11.39. After removal from the reactor and insertion in the storage pool, all of the radionuclides contained in the fuel continue the decay process. Short and moderately short half-life fission products become insignificant after a few months. Therefore, a cooling period of 150 days is a useful point of reference, which just happens to be the intended cooling period when reprocessing was planned.

11.40. The major contributions to the radioactivity of spent reactor fuel after 150 days of cooling are given in Table 11.1. The values, in curies and becquerels per metric ton (1000 kg) of uranium (initially free from plu­tonium) charged to the reactor, were calculated for a hypothetical LWR having a thermal power of 3300 MW and a fuel burnup (§10.15) of 2.85 x 1012 J (thermal)(2.85 TJ) per kilogram of uranium in the original fuel.[18] Somewhat different activities would be applicable to other operating con­ditions, but those in the table are fairly typical. An indication of the re­spective activities at times after 150 days can be obtained from the half­lives (see also Fig. 11.6). It is seen that for a cooling period of 150 days (or more) a relatively few fission products, namely, strontium, zirconium, niobium, ruthenium, cesium, and some rare-earth elements, are respon­sible for nearly all of the radioactivity. These are the most important elements from which the uranium and plutonium would be separated in spent fuel reprocessing.

11.41. The manner in which buildup of isotopes of the heavy elements occurs during reactor operation in a fuel consisting of uranium-235 and uranium-238 is illustrated in Fig. 11.2. Horizontal arrows pointing to the right represent (n, y) reactions and those pointing to the left are for (n, 2n) reactions with fast neutrons. Vertical arrows indicate beta decay. Where vertical arrows are absent, the nuclides are alpha-particle emitters. The

TABLE 11.1. Major Contributions to Radioactivity of Spent LWR Fuel After 150 Days Cooling

Main Activity

Half-Life Decay

Nuclide (years) Mode Ci/1000 kg U Bq/1000 kg U

Fig. 11.2. Heavy-isotope buildup in uranium. (Unless otherwise indicated, the nuclides are alpha-particle emitters.)

Urn’ll’

243Am ‘li! 244Am~

4 4

238Pu^ 242pu !U> 243Pu_

d 4 d

*>2Np,^238NP<.^ 239Np(^rl 240Np.._

4, , 4 4

235uln-|Tl 236y ln,|)’1 237у ZZ 238и |П’,’1’1 239y <П’?1 240g_.<_

(n.2n)

decay product is then a nuclide with an atomic number two units less and a mass number of four units less than the parent. Alpha-particle decays are of minor importance in the cooling period, but they affect the buildup of heavy isotopes at a later stage (§11.87).

11.42. Of immediate interest is uranium-237 (half-life 6.75 days) which is formed by two (n, y) stages starting with uranium-235 or by the (и, 2n) reaction with uranium-238. Any uranium-237 remaining in the spent fuel will be associated with the recovered uranium. Since uranium-237 is a gamma-ray emitter with a fairly short half-life, its presence makes the product difficult to handle. After a 150-day cooling period, however, the uranium-237 will have decayed to such an extent that the gamma activity is small enough to be tolerable. Moreover, the beta decay product, nep­tunium-237, will be separated from the uranium during the reprocessing operation.

11.43. A chart similar to Fig. 11.2 showing the buildup of heavy isotopes when thorium-232 is included in the fuel is shown in Fig. 11.3. With thor­ium-232 as fertile material, a long cooling time would be required to permit decay of the intermediate protactinium-233 (half-life 27 days) to the fissile uranium-233. During this period, the activity of thorium-234 (half-life 24 days) would also decrease to a permissible level. However, at the same time, thorium-238 (half-life 1.9 years) would accumulate as a result of the alpha decay of uranium-232 (Fig. 11.3). Since thorium-228 has strong gamma — ray emitters among its daughter products, remote handling of spent fuel would appear to be necessary regardless of the cooling time.