Neutron absorption capability

Hafnium neutron absorption capabilities have caused its alloys to be proposed as separator sheets to allow closer spacing of spent nuclear fuel rods in interim holding ponds. Hafnium is the preferred material of constmction for certain critical mass situations in spent fuel reprocessing plants where hafnium s excellent corrosion resistance to nitric acid is also important. 

Only two Marie 22, Mark 16B, Nfork 42 fiiel bundles are allowed b a shipping cask. There are five sections b a cask. Three portions are physically blodoed with blanks 0.e., postions 2,3, and 4) and two are open for fuel storage (i.e., positions 1 and S). One fiid bundle is placed in each section (Ref. 2-2<. Cadmium separator plates are installed b tiiese cask sections to provide neutron absorption capabilities. They are also used to mamtab the barriers between the cask... 

The light weight, high modulus and high sound velocity of boron carbide (Table 11.1) are utilized in a large variety of military and personnel armor applications (see Section 7.1.4.3). The high thermal neutron capture cross-section of 600 barns (6 X 10 m ) provides neutron absorption capability for wet and dry spent nuclear fuel storage and transport applications, and possibly for the first wall protection of nuclear fusion reactors (Van der Laan et al., 1994 Buzhinskij et al., 2009). 

It is an object of the present invention, then to provide a chain reacting system having a neutron moderator characterized by very high slowing capability while at the same time having very low neutron absorption so that neutron losses in the slowing medium are reduced to a minimum. 

This invention relates to an improved apparatus for the measurement of neutron absorption characteristics of materials. More specifically the invention relates to an improved apparatus for measuring the effect of the presence of the sample under measurement on the neutron reproduction factor, and thus the power output, of a neu-tronic reactor capable of sustaining a nuclear fission chain reaction. 

The choice of the moderator material for a central-station powerplant is generally based on the economics involved. Obviously, many factors other than the cost per unit weight or volume, per se, enter into the economics. The neutron slowing-down capability of the material has an important effect on the size of the reactor core and, therefore, the capital cost of the plant, because of the investment in moderator, pressure vessel, shielding, etc. Containment requirements for the moderator (particularly liquid moderators) can affect both the capital cost of the plant and the fuel cycle economics, the latter because of possible neutron losses. Integrity and stability of the moderator material can, of course, have important implications on other aspects of the reactor design. The neutron absorption behavior of the moderator itself affects the potential conversion ratio of the reactor and, therefore, the fuel cycle economics of the reactor. The properties of the more important moderators and the implications of these properties on the choice and performance characteristics of gas-cooled reactors will be reviewed in this section. 

For gas-cooled reactors, the coolant which has been most widely used is carbon dioxide, which is readily available and has a low neutron absorption cross section. The poor heat transfer capability of a gas as compared to a liquid coolant requires the gas-cooled reactor to be operated at a high pressure (about 600 psi) in order to provide the required heat removal capacity. 

The classical approach for determining the structures of crystalline materials is through diflfiaction methods, i.e.. X-ray, neutron-beam, and electron-beam techniques. Difiiaction data can be analyzed to yield the spatial arrangement of all the atoms in the crystal lattice. EXAFS provides a different approach to the analysis of atomic structure, based not on the diffraction of X rays by an array of atoms but rather upon the absorption of X rays by individual atoms in such an array. Herein lie the capabilities and limitations of EXAFS. 

Atomic absorption spectroscopy of VPD solutions (VPD-AAS) and instrumental neutron activation analysis (INAA) offer similar detection limits for metallic impurities with silicon substrates. The main advantage of TXRF, compared to VPD-AAS, is its multielement capability AAS is a sequential technique that requires a specific lamp to detect each element. Furthermore, the problem of blank values is of little importance with TXRF because no handling of the analytical solution is involved. On the other hand, adequately sensitive detection of sodium is possible only by using VPD-AAS. INAA is basically a bulk analysis technique, while TXRF is sensitive only to the surface. In addition, TXRF is fast, with an typical analysis time of 1000 s turn-around times for INAA are on the order of weeks. Gallium arsenide surfaces can be analyzed neither by AAS nor by INAA. 

