Fission models

Kozlovsky, Y., Kozlov, M.M. Membrane fission Model for intermediate structures. Biophys. J. 2003, 85, 85-96. 

Choices of pr(jl) corresponding to phase-space models have been considered in the paper by Miller, while the pXjp) have been parametrized by Herschbach and collaborators (see e.g. Miller et al 1967) in accordance with a fission model, which leads to... 

The single-well curve in Figure 14.14a predicts symmetric fission whereas the double-well curve (Fig. 14.14b) leads to the correct prediction of asymmetric fission and a thin neck. Incorporation of shell effects in the fission model also leads to the prediction that the half-lives of very heavy nuclides (Z 106) must be longer than the simple liquid-drop model would indicate. This has led to a search for "super heavy" elements with Z = 110-118. 

FIGURE 8.13 Comparison of the fission half lives calculated in the fission model (upper figure—see also Figure 8.11) and in the Preformation Cluster Model [18]. In both models the deformation of the fission fragments is not included completely. 

In contrast to the solid-state, solution-based systems can provide tight control of inter-chromophore geometries, diffusion controlled singlet fission rates, and simplified, two-molecule fission models - all of which can aid analysis via time-resolved spectroscopy. As more attention is focussed towards developing new singlet fission chromophores, solution-based experimental work could become an important tool to understanding the interchromophore interactions involved in singlet fission. 

The nuclear chain reaction can be modeled mathematically by considering the probable fates of a typical fast neutron released in the system. This neutron may make one or more coUisions, which result in scattering or absorption, either in fuel or nonfuel materials. If the neutron is absorbed in fuel and fission occurs, new neutrons are produced. A neutron may also escape from the core in free flight, a process called leakage. The state of the reactor can be defined by the multiplication factor, k, the net number of neutrons produced in one cycle. If k is exactly 1, the reactor is said to be critical if / < 1, it is subcritical if / > 1, it is supercritical. The neutron population and the reactor power depend on the difference between k and 1, ie, bk = k — K closely related quantity is the reactivity, p = bk jk. i the reactivity is negative, the number of neutrons declines with time if p = 0, the number remains constant if p is positive, there is a growth in population. 

The simplest model of time-dependent behavior of a neutron population in a reactor consists of the point kinetics differential equations, where the space-dependence of neutrons is disregarded. The safety of reactors is greatly enhanced inherently by the existence of delayed neutrons, which come from radioactive decay rather than fission. The differential equations for the neutron population, n, and delayed neutron emitters, are... 

The Model 412 PWR uses several control mechanisms. The first is the control cluster, consisting of a set of 25 hafnium metal rods coimected by a spider and inserted in the vacant spaces of 53 of the fuel assembhes (see Fig. 6). The clusters can be moved up and down, or released to shut down the reactor quickly. The rods are also used to (/) provide positive reactivity for the startup of the reactor from cold conditions, (2) make adjustments in power that fit the load demand on the system, (J) help shape the core power distribution to assure favorable fuel consumption and avoid hot spots on fuel cladding, and (4) compensate for the production and consumption of the strongly neutron-absorbing fission product xenon-135. Other PWRs use an alloy of cadmium, indium, and silver, all strong neutron absorbers, as control material. 

In view of the success of von Neumann s machine-based hydrodynamics in 1944, and at about the time when the fission bomb was ready, some scientists at Los Alamos were already thinking hard about the possible design of a fusion bomb. Von Neumann invited two of them, Nicholas Metropolis and Stanley Frankel, to try to model the immensely complicated issue of how jets from a fission device might initiate thermonuclear reactions in an adjacent body of deuterium. Metropolis linked... 

The rest of the less volatile fission products along with constituents of zircalloy, stainless steel, and the control rods are assumed to be in condensed form as inert aerosols that are treated together in TRAPMELT as "other aerosols." The aerosols are modeled as agglomerating and depositing on surfaces by several mechanisms (e.g., gravitational settling). 

This section reflects on the limitations of the PSA process and draws extensively from NUREG-1050. These subjects are discussed as plant modeling and evaluation, data, human errors, accident processes, containment, fission product transport, consequence analysis, external events, and a perspective on the meaning of risk. 

As a result of various side reactions, the yields are relatively low. However, in no case was ring fission found during the oxidations. Specially noteworthy is the ease with which the two methine groups in the 5-position of the 2-hydrazino-selenazoles are coupled together. Reference to models indicates that the quinonoid dyes exist in the trans form. 

Summary.—The assumption that atomic nuclei consist of closely packed spherons (aggregates of neutrons and protons in localized Is orbitals—mainly helions and tritions) in concentric layers leads to a simple derivation of a subsubshell occupancy diagram for nucleons and a simple explanation of magic numbers. Application of the close-packed-spheron model of the nucleus to other problems, including that of asymmetric fission, will be published later.13... 

The close-packed-spheron theory of nuclear structure may be described as a refinement of the shell model and the liquid-drop model in which the geometric consequences of the effectively constant volumes of nucleons (aggregated into spherons) are taken into consideration. The spherons are assigned to concentric layers (mantle, outer core, inner core, innermost core) with use of a packing equation (Eq. I), and the assignment is related to the principal quantum number of the shell model. The theory has been applied in the discussion of the sequence of subsubshells, magic numbers, the proton-neutron ratio, prolate deformation of nuclei, and symmetric and asymmetric fission. 

Nurse Work in starfish suggested that Cdk activity remained quite high, so that is the simplest model, and that is likely to be the case in fission yeast as well. We are cataloguing all the different components to see if we can work out the regulation of Cdk. It is consistent with but not yet proven that cdk regulation could regulate S phase between meiosis I and meiosis II. But I am worried about this mouse observation. Did you look at total HI histone kinase ... 

The elder model of ion formation, the charged-residue model (CRM), assumes the complete desolvation of ions by successive loss of all solvent molecules from droplets that are sufficiently small to contain just one analyte molecule in the end of a cascade of Coulomb fissions. [9,42,84] The charges (protons) of this ultimate droplet are then transferred onto the molecule. This would allow that even large protein molecules can form singly charged ions, and indeed, CRM is supported by this fact. [23]... 

B. Novak, Z. Pataki, A. Ciliberto, and J. J. Tyson, Mathematical model of the cell division cycle of fission yeast. Chaos 11, 277-286 (2001). 

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