Experimental and Theoretical Evidence for a Short Effective Fuel Time Constant in a Boiling Water Reactor

The effective time constant related to heat transfer from fuel to coolant is a very important parameter for the dynamic behavior and thus the stability of a nuclear reactor. Usually a single time constant of a lumped parameter model is used. Both experimentally, via two independent methods of analysis, and theoretically, it is determined that a more elaborate model, using two or three time constants, is necessary. Heat transfer for high frequencies is governed by the small fuel time constants that stem from the outer region of the fuel. The bulk follows slow variations with time constants of >5 s.