Fast fission neutrons

Deuterium oxide is available in ton quantities and is used as a moderator in nuclear reactors, both because it is effective in reducing the energies of fast fission neutrons to thermal energies and because deuterium has a much lower capture cross-section for neutrons than has hydrogen and hence does not... 

Calculate the number of collisions required to reduce a fast fission neutron ( = 2 MeV) to thermal energy ( 0.025 eV) in a light-water-moderated reactor, assuming that the data in Table 19.3 are valid. 

However, in order for the premise to be fulfilled that the fast fission neutrons be slowed to thermal energies in a slowing medium without too large an absorption in the U isotope of the uranium, certain types of physical structure should be utilized for the most efficient reproduction of neutrons, since unless precautions are taken to reduce various neutron losses and thus to con-serve neutrons for the chain reaction the rate of neutron reproduction may be lowered and in certain cases lowered to a degree such that a self-sustaining system is not attained. 

Fio. 10.18 Spatial distribution of successive generations of fast-fission neutrons. 

The resulting thermal neutrons will diffuse through the core and eventually be absorbed or escape from the system. Those neutrons absorbed in the fuel will cause fissions and produce more fast fission neutrons. The fission neutrons in question are prompt neutrons, for the delayed neutrons are now stored in their respective precursors whose probability of decay is quite small over the time scale of the experiment. The fission neutrons now suffer the same fate as the source neutrons they are ther-malized and then absorbed or lost from the system. The thermal-neutron population in the reactor will decrease with time since the reactor is subcritical and because there is a negligible number of delayed neutrons entering the system. The population will decay exponentially, and the decay rate will be related to the reactivity of the system measured from prompt critical. 

Fast Reactor An advanced technology nuclear reactor that uses a fast fission process utilizing fast neutrons that would split some of the U-258 atoms as well as transuranic isotopes. The goal is to use nuclear material more efficiently and safely in the production of nuclear energy. 

The geochemistry of uranium and thorium has excited considerable interest on accoimt of their strategic importance. Smales determined uranium in rocks by neutron activation followed by isolation of fission product Ba (81). Interference from the fast fission of any thorium present in the sample and from beta-emitting barium isotopes formed by (n,y) reaction is discussed and methods of overcoming the diflSculties are described. The uranium content of two iron meteorites was determined by... 

The nuclide, " 92 U, is fissionable by fast-moving neutrons but is not fissile. 

Fission The splitting of a large atomic nucleus into two or more smaller parts. This is usually the result of a collision between a fast-moving neutron and a nucleus. When the nucleus breaks apart, a small amount of matter is converted to energy. This is the basis for nuclear power plants and for the first nuclear weapons. 

Uranium-235 in fuel rods produces fast-moving neutrons and heat in a fission chain reaction. The neutrons are slowed down by a moderator such as water or graphite so that they are not moving too quickly to be absorbed by other uranium-235 nuclei. The rate of the reaction is maintained using control rods that absorb some of the neutrons. These rods can be raised or lowered in the reaction chamber to slow or speed the reaction, respectively. 

Tabto 21.2 Delayed neutron parametera for fast fission ... 

Those fast neutrons that have energies greater than about 1 MeV may cause a limited amount of fission of fertile material. To account for this, the reactor designer usually specifies a quantity e, called the fast-fission factor, which is defined as the ratio of the net rate of production of fast neutrons to the rate of production of fast neutrons by thermal fission. The fraction e — 1 of the fast neutrons comes from fission of fertile material with fast neutrons e — 1 may be of the order of a few hundredths in a thermal power reactor. The net production rate of fast neutrons from fission is er N a 4>-... 

Neutron activation has been used to study mercury levels in some crude oils (19). The choice of isotope used for measurement depends on the interfering elements in the samples. With [196Hg (n,y) 197Hg, 77 KeV, Ti/a — 65 hr], a 10 ng/g detection limit is readily achieved with 1-hr irradiation at a neutron flux of 1012 n cm"2 sec -1. However, inherent sulfur or chloride (from entrained formation water) create serious interferences in some crudes. The fission neutrons (fast) from these elements produce 32P (T1/2 = 14 days) and 35S (Ti/2 = 87 days), both of which contribute... 

The n-capture in the r-process has been suggested to go up to Z about 100 and N 184. In the intense neutron field a considerable amount of (mainly fast) fission of the newly synthesized heavy elements probably also occurs. This partly explains the peaks at IV = 50 and 82 in Figure 17.7b, which also correspond to maximum yields at 4 = 95 and 140 in thermal hssion. Some stars are unique in that they have an imusually high abundance of fission products spectral lines from heavy actinides, like americium and curium, have also been observed in such stars. 

It is obvious that the neutron energy spectrum of a reactor plays an essential role. Figure 19.4 shows the prompt (unmoderated) fission neutron spectrum with 2 MeV. In a nuclear explosive device almost all fission is caused by fast neutrons. Nuclear reactors can be designed so that fission mainly occurs with fast neutrons or with slow neutrons (by moderating the neutrons to thermal energies before they encounter fuel). This leads to two different reactor concepts - the fast reactor and the thermal reactor. The approximate neutron spectra for both reactor types are shown in Figure 19.4. Because thermal reactors are more important at present, we discuss this type of reactors first. 

A rough estimate of the critical radius of a homogeneous unreflected reactor may be obtained simply by estimating the neutron mean free path according to (14.6). Assuming metal with a density of 19 g cm and a fast fission cross section of 2 X crn, one obtains = 10 cm. A sphere with this radius weighs 80 ks. For an unreflected metal sphere containing 93.5% the correct value is 52 kg. Pu has the smallest unreflected critical size for Pu (5-phase, density 15.8 g cm ) it is 15.7 kg ( 6 kg reflected), and for 16.2 kg ( 6 kg reflected). 

When fabrication and handling of metal nuclear fuels is contemplated, a different type of design curve is required. Moderation of fission neutrons is absent and conditions for fast-neutron fission exists. Critical values are larger under these conditions (Fig. 10-11). 

One of its attractive features of rhenium is that it is a spectral shift absorber (SSA), which means that it has a low relative absorption cross section for fast neutrons while in the thermal spectrum its absorption cross section increases dramatically. This has safety applications for the reactor design in accident scenarios. Rhenium has an absorption cross section of in the fast spectrum, however the magnitude of the difference between the absorption cross section and the fast fission cross section of is low compared to the difference at a thermal spectrum. It also provides a barrier that protects Niobium 1% Zirconium from nitrogen attack and damage caused by other fission products that outgas from the fuel. Most of the other SSA materials have a relatively low melting point, making them less attractive. 

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