Experimental fuel rod stored energy determination. STEED I project

The objective of the STEED I (STored Energy/Enthalpy Determination) project was to evaluate an experimental method for producing accurate and reliable data concerning the stored energy in fuel rods during operation. The STEED data should provide useful information for LOCA evaluation, fuel design and thermo-mechanical modelling. Stored energy refers to the amount of heat, which at a certain time is stored within the fuel. Physical properties of the fuel that affect the quantity of stored energy are radial power profile, burnup, fuel geometry, fuel density and thermal conductivity and heat capacity of the fuel pellet, and the gas gap conductance. The quantity of stored energy is conveniently studied under transient conditions when all, or part of the stored heat is released. This work describes determination of the stored energy by evaluating scram tests. The R2 test reactor is well suited for this type of experiments, where the thermal response of different types of fuel rods can be evaluated and compared. Scrams have been performed with the intent to evaluate the fuel rod stored energy before the scram. Methods have been developed for evaluation of the stored energy from the scram response It was found that the time dependence for a large part of the heat release from the rod could be described by a single time constant. Evaluations of the time constant have been made from the data in different ways. The stored energy has been evaluated integrating the exponential decay. The integral of the exponential decay is the initial power multiplied by the time constant. This means that differences in the stored energy due to, for instance, rod properties or rod power dependence are best studied using the same time constant. The scram response was modelled with the TOODEE2 transient code. The calculations gave a time constant of about 4 s and very little power dependence. The experimental result is a time constant around 4 s. The small differences in the measurement results are within the uncertainties and it is not possible to deduct a power dependence from them. Noise and zero level determination uncertainties made it difficult to obtain a high degree of accuracy in the experimental time constant and stored energy determination. If the 2 Hz noise with an amplitude of about 100 W can be handled and if the zero level correction can be mastered then the time constant and the stored energy can be determined using scrams