It is seen by examination of Table 1.11(b) that a wide variety of techniques have been employed including spectrophotometry (four determinants), combustion and wet digestion methods and inductively coupled plasma atomic emission spectrometry (three determinants each), atomic absorption spectrometry, potentiometric methods, molecular absorption spectrometry and gas chromatography (two determinants each), and flow-injection analysis and neutron activation analysis (one determinant each). Between them these techniques are capable of determining boron, halogens, total and particulate carbon, nitrogen, phosphorus, sulphur, silicon, selenium, arsenic antimony and bismuth in soils. 

Since the mid-1960s, a variety of analytical chemistry techniques have been used to characterize obsidian sources and artifacts for provenance research (4, 32-36). The most common of these methods include optical emission spectroscopy (OES), atomic absorption spectroscopy (AAS), particle-induced X-ray emission spectroscopy (PIXE), inductively coupled plasma-mass spectrometry (ICP-MS), laser ablation-inductively coupled plasma mass spectrometry (LA-ICP-MS), X-ray fluorescence spectroscopy (XRF), and neutron activation analysis (NAA). When selecting a method of analysis for obsidian, one must consider accuracy, precision, cost, promptness of results, existence of comparative data, and availability. Most of the above-mentioned techniques are capable of determining a number of elements, but some of the methods are more labor-intensive, more destructive, and less precise than others. The two methods with the longest and most successful histoty of success for obsidian provenance research are XRF and NAA. 

Some comments should be made on areas which are not included. No discussion is given on specific instrumentation. There is virtually no discussion of structural characterization of surfaces, the emphasis being on elemental and chemical composition analysis. For this reason, several techniques which are primarily structural tools, are not discussed at all (e.g., Low Energy Electron Diffraction, LEED (1), Surface Extended X-Ray Absorption Fine Structure SEXAFS (2), and neutron scattering (3)), and the structural analysis capabilities of XPS (4), SIMS (5), and Ion Scattering (6) are not covered. 

Techniques can be classified into two main categories those that detect total metal concentrations and those that detect some operationally defined fraction of the total. Methods which detect total concentrations such as inductively coupled plasma spectrometry, neutron activation analysis, atomic absorption spectrometry and atomic emission spectrometry have no inherent speciation capabilities and must be combined with some other separation technique(s) to allow different species to be detected (approach A in Fig. 8.2). Such separation methods normally fractionate a sample on the basis of size, e.g. filtration/ultrafiltration, gel filtration, or a combination of size and charge, e.g. dialysis, ion exchange and solvent extraction (De Vitre et al., 1987 Badey, 1989b Berggren, 1989 1990 Buffle et al., 1992). In all instances the complexes studied must be relatively inert so that their concentrations are not appreciably modified during the fractionation procedure. 

The most common neutron detectors are of the proportional gas type. Since neutrons themselves have no charge and are non-ionizing, they are harder to detect than X-rays. Detection relies on the absorption of the neutron by an atomic nucleus with the simultaneous emission of a y-ray photon, often referred to as an (n,y) reaction. Since the absorbing material must absorb neutrons and be capable of existing in gaseous form, the choice of substances is limited. The most common is He gas, which relies on the reaction ... 

A number of instrumental methods have been used to determine ppb levels of cobalt in water (4,5,6), biological tissues (7,8), and air particulates (9, 10). Kinetic methods are capable of measuring sub-parts-per-billion (11,12). Potentially any of these techniques could be used in the analysis of petroleum, but only neutron activation analysis (I, 3) and atomic absorption spectroscopy (13,14) have been applied to any appreciable extent. Flame and heated vaporization atomic absorption techniques were selected for more detailed study by the Project because atomic absorption is sensitive, subject to relatively few interferences, and is rather generally available. 

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