Probability of Fission

Let us begin with a discussion of the probability of fission. For the first approximation to the estimation of the fission barrier, we shall use the liquid drop model (Chapter 2). We can parameterize the small nonequilibrium deformations, that is, elongations, of the nuclear surface as... 

In analogy to chemical reactions, we might expect the probability of fission as expressed in the fission width, Tf(=h/r) to be given as... 

We should note that once again, the probability of fission is more complicated than the simple relation given above would indicate. In a paper written shortly... 

Up to this point, we have focused on describing the factors that control the probability of fission to occur. Now we will focus our attention on the distributions of the products in mass, energy, charge, and so forth. In doing so, we will mostly be discussing scission point or postfission phenomena. Our treatment of these phenomena is, of necessity, somewhat superficial, and the reader is referred to the excellent monograph of Vandenbosch and Huizenga (1973) for a more authoritative account. 

The main features of cold fusion reactions with the spherical nuclei of Pb or ° Bi as targets are low excitation energies of the compound nuclei (Ex w 15 to 20 MeV) with the consequence of emission of only one or two neutrons, low probability of fission, and relatively high fusion cross sections otus- On the other hand, the reaction products have relatively small neutron numbers and short half-fives. Suitable projectiles are neutron-iich stable nuclei, such as " Ca, Ti, " Cr, Fe, Ni, Zn, and Kr. 

Hot fusion reactions with the deformed nuclei of the actinides (e.g. or Pu) as targets lead to high excitation energies of the compound nuclei (Ex 50 MeV), emission of about 4 to 5 neutrons, high probability of fission and relatively low values of i7fus. On the other hand, the neutron numbers of the reaction products are relatively higli and the half-lives relatively long. 

If any of the daughter ganglia does not become immediately stranded, but keeps moving, it, in turn, has a high probability of fissioning, unless it gets entrapped before that. These observations indicate clearly the crucial role played by coalescence the outcome of an immiscible flood is the net product of the competition of mobiliza-tion/collision/coalescence on one hand and breakup/stranding on the other. A quantitative treatment of this process was developed by Payatakes et al. (4,11,12). 

Conventional LWRs alone cannot be used to transmute minor actinides because thermal neutrons are not as effective for inducing the fission reaction. As a result, nfinor actinides (especially, non-fissile even isotopes of plutonium) build up as a function of time. Therefore, thermal reactors tend to preferentially produce minor actinides. Fast reactors or fast neutron spectrum devices on the other hand tend to more effectively destroy the minor actinides because the probability of fission for both the even and odd isotopes of plutonium fission with fast neutrons is considerably higher than with thermal neutrons. Thus, last reactors are heavily preferred for the recycle of plutonium and the ultimate complete destruction of all of the minor actinides. 

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