Page "Nuclear reactor" Paragraph 73

Most BWR and PWR commercial reactors use uranium enriched to about 4 % U-235, and some commercial reactors with a high neutron economy do not require the fuel to be enriched at all ( that is, they can use natural uranium ).

According to the International Atomic Energy Agency there are at least 100 research reactors in the world fueled by highly enriched ( weapons-grade / 90 % enrichment uranium ).

Theft risk of this fuel ( potentially used in the production of a nuclear weapon ) has led to campaigns advocating conversion of this type of reactor to low-enrichment uranium ( which poses less threat of proliferation ).