Impact of Auto-irradiation on the Thermophysical Properties of Oxide Nuclear Reactor Fuels

The nuclear fuel assemblies are used in the reactor for 3 to 6 years, and then are stored underwater at relatively low temperature. During this cooling phase, radioactive decay damage and helium begin to accumulate in spent fuel. After a few years of cooling, when the radioactivity level has sufficiently decreased samples of spent fuel are extracted for post-irradiation examination. In order to assess the in-pile value of thermophysical properties using out-of pile measurements, the effect of decay damage has to be distinguished from that caused by fission during reactor operation. This characterisation is also necessary in order to provide information on the evolution of the state of spent fuel under final or interim storage conditions. The characterisations performed on fuel that has accumulated damage during years of storage reveal that the damage has significant effects on the thermophysical properties and that some of these effects can be progressively annealed-out by heat treatments. This makes the interpretation of the experiments difficult, and specific methods and verifications have to be applied. The effects of -damage on the thermophysical properties of the fuel were investigated at JRC/ITU using samples of UO2 doped with ~0.1 and ~10 wt% 238Pu. These samples were stored for different times, under conditions where -damage dose resulting from the decay of 238Pu could be accurately calculated. Some specimens were annealed to as-fabricated state (t = 0) and prepared for periodical X-ray diffraction to monitor the lattice parameter evolution. Knudsen-cell helium release experiments and transmission electron microscopy examinations were performed after four years of damage accumulation. After five years, the degradation and recovery of the thermal diffusivity was measured with a laser flash technique during thermal annealing programs. Six months later, the apparent heat capacity, Cp*, was measured by differential scanning calorimetry. The deviation of the measured Cp*(T) from the real heat capacity, Cp(T), is related to the recovery of the latent heat of the lattice defects during thermal healing. Each recovery stage observed on the Cp*(T) curve was analysed and attributed to a certain kind of defect (oxygen Frenkel pair recombination, uranium vacancy/interstitial clusters recombination, dislocation loop growth, void growth). It was observed that the recovery process in the apparent heat capacity did not affect individual laser flash heat transport measurements, since these are characterized by very short laser pulse duration (and very small temperature increase) compared to the kinetics of the Cp* recovery. Repeated laser flash measurements did not induce any change in the apparent heat capacity. The thermal diffusivity, measured during annealing cycles, displayed three annealing stages. Comparison between the 0.1 and 10 wt% 238Pu samples shows that the degradation of the diffusivity with increasing -dose is not linear, and that saturation occurs at relatively low doses. As a result, the thermal conductivity degradation of stored spent-fuel can be expected to level off after a few years of storage. A correlation quantifying this degradation is proposed.

STAICU Dragos;  WISS Thierry;  SHEINDLIN Mikhail;  RONDINELLA Vincenzo;  HIERNAUT Jean-Pol;  RONCHI Claudio; 

2006-03-17

Springer

JRC30258