Parasitic neutron capture

In the thermal breeder reactor, is produced from Th. The reactor may be designed either with a core containing a mixture of Th and or with a central zone core) of surrounded by an outer layer blanket) of Th. However, it is necessary to minimize parasitic neutron capture in structmal materials including monitoring systems, control rods, etc. Calculations have shown that a conversion ratio of 1.06 should be possible. 

Unlike other fuel forms, the ratio of fuel and moderator to liquid coolant is fixed in a pebble-bed reactor. This places major constraints on the choice of coolant (it most likely will require the use of a salt with enriched lithium-7 and beryllium) and other core design parameters. While this salt is more expensive, it has very low parasitic neutron capture, which combined with the very small excess reactivity and large cylindrical core, would provide high fuel utilization. Initial studies on a liquid-salt-cooled pebble-bed reactor have been conducted at Delft University in the Netherlands. 

Besides neutron capture in the fuel, other parasitic reactions play a role. They will be discussed in the following with respect to the practical case of reactor construction and operation. In this context a criticality factor is defined. 

Bohr s logical distinction between U238 and thorium on the one hand and U235 on the other ruled out (1) U238 was not fissioned by slow neutrons. (2) was inefficient because of scattering and the parasitic effects of the capture resonance of U238. (3) was possibly applicable to power production but too slow for a practical weapon. But what about (4) Apparently no one in Britain, France or the United States had asked the question quite that way before. 

The moderator temperature reactivity coefficient is also important for safety. In fact, when the moderator temperature increases, its density decreases and, as a consequence, the moderating effectiveness also decreases. This decrease causes an increase in the loss of neutrons from the core and an increase in the parasite captures, so that the reactivity tends to decrease. 

A completely different scenario involves the utilization of the neutrons otherwise lost by parasitic capture in the control materials of the "client" fission reactors. For example, using °Li and/or He (instead of etc.) as control materials in Light Water Reactors, one could arrive at y s (y = No. of tritons produced per average fusion neutron) close to 0.75.[43] This would support a SCD-T mode of operation having about 5 fold the power density of the corresponding SCO plasma. 

Answer By withdrawing the neutron-absorbing control rods, the parasitic capture of neutrons is reduced so lliat k ff becomes greater than one and the rising chain reaction causes the neutron density to increase until the desired power level is attained. The control rods are then partially inserted to absorb enou neutrons to make keff equal to unity the chain reaction now becomes steady and the power level remains constant. 

The neutron absorption cross-sections of any liquid salt for reactor applications must be low to avoid excessive parasitic capture of neutrons. For thermal and intermediate neutron spectrum reactors, this probably eliminates chloride salts with their higher nuclear cross sections, even if the high cross section Cl is removed. Only fluoride salts are candidates. A wide variety of atoms have low cross sections however, the realistic candidates are also restricted by the requirements of thermodynamic stability to ensure viable materials of construction for the container. Table XXVI-5 shows the primary salt options and their cross sections. If either lithium or boron is used as a salt component, isotopically separated lithium and boron are required to have a salt with a low absorption cross section. 

Uranium-233 is a superior fuel for use in molten fluoride-salt reactors in almost every respect. The fission cross section in the intermediate range of neutron energies is greater than the fission cross sections of U and Pu239 Thus initial critical inventories are less, and less additional fuel is required to override poisons. Also, the parasitic cross section is substantially less, and fewer neutrons are lost to radiative capture. Further, the radiative captures result in the immediate formation of a fertile iso-... 

Neutron balance and miscellaneous details. Details of the neutron economy of a reactor fueled with plutonium are given in Table 14-7. Parasitic captures in Pu are relatively high y is 1.84, compared with a, v oi 2.9. The neutron spectrum is relatively soft almost 60% of all fissions are caused by thermal neutrons and, as a result, absorptions in lithium are high. 

Intermediate states. On the basis of the average value of a of Pu , it is estimated that Pu will accumulate in the system until it captures, at equilibrium, about half as many neutrons as Pu . While these captures are not wholly parasitic, inasmuch as the product, Pu , is fissionable, the added competition for neutrons will necessitate an increase in the concentration of the Pu . Likewise, the ingrowth of fission products... 

The limitations of motallurgical knowledge at present lead to the conclusion that tantalum will be one of the be.st container materials for those plutonium alloys. The high-temperature strength properties and the heat-transfer properties of tantalum are excellent moreover, it is weldable. The parasitic capture cross section of tantalum would be intolerable in an epithermal or thermal power breeder reactor and, although relatively large in a fast spectrum, its effect on neutron economy in a fast reactor can be made small, if not minor, by careful design. 

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