The application of POSIX threads and OpenMP to the U. S. NRC neutron kinetics code PARCS POSIX Threads and OpenMP were used to implement parallelism in the nuclear reactor transient analysis code PARCS on multiprocessor SUN and SGI workstations. The achievable parallel performance for practical applications is compared for each of the code modules using POSIX threads and OpenMP. A detailed analysis of the cache misses was performed on the SGI to explain the observed performance. Considering the effort required for implementation, the directive based standard OpenMP appears to be the preferred choice for parallel programming on a shared memory address machine.