Code of Federal Regulations Title 10. Energy § 10.150.11 Critical mass

(a) For the purposes of this part, special nuclear material in quantities not sufficient to form a critical mass means uranium enriched in the isotope U–235 in quantities not exceeding 350 grams of contained U–235;  uranium–233 in quantities not exceeding 200 grams;  plutonium in quantities not exceeding 200 grams;  or any combination of them in accordance with the following formula:  For each kind of special nuclear material, determine the ratio between the quantity of that special nuclear material and the quantity specified above for the same kind of special nuclear material. The sum of such ratios for all kinds of special nuclear materials in combination shall not exceed unity. For example, the following quantities in combination would not exceed the limitation and are within the formula, as follows:

(175 (grams contained U–235/350)+(50 grams U–233)/200)+(50 grams Pu/200)=1

(b) To determine whether the exemption granted in § 150.10 applies to the receipt, possession or use of special nuclear material at any particular plant or other authorized location of use, a person shall include in the quantity computed according to paragraph (a) of this section the total quantity of special nuclear material which he is authorized to receive, possess or use at the plant or other location of use at any one time.