parafish: A parallel FE–PN neutron transport solver based on domain decomposition. A new core solver named parafish is presented for the solution of large neutron transport core calculations. The second-order even-parity form of the time-independent Boltzmann transport equation is solved using an innovative algebraic domain-decomposition method. The spatio-angular discretization is performed using non-conforming finite elements and spherical harmonic expansions (PN method). The parafish code allows one processor to handle more than one domain. This enables proper evaluations of the speed-up. Also, this enables to show that the domain-decomposition method not only performs well in parallel calculations, but also has an inherent acceleration potential. That is, it yields acceleration even without increasing the number of processors.