Second Barrier Fuel Cladding

The fuel cladding is the second barrier, and (from the viewpoint of normal operation) the most significant barrier preventing the release of fission products into the reactor coolant. Regular and proper leak testing of fuel assemblies is an inevitable requirement that is closely related to not only operational regulations but also attempts to assure optimal operation from a radiation protection point of view. 

By monitoring the activity concentrations of several short-lived radionucUdes (e.g., iodine and noble gases) in the primary coolant, fuel defects can be identified, and the defect type (tight or open) and number of leaking elements can be assessed. Major nuclear characteristics of the defective element, such as bum-up and the original enrichment, can be determined by measuring the activity ratios of certain long-lived nuclides (e.g., cesium, transuranium), so failed elements can be localised. 

The concentrations of the radionuclides originating from both the uranium concentration of the core and defective fuels are measured regularly. Iodine and cesium nuclides are measured by gamma spectroscopy. Occasionally, alpha and beta spec-trometric analyses are performed to determine transuranium and strontium isotopes, respectively. 

Fuel leakages are qualitatively indicated by a sudden increase in the concentrations (spiking) of fission products, especially noble gases and iodine nuclides. The appearance of less volatile nuclides in the coolant (e.g., transuranium nuclides) suggests the occurrence of open defects. 

The following mathematical approaches can be used for quantitative fuel characterisation. 

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