Useful Fission Product Yields

The yields of the fission product Cs, Sr, Pm and Ce are shown in Table 3 7 3 1 for 36 and 116 day continuous Irradiations at 4000 The yields are given as present per ton of fuel at one day following reactor shutdown assuming all fuel at the same exposure. The exposures corresponding to 33 and 116 days are roughly 600 and 1200 MWD/T 387 tons of uranium are assumed in the loading. 

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The yield of primary fission products in relation to their mass number is shown in Fig. 8. Most of the radioactive fission products have short half-lives, but some decay slowly technetium-99 has 2.2 x 10 yr and promethium-147, 2.6 yr. Two of the most useful fission products obtained in the operation of a pile are caesium-137 (/ -, 33 yr) and strontium-90 ( +, 22 yr). The former is used in medicine and the latter in medicine and industry both are separated from shorter-lived material. 

An evaluation of the available measured nuclear data is necessaiy to produce a recommended file of cross-section data for a nuclide, or an evaluation of fission product yields or the radioactive decay data for a nuclide. This is because there are often several sets of measurements or gaps in the measured data which must be filled using theory. Inconsistent data must be examined and a best estimate made, taking into accoimt any theoretical constraints. There are several evaluation projects which coordinate the efforts of the scientists working in the field and make plans to meet the data requirements. All cross-section... 

Sepa.ra.tion of Plutonium. The principal problem in the purification of metallic plutonium is the separation of a small amount of plutonium (ca 200—900 ppm) from large amounts of uranium, which contain intensely radioactive fission products. The plutonium yield or recovery must be high and the plutonium relatively pure with respect to fission products and light elements, such as lithium, beryUium, or boron. The purity required depends on the intended use for the plutonium. The high yield requirement is imposed by the price or value of the metal and by industrial health considerations, which require extremely low effluent concentrations. 

A very interesting result on ruthenocene showed that when fission product ruthenium was projected into dimeric cyclopentadiene, the yield of ruthenocene was quite low, while when monomeric cyclopentadiene was used, the yield was close to 100%. This was interpreted as involving a thermal reaction between the ruthenium atom and a cyclopentadiene monomer molecule, likely the simple displacement of an acid hydrogen. 

Just as earlier we were able to observe mass-yield distributions of the fission products from the fissionable nuclide used in the Chinese nuclear device, it is possible to see part of the mass-yield curve from the fission of 244Pu, which was synthesized originally in a supernova. Figure 6 shows the mass-yield distribution of the excess fissiogenic xenon observed in the meteorite Pasamonte (15). 

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. 

The cyclization is therefore a low-yield secondary reaction and not a useful route to 3-phenyl benzofurans. a-Phenoxypropiophenones and a-(3-methoxyphenoxy)acetophenones give fission products only.251 Benzofuryl-Bz-oxyacetophenones yield 5% of furobenzofurans only.255 a-(3-Methoxyphenoxy)propiophenones resist thermal cracking under the same conditions.245... 

There was also great uncertainty of the production yields for SHEs. Closely related to the fission probability of SHEs in the ground-state, the survival of the compound nuclei formed after complete fusion was difficult to predict. Even the best choice of the reaction mechanism, fusion or transfer of nucleons, was critically debated. However, as soon as experiments could be performed without technical limitations, it turned out that the most successful methods for the laboratory synthesis of heavy elements are fusion-evaporation reactions using heavy-element targets, recoil-separation techniques, and the identification of the nuclei by generic ties to known daughter decays after implantation into position-sensitive detectors [13-15],... 

Aqueous reprocessing methods have been developed to effect an efficient and thorough separation of fissile elements from the contaminating fission products in spent fuel( l). While these processes may be altered to yield a proliferations-resistant product by coprocessing or by the addition of radioactive material that will contaminate the clean fissile material, it still is necessary to safeguard some of the process steps to ensure that material useful in nuclear weapons will not be diverted (3). The safeguard requirements and the ease of subversion of such provisions make many versions of the conventional processes subject to unacceptable proliferation risks. 

We are limited in this modeling process by the accuracy with which measurements can be made and by the accuracy of the fission yields and neutron reaction cross sections which are used to interpret the results. As an example consider the Nd- Nd fission product pair, which has been used as an indicator of thermal neutron fluence because the capture cross section for the former is large and for the latter is small. The thermal cross section for l53Nd has recently been listed as 325 ( 10) barns (20), and more recently as 266 barns (11). Using the 325-barn value we deduce an age of about 2 to 27T billion years from neodymium to uranium ratios in the Oklo reactors, while an age of about 1.8 billion years is obtained using the 266-barn figure. 

To convert olefins into carboxylic acids in this way it is preferable to oxidize the primary fission products of the ozonide in a subsidiary reaction. Particularly good results were obtained by Asinger using a hot suspension of silver oxide116 and by Wilms using peracetic acid 117 Wilms thus obtained adipic acid in yields of about 90% from cyclohexene ... 

A technique for the separation of pertcchnetalc from mixed fission products by solvent extraction with TBP was described. The extraction was almost quantitative from a sulphuric acid solution. Sodium fluoride was used to provide the zirconium-niobium decontamination and a cation exchange column ensured the decontamination from metallic ions. Tc yields of 92 % were obtained [123. ... 

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