Other fissioning nuclides

Modifications to this process can be made to effect recovery of neptunium, americium, curium, californium, strontium, cesium, technetium, and other fission nuclides. The efficient production of specific transuranic... 

Activation analysis by fission counting Is of value only If one fissioning nuclide Is present or if the amounts of other fissioning nuclides present are known and corrections can be made for them. The same Is true for the Isolation and determination of fission products. Ui anlum Isotopes that are... 

In discussing the fission-product composition of fallout samples it is advantageous to choose some fission product as a reference nuclide, j, and express the composition of the other fission products i by a set of fij ratios. For local fallout from land-surface bursts the choice of 95Zr as reference nuclide has proved convenient. A ratio of particular interest is 7 89,95 since 89Sr and 95Zr generally fractionate from each other about as severely as any pair of nuclides. Thus, r89,95 indicates approximately the maximum extent of fractionation that will be observed in the sample. 

Multichannel y-ray spectrometry of the counting samples indicated that measurable quantities of 134Cs or other fallout nuclides were not present, confirming that this procedure gives good decontamination from other fission products. Previous studies have failed to demonstrate the presence of 135Cs in cattle (5). 

The decay of extinct 244Pu is deduced from excess abundances of the nuclides 136Xe, 134Xe, and 132Xe, produced by the spontaneous fission of 244Pu. Uncertainties arise because there is no stable isotope of Pu that can be used in the way that 127I is used in Equation (3.60) and the use of other heavy nuclides U or Th as substitutes leads to difficulties due to differences in primordial production and chemistry. 

In one mode of fission of Pu, three neutrons are observed, and Xe is one of the fission products. What nuclide is the other fission product ... 

In thermal-neutron reactors has an important advantage over or Pu in that the number of neutrons produced per thermal neutron absorbed, tj, is higher for than for the other fissile nuclides. Table 6.1 compares the 2200 m/s cross sections and neutron yields in fission of these three nuclides. Thorium has not heretofore been extensively used in nuclear reactors because of the ready avaUabihty of the U in natural or slightly enriched uranium. As natural uranium becomes scarcer and the conservation of neutrons and fissile material becomes more important, it is anticipated that production of U from thorium will become of greater significance. 

Pu. The isotope Pu is produced by neutron capture in Pu. It is not fissionable by thermal neutrons, but, like all other plutonium isotopes, it fissions with fast neutrons. Pu is converted to a fissionable nuclide by neutron capture. Therefore, like Th and it is a fertile material. It undergoes alpha decay, with a half4ife of 6580 years, to form which then decays to Th, the parent of the 4n decay series discussed in Chaps. 6 and 8. Like the other even-mass plutonium isotopes, Pu produces neutrons by spontaneous fission. It is present in greater concentration in reactor plutonium than any of the other even-mass plutonium isotopes. 

Tc is available through the /l -decay of Mo (Fig. 2.1.B), which can be obtained by irradiation of natural molybdenum or enriched Mo with thermal neutrons in a nuclear reactor. The cross section of the reaction Mo(nih,v) Mo is 0.13 barn [1.5], Molybdenum trioxide, ammonium molybdate or molybdenum metal are used as targets. This so-called (n,7)-molybdenum-99 is obtained in high nuclidic purity. However, its specific activity amounts to only a few Ci per gram. In contrast, Mo with a specific activity of more than in Ci (3.7 10 MBq) per gram is obtainable by fission of with thermal neutrons in a fission yield of 6.1 atom % [16]. Natural or -enriched uranium, in the form of metal, uranium-aluminum alloys or uranium dioxide, is used for the fission. The isolation of Mo requires many separation steps, particularly for the separation of other fission products and transuranium elements that arc also produced. 

This latter explanation of fine structure is a plausible one. However, complete fission yield curves for the neutron-induced fission of other fissile nuclides are needed to add further light on this problem. 

Although other technologies are now coming into use for this purpose, gaseous diffusion has played an important role in the enrichment of uranium for use in nuclear reactors. Natural uranium is mostly 2, which cannot be fissioned to prodnce energy. It contains only about 0.7% of the fissionable nuclide IfU. For uranium to be useful as a nuclear fuel, the relative amount of IfU must be increased to abont 3%. In the gas diffusion enrichment process, the natnral nraninm (containing IfU and a small amount of 9iU) reacts with fluorine to form a mixtnre of and UFg. Because these... 

