This paper describes the implementation of the multi-group pin-by-pin SP3 method in the Pennsylvania State University's (PSU) Nodal Expansion Method (NEM) core transient simulator code. The presented work is a part of the cooperative research between PSU and Polytechnic of Turin (PT) and it was performed in a manner that the existing expertise at both institutions complemented each other.Comparison was made to the VENUS II international benchmark for SP3 and eight groups. The results demonstrated a deviation in the predicted keff as compared to experimental value of 1.00000 is 250 pcm and deviations in the prediction of pin powers as compared to reference values in the range of + or - 2.5%. The lowest order two-group diffusion calculations predicted well the eigenvalue but showed large errors in pin power predictions especially in the MOX assembly (about 9%). The two-group calculations are unable to model accurately neutron transport in the region where the flux is changing rapidly (at the core/reflection interface where MOX assemblies are located). The pin power error is reduced significantly with eight group calculations where in the diffusion case the maximum pin power error values is reduced to about 4.6% and in the SP3 case to 2.5%.
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