The influence of natural trace element distribution on the mobility of radionuclides. The exemple of nickel in a clay-rock

•Ni distribution in the Callovian–Oxfordian formation (Bure, France) was investigated.•Ni is mainly borne by pyrite (Py), chlorite and organic matter (OM).•63Ni sorption on clays is hindered by competition with trace metals (e.g. Zn, Co).•Additional sorption mechanisms are necessary to explain 63Ni long-term retention.•These may be isotopic exchange and structural incorporation (into calcite, OM or Py).