Contamination of the plutonium-bearing materials with other spontaneous fission nuclides (e.g., Cm) will strongly interfere with the neutron assay. Cm is the strongest neutron emitter and accounts for more than 95% of the neutrons in spent fuel although the share of curium is only about 0.5% of the plutonium content. 

The determination of the slowly time-varying nuclide densities that occur in an operating reactor (referred to as "bumup"), which includes the build-in of other fissionable isotopes (referred to as "conversion" or "breeding")... 

The other fission product nuclides, such as the isotopes of cesium, the alkaline earths and the rare earth elements, as well as tellurium and the actinides, show an... 

If the interference is not too important it can be corrected for. If uranium or other fissionable material is present in the sample, the fission products with high fission yields (Sr, Mo, Zr, Ce, Ba) can induce important positive errors. The interference can easily be estimated when the uranium content is known. In FNAA, secondary interference reactions may occur when fast neutrons interact with other elements and produce particles that induce a nuclear reaction that forms the same indicator nuclide. These particles are usually protons ejected by fast neutrons from a matrix with a high hydrogen content. Examples are ... 

Anthropogenic nuclides mainly Cs, from nuclear weapon and Chernobyl fall-out, and °Co from steel manufacture. In particular circumstances, there could be other fission product nuclides. 

FISSION PRODUCT One of the many nuclides created by nuclear fission, including nuclides farmed by decay of other fission products. 

Citric acid and nitriloacetic acid (NTA) lanthanide complexes were used in the earliest ion exchange separations of lanthanides from fission product mixtures (Kf = 3.2 for Ce(H3 Cit.)3 and Kf = 10.8 for CeNTA2) (Sillen and Martell, 1964). More recently such polyamino-polycarboxylic acids as ethylenediaminetetraacetic acid (EDTA), 1,2-diaminocyclohexaneacetic acid (DCTA), and diethylenetriaminepentaacetic acid (DTPA) have been prepared. Their lanthanide complexes are very stable (Table 3) and have been widely used in analysis and separation of lanthanide mixtures. They have also been used experimentally to remove internally-deposited 144Ce and other radioactive lanthanide nuclides from animals and man (Foreman and Finnegan, 1957 Catsch, 1962 Balabukha et al., 1966 Palmer et al., 1968 among others). 

Numerous studies by other workers (I, 10) have shown that the releases of iodine and the noble-gas fission products from pyrolytic carbon-coated fuel particles are controlled by diffusion of these nuclides through grain boundaries, cracks, and defects in the isotropic pyrolytic carbon coating. When coatings are intact, however, the release of these fission product nuclides is low. However, the pyrolytic carbon coating constitutes only a delaying barrier to the metallic nuclides barium and strontium through which they diffuse with diffusion coefficients of the order of 10 9 cm.2/sec. (at — 1400°C.). The steady-state release of these metallic nuclides is controlled instead by diffusion out of the fuel kernel,... 

Table II shows the result of the analysis. In this event only fission-product radionuclides were considered. As before, four nuclides are significant at well above the 99% confidence level all other nuclides have a level of significance at or below 30%. Since the estimated experimental errors are probably not realistic, a level of significance below 50% is probably not meaningful. Four nuclides are then necessary and sufficient.

For a nuclear weapon hurst in air. all materials in the fireball are vaporized. Condensation of fission products and other bomb materials is then governed by the saturation vapor pressures of the most abundant constituents. Primary debris can combine w ilh naturally-occurring aerosols, and almost all of (he fallout becomes tropospheric or stratospheric. If the weapon detonation takes place within a few hundred Icet of (either above or below) a land or water surface, large quaniilies of surface materials are drawn up or thrown into the air above Ihe place ol detonation. Condensation of radioactive nuclides in this material then leads in considerable quantities of local fallout, but some of the radioactivity still goes into tropospheric and stratospheric fallout. If the hurst occurs sufficiently fur underground, the surface is not bruken and no fallout results. 

---