A subcritical physics testing program which utilizes vanadium self-powered incore instrumentation thimble assemblies to provide an actual measured powered distribution that is used to confirm that the core will operate as designed The signals received from the incore detector elements are integrated until a fractional uncertainty is less than a specified level The measured power distribution is then compared against a predicted power distribution for a given rod position or temperature difference. If the measured power distribution is within a specified tolerance to the predicted power distribution, then the core is expected to behave as predicted.