Radioactivity from neutron activation

In addition to tritium produced by ternary fission, as shown in Table 8.1, tritium is also produced in reactors by neutron reactions with lithium, boron, and deuterium. Reactors can be designed to produce tritium by irradiating lithium targets with thermal neutrons, resulting in the (/I, a) reaction  

Although relatively little tritium is produced from natural lithium contaminant in thermal reactors by reactions (8.48) and (8.49), the li source of tritium is also produced by the (n, a) reaction with boron used for reactivity control  

The cross section for reaction (8.50) is 3837 b for 2200 m/s neutrons. Boron also reacts with high-energy neutrons in reactors to produce tritium by the reactions  

The cross section for reaction (8.51) can be interpreted as the spectrum-averaged value for neutrons of energy greater than 1 MeV. The threshold neutron energy for reaction (8.52) is 

The flux of neutrons with energies above this threshold is negligible in flssion reactors, so tritium production from reaction (8.52) is negligible. 

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The determination of 129I in low-level radioactive waste was accomplished by radioactive instrumental neutron activation analysis [3]. A different group reported the determination of both 129I and 127I by neutron activation analysis and inductively coupled plasma mass spectrometry [4]. The method was very rapid - a sample could be analysed in three minutes. However, interference from 129Xe resulted in limited sensitivity for 129I detection. 

This can result in a radioactive product from the A(n, t)A reaction where A is the stable element, n is a thermal neutron, A is the radioactive product of one atomic mass unit greater than A, and y is the prompt gamma ray resulting from the reaction. A is usually a beta and/or gamma emitter of reasonably long half-life. Where access to a nuclear reactor has been convenient, thermal neutron activation analysis has proven to be an extremely valuable nondestructive analytical tool and in many cases, the only method for performing specific analyses at high sensitivities... 

Neutron activation analysis (NAA) is a technique for the qualitative and/or quantitative determination of atoms possessing certain types of nuclei. Bombarding a sample with neutrons transforms some stable isotopes into radioactive isotopes measuring the energy and/or intensity of the gamma rays emitted from the radioactive isotopes created as a result of the irradiation reveals information on the nature of the elements in the sample. NAA Is widely used to characterize such archaeological materials as pottery, obsidian, chert, basalt, and limestone (Keisch 2003). 

Nuclear bombardment reactions in which the product is radioactive constitute the basis of radioactivation analysis (p. 456). Although in principle any bombardment-decay sequence may be used the analyst is largely concerned with thermal neutron activation. Equation (10.13) relates the induced activity to the amount of the parent nucleide (analyte). However, practical difficulties arise because of flux inhomogeneities. It is common therefore to irradiate a standard with very similar characteristics alongside the sample, e.g. for a silicate rock sample a standard solution would be evaporated on to a similar amount of pure silica. On the assumption that identical specific activities for the analyte are then induced in the sample and standard, the amount w2 of analyte is readily calculated from... 

A method has been developed for the determination of technetium-99 in mixed fission products by neutron activation analysis Tc is separated from most fission products by a cyclohexanone extraction from carbonate solution, the stripping into water by addition of CCI4 to the cylohexanone phase, and the adsorption on an anion exchange column. Induced Tc radioactivity is determined using X-ray spectrometry to measure the 540 and 591 keV lines. The sensitivity of the analysis under these conditions is approximately 5 ng. The method has been successfully applied to reactor fuel solutions. 

One of the more important factors affecting the isolation of radioactive waste is the rate of release of the radioactivity from the solid waste form to the environment. The most probable mechanism for release and transport of radioactivity from a solid waste form is by leaching of radioactive isotopes with groundwater. The objective of leach-testing various waste forms is to evaluate the rate at which specific hazardous radionuclides migrate from waste if and when the waste form comes in contact with groundwater. In this paper, measurement of leach rates of radioactive waste by a method which incorporates neutron activation is described. 

Impurities in the water and water activation products also contribute to the radioactivity of the coolant water. Tritium is produced as a low yield ( 0.01%) fission product that can diffuse out of the fuel, by activation of boron or fiLi impurities in PWRs. 24Na and 38C1 are produced by neutron activation of water impurities. In BWRs, the primary source of radiation fields in the coolant and steam systems during normal operations is 7.1s 16N. This nuclide is produced by 160(n, p)16N reactions from fast neutrons interacting with the coolant water. This 16N activity can exist as N07, NO in the coolant and NHj in the steam. 

Stainless steel contains iron and nickel—important materials in nuclear power reactors and possible constituents of the materials used to construct nuclear test devices or their supporting structures.8 9 During nuclear weapons tests, stable Fe and Ni isotopes are neutron activated, giving rise to radioactive Fe and Ni along with fission products. In nuclear power plants, moreover, stable Fe and Ni isotopes are released from stainless steel through corrosion, become activated, and are transported to different parts of the reactor system. 

One of the severe problems associated with material selection for use in the presence of neutrons is the production of radioactive nuclides formed by neutron activation. It is therefore important to investigate radioactive species produced from inorganic fillers in composites. 

Neutron activation analysis is a two-step technique (3). The sample to be analysed is first irradiated with the neutron flux of a nuclear reactor. The nuclei of a small bit predictable fraction of the atcms of each element in the sample will capture neutrons and beocme radioactive. These will subsequently emit radiation at a time which depends on the half-life of the radioactive isotope. At the end of the pre-determined irradiation time the sample is removed from the reactor it is now radioactive and the activity will decrease with time as the individual atcms emit radiation and... 

In the characterization of samples containing >6 x 10 protons, adequate spectra can be obtained on a timescale ranging from several hours to about one day. For quantitative and semi-quantitative comparison, the INS spectra can be normalized to the amount of sample in the beam. Most catalyst characterization experiments are difference measurements the spectrum of the catalyst with and without an adsorbate present is recorded and the difference taken. After the neutron experiment, safety requires that the samples be stored on site, as a consequence of neutron activation. The radioactivity decreases to background levels within a period ranging from a few hours to several months, depending on the atomic number and isotopes of the catalyst components. 

For the investigation of meteorites various experimental methods are applied, in particular mass spectrometry, neutron activation analysis, measurement of natural radioactivity by low-level coimting and track analysis. The tracks can be caused by heavy ions in cosmic radiation, by fission products from spontaneous or neutron-induced fission and by recoil due to a decay. Etching techniques and measurement of the tracks give information about the time during which the meteorites have been in interstellar space as individual particles (irradiation age). 

The concentration of natural lithium (tritium-precursor) in un-irradiated Magnox RPV steel has been estimated directly using SIMS-MS and SEM-EDX and indirectly from measuring tritium induced by neutron activation of mainly Li. Tritium was measured in surveillance specimens irradiated during the 26 year operational life of a Magnox reactor followed by radioactive decay of approximately 15 years. For comparison purposes, inactive archive RPV steel was also irradiated in the CONSORT reactor followed immediately by radioanalysis for tritium. In this way, the possibility of diffusive transfer of tritium into or out of the steel during residence in the reactor could be evaluated. 

Neutron activation This is the term used for neutron reactions that result in the formation of radioactive nuclides. This type of reaction is used in solid-state use-once devices containing one or more materials that are activated by neutron radiation. The induced activity of each material can be measured and the neutron exposure can then be calculated from these activities. 

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