def make_fuel_mat_str(values):
    """Make a string for fuel material cards in MCNP format.

    This function takes burnup material inventory data from the main 'values'
    dictionary and makes fuel material cards as a string in MCNP format.

    Arguments:
        values (dict)[-]: Main dictionary storing processed values.

    Returns:
        mat_card (str)[-]: A string of MCNP material cards.
    """
    mat_card = ''

    # Write new material cards with inventories of the last burnup step.
    final_step = 'step_{0}'.format(values['burnup_summary']['step'][-1])

    sorted_keys = sorted(list(values['inventory']),
                         key=lambda x: (str(x[0]), x[1]))
    for (mat, step) in sorted_keys:
        if mat != 'sum' and step == final_step:
            mat_obj = values['inventory'][(mat, step)]
            mat_obj.metadata['mat_number'] = mat
            mat_card += mat_obj.mcnp()
    return mat_card