Cross-Checking of the Operator Data Used for Burn Up Measurements

Taking into account of the loss of reactivity of fuels at the end of their irradiation is known under the
term burnup credit (BUC). It is a question of dimensioning in a less penalizing way the devices of transport,
storage or of processing with respect to the risk of criticality. In the context of nuclear criticality safety a better
realism cannot be obtained at the price of conservatism. As a result the regulator requires measurements make it
possible to validate the adequacy between real fuels and the design assumptions. The sophistication of the
measures recommended by the regulator can vary from a simple control of the irradiation to a complete
characterization of fuels. It is within the latter framework that this study fits of which the goal is to describe the
methods by which some of the data necessary to interpretation of the measurements and provided by the operator
could be validated. The interest is that if the method of measurement makes it possible to detect a basic data
error, its effects on the result do not have to be integrated into the assessment of uncertainty of measurement.
This paper exposes the mechanisms making it possible to validate the data input used by calculations associated
with interpretation in term with burnup rate with passive neutron measurements. In the majority of the cases, the
time of cooling as well as the history of irradiation could be validated in a reliable way if the precision of
measurements of gamma spectrometry makes it possible to consider the relationship isotopic with a precision of
about 10%. It remains that in practice, the reliability of the methods described in this document depends
primarily on the performances of the instrumentation, which will be used. A work of evaluation of these
performances on the basis of characteristic of the instrumentation remains to be made for each project. A study
of sensitivity based on sets of incorrect data but plausible will be carried out in order to specify the performances
of the measuring instruments necessary to the detection of each anomaly. Contrary to the time of cooling and
history of irradiation, the checking by gamma spectrometry of the initial composition of fuels MOX and UOX
remains problematic because, on the one hand, the transmitters gamma are not sensitive to the initial isotope of
the plutonium used in fuels, and on the other hand, the sensitivity to the initial content plutonium or initial
enrichment in 235U remains low. In any event, very accurate gamma spectrometry measurements should be used
for checking initial contents. It is however significant to note that the initial composition can be checked
independently of the operator and that thus the probability of an error is less than for the operating data.