Other Fission Processes

5 Other Fission Processes - The results of the irradiation of benzyl acetate and 3,5-dimethoxybenzyl acetate have been analysed using membrane introduction mass-spectrometry. Irradiation of the acid derivative (136) under nitrogen in methanol brings about the formation of the corresponding ester. The authors suggest that this process is the result of CO bond fission and the liberation of a hydroxy radical. Subsequently this is trapped by methanol as the ester (137). 

Further proof for the intermediacy of such a radical is obtained by its trapping with oxygen to give the adduct (138). 

Irradiation of the cyclopentanecarboxylate derivatives (139) results in cycliza-tion to afford the spiro compounds (140). This process arises from Se-C bond cleavage, addition of the resultant radical to the pendant alkene group, and readdition of the PhSe radical. Several examples of the process are described as a synthetic path to novel bakkenolides. Irradiation of the stannane (141) in benzene brings about C-Sn bond cleavage and the formation of the tripropyltin radical. The irradiation of telluro derivatives such as (142) can be used as a source of a variety of alkyl and acyl radicals formed by the fission of the C-Te bond.  

Fasani, M. Mella, S. Monti, S. Sortino and A. Albini, J. Chem. Soc., Perkin Trans. 2,1996,1889. 

Steiner, P. Wessig and K. Polbom, Helv. Chim. Acta, 1996, 79, 1843 Chem. Abstr., 1997,126,19198). 

4 Other Fission Processes. - The photochemically induced fission reactions of a series of naphthylmethyl alkanoates have been described. Some factors that control the ease of bond cleavage within the esters were identified. Irradiation of (200) in a NaY zeolite using 308 nm light results in C-O bond fission and the formation of an acetyl radical. Irradiation of perfluoroacetyl fluoride at 254 nm in the gas phase brings about fission of CO-F bond with the formation of fluorine atoms and perfluoroacetyl radicals.  

Laser-flash photolysis of (203) in solution affords the ketene (204) and this is the first time that this ketene has been observed using time-resolved IR spectroscopy, Although other workers have previously reported the formation of this species. The present work reports the kinetics of the reaction of the ketene with water, methanol and diethylamine. The products obtained from the irradiation of a series of AT-acetoacetyl-a-amino acids have been identified by gas chromatography and chemical ionization mass spectrometry.  

Lindemann, D. Wolff-Molder and P. Wessig, J. Photochem. Photobioi A-Chem., 1998,119, 73. 

Suzuki and T. Ichimura, Chem. Phys. Lett., 1998, 293, 436 Chem. Abstr., 1998,129, 302268). 

Keating and M. A. Garcia-Garibay, Mol Supramol Photochem., 1998, 2(Organic and Inorganic Photochemistry), 195, Chem. Abstr., 1999,130, 58927). 

The acetone-sensitized decarbonylation of -(lS),2(S) (125a) has been studied. The principal reaction is the formation of the 2(S),3(R)-cyclopropane (126a). Other products (127a), (128a) and (129a) are also formed in low yield. The reaction arises from the triplet state and this was confirmed by using Michler s ketone as the sensitizer and by quenching experiments. A similar selectivity is 

Yamashita, N. Sato, M. Anpo, T. Nakajima, M. Hada and H. Nakatsuji, Stud. Surf Sci. Catal, 1997,105B, 1141 Chem. Abstr., 1998,128, 183387). 

Wessig, J. Schwarz, D. WulfT-Molder and G. Reck, Monatsh. Chem., 1997, 128, 849 Chem. Abstr., 1997,127, 331378). 

Lindemann, G. Reck, D. Wulff-Molder and P. Wessig, Tetrahedron, 1998, 54, 2529. 

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With simple aliphatic amines, the initial diazonium cations (56) will break down extremely readily to yield carbocations (cf. p. 107) which are, for reasons that are not wholly clear, markedly more reactive than those obtained from other fission processes, e.g. RBr- R Bre. Where the prime purpose is the formation of carbocations, the nitrosation is better carried out on a derivative of the amine (to avoid formation of H20) under anhydrous conditions ... 

Other Fission Processes - Considerable use is being made of radical reactions in organic synthesis. For example, Deng and Kutateladze have described a novel method for the synthesis of esters. This involves the irradiation of the ester (124) in the presence of terminal alkenes. Unfiltered light from a medium pressure mercury arc lamp results in the fission of the S-methylene bond and the formation of a radical which adds to the alkenes. The yields obtained are reasonable with acetonitrile as the solvent. Other solvents such as methanol or ethanol/water can also be used. Yields of the products obtained are shown in Scheme 7. 

The main drawback to nuclear power is the production of radioactive waste. Spent fuel from a nuclear reactor is considered a high-level radioactive waste, and remains radioactive for a veiy long time. Spent fuel consists of fission products from the U-235 and Pu-239 fission process, and also from unspent U-238, Pu-240, and other heavy metals produced during the fuel cycle. That is why special programs exist for the handling and disposal of nuclear waste. 

The two nuclei on the right side are just two of the many possible products of the fission process. Since more than one neutron is released in each process, the fission reaction is a self-propagating, or chain reaction. Neutrons released by one fission event may induce other fissions. When fission reactions are run under controlled conditions in a nuclear reactor, the energy released by... 

