  Evaluating the neutronic state of the whole nuclear core is a very important
topic that have strong implication for nuclear core management and for security
monitoring. The core state is evaluated using measurements. Usually, part of
the measurements are used, and only one kind of instruments are taken into
account. However, the core state evaluation should be more accurate when more
measurements are collected in the core. But using information from
heterogeneous sources is at glance a difficult task. This difficulty can be
overcome by Data Assimilation techniques. Such a method allows to combine in a
coherent framework the information coming from model and the one coming from
various type of observations. Beyond the inner advantage to use heterogeneous
instruments, this leads to obtain a significant increasing of the quality of
neutronic global state reconstruction with respect to individual use of
measures. In order to present this approach, we will introduce here the basic
principles of data assimilation focusing on BLUE (Best Unbiased Linear
Estimation). Then we will present the configuration of the method within the
nuclear core problematic. Finally, we will present the results obtained on
nuclear measurement coming from various instruments.
