Criticality safety analysis devoted to spent-fuel storage and transportation has to be conservative in order to be sure no accident will ever happen. In the spent-fuel storage field, the assumption of freshness has been used to achieve the conservative aspect of criticality safety procedures. Nevertheless, after being irradiated in a reactor core, the fuel elements have obviously lost part of their original reactivity. The concept of taking into account this reactivity loss in criticality safety analysis is known as burnup credit. To be used, burnup credit involves obtaining evidence of the reactivity loss with a burnup measurement.

Many nondestructive assays (NDA) based on neutron as well as on gamma-ray emissions are devoted to spent-fuel characterization. Heavy nuclei that compose the fuels are modified during irradiation and cooling. Some of them emit neutrons spontaneously, and the link to burnup is a power link. As a result, burnup determination with passive neutron measurement is extremely accurate.

Some gamma emitters also have interesting properties in order to characterize spent fuels, but the convenience of the gamma spectrometric methods is very dependent on the characteristics of the spent fuel. In addition, contrary to the neutron emission, the gamma signal is mostly representative of the peripheral rods of the fuels.

Two devices based on neutron methods but combining different NDA methods which have been studied in the past are described in detail:

1. The PYTHON device is a combination of a passive neutron measurement, a collimated total gamma measurement, and an online depletion code. This device, which has been used in several nuclear power plants in western Europe, gives the average burnup within a 5% uncertainty and also the extremity burnup.

2. The NAJA device is an automatic device that involves three nuclear methods and an online depletion code. It is designed to cover the whole fuel assembly panel (active neutron interrogation, passive neutron counting, and gamma spectrometry).