Nuclear decay heat is a crucial issue for PWR in-core safetyafter reactor shutdown and back-end cycle. It is a dimensioning parameterfor safety injection systems (SIS) to avoid a dewatering of the reactor core.The decay heat uncertainty needs to be controlled over the largest range ofapplications. The assimilation of the MERCI-1 experiment was studied toprovide feedbacks on nuclear data. This experiment consisted in themeasurement of the decay heat of a PWR UOX fuel sample irradiated inthe OSIRIS reactor, for cooling times between 45 minutes and 42 days.More specifically, the consideration of several experimental values ofMERCI-1 at different cooling times was tested. This raised issues aboutcorrelations to consider between different measurements. Besides, theimpact of considering correlations between independent fission yields incovariance matrices on the decay heat uncertainty calculation and on thefeedbacks on nuclear data is discussed.