Neutrons cross sections

Niobium is also important in nonferrous metallurgy. Addition of niobium to tirconium reduces the corrosion resistance somewhat but increases the mechanical strength. Because niobium has a low thermal-neutron cross section, it can be alloyed with tirconium for use in the cladding of nuclear fuel rods. A Zr—l%Nb [11107-78-1] alloy has been used as primary cladding in the countries of the former USSR and in Canada. A Zr—2.5 wt % Nb alloy has been used to replace Zircaloy-2 as the cladding in Candu-PHW (pressurized hot water) reactors and has resulted in a 20% reduction in wall thickness of cladding (63) (see Nuclear reactors). 

Boron-10 has a natural abundance of 19.61 atomic % and a thermal neutron cross section of 3.837 x 10 m (3837 bams) as compared to the cross section of 5 x 10 m (0.005 bams). Boron-10 is used at 40—95 atomic % in safety devices and control rods of nuclear reactors. Its use is also intended for breeder-reactor control rods. 

Account must be taken in design and operation of the requirements for the production and consumption of xenon-135 [14995-12-17, Xe, the daughter of iodine-135 [14834-68-5] Xenon-135 has an enormous thermal neutron cross section, around 2.7 x 10 cm (2.7 x 10 bams). Its reactivity effect is constant when a reactor is operating steadily, but if the reactor shuts down and the neutron flux is reduced, xenon-135 builds up and may prevent immediate restart of the reactor. 

Several of the reactor physics parameters are both measurable and calculable from more fundamental properties such as the energy-dependent neutron cross sections and atom number densities. An extensive database. Evaluated Nuclear Data Files (ENDF), has been maintained over several decades. There is an interplay between theory and experiment to guide design of a reactor, as in other engineering systems. 

Fig. 2. Total neutron cross sections of silver (—) and cadmium (----) (10). To convert pm to bams, multiply by 10 ...

Laser stimulation of a silver surface results in a reflected signal over a million times stronger than that of other metals. Called laser-enhanced Raman spectroscopy, this procedure is useful in catalysis. The large neutron cross section of silver (see Fig. 2), makes this element useful as a thermal neutron flux monitor for reactor surveillance programs (see Nuclearreactors). 

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). 

For the deterrnination of trace amounts of bismuth, atomic absorption spectrometry is probably the most sensitive method. A procedure involving the generation of bismuthine by the use of sodium borohydride followed by flameless atomic absorption spectrometry has been described (6). The sensitivity of this method is given as 10 pg/0.0044M, where M is an absorbance unit the precision is 6.7% for 25 pg of bismuth. The low neutron cross section of bismuth virtually rules out any deterrnination of bismuth based on neutron absorption or neutron activation. 

An alloy containing 80% Ag, 15% In, and 5% Cd is used in control rods in nuclear reactors because it has a high neutron cross-section and good mechanical strength (see Nuclearreactors). 

SiHcon carbide s relatively low neutron cross section and good resistance to radiation damage make it useful in some of its new forms in nuclear reactors (qv). SiHcon carbide temperature-sensing devices and stmctural shapes fabricated from the new dense types are expected to have increased stabiHty. SiHcon carbide coatings (qv) may be appHed to nuclear fuel elements, especially those of pebble-bed reactors, or siHcon carbide may be incorporated as a matrix in these elements (153,154). 

Niobium-Vanadium The presence of vanadium reduces niobium s corrosion resistance to most media. The alloy containing 12 6 at. Vo V however has excellent resistance to high-temperature water and steam, and this property and the alloy s relatively low neutron cross section give it considerable potential for nuclear applications. 

Metal A lomic number Atomic weight Lattice structure Density at 20°C (g/em ) Melting point (°C) Thermal conductivity at 0-l00°C (W/m°C) Specific heat at 0°C (J/kg C) Coefficient of linear expansion at 20-iOO°C X 70 Thermal neutron cross-section (barns) (10-- m ) Resistivity at 0°C (fiil em) Temperature coefficient of resistance o-ioo°c X 10 ... 

A large neutron cross section of 235U for fission (5.8 x 10 26 m2), a high fission yield (6%) for "Tc, and a long half-life of the resulting "Tc (2.1 x 105 yr) make this radionuclide one of the principal nuclear wastes. Fig. 1 shows radioactivity of nuclear wastes plotted against cooling time in years. Tc activity is very important in the time interval 104-106 years. 

Mughabghab SF (1984) Neutron Cross Sections Vol. I Part B. Academic Press, New York... 

Zirconium is used for structural parts in the core of water moderated nuclear reactors to this end Zr has several good properties and especially it has low thermal neutron cross-section. Hf, on the contrary, has a high thermal neutron absorption coefficient, so it is necessary to be able to prepare Hf-free zirconium. On the other hand, in some cases the Hf properties too may be useful in nuclear technology, in the control rods of submarine reactors. 

Stehn, J. R. Goldberg, M. D. Mayumo, B. A. Wiener-Chasman, R. Neutron Cross Section (BNL 325) Brookhaven National Laboratory 1964. 

Figure 1 shows the energy dependence of the neutron cross section for representative materials/reactions. Some reactions are seen to exhibit a threshold... 

Rgure 1 Variability of the neutron cross sections for different isotopes. 

Figure 2 High-energy resonance behavior for some neutron cross sections.

