An important contribution to the damage in nuclear fuels derives from fission fragments. There are two groups of fission products: one group with atomic number near 42 (Mo) and energy «100 MeV and the other with atomic number near 56 (Ba) and energy «70 MeV. The maximum electronic stopping powers of these energetic particles, «18keVnm~ for the heavier and 22 keV nm-1 for the lighter, are far greater than their respective nuclear stopping powers. Similar to ion irradiation studies described above, where the primary recoil spectrum can be systematically varied, the masses and energies of ions can be varied to examine effects of electronic stopping power. An example is shown in Figure 18 where the electronic stopping power is plotted as a function of energy (per nucleon) for different ion irradiations of UO2. The two boxes in the figure indicate stopping powers associated with the fission fragments and the heavy particle recoils of a emit­ters. One of the questions addressed by such studies

40 cN D) C Ф
20 15 10 5 0
20
10
20
10
0
10 20 HiapP^bP3
0
(b)
(a)

Figure 17 Cavity volume fraction (a) and cavity density (b) in pure vanadium irradiated with 12 MeV Ni3+ ions to 30dpa at 873 K with and without simultaneous irradiation of He and H. Reproduced from Sekimura, N.; Iwai, T.; Arai, Y.; etal.

J. Nucl. Mater. 2000, 283-287, 224-228.

has been the formation of fission fragment tracks. Tracks have not yet been observed in the bulk of UO2 due to fission; however, by using ion irradiation, the stopping powers could be increased. The dashed line at 29keVnm-1 in Figure 18 represents the threshold stopping power for track formation.37 This value is «30% greater than the maximum for fission fragments, thus helping to explain why fission fragment tracks are not seen in the bulk. Such tracks are observed, however, close to the surface. They are explained by fission products passing near or parallel to the surface and creating shock waves which inter­act with the surface.38 These studies have also been useful in gaining important data for understanding fission gas evolution in nuclear fuels. For example, 72MeV iodine ions (see Figure 18), approximate very closely the stopping power of fission fragments. Such studies have shown that 72 MeV I irradiations cause Kr atoms preimplanted into UO2 to nucleate into bubbles, and preformed bubbles to undergo res­olution. A radiation-enhanced diffusion coefficient for the Kr was estimated from these studies to be D « 1.2 x 10~30cm5 x F, where F is the fission rate per cubic centimeter, and found independent of tem­perature below «500 °C (see Matzke etal.37 for details). The importance of such studies as these is that the basic processes in complex nuclear fuels can be elucidated by studies that carefully control singly the irradiation conditions and materials parameters in the fuel, such as fission gas concentration, damage, etc.