Parasitic neutron absorption

The net gain in neutrons provided by the low neutron absorption in the heavy water may be used up, for example, by the insertion of more parasitic neutron absorbers into the system than could be tolerated with other... 

The reactivity difference between an air (nitrogen) and a helium atmosphere in the reactor has been calculated to be 0.l6. The principal effect of nitrogen in the lattice is to Increase the parasitic absorptions and thus reduce the thermal utilization, f The thermal neutron absorption cross section of helium is zero. 

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. 

Most of these problems arise from the need to minimize parasitic neutron absorption In the pressure tubes. As well as limiting directly the choice of alloy systems of interest in this application, it also makes It desirable that the material shows a high strength and Is able to tolerate the chemical. Irradiation and thermal environments without the need for large corrosion allowances, shielding or Insulation. It Is such considerations which have led to the use of zirconium alloys for pressure tubes in water reactors. 

One of the principal design goals for the SSR is to provides power on an as-needed basis and shut down automatically when there is no demand. This is accomplished by designing the reactor to have a negative temperature coefficient. As the heat extraction is decreased (power demand is reduced), the reactor temperature will increase causing an increase in parasitic neutron absorption in the fuel due to the Doppler effect. The power level of the reactor will then drop to the level where the thermal heat production balances the thermal heat removal. 

Reduced parasitic neutron absorption. The neutron economy of the TRICON-440 fuel assembly is significantly better than currently operating fuel assemblies due to the absence of stainless steel in the core and the reduced amount of Zry-4 compared to the amount of ZrlNb in current fuel assemblies. The very small... 

Refractory and Rare Earth Parasitic Neutron Absorption - Fast Range... 

We were inclined to conclude that... the water-uranium system would sustain a chain reaction. Placzek said that our conclusion was wrong because in order to make a chain reaction go, we would have to eliminate the absorption of [neutrons by the] water that is, we would have to reduce the amount of water in the system, and if we reduced the water in the system, we would increase the parasitic absorption of [neutrons by] uranium [because with less water fewer neutrons would be slowed]. He recommended that we abandon the water-uranium system and use helium for slowing down the neutrons. To Fermi this sounded funny, and Fermi referred to helium thereafter invariably as Placzek s helimn. 

The breeder is an arrangement in which 23 undergoes flssion and the neutrons produced thereby are absorbed by thorium. When this thorium decays it produces again 23 via Pa. Since the rj of 23 is, according to Anderson and May, 2.37 the net increase in the number of neutrons per 23 destroyed is 1.37. If all these neutrons were absorbed in thorium, we would obtain 1.37 23 atoms per 23 destroyed. It is well to remember, however, that if only as many as 15% of the 2.37 neutrons emitted are lost by escape from the system or parasitic absorption by other materials or otherwise, the efficiency of the breeder goes down to 1 and no increase in the amount of 23 results. 

The use of the reactor surrounded by the absorbing zone, except for the solution inlet and outlet, reduces the escape of neutrons in the directions in which there 10 is absorption to a minimum. The reactor in Figs. 1 and 2 may be said to be, in one sense, totally surrounded by reflecting material in that water is present in both inlet and outlet pipes 11 and 12, respectively. Parasitic neutron escape completely out of the system therefore is re-15 duced. 

It has been found in this connection that the 17 of or, in other words, the number of neutrons emitted per fission of a atom is about 2.37, thus providing a net 35 increase of 1.37 neutrons per fission. If all of these neutrons are absorbed in fertile material such as, fer example, thorium atoms, 1.37 atoms are produced for each atom consumed or destroyed by the chain reaction. However, it will be noted that the loss of as 0 much as 15 percent of the 2.37 neutrons by escape from the system or by parasitic absorption of other materials than the thorium results in a net increase of zero thus preventing the breeding of thermally fissionable material. An enumeration of the losses follows ... 

Stainless-steel cladding results in parasitic absorption of neutrons, which reduces the neutron economy of the reactor. 

Because of the small reactivity margin available for breeding in a thermal reactor, the use of the thorium cycle has mainly been associated with reactors with very good neutron economy based on low parasitic absorption, such as the high-temperature gas-cooled reactor, where graphite is used in place of metal for the fuel cladding, or heavy water reactors, with very low moderator absorption. A special case is the molten salt breeder reactor, where circulation of the fissile and fertile materials allows continuous removal not only of Pa but also of fission products. 

In addition, in the top parts of the fuel rods of the control assembly, depleted uranium is used instead of steel to avoid high local power peaks in the uppermost fuel pellets, and to improve economics by reducing parasitic absorption (which would occur in the steel) and reducing axial neutron leakage ... 

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. 

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