Ternary fission

Tritium is also produced in ternary fission and by neutron-induced reactions with 6Li and 10B. Tritium is a very low energy (3 emitter with a half-life of 12.33 y. The global inventory of naturally produced tritium is 9.6 x 1017 Bq. Tritium is readily incorporated in water and is removed from the atmosphere by rain or snow. Its residence time in the stratosphere is 2-3 y after reaching the troposphere it is removed in 1-2 months. The natural concentration of 3H in streams and freshwater is 10 pCi/L. 

Tritium is also formed as a product of ternary fission in power reactors. Yields in thermal fission of 235U and 239Pu are about 1 x 10-4 and 1.5 x 10 4 respectively (NCRP, 1979). Most of the tritium is retained in the fuel, but some may be released to atmosphere as HTO during reprocessing. At present, no fuel is reprocessed in the USA. The NCRP (1979) report included speculative estimates that reprocessing in Europe, excluding the USSR, may release 0.4 kg a-1. This is small compared with the release in atmospheric thermonuclear tests. 

Minor amounts of 4He and 3He are produced in ternary fission. Spontaneous fission of 2 i2Th has also been observed (Whetherill, 1953), as has induced fission of 23,Pu in the Oklo natural chain reactor (Drozd et al., 1974). b Except as noted, data from review/compilation by Hyde (1974). 

Predominant P—O Fission. In the absence of Zn2+ ion, the reactions of PPS and PCA were very slow. Therefore, Zn2+ ion is essential for faster reaction. The kinetics described later indicate that the reaction proceeds through the formation of ternary complex (A) as illustrated in Figure 13. The oximate anion in A may either attack phosphorus (Path a) or sulfur (Path b). Inorganic sulfate was obtained quantitatively. This itself is not proof of Path a, because C (prepared separately) was found to be hydrolyzed readily to give sulfate under the same reaction conditions. However, the other isolated major product was B instead of the oxime catalyst that would be regenerated from C. The product B gave methylphenylphosphate when solvolyzed in methanol in the presence of Zn2+ ion. Methylphenylphosphate also was obtained directly from A in the reaction in methanol, whereas the formation of methylsulfate was not detected. Thus, these results all indicate that the Zn2+PCA complex promotes predominant P—O fission. 

The yields of selected mass chains that result from fission of U-235 and Pu-239 are shown in Table 21.8. In addition to the fission products with masses roughly half that of U-235, neutrons, tritium, helium, and beryllium are products of ternary fission. There are significant differences in some of the yields from U-235 and Pu-239. 

Ternary fission into three fragments of similar mass may proceed in two different ways (Fig. 8.18) (a) is a sequence of two binary fissions, also called cascade fission, whereas (b) illustrates a true ternary fission.. Theoretical considerations lead to the conclusion that, in general, cascade fission is more probable, with the exception of ternary fission at low Z /A values and at low excitation energies. 

Figure 8.18. The two different modes of fission into three fragments (schematic) (a) cascade fission (b) true ternary fission.

Deuterium is in very low concentration. Lithium has an atomic weight of 6.94 and the abundance of Li is around 7% in natural Li. The main reaction product of B is Li which does not generate but there are other, minor reactions that do. Except in boron steels, the activation of Li predominates. Another source of in fission reactors is the low yield, ternary fission of fuel (-130 x 10 atoms per fission product pair). In Magnox gas-cooled reactors, from ternary fission is mainly retained in the metallic uranium fuel and its cladding but some is released into the coolant circuits, where it may possibly diffuse into structures within the primary vessel. Tritium is a low energy /5 emitting radionuclide of low radio-toxicity and with a half life of 12.3 years. 

The intention of this work was to measure the concentration of Li in examples of unirradiated reactor steel from which the likely activity concentration could be calculated using suitable values for the neutron flux. Calculated values could then be compared with the activities measured in examples of steel irradiated in a Magnox reactor. From this it was hoped to ascertain whether, given the concentration of Li, the activity could be reliably calculated or whether the effects of formation from ternary fission and subsequent diffusion would prevent this. Whilst the activity in activated steel was measured in a straight forward manner, the measurement of Li in steel was difficult. The conventional analytical techniques of ICP-OES and ICP-MS respectively failed to achieve an adequate detection limit, so alternative techniques were tried. SIMS was sensitive enough to detect Li but is a microscopic technique and so was prone to large uncertainties when used to predict the bulk concentration. NAA followed by radiochemical determination allowed the determination of but initially suffered interference from another activated radionuclide ( S). 

Kugler, G. and Clarke, W.B., 1972. Mass spectrometric measurements of H, He and He produced in thermal-neutron ternary fission of evidence for short range He. Phys. Rev., Ser. C., 5 551-560. 

Tritium is produced by ternary fission and resides in the moderator. It is a soft p-emitter and is responsible for about 40% of man-rems exposure in the reactor. 

