Neutron absorbers characteristics

Credit shall not be claimed for neutron absorbing parts or components of the facility unless they are fixed, their neutron absorbing ciqiabilities can be determined, and they are unlikely to be degraded by any postulated initiating events. Consideration of the neutron absorbing characteristics of the fuel assembly may be included. 

Boron trifluoride is also employed in nuclear technology by uti1i2ing several nuclear characteristics of the boron atom. Of the two isotopes, B and B, only B has a significant absorption cross section for thermal neutrons. It is used in " BF as a neutron-absorbing medium in proportional neutron counters and for controlling nuclear reactors (qv). Some of the complexes of trifluoroborane have been used for the separation of the boron isotopes and the enrichment of B as (84). 

Deuterium has an extremely low neutron absorption characteristic for thermal neutrons i. e., thermal neutrons 0Q can ( use in the deuterium with relatively little danger of being absorbed by deuterium nuclei. For this reason heavy water is an ideal moderator. 

Diphenyl has a lower absorbing characteristic for thermal neutrons than has light water for example, a loss of neutrons by absorption due to a diphenyl cooling layer 4 millimeteia corresponds to the loss due to a 2.2... 

This theory allows us to estimate perturbations in some characteristics of interest in noncritical systems. For example, the thermal utilization / is the ratio of neutrons absorbed in the fuel (with probability h/v) to the neutrons absorbed throughout the cell (with probability L v), Thus, f = j 1,/vN dxjj If vN dx. Let... 

The neutron-physical characteristics and the efficiency of the reactivity control system are such that at any moment in the reactor life cycle, cold subcriticality, with no dissolved boron is assured, even in the case of the most effective rod being stuck in its upper position. The liquid absorber injection system is used only in beyond design accidents. 

The Advanced Fuel Recycle Program is concerned with the safe reprocessing of mixed plutonium and uranium oxide fuels, characteristic of fast reactors. The safe handling and storage of these fuels hin on calculations and these in turn depend on clean, well-deflned experiment data for validation. Benchmark experiment data have been. acquired for fast test reactor (I R)-type fuels for impoisoned systems and systems intermixed with soluble poisons. However, there, are no data now available, fliat explore the criticality of these fuels intermixed with solid neutron absorbers (poisons). In this paper, we will present the results of experiments performed at the Pacific Northwest Laboratory (Critical Mass Laboratory) on fast test reactor fuel elements intermixed with solid neutron absorbers. The isons used were Bbral and cadmium plates and gadoliniuth cylindrical rods. Each absorber was separately examined to see its reactivity effect on lattices of FTR fuel pins in water none was intermixed. 

What two characteristics of Xenon-135 result in it being a major reactor poison/neutron absorber, compared to other fission products ... 

A comprehensive presentation of the Generation IV project is made in Chapter 1. Here, we only give the main characteristics of the planned systems in order to draw the main requirements the neutron absorber materials should fulfill. As compared to generation n and in reactors, the Generation IV projects are intended to propose breakthrough solutions to safety, competitiveness, sustainability, nonproliferation, and waste management [1]. In that frame, and taking into account the previous experience of the GIF forum participants, six different systems are considered ... 

Some of their characteristics (temperature, thermal volume power, neutron spectra, cooling media, etc.), which strongly impact the neutron absorber material choice and the Control Element Assemblies (CEA) design, are reported in Table 15.2. The relations between the systems characteristics and the CEA choice and design are as following ... 

Table 15.2 Some characteristics of the Generation IV projects to be taken into consideration for the choice of the neutron absorber materials... 

Characteristics of FPN poisoning. Owing to the fact that they include no nuclides which are particularly high neutron absorbers, the FPN can be allowed to build up to relatively high concentrations in the fuel. The two worst poisons are Tc with a 19-barn thermal cross section, and stable Rh - with 1.50. For the reactor conditions described earlier, the poisoning effect of the FPN (less Mo) is essentially proportional to their concentration or average residence time in the fuel. The relationship is... 

Characteristics of soluble and solid neutron absorbers Bumup characteristics of fuel normally handled in casks or stored in facility... 

Another major difference between the use of X rays and neutrons used as solid state probes is the difference in their penetration depths. This is illustrated by the thickness of materials required to reduce the intensity of a beam by 50%. For an aluminum absorber and wavelengths of about 1.5 A (a common laboratory X-ray wavelength), the figures are 0.02 mm for X rays and 55 mm for neutrons. An obvious consequence of the difference in absorbance is the depth of analysis of bulk materials. X-ray diffraction analysis of materials thicker than 20—50 pm will yield results that are severely surface weighted unless special conditions are employed, whereas internal characteristics of physically large pieces are routinely probed with neutrons. The greater penetration of neutrons also allows one to use thick ancillary devices, such as furnaces or pressure cells, without seriously affecting the quality of diffraction data. Thick-walled devices will absorb most of the X-ray flux, while neutron fluxes hardly will be affected. For this reason, neutron diffraction is better suited than X-ray diffraction for in-situ studies. 

Another characteristic that makes zirconium useful is the production of zircaloy, which does not absorb neutrons as does stainless steel in nuclear reactors. Thus, it is ideal to make nuclear fuel tubes and reactor containers. Zircaloy is the blend (alloy) of zirconium and any of several corrosion resistant metals. 

A problem of obtaining zirconium with lowest possible contents of hafnium comes from construction requirements when using zirconium and its alloys in building nuclear reactors. The construction material must have good mechanical properties and must be resistant to corrosion in contact with heat carriers. Since reactor power is proportional to the quantity of neutrons, their absorption into construction materials should be as small as possible. Zirconium and its alloys are unique materials that satisfy these requirements. However, hafnium has approximately the same chemical characteristics as zirconium but it absorbs neutrons strongly. 

NAA is gamma ray spectroscopy that uses the slow thermal neutrons from a nuclear reactor to excite the nucleus of an atom. When an atom absorbs a thermal neutron, its atomic mass increases by one and the nucleus becomes unstable. One or more nuclear reactions then take place that release gamma-rays with energies characteristic of the particular nuclear decay reactions, along with other radiation (Fig. 4.14). While... 

---