Nuclear power plants involve large numbers of fuel elements arranged in a core lattice. These fuel elements produce power and substantial amounts of radiation in the form of neutrons, gamma rays and charged particles. By monitoring the radiation, it is possible to relate the power produced to in-core instrumentation signals from such devices as fission chambers or gamma radiation detectors. The GE BWR power plants have incorporated "in-core" instrumentation and computer-based analyses to provide real time monitoring of core thermal limits and fuel behavior. The GE BWR power plants were originally outfitted with Honeywell 4000 series process computer systems to do the monitoring and computations. These machines have been eclipsed by the substantial improvements in hardware and software of the modern mini-computer systems. Speed and memory performance have easily improved two orders of magnitude in the past 10 years. This allows more intricate analyses to be performed. It is now feasible to perform first principles nuclear simulation of BWR cores with the modern computer systems. Such a simulation system has been developed for a DEC VAX 11/780 and applied at the Duane Arnold Energy Center (DAEC) at Cedar Rapids, Iowa.