The fuel element of the MARIA reactor consists of 6 (MR type) or 5 (MC type) concentric tubes, 2 mm thick, separated by a 2.5 mm water gap. The water in the gaps between the fuel tubes serves a dual role. Firstly, it acts as a neutron moderator (decelerator), needed to slow down the fast neutrons produced during fissions to thermal energies at which subsequent fissions of 235U nuclei efficiently take place. Water also acts as a coolant, i.e. it serves to remove the heat generated in the fuel.

There is up to 485 g of 235U in a single fuel element. The fuel material used in the MARIA reactor is a dispersion of uranium oxide (Russian MR fuel) or uranium silicide (French MC fuel) in aluminium. The 0.8-mm-thick fuel layer is sandwiched between two layers of what is known as a 'fuel jacket', which prevents the release of fission products into the coolant.

The MARIA reactor core holds between 6 and 7.5 kg of uranium-235, an amount that depends on the core configuration, fuel burn-up and the current operating programme. Fuel elements are removed from the core when ²³⁵U burnup has reached between 40-60%. Further fuel burn-up is economically disadvantageous.

After operation in the core, the fuel is transferred to a storage pool from where it is taken back by the manufacturer after a cooling down period of several years.