sub-group weights and sub-group cross-sections are calculated by fitting method using the NJOY processed multi-group constant under various dilutions. Elastic scattering Resonance for heavy isotopes is taken into account. Space-dependent sub-group flux is then calculated by fix source sub-group transport calculation. Spatially-dependent self-shielded multi-group cross-sections and multiplication factor can be obtained. Calculations are carried out on the NEA-CRP proposed standard problems for fissile pellets in fissile solution. Results show that the sub-group method is a very promising deterministic method for the self-shielding calculation in the spent fuel dissolution.

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