Fission product fractions

Of course, Equations la and lb can be reversed to consider fractional uptakes in an atmosphere that at equilibrium would lead to a concentration C0 throughout the particle. This amounts to only a change in initial conditions, and the fractional uptakes are given by F — 1 — C(t)/C0. This form of Equations la and lb is more useful in calculations of fission product fractionation during fallout formation as discussed later. 

During release and initially after deposition of fission products, fractionation between volatile elements associated with condensed particles ( Cs, °Sr) and refractory elements associated with fuel particles Nb) may take place. The behavior of Tc in the environment will, therefore, depend on whether technetium is released as such or formed from precursors after deposition. Technetium-99 is believed to be released from the nuclear fuel cycle as the volatile heptaoxide (TcaOy) in air emissions or as soluble and highly mobile pertechnetate (TcO T) in effluents. 

The Natural Reactor. Some two biUion years ago, uranium had a much higher (ca 3%) fraction of U than that of modem times (0.7%). There is a difference in half-hves of the two principal uranium isotopes, U having a half-life of 7.08 x 10 yr and U 4.43 x 10 yr. A natural reactor existed, long before the dinosaurs were extinct and before humans appeared on the earth, in the African state of Gabon, near Oklo. Conditions were favorable for a neutron chain reaction involving only uranium and water. Evidence that this process continued intermittently over thousands of years is provided by concentration measurements of fission products and plutonium isotopes. Usehil information about retention or migration of radioactive wastes can be gleaned from studies of this natural reactor and its products (12). 

Computer sensitivity studies show that hole size strongly affects the fraction of fission products released from the containment. The failure location determines mitigation due to release into another building in which condensation and particulate removal occur. The quantity released depends on the time of containment fails relative to reactor vessel failure. If containment integrity is maintained for several hours after core melt, then natural and engineered mechanisms (e.g., deposition, condensation, and filtration) can significantly reduce the quantity and radioactivity of the aerosols released to the atmosphere. 

Baumgartner and Reichold prepared carrier-free Mo(CO)g in high yield by neutron irradiation of powdered mixtures of UjOg and Cr(CO)g. As with their preparation of ° RuCp2, the Cr(CO)g acted only as a catcher for fission-product molybdenum (and for its precursors niobium and zirconium). The yield of 60% found for Mo(CO)6 is higher than the fractional chain yield of Mo in fission, so that the reaction must be partly thermal, starting with molecular fragments which survive j8 decay. 

The final answer came from the atomic pile. J. A. Marinsky, L. E. Glendenin, and C. D. Coryell at the Clinton Laboratories at Oak Ridge (20) obtained a mixture of fission products of uranium which contained isotopes of yttrium and the entire group of rare earths from lanthanum through europium. Using a method of ion-exchange on Amberlite resin worked out by E. R. Tompkins, J. X. Khym, and W. E. Cohn (21) they were able to obtain a mixture of praseodymium, neodymium, and element 61, and to separate the latter by fractional elution from the Amberlite column with 5 per cent ammonium citrate at pH 2.75. Neutron irradiation of neodymium also produced 61. 

FAA FA FBC FC FEBEX FFFF FGD FP FSU FT FTIR FUETAP Flame atomic absorption Fly ash Fluidized bed combustion Filter cake Full-scale engineered barriers experiment (in crystalline host rock) Flow-field flow fractionation Flue gas desulphurization Fission products Former Soviet Union Fourier transforms Fourier transformed infrared spectroscopy Formed under elevated temperature and pressure... 

Fractionation of fission products during fallout formation was recognized by Freiling (4) in early studies of fallout particles. He also recognized that this phenomenon involved the volatility of the fission products. In an attempt to describe fractionation quantitatively, Miller (9) devised... 

This calculational scheme has been applied to the Small Boy detonation. The test was made to compare fractionation data from this well-studied event (2) with calculated values. Using 25 particle size fractions that approximate the fallout from the event and the time-temperature history given in Table III, which was derived from generalized time-temperature event histories, the fission product contents of the nuclide chains 80 through 150 in the particle size fractions were calcu-... 

An attempt to investigate more realistic fuel particles and fission product loadings has been made. A recoiled fission product-absorbing buffer carbon layer has been included in the particle, and fractional releases with recoil into this layer have been considered. Another factor... 