The heart of the nuclear reactor boiler plant system is the reactor core, in which the nuclear fission process takes place. Nuclear fission is the splitting of a nucleus into two or more separate nuclei. Fission is usually by neutron particle bombardment and is accompanied by the release of a very large amount of energy, plus additional neutrons, other particles, and radioactive material. The generation of new neutrons during fission makes possible a chain reaction process and the subsequent... 

This reaction exemplifies the important process of methyl removal, which becomes even more significant in the case of multiply branched paraffins. The rather large exothermicity of Reaction 11 results from the fact that a secondary carbonium ion is formed. The beta fission process can be illustrated using reactions in 8-ra-hexylpentadecane (compound 2) as an example. Table II shows that the ions formed by C-C fission at a branch point (Ci4+, m/e = 197 and Ci5+, m/e = 211) have intensities appreciably larger than the other alkyl ions in the same region... 

Some other fission products were considered as suitable for radiation sources (heat sources, radiography, etc.). Thus, processes were designed to isolate these from the rest of the fission products. Today, there is little interest in such applications. 

Tc-99, which has a half life of 2.12 x 10 years, can be recovered from nuclear fission waste in kilogram quantities. Solvent extraction, ion exchange, and volatilization processes are employed to separate it from the numerous other fission products. Because of its long half life and its emission of a soft (low energy) beta particle, it can be safely handled in milligram quantities. Almost all chemical studies of the element have been carried out with this isotope. 

Spent fuel from a reactor contains unused uranium as well as plutonium-239 which has been created by bombardment of neutrons during the fission process. Mixed with these useful materials are other highly radioactive and hazardous fission products, such as cesium-137 and strontium-90. Since reprocessed fuels contain plutonium, well suited for making nuclear weapons, concern has been expressed over the possible capture of some of this material by agents or terrorists operating on behalf of unfriendly governments that do not have a nuclear weapons capability. 

In tlie PUREX process, the spent fuel and blanket materials are dissolved in nitric acid to form nitrates of plutonium and uranium. These are separated chemically from the other fission products, including the highly radioactive actinides, and then the two nitrates are separated into tv/o streams of partially purified plutonium and uranium. Additional processing will yield whatever purity of the two elements is desired. The process yields purified plutonium, purified uranium, and high-level wastes. See also Radioactive Wastes in the entry1 on Nuclear Power Technology. Because of the yield of purified plutonium, the PUREX process is most undesirable from a nuclear weapons proliferation standpoint,... 

Of the fast neutrons produced in fission, some of them will be moderated to thermal energies and will induce other fission reactions while others will be lost. The ratio of the number of neutrons in the next generation to that in the previous generation is called the multiplication factor k. If the value of k is less than 1, then the reactor is subcritical and the fission process is not self-sustaining. If the value of k is greater than 1, then the number of fissions will accelerate with time and the reactor is supercritical. The goal of reactor operation is to maintain the system in a critical state with k exactly equal to 1. The extreme upper limit for the multiplication factor would correspond to the mean number of neutrons per fission ( 2.5 for 235U(n,f)) if each neutron produces a secondary fission. 

The ZEALEX Process Researchers from KRI have shown that the zirconium salt of dibutyl phosphoric acid (ZS-HDBP) was soluble in Isopar-L in the presence of 30% TBP. This super PUREX solvent, known as ZEALEX, extracts actinides (Np-Am) together with lanthanides and other fission products, such as Ba, Cs, Fe, Mo, and Sr from nitric acid solutions. The extraction yields depend on both the molar ratio between Zr and HDBP in the 30% TBP/Isopar-L mixture and the concentration of HN03 (232). Trivalent transplutonium and lanthanide elements can be stripped together from the loaded ZEALEX solvent by a complexing solution, mixing ammonium carbonate, (NH4)2C03, and ethylenediamine-N.N.N. N -tetraacetic acid (EDTA). An optimized version of the process should allow the separation of... 

The element uranium is the element used for almost all fission processes. It has two natural isotopes. One of them is 238CI which, constitutes 99.3% of uranium ore, and the other is 235CJ, which constitutes 0.7% of uranium ore. Fissionable nuclei such as 235CJ and 239Pu are called fissile. Nuclear fission reactions occur... 

Photodecomposition of the benzenesulfonamide derivative 183 in aqueous solution was investigated by irradiation from a 254 nm low pressure lamp under a helium atmosphere. Homolytic fission of the S-N bond gives arylsulfonyl and arylsulfonylamino radicals. Gyclization from the latter radical via C-N bond formation and hydrogen atom abstraction afforded the /3-sultam 47 in 7% yield. Other photodissociation processes resulted in the formation of compounds 184-186 (Scheme 59) <2002JPH109>. 

Prior to beginning actual plant processing, it was necessary to flush all tanks and pipes extensively to avoid contamination of the plutonium and the americium with plutonium of a different isotopic composition or with fission products or other impurities. Processing began only after analyses of the flush solutions confirmed the product contamination would be acceptably low. 

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