Small fission chambers manufactured by Photonis, Reuter-Stokes, CEA and Centronic are routinely calibrated in standard neutron fields of the BR1 reactor at SCK CEN in Mol, Belgium. Two irradiation fields are available: fast spectrum in the MARK3 convertor and thermal spectrum in the Empty Cavity. In this work, for the first time the calibration procedure of larger fission chambers (Photonis CFUL01 and CFUM21 type) with the deposit length exceeding the length of the convertor is presented. Spatial-energy correction factors taking into account the neutron flux shape beyond the convertor as well as radial gradient of the neutron flux were calculated with MCNP and experimentally validated. The combination of calibration (i.e. effective mass measurement) in fast and thermal irradiation fields allows for determination of the purity of the 238U deposit. This is demonstrated on three CFUL01 type fission chambers with purity between 99.8 % and 99.998 %.