The principal actinides involved in using thorium-uranium fuel are shown in the actinide chains of Fig. 8.11. The important reactions are the fission of 233U and 235U and the absorption of neutrons in 232 Th to form 233 U.

The relatively long 27.0-day half-life of 233Pa, the precursor of 233U, affects the time that irradiated fuel must be stored prior to reprocessing. If the discharged fuel is stored only for 150

Figure 8.11 Actinide chains in thorium fuel.

days, as is frequently specified for sufficient decay of 1311, some of the 233 Pa will remain during reprocessing. Protactinium is one of the more difficult elements to separate from uranium, and the high radioactivity of protactinium will contribute to the problem of decontaminating the uranium product after it is separated from the fission products and thorium. Also, if protactinium is not recovered, the loss of undecayed 233Pa will represent some loss in the production of 233U for recycle.

Another problem of the thorium fuel cycle results from the radioactivity of 72-year 232 U, and its daughters. 232U is formed by (n, 2n) reaction with 232Th according to

232 Th 231 Th 231 Pa M2Pa __J1————— > 232 и (8.13)

25.5 h 1.31 days v ‘

and by

233 jj n’2n, 232 у

Also, many thorium ores as well as thorium, which is obtained as a by-product of uranium mining, contain traces of 230Th, a radionuclide in the decay chain of 238U. Neutron absorption in 230Th also results in the formation of 232 U:

Although significant alpha activity results from 232 U in the 233 U to be recovered and recycled, more of a problem results from the 232U daughters. The 232U decay daughter is 1.91-year 228Th, a radionuclide that is also formed by the radioactive decay of 232Th. As shown in Table 6.3, the decay daughters of 228Th are all short-lived, so they reach secular equilibrium with 228Th after a delay time of only a few days. The decays of 212Bi and 208T1 are accompanied by very energetic and penetrating gammas, so gamma shielding is required when fabricating fuel from recycled uranium containing 232U.

Although chemical reprocessing yields essentially pure uranium, storage after separation and time elapsed in shipping to fabrication allow the buildup of 228Th and its decay daughters. Consequently, the gamma activity in separated uranium containing 232 U increases continuously with storage time, until it reaches a maximum at about 10 years after separation. Once uranium has been separated from thorium, there is considerable incentive to complete the uranium purification and fuel fabrication quickly to avoid the increasing gamma radiation due to the buildup of 228Th. Hydrogenous shielding is also necessary because of the high-energy neutrons from alpha decay in recycled uranium. The alphas from the decay of 233U, 232U, and 228Th interact with light elements such as oxygen and carbon to form neutrons, so the neutron activity also increases with storage time.

The 228Th and 234Th appearing with irradiated thorium fuel results in appreciable radioactivity in the separated thorium. Consequently, as discussed in Sec. 2.9, it may not be practicable to recycle the recovered thorium until it has been stored for about 5 to 20 years.

When 235 U is used as fissile makeup in the thorium cycle, as in the reference high-temperature gas-cooled reactor (HTGR) fuel cycle, the high bumup and uranium recycle result in considerable production of “’’Np, according to the reactions shown in Fig. 8.11. The 237Np then forms a relatively large activity of 238Pu. These plutonium activities are important because of the problems of decontaminating uranium from plutonium when reprocessing the uranium. Also, even though fissile plutonium is formed by neutron absorption in the 238 U
accompanying the highly enriched 23SU makeup, the high activities of 238Pu may discourage the utilization of the fuel value of plutonium in the discharge fuel.

Relatively little 239Pu, 240 Pu, 241 Pu, americium and curium are formed in the irradiation of thorium-uranium fuel with 235 U fissile makeup. However, when plutonium is used as fissile makeup for a thorium fuel cycle, considerable quantities of americium and curium are formed. As discussed in Sec. 2.4, these are the radionuclides that are the greatest contributors to radioactivity and ingestion toxicity after about 600 years of waste isolation, when the fission products have decayed.

Material quantities and activities of the actinides calculated [HI, P3] in the cooled discharge fuel from the uranium-thorium-fueled HTGR (cf. Fig. 3.33) are listed in Table 8.6. The natural thorium is assumed to contain 100 ppm 230Th, so the quantities of 228Th and 232U in the discharge fuel are greater than would occur for thorium consisting of pure 232 Th. The strongest actinide beta source is 233Pa, which contributes 7.58 X 106 Сі/year after 150 days of cooling. In the uranium, which is to be recovered and fabricated into recycle fuel, the main contributors to alpha activity are 232 U and 233 U. Both are important as potential environmental contaminants, but the activity of the 232U daughters, which grow into separated uranium prior to fabrication, dictate the requirements for semiremote and remote fabrication. By comparison with the data in Table 8.5, the total alpha activity of 5.16 X 103 Сі/year in the uranium to be fabricated as recycle HTGR fuel is much less than the 1.70 X 10s Сі/year of alpha activity in the plutonium to be fabricated for recycle in a 1000-MWe LWR.

The total alpha activity in the plutonium in the HTGR discharge fuel is within 20 percent of the total alpha activity in plutonium from the uranium-fueled LWR (Table 8.4). In both cases the plutonium alpha activity is dominated by 238 Pu. However, the HTGR plutonium consists of 66 percent 238Pu, and the high alpha activity, the high heat generation rate, and the low fissile content mitigate against the recycle of HTGR plutonium.

Because of the relatively small amount of high-mass plutonium nuclides produced in uranium-thorium fueling, the amounts of americium and curium produced are about two orders of magnitude less than in a uranium-fueled reactor with plutonium recycle.