def def_commodities():
    """
    Produces the default commodities for a rickshaw simspec.

    Returns
    ----------
    spec : dict
        Dictionary representation of the niche-link to commodity setup.
    """
    commods = {('mine', 'enrichment'): 'natural_uranium', ('mine',
        'reactor:hwr'): 'natural_uranium_fuel', ('enrichment', 'fuel_fab'):
        'low_enriched_uranium', ('enrichment', 'reactor'):
        'low_enriched_uranium', ('enrichment', 'repository'):
        'enrichment_waste_stream', ('fuel_fab', 'reactor'): 'fresh_fuel', (
        'fuel_fab:uo2', 'reactor:lwr'): 'fresh_uox', ('fuel_fab:triso',
        'reactor:pb'): 'fresh_triso', ('reactor', 'storage'): 'used_fuel',
        ('reactor', 'repository'): 'used_fuel', ('reactor:lwr', 'storage'):
        'used_uox', ('reactor:pb', 'storage'): 'used_triso', ('reactor',
        'separations'): 'used_fuel', ('reactor:lwr', 'separations'):
        'used_uox', ('storage', 'separations'): 'stored_used_fuel', (
        'storage', 'repository'): 'stored_used_fuel', ('separations',
        'fuel_fab'): 'separated_product', ('separations', 'storage'):
        'separated_waste', ('separations', 'repository'): 'separated_waste'}
    return commods