Touring the formation of radioactive fallout particles, one of the most important processes is the uptake, in the cooling nuclear fireball, of the vaporized radioactive fission products by particles of molten soil or other environmental materials. Owing to the differences in the chemical nature of the various radioactive elements, their rates of uptake vary, depending upon temperature, pressure, and substrate and vapor-phase composition. These varying rates of uptake, combined with different residence times of the substrate particles in the fireball, result in radiochemical fractionation of the fallout. This fractionation has a considerable effect on the final partition of radioactivity, exposure rate, and radionuclides between the ground surface and the atmosphere. 

Equation 13 reduces to the Rayeigh equation (3) when the ratio of the gas-phase diffusivities, , is unity. Since gas-phase diffusivity is inversely proportional to the square root of the reduced mass, in the case of fission product-sodium systems where sodium has the smallest molecular weight, the above diffusivity ratio is less than unity. Therefore, the Rayleigh equation, which was derived on the basis of equilibrium vaporization, in fact represents an upper limit for the fractional fission-... 

Equilibrium Vaporization. The cesium release results presented in this chapter may also be used to demonstrate our earlier conclusion that equilbirium vaporization represents the upper limit for the fractional fission-product release as a function of sodium vaporization. Figure 6 shows three cesium release curves. Curve A was calculated from the Rayleigh Equation in conjunction with the partial molar excess free energy of mixing of infinitely dilute cesium—sodium solutions reported... 

Values a and b for the fission product isotopes and the partition factors ai and a2 are listed in Table V au for a given isotope, is the fraction which was retained by the local fallout glass particles, and < > is the fraction released to the cloud. Thus, from Table V, i137 is 0.153 which indicates that 15.3% of the 137Cs is retained by the local glass particles. It is interesting to note that the independent yield of cesium in the 137 mass chain is approximately 17%—the balance of the chain is formed as tellurium, iodine, and xenon. 

Data relating to radionuclide deposition (fallout) within a few miles of the Danny Boy, Sedan, and Palanquin nuclear cratering shots are examined for evidence of fractionation. The fractionation index is computed for several fission-product mass chains produced in each event. For the three events studied only Danny Boy showed unambiguous evidence of fractionation in the early fallout, and the degree of fractionation was small. In Danny Boy there was only a factor of four difference between most enriched and most depleted species, compared with the factors of several hundred that have been observed in many late time samples of airborne debris. If this small amount of fractionation proves to be true in general for cratering shots, predictions of early-fallout gamma-radiation patterns will be simplified. 

The fractionation index U is calculated for each measured fission product at each tray location in the fallout field as follows ... 

As a rough check on the magnitude of the indices in Table I, a theoretical estimate of the fractionation index for unfractionated fission products can be calculated as follows. The index has the units kiloton-hr./ft.2 roentgens if we use the most probable value of 2500 roentgens /hr. for 1 kiloton/sq. mile over Nevada Test Site terrain for unfractionated fission products, we obtain (since 1 square mile = 2.78 X 107 ft.2)... 

The large body of radiochemical data available from the low yield land-surface event Small Boy has been re-examined for internal consistency by a variety of methods. It was possible to show that certain portions of the data are not sufficiently reliable to be useful for establishing the fractionation behavior of fission-product radionuclides in the nuclear debris. Certain other parts of the data are shown to need adjustment for calibration differences which existed between the four laboratories which performed the analyses. Despite the shortcomings of the data, it was possible to establish that the relationships which existed among the fractionating radionuclides were qualitatively similar to those previously observed for other events. Other features of the data appear to be unique to Small Boy. 

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. 

The results of correlation studies reported in Refs. 2, 4, and 7 indicated that the fractionation behavior of most fission products was remarkably similar for coral surface bursts, bursts on the surface of deep and shallow seawater, and bursts at altitudes sufficiently great to avoid entrainment of soil or water in the fireball and cloud. Furthermore, the correlations showed no clear-cut dependence on the explosive yield of the device. This report extends the treatment to a near-surface event on silicate soil. 

The large body of radiochemical data reported on samples from event Small Boy contained a considerable proportion of unreliable results and calibration factors. For successful use of the data to establish the fractionation behavior of fission products in nuclear debris, it was necessary to examine the individual values by all available methods, to discard those which fail to show internal consistency, and to revise the calibrations. The authors of this report have undertaken such an evaluation, and a number of meaningful relationships have already resulted. 

---