Supercritical nuclear processes

Figure 9 A proposed dry supercritical fluid process for reprocessing spent nuclear fuels. (From Ref. 31.)... 

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. 

Actually, the kinetics study of the redox potential of transient clusters (Section 20.3.2) has shown that beyond the critical nuclearity, they receive electrons without delay from an electron donor already present. The critical nuclearity depends on the donor potential and then the autocatalytic growth does not stop until the metal ions or the electron donor are not exhausted (Fig. 8c). An extreme case of the size development occurs, despite the presence of the polymer, when the nucleation induced by radiolytic reduction is followed by a chemical reduction. The donor D does not create new nuclei but allows the supercritical clusters to develop. This process may be used to select the cluster final size by the choice of the radiolytic/chemical reduction ratio. But it also occurs spontaneously any time when even a mild reducing agent is present during the radiolytic synthesis. The specificity of this method is to combine the ion reduction successively ... 

The copper-chloride hybrid thermochemical cycle is one of the best potential low temperature thermochemical cycles for the massive production of hydrogen. It could be used with nuclear reactors such as the sodium fast reactor or the supercritical water reactor. Nevertheless, this thermochemical cycle is composed of an electrochemical reaction and two thermal reactions. Its efficiency has to be compared with other hydrogen production processes like alkaline electrolysis for example. 

Supercritical reaction (nuclear) a reaction in which more than one neutron from each fission event causes another fission event. The process rapidly escalates to a violent explosion. (21.6) Superheating the process of heating a liquid above its boiling point without its boiling. (16.10)... 

As for the nuclear-heated steam reforming of synthetic crude, the medium temperature recirculation-type membrane reforming process (Ref. 10) can be applied, where either SFR (sodium fast reactor) or SCWR (supercritical water reactor) could be adopted as medium-temperature heat source. 

AECL is developing a supercritical heavy water moderated nuclear reactor (SCWR) [2] based on its successful CANDU reactor system currently deployed around the world. Since the Mark 2 [2] version of the heavy water moderated SCWR can satisfy the temperature requirements of the hybrid Cu-Cl cycle, AECL is collaborating with ANL in the development of this cycle. Also, AECL is particularly interested in this process since some of its hydrogen-economy related technologies are a good match for the developmental needs of this process, in particular for the development of the electrochemical step involved. 

If the mass is larger than this so that few neutrons escape, the reaction is called supercritical. The chain reaction then multiplies the number of fissions that occur and the violent explosion of an atomic bomb will take place if the process is not stopped. The concentration of fissile material in nuclear power plants is sufficient for a critical reaction to occur but too low for a supercritical reaction to take place. 

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