BURN-UP MEASUREMENTS AT TRIGA FUEL-ELEMENTS CONTAINING STRONG BURNABLE POISON

The reactivity method of determining the burn-up of research reactor f uel elements is applied to the highly enriched FLIP elements of TRIGA reactors. In contrast to other TRIGA fuel element types, the reactivit y of FLIP elements increases with burn-up due to consumption of burnab le poison. 33 fuel elements with burn-up values between 3% and 14% wer e investigated The experiments showed that variations in the initial f uel composition significantly influence the reactivity and, consequent ly, increase the inaccuracy of the burn-up measurements. Particularly important are variations in the initial concentration of erbium, which is used as burnable poison in FLIP fuel. A method for reducing the ef fects of the material composition variations on the measured reactivit y is presented If it is applied, the accuracy of the reactivity method for highly poisoned fuel elements becomes comparable to the accuracy of other methods for bum-up determination.