HEXAGONAL CMFD FORMULATION EMPLOYING TRIANGLE-BASED POLYNOMIAL EXPANSION NODAL KERNEL

A triangle-based polynomial expansion nodal (TPEN) method is developed such that it can be readily implemented into a hexagonal coarse mesh finite difference (CMFD) formulation. This method combines the higher order polynomial expansion nodal (HOPEN) method and the nodal expansion method (NEM) by decoupling the three-dimensional neutron diffusion equation into a radial and an axial one. For the implementation into the hexagon-based CMFD formulation, the TPEN method is derived such that it solves one hexagonal node problem consisting of six triangular nodes. The coupled solution for the six triangles is obtained concurrently by algebraic direct elimination. The inefficient point flux computation scheme of the original HOPEN method is improved by using a 13 term polynomial expansion for the intranodal flux within a hexagon. The computational speed of the TPEN based CMFD is about 5 times faster than the original HOPEN accelerated with coarse mesh rebalancing. The accuracy of this method is very good in that the errors in the eigenvalue and power distribution are within 15 pcm and 1%, respectively, for various eigenvalue benchmark problems, and in that the transient core power behaviors for two VVER rod ejection benchmark problems also agree well with the references. 1