[en] At FCA, the Doppler effect is measured up to 2000degC by using the combination of measurement methods; one method is a sample reactivity measurement and the other is a foil activation measurement with laser hearing. In these measuring devices, relatively large amount of tungsten is used as the high temperature structural material. For analyzing the experimental data, it is important to evaluate the resonance interaction effect between ²³⁸U of the Doppler sample and tungsten structural material of the sample capsule and between ²³⁸U in fuel and ²³⁸U of heated sample. A new cell code, PEACO-X, was developed to obtain effective cross sections in the resonance energy range where the effects of overlapping between resonances is significant. The ultra-fine group constants (MCROSS library) required for the detailed analysis of resonance overlapping were generated from the JENDL-3 data by using both the processing codes, TIMS-1 for heavy nuclei and PROF GROUCH-G for light and intermediate nuclei. The PEACO-X code calculates the regional neutron flux in a heterogeneous cell with ultra-fine group structure (Δu = 0.25 ∼ 4 x 10^-4). For the calculation of the neutron spectrum, the RABBLE method was adopted. The PEACO-X code can handle 20 nuclides, all of which can be resonant absorbers, 25 compositions and 50 regions. The PEACOX-EDIT code was also developed to calculate regional reaction rates by using both the neutron fluxes obtained from the PEACO-X and the cross section data in MCROSS library, and generate effective cross sections with the JFS-3 type group structure. By using the PEACO-X code for the analysis of Doppler effect measurement in a fast reactor core, the detail cell calculation of the effective cross section of Doppler sample and the precise evaluation of resonance interaction effects are obtained. (author)