Experimental investigation of the distribution of the gaseous fission products in fuel rods

At the laboratories of High and Medium Activity (LHMA) of the Nuclear Research Centre at Mol, several destructive techniques are existing to determine the distribution of the fission gases krypton, xenon and tritium between the cladding, the plenum and the fuel of nuclear rods. A puncture device allows to measure quantitatively the free gas contents of the plenum. Aliquots of this gas are sampled for analysis. The gas components as well as the isotopic composition of the noble gases are determined by mass spectrometry. The tritium content is found by oxidation of an aliquot to tritiated water followed by liquid scintillation counting. A hot vacuum extraction method is used to extract quantitatively the tritium out of the cladding material. The quantity of 3H gas is also measured by liquid scintillation counting after oxidation. A dissolution technique is used to determine both the tritium and noble gases contents of the fuel. Dissolution of the fuel in a boiling mixture of HNO3-HF converts all 3H into tritiated water. This water is separated and purified by several successive distillations so that it can be measured for its 3H content by liquid scintillation counting. The same dissolution process also results in a release of the entrapped Xe and Kr molecules. After passage through a purification chain they are sampled and analysed for their isotopic composition by mass spectrometry. (author)