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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 (different materials, designs, burnups etc) can be evaluated and compared.
Scrams were performed with the intent to evaluate the fuel rod stored energy before the scram. Methods were developed for evaluation of the stored energy from the scram response. It was found that the time dependence for a large part of the stored heat release from the rod could be described by a single time constant.
Evaluations of the time constant were made from the data in different ways. The stored energy was evaluated by integrating 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 time constant.
The fuel rodlets used were manufactured by ABB Atom. The cladding was of the type PWR 17x17 and the rodlets were filled with He-gas at a pressure of 3.0MPa.
The report describes the use of fuelled rods, empty rods and one rod with the fuel column replaced by a stainless steel rod. These were necessary to make corrections:
A) Stored heat outside the fuel at the time of the scram, that is gamma heating contribution from the R2 reactor in the ramp rig structural material, the rig coolant and the non-fuel parts of the rodlet. This heat release is measured by scram testing of an empty rodlet. Steady state power was also determined by using the empty rodlet to correct for gamma heating from the R2 reactor.
B) Power generation in the fuel after the scram. This does not belong to the stored energy in the rod before the scram. The major contribution is the decay heat (gamma). This does not only originate from the fuel rodlet but also from the surrounding R2 driver fuel. Just after the scram there is also a contribution from fissions due to delayed neutrons.
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Keywords: fuel design, fuel rods, loss-of-coolant accident, modelling, stored energy, thermal mechanical.