Thermal fissions

This stems from the weakness, i.e. ease of thermal fission, of the Pb—R bond, and radicals may be generated in solution in inert solvents, as well as in the vapour phase, through such thermolysis of weak enough bonds, e.g. those with a bond dissociation energy of < w 165 kJ (40kcal)mol 1. Such bonds very often involve elements other than carbon, and the major sources of radicals in solution are the thermolysis of suitable peroxides (O+O) and azo compounds (C+N). Relatively vigorous conditions may, however, be necessary if the substrate does not contain substituents capable of stabilising the product radical, or... 

The essential ingredients for producing heat in a thermal fission nuclear reactor are the fuel and a moderator. A heat transport system with its coolant is necessary to convey the heat from the reactor to boilers where steam is produced to drive the turbogenerator. The natural materials available for fuel and moderator are uranium ore and water natural uranium extracted from the ore comprises the fissionable isotope uranium-235 and water contains hydrogen which is a good moderator. (Table I)... 

Phosphorus ylides are useful synthons77-79 and there is great interest in the bond between phosphorus and carbon80-82. Today, the zwitterionic character of this bond is undisputed82. After an early indication of a thermal fission of alkoxy-substituted al-kylidenephosphoranes83, there have been few subsequent studies that have suggested decomposition of the phosphorus ylides into phosphanes and carbenes84-88. 

Isomeric states are denoted by the symbol "m after the mass number and are given in the order of increasing excitation energy. The 235U thermal fission products, with fractional cumulative yields>10, are italicized in the table. The information on fission products is taken from the ENDF/B—VI fission products file [8],... 

Depolymerization is a special case of thermal degradation. It can be observed particularly in polymers based on a, a -disubstituted monomers. In these, degradation is a reversal of the synthesis process. It is a chain reaction during which the monomers are regenerated by an unzipping mechanism. This is due to the low polymerization enthalpy of these polymers. For the thermal fission of polymers with secondary and tertiary C-atoms, higher energies are required. In these cases elimination reactions occur. This can be seen very clearly in PVC and PVAC. 

Thermal fission products, Cs and Sr to reduce the costs of the underground repository... 

Croammeters to measure w eak light intensities. Background thermal fission can be minimised by cooling the photomultiplier. The schematic... 

Reagents i, perbenzoic acid ii, aq. Me2NH iii, 30% H202 iv, thermal fission v, HCl-MeOH vi, lithium alanate vii, MeS02Cl viii, KOH in glycol monomethyl ether... 

Fig. 2.1. Chain yield of fission products after thermal fission of 235U (x) and 239Pu (O).

Tritium is also formed as a product of ternary fission in power reactors. Yields in thermal fission of 235U and 239Pu are about 1 x 10-4 and 1.5 x 10 4 respectively (NCRP, 1979). Most of the tritium is retained in the fuel, but some may be released to atmosphere as HTO during reprocessing. At present, no fuel is reprocessed in the USA. The NCRP (1979) report included speculative estimates that reprocessing in Europe, excluding the USSR, may release 0.4 kg a-1. This is small compared with the release in atmospheric thermonuclear tests. 

The parent 3-piperideine (3) was prepared from l-acetyl-4-piperi-done by conversion into the p-toluenesulfonylhydrazone and thermal fission of the latter in alkaline media (the Bamford-Stevens reac-... 

In Fig. 8.13 the yield of fission products obtained by thermal fission of is plotted as a function of the mass number A (mass distribution). The maxima of the yields are in the ranges of mass numbers 90-100 and 133-143. In these ranges the fission yields are about 6%, whereas symmetrical fission occurs with a yield of only about 0.01%. The peaks in the mass distribution curve A = 100 and at 4 = 134 are explained by the fact that formation of even-even nuclei is preferred in the fission of the even-even compound nucleus It should be taken into account that the sum of the fission yields is 200%, because each fission gives two fission products. 

BRn3, = the cumulative yield branching ratio for thermal fission of leading to Xe = 0.067 = barometric pumping factor ratio = 1. 

The pyrolysis Is Initiated by thermal fission of the CHi-0 bond (reaction 1). The resulting oxynaphthyl radical and methyl radical abstract H-atoms from the parent forming naphthol, CH% and the methylene-naphthylether radical (reactions 2 and 3). 

