Robust multipole approach for continuous nuclear Data: RKFIT implementation for X2 VVER-1000 reactor benchmark

The windowed multipole method stands out as a promising approach for effectively conducting on-the-fly Doppler-broadening for continuous nuclear cross-section data. Its implementation was predominantly relied on the conversion of the resolved resonance parameters and vector-fitting technique into the multipole representation. Recently, progress has been made in deriving multipole representations from continuous cross-section data, with one notable method being RKFIT mothed, that is a robust least square fitting method. The advantages and disadvantages of this method have been thoroughly investigated. Detailed instructions on utilizing this method are provided within the OpenMC Monte Carlo calculation code. The experimental reactor physics results of the X2 reactor (a VVER-1000 type reactor) have been used to benchmark the effectiveness of the continues nuclear data generated by this approach via the OpenMC code simulations. Also, the simulation results are compared with those obtained using continuous cross-section libraries and other multipole libraries.