Fast reactors cross sections

R. J. ARCHIBALD and D. R. MATHEWS, "The GAF/ GAR/GAND Fast Reactor Cross Section Preparation System Volume I, GAF/GAR - A Program for the Calculation of Neutron Spectra and Group-Averaged Cross Sections, GA-7542, Vol. I, Gulf General Atomic (1968). ... 

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). 

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. 

In the case of concentrated reactant solutions, we can observe a sharp temperature increase during acidic media neutralisation. The temperature field in the reaction zone can be adjusted by using small tubular turbulent reactors of cylinder or diffuser-confusor design. There are several options for temperature adjustment [16], e.g., the change of the device radius and reactant flow rate, allows the application of the zone model for fast chemical processes and the use of shell-and-tube reactors with pipe columns of small radius comprising the same reactor cross section in total [17],... 

The technologically most important isotope, Pu, has been produced in large quantities since 1944 from natural or partially enriched uranium in production reactors. This isotope is characterized by a high fission reaction cross section and is useful for fission weapons, as trigger for thermonuclear weapons, and as fuel for breeder reactors. A large future source of plutonium may be from fast-neutron breeder reactors. 

Sodium is used as a heat-transfer medium in primary and secondary cooling loops of Hquid-metal fast-breeder power reactors (5,155—157). Low neutron cross section, short half-life of the radioisotopes produced, low corrosiveness, low density, low viscosity, low melting point, high boiling point, high thermal conductivity, and low pressure make sodium systems attractive for this appHcation (40). 

Nuclear and magneto-hydrodynamic electric power generation systems have been produced on a scale which could lead to industrial production, but to-date technical problems, mainly connected with corrosion of the containing materials, has hampered full-scale development. In the case of nuclear power, the proposed fast reactor, which uses fast neutron fission in a small nuclear fuel element, by comparison with fuel rods in thermal neutron reactors, requires a more rapid heat removal than is possible by water cooling, and a liquid sodium-potassium alloy has been used in the development of a near-industrial generator. The fuel container is a vanadium sheath with a niobium outer cladding, since this has a low fast neutron capture cross-section and a low rate of corrosion by the liquid metal coolant. The liquid metal coolant is transported from the fuel to the turbine generating the electric power in stainless steel... 

In addition, the PFR model assumes that mixing between fluid elements at the same axial location is infinitely fast. In CRE parlance, all fluid elements are said to be well micromixed. In a tubular reactor, this assumption implies that the inlet concentrations are uniform over the cross-section of the reactor. However, in real reactors, the inlet streams are often segregated (non-premixed) at the inlet, and a finite time is required as they move down the reactor before they become well micromixed. The PFR model can be easily... 

Reactions with fast neutrons, such as (n, 2n), (n, p) and (n, a) reactions, are only of minor importance for production of radionuclides in nuclear reactors. However, special measures may be taken for irradiation of samples with high-energy neutrons. For instance, the samples may be irradiated in special fuel elements of ring-like cross section as shown in Fig. 12.1, or they may be irradiated in a receptacle made of enriched uranium. In both cases, the fast neutrons originating from the fission of enter the samples directly and their flux density is higher by about one order of magnitude than that at other places in the reactor. 

The more important of these reactions will be considered below. Reactions 3, and 4 are brought about by the fast neutron flux in the pile, which flux Mellish et al. 59) calculated to be 0.17 of the slow flux in the center of the Harwell pile (BEPO). Fortunately, however, the cross sections for these reactions are usually considerably lower than those for normal (TO,y) reactions induced by thermal neutrons. Contributions brought about by n,p) and (to, ) reactions can often be greatly reduced, as mentioned previously, by irradiating in the thermal column of the reactor, with some loss of sensitivity. Reaction 4, in,2n), is produced by... 

There are three fast-flux reactors proposed for development the sodium cooled, the gas cooled, and the lead cooled. The fission cross sections for fast neutrons (high-energy spectrum neutrons) for all of the fissile actinides are nearly the same so the fast-flux reactors use all of the fissile actinides as fuel. The fast-flux isotopic fission cross sections are smaller than for thermal neutrons so the fraction of fissile isotopes (e.g., 235u 239pu, range of... 

A fast reactor is one in which the average speed of neutrons is near that which they have at the moment of fission, around 15 million m/s. At these high speeds the probability of a neutron s being absorbed by a fissionable atom is low, and the neutron-absorption cross section, which is a measure of this probability, is small. 

A thermal reactor is one in which the neutrons have been slowed down until they are in thermal equilibrium with reactor materials in a typical power reactor, thermal neutrons have speeds around 3000 m/s. At these lower speeds, the neutron-absorption cross sections are much larger than for fast neutrons. 

The critical mass of fissile material required to maintain the fission process is roughly inversely proportional to the neutron-absorption cross section. Thus the critical mass is lowest for plutonium in thermal reactors, larger for the uranium isotopes in thermal reactors, and much greater in fast reactors. For this reason, as well as others, thermal reactors are the preferred type except when breeding with plutonium is an objective then a fast reactor must be used. 

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