In addition to tritium produced by ternary fission, as shown in Table 8.1, tritium is also produced in reactors by neutron reactions with lithium, boron, and deuterium. Reactors can be designed to produce tritium by irradiating lithium targets with thermal neutrons, resulting in the (/I, a) reaction ... 

Tritium collection. Tritium in air is usually in the form of water vapor and less commonly in the elemental or organic-bound forms. It is generated in nature by cosmic-ray interactions, and at nuclear reactors and tritium-production facilities by ternary fission and neutron activation. Tritium as HT tends to oxidize to water vapor in air. Conversion to and from organic-bound tritium occurs in biota (NCRP 1979). 

Abstract This chapter first gives a survey on the history of the discovery of nuclear fission. It briefly presents the liquid-drop and shell models and their application to the fission process. The most important quantities accessible to experimental determination such as mass yields, nuclear charge distribution, prompt neutron emission, kinetic energy distribution, ternary fragment yields, angular distributions, and properties of fission isomers are presented as well as the instrumentation and techniques used for their measurement. The contribution concentrates on the fundamental aspects of nuclear fission. The practical aspects of nuclear fission are discussed in O Chap. 57 of Vol. 6. 

Alpha particles are by far the most prominent ternary fragments. Other lighter and heavier fragments are, however, also emitted. The extreme sensitivity of the mass separator Lohengrin has allowed to measure yields of ternary fragments down to values of 10 %. A survey of yields of ternary products as a function of their atomic number (Z) is given in O Fig. 4.29 for the thermal-neutron-induced fission of and... 

It is interesting to note that the spectra of ternary particles extend to different atomic numbers, which coincide nearly with the size of the neck that results from the postulate extracted from the mass yield and nuclear charge distributions. To correlate the data, it has been postulated that the two spheres of the dumbbell configuration shown in O Fig. 4.11 for the fission of are practically the same for all asymmetrically fissioning nuclei from Z= 90 to about 99 and, consequently, that the variation in the neutron/proton numbers of the different compound nuclei must be connected with the size of the neck. O Fig. 4.29 is the direct experimental proof for this assumption in the fission of uranium, the neck size is (92 — 82 =) 10 protons in the fission of californium, the neck size is (98 — 82 =) 16 protons. The situation is similar for neutrons and for the total mass. This is, however, less convincing due to prompt neutron emission. 

Wdstheinrich M, Pfister R, Gonnenwein F, Denschlag HO, Faust H, Oberstedt S (1998) Ternary partides fi-om the reactions (nth, U(nth, f) and Pu(nth, f). In Fioni G, Faust H, Oberstedt S, Hambsch FJ (eds) Nudear fission and fission-product spectroscopy. American Institute of Physics (AIP), Woodbury, NY), pp 330-337 YCALC (2003) http //wwwJcemchemie.uni-mainz.de/ institut/links.html... 

Recent studies of SF of Cf with large y-ray detector arrays, Gammasphere (Lee 1990), revealed a new fission mode, cold fission with no neutron emission in the pair fragments of Zr-Ce and Mo-Ba (Ter-Akopian et al. 1994 Hamilton et al. 1995). Cold ternary SF, as Cf Ba + Sr -l- °Be, has also been identified by measuring the various y-transitions of these nuclei in coincidence (Ramayya et al. 1998). The use of the high-resolution triple y-coincidence technique can provide important information on very rare events in fission. 

Tritium was produced in large quantities by ternary fission. Although their yields are smaller. 

Most proposed MSR designs call for the use of enriched lithium-7 and/or beryllium such as in the 70.7%T ,iF-17%BeF2-12%ThF4-0.3%UF4 fuel salt of the 1970s MSBR program. This carrier salt is often termed FLiBe. The presence of either Li or Be leads to significant production of tritium, on par with production levels in heavy water reactors such as CANada Deuterium Uranium (CANDU). For example in the 1970s, the 1000 MWe MSBR projected 2420 curies per day (ORNL 4541,1971), 98.3% from lithium and beryllium, and the remainder of 0.4% from fluorine and 1.2% from ternary fissions. For comparison, CANDU operations typically experience a tritium release rate of less than 24 curies per day (CNSC INFO-0793,2009). 

In 0.2 to 0.3% of all fissions a third, light fragment beside the two medium-weight products is generated. The light product of ternary fissions with the greatest significance in reactor radiochemistry is tritium H, the yields of which are approximately... 

Ternary fissions resulting in the formation of three fragments with approximately equal masses are very seldom, with a probability of about 10 o. 

As was described in Section 3.2.1., tritium is produced in irradiated nuclear fuel as a product of the ternary fission of and Pu with the comparatively... 

As can be seen from Table 3.11., in a typical LWR fuel rod about one-half to two-thirds of the produced by ternary fission is released from the fuel to the Zircaloy cladding. The resulting axial profile in the cladding is virtually parallel to the y-scan data this means that the relative fraction of total production... 

---