and control of access (as with vaults for precious materials); multiple checking of shipment authorizations and deliveries to shipping; and, since the product fissile material is still quite radioactive, multiple radiometric monitoring of access points and the personnel using them.

Radioactivity of Reprocessed Fuel

Recovered uranium from slightly enriched reactor fuel has more 236U than natural uranium, but after purification from fission products and plutonium, it can still be handled essentially as virgin material. Recovered thorium is highly contaminated with 228Th and its radioactive daughters. It must be stored for 10–20 years before it can be reused. By that time, the activity from 228Th will have decayed to its low natural level.

The plutonium produced from 238U and the uranium produced from 232Th are much more radioactive. Typical plutonium recovered from spent LWR fuel contains 3 percent 238Pu, 57 percent 239Pu, 23 percent 240Pu, 12 percent 241Pu, and 5 percent 242Pu. After several recycles, concentrations might be 5 percent 238Pu, 31 percent 239Pu, 27 percent 240Pu, 17 percent 241Pu, and 20 percent 242Pu.97 Representative numbers from thorium cycles are less firm, since there is far less experience with these fuels. The main radioactive constituent of 233U, however, is 232U, and it has been calculated that 233U from HTGR irradiation could have 300–1000 ppm (i.e., 0.03–0.1 percent) of 232U.98

Each kilogram of the mixture of plutonium isotopes coming from spent reactor fuel has typically over 600 curies (Ci) of alpha activity and more than 10,000 Ci of beta activity. (One curie is 3.7×1010 disintegrations per second, the radioactivity of 1 g of purified radium.) While these radiations are readily shielded by relatively thin containers, they are accompanied by penetrating gamma rays and neutrons. A kilogram of reactor plutonium emits 3×108 gamma rays per second with energy greater than 300 keV, considered “hard” gammas, and 3×105 fast neutrons per second from spontaneous fission of 240Pu. These radiations deliver a dose of about 140 rads/hr, on contact (as in a pocket), that is dangerous and easily detectable, even with shielding. Only a minute quantity could escape detection unless lead or other heavy shielding is used in its removal, and these materials are readily sensed by other means.

Freshly reprocessed 233U has less radiation; its alpha activity, mostly from 232U, is only about 30 Ci/kg, and the penetrating gamma radiation that accompanies it is one tenth that of plutonium. This is probably detectable in large quantities. However, it may be desirable to “age” 233U in order to increase its detectability, as illustrated below.

If the fissile material is not used after reprocessing, its short-lived isotopes (238Pu, 241Pu, and 232U) begin to decay. From plutonium, the

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