Soft, lustrous metal silver-like appearance close-packed hexagonal crystal system density 8.78 g/cm paramagnetic magnetic moment 11.2 Bohr magnetons melts at 1,472°C vaporizes at 2,694°C electrical resistivity 195 microhm-cm at 25°C Young s modulus 6.71xl0n dynes/cm2 Poisson s ratio 0.255 thermal neutron cross section 64 barns insoluble in water soluble in acids (with reactions). 

Silvery white metal soft and malleable hexagonal closed pack crystal system transforms to face-centered cubic crystals at 310°C which further transforms to a body-centered cubic allotropic modification at 868°C density 6.166 g/cm3 Brinnel hardness (as cast) 37 melts at 918°C vaporizes at 3,464°C vapor pressure 1 torr at 2,192°C electrical resistivity 56.8 x 10 ohm-cm at 25°C Young s modulus 3.84 x lO- dynes/cm Poisson s ratio 0.288 thermal neutron cross section 8.9 bams. 

Silvery-white metal hexagonal close-packed structure density 9.84 g/cm melts at 1,663°C vaporizes at 3,402°C electrical resistivity 59 microhm-cm slightly paramagnetic thermal neutron cross section 108 barns soluble in acids. 

Silvery-white, soft maUeable metal exists in two aUotropic forms an alpha hexagonal from and a beta form that has body-centered cubic crystal structure the alpha allotrope converts to beta modification at 868°C paramagnetic density 7.004 g/cm compressibility 3.0x10 cm /kg melts at 1024°C vaporizes at 3027°C vapor pressure 400 torr at 2870°C electrical resistivity 65x10 ohm-cm (as measured on polycrystalline wire at 25°C) Young s modulus 3.79xl0 ii dynes/cm2 Poisson s ratio 0.306 thermal neutron cross section 46 barns. 

Silvery-white lustrous metal face-centered cubic crystal structure ductile ferromagnetic density 8.908 g/cm at 20°C hardness 3.8 Mohs melts at 1,455°C vaporizes at 2,730°C electrical resistivity 6.97 microhm-cm at 20°C total emissivity 0.045, 0.060 and 0.190 erg/s.cm2 at 25, 100 and 1,000°C, respectively modulus of elasticity (tension) 206.0x10 MPa, modulus of elasticity (shear) 73.6x10 MPa Poisson s ratio 0.30 thermal neutron cross section (for neutron velocity of 2,200 m/s) absorption 4.5 barns, reaction cross section 17.5 barns insoluble in water dissolves in dilute nitric acid shghtly soluble in dilute HCl and H2SO4 insoluble in ammonia solution. Thermochemical Properties... 

Silvery-white metal face-centered cubic crystalline structure density 12.02 g/cm Vickers hardness, annealed 37-39 melts at 1,554°C vaporizes at 2,970°C electrical resistivity 9.93 microhm-cm at 0°C Poisson s ratio 0.39 magnetic susceptibility 5.231x10 cm /g thermal neutron cross section 8... 

Silvery-white lustrous metal remains bright at all temperatures face-centered cubic crystal density 21.5g/cm3 Vickers hardness, annealed 38-40 melts at 1,768.4°C vaporizes at 3,825°C vapor pressure at melting point 0.00014 torr electrical resistivity 9.85 microhm-cm at 0°C magnetic susceptibility 9.0x10— cm /g Poisson s ratio 0.39 thermal neutron cross section 8 barns insoluble in water and acids soluble in aqua regia... 

Xenon occurs in the atmosphere at trace concentrations. It also occurs in gases from certain mineral springs. Xenon also is a fission product of uranium, plutonium, and thorium isotopes induced by neutron bombardment. The radioactive fission product, xenon-135, has a very high thermal neutron cross-section. The element has been detected in Mars atmosphere. 

The Physical Properties are listed next. Under this loose term a wide range of properties, including mechanical, electrical and magnetic properties of elements are presented. Such properties include color, odor, taste, refractive index, crystal structure, allotropic forms (if any), hardness, density, melting point, boiling point, vapor pressure, critical constants (temperature, pressure and vol-ume/density), electrical resistivity, viscosity, surface tension. Young s modulus, shear modulus, Poisson s ratio, magnetic susceptibility and the thermal neutron cross section data for many elements. Also, solubilities in water, acids, alkalies, and salt solutions (in certain cases) are presented in this section. 

In some cases, thermal neutrons can also be used to measure the absolute abundances of other elements. Transforming the neutron spectrum into elemental abundances can be quite involved. For example, to determine the titanium abundances in lunar spectra, Elphic et at. (2002) first had to obtain FeO estimates from Clementine spectral reflectances and Th abundances from gamma-ray data, and then estimate the abundances of the rare earth elements gadolinium and samarium from their correlations with thorium. They then estimated the absorption of neutrons by major elements using the FeO data and further absorption effects by gadolinium and samarium, which have particularly large neutron cross-sections. After making these corrections, the residual neutron absorptions were inferred to be due to titanium alone. 

Os-191 is produced by neutron irradiation of isotopically enriched 0s-190 (isotopic composition 0s-190, 97.8 o Os-188, 0.47 o Os-192, 1. 02 o). Irradiations are currrently performed at the Oak Ridge National Laboratory in the High Flux Isotope Reactor (HFIR) at a neutron flux of 2.5 x 10 n/cm -s. The routes to the various nuclides produced during irradiation of the 0s-190 target and the neutron cross-section values (2 ) are summarized below (Scheme I). 

Mughabghab, S.F. Garber, D.I. "Neutron Cross Sections" Vol. 1, Resonance Parameters, BNL 325, Third Edition, Brookhaven National Laboratory Uptown, NY, 1973. 

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