One of the basic requirements is to synthesize the desirable compounds and to rapidly transport them to the equipment for chemical experiments. The time spent to accomplish such processes and its probability distribution can be properly determined only when a radioisotope of the element under study can be produced in a quantity that can be easily and accurately measured. It has never been the case for the transactinoid elements. Rough estimation of the chlorination time of Zr and Mo was done in the model experiments described in the above Section (see also Fig. 1.2). The ampoule was filled with pure inert gas, then closed and bombarded for some time with neutrons at ambient temperature. Thus, the thermalized fission products were accumulated on the walls. Then the ampoule was heated and flushed for a short time with the gas containing a reagent. Most of the activity got transferred into the nap in 30 seconds or so it involved a mean gas hold-up time of 15 seconds, so that the actual upper limit of the chlorination time could be set as 15 seconds. 

Table 21.1 Delayed neutron parameters for thermal fission...

Table 21.1 gives values of delayed neutron parameters, Pi, ki, for thermal fission, while Table 21.2 gives... 

To give another example, the number of delayed neutron groups for thermal fission of uranium-235 could be reduced from six to three by combining groups 3, 4, 5 and 6 = 0.005247 and X3 = 0.2463,... 

Table 2.11 Decay-heat power from fission products from thermal fission of and for near-infinite reactor operating time ...

Ratio based on thermal fission for 4 years, no depletion, typical spectrum for light-water reactor. 

Those fast neutrons that have energies greater than about 1 MeV may cause a limited amount of fission of fertile material. To account for this, the reactor designer usually specifies a quantity e, called the fast-fission factor, which is defined as the ratio of the net rate of production of fast neutrons to the rate of production of fast neutrons by thermal fission. The fraction e — 1 of the fast neutrons comes from fission of fertile material with fast neutrons e — 1 may be of the order of a few hundredths in a thermal power reactor. The net production rate of fast neutrons from fission is er N a 4>-... 

The simplest alkyl isocyanates are often best prepared by thermal fission of various urea or urethane derivatives. 

Isothiocyanates can be prepared by the analogous thermal fission of thioureas, and analogously the imidazole process using l,l -(thiocarbonyl)diimid-azole634 has been useful. 

The low thermal stability of mercaptols can be utilized for their thermal fission to yield thioketones ... 

Diphenylketene is accessible by thermal fission of a-azobenzyl phenyl ketone, N2C(C6H5)COC6H5.174... 

Dilorio, G. J., Direct Physical Measurement of Mass Yields in Thermal Fission of Uranium 235, Ph.D. thesis. University of Illinois at Urbana-Champaign, 1976. 

Thermal fission of leads to a yield of 6.183% for the. 4 = 137 chain. In a small number of cases the decay chains passes nuclides which emit a neutron after jS -decay, so called delayed neutrons (e.g. 0.016% of all neutrons from thermal fission of are delayed neutrons). Neutrons are emitted in 2% of the Te j3-decays and in 6.4% of the jS-decays. The existence of delayed neutrons is inqmrtant for nuclear reactor control, see Ch. 19. 

The light mass peak of double-humped fission yield curves shifts towards heavier masses when heavier nuclides undergo fission, but the position of the heavier mass peak remains almost constant. The Zp-value increases somewhat with increasing charge of the fissioning nucleus. As an example the average mass of the light and heavy mass yield peaks in thermal fission of Pu and Pu are 96.57, 100.34, 102.58 and 139.43, 139.66, 139.42 u, respectively, c.f. Fig. 14.9. 

TABLE 14.1. Data for energy distribution in thermal fission of V in MeV... 

The DT reactor needs several kg tritium as starting material. A likely technique involves the irradiation of a Li-Al alloy in a high flux thermal fission reactor which produces both tritium and He (17.43) These can be separated on the basis of their different vapor pressures, different permeability through palladium, or through their different chemical reactivities. 

Using effective cross-sections and yield values the amounts and radioactivities in Figure 21.7 and Table 21.2 were calculated. It is seen that Xe, Zr, Mo, Nd, Cs, and Ru, which are the elements formed in largest amounts in thermal fission (both by mole percent and by weight), constitute about 70% of the fission product weight after a cooling time of 10 y. 

Nuclide Half-life (yeani) Decay 0 (energy MeV) Thermal fission yield(%) activity (TBq/t U) ... 

Thermal fission of (fission of U, fission of Pu isotopes and n.y-reactions are important effects in a nuclear reactor). 

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