Neutron irradiations

Fm and heavier isotopes can be produced by intense neutron irradiation of lower elements, such as plutonium, using a process of successive neutron capture interspersed with beta decays until these mass numbers and atomic numbers are reached. 

Fig. 1. Nuclear reactions for the production of heavy elements by intensive slow neutron irradiation. The main line of buildup is designated by heavy...

Available only in very small amounts from neutron irradiations ia thermonuclear explosions. 

Most chemical iavestigations with plutonium to date have been performed with Pu, but the isotopes Pu and Pu (produced by iatensive neutron irradiation of plutonium) are more suitable for such work because of their longer half-Hves and consequendy lower specific activities. Much work on the chemical properties of americium has been carried out with Am, which is also difficult to handle because of its relatively high specific alpha radioactivity, about 7 x 10 alpha particles/(mg-min). The isotope Am has a specific alpha activity about twenty times less than Am and is thus a more attractive isotope for chemical iavestigations. Much of the earher work with curium used the isotopes and Cm, but the heavier isotopes... 

A PWR can operate steadily for periods of a year or two without refueling. Uranium-235 is consumed through neutron irradiation uranium-238 is converted into plutonium-239 and higher mass isotopes. The usual measure of fuel bumup is the specific thermal energy release. A typical figure for PWR fuel is 33,000 MWd/t. Spent fuel contains a variety of radionucHdes (50) ... 

Water as coolant in a nuclear reactor is rendered radioactive by neutron irradiation of corrosion products of materials used in reactor constmction. Key nucHdes and the half-Hves in addition to cobalt-60 are nickel-63 [13981 -37-8] (100 yr), niobium-94 [14681-63-1] (2.4 x 10 yr), and nickel-59 [14336-70-0] (7.6 x lO" yr). Occasionally small leaks in fuel rods allow fission products to enter the cooling water. Cleanup of the water results in LLW. Another source of waste is the residue from appHcations of radionucHdes in medical diagnosis, treatment, research, and industry. Many of these radionucHdes are produced in nuclear reactors, especially in Canada. 

A large number of radiometric techniques have been developed for Pu analysis on tracer, biochemical, and environmental samples (119,120). In general the a-particles of most Pu isotopes are detected by gas-proportional, surface-barrier, or scintillation detectors. When the level of Pu is lower than 10 g/g sample, radiometric techniques must be enhanced by preliminary extraction of the Pu to concentrate the Pu and separate it from other radioisotopes (121,122). Alternatively, fission—fragment track detection can detect Pu at a level of 10 g/g sample or better (123). Chemical concentration of Pu from urine, neutron irradiation in a research reactor, followed by fission track detection, can achieve a sensitivity for Pu of better than 1 mBq/L (4 X 10 g/g sample) (124). 

Compaction has also been observed as a result of neutron irradiation and extended exposure to iatense uv (excimer) laser light (131,132). The compaction tends to relax over months at room temperature and can be reversed quickly by annealing at sufficiently high temperatures (133). 

It has been estimated that using available neutron intensities such as 10 neutrons/(cm -s) concentrations of B from 10—30 lg/g of tumor with a tumor cell to normal cell selectivity of at least five are necessary for BNCT to be practical. Hence the challenge of BNCT ties in the development of practical means for the selective deUvery of approximately 10 B atoms to each tumor cell for effective therapy using short neutron irradiation times. Derivatives of B-enriched /oj o-borane anions and carboranes appear to be especially suitable for BNCT because of their high concentration of B and favorable hydrolytic stabiUties under physiological conditions. 

Production in Target Elements. Tritium is produced on a large scale by neutron irradiation of Li. The principal U.S. site of production is the Savaimah River plant near Aiken, South Carolina where tritium is produced in large heavy-water moderated, uranium-fueled reactors. The tritium may be produced either as a primary product by placing target elements of Li—A1 alloy in the reactor, or as a secondary product by using Li—A1 elements as an absorber for control of the neutron flux. 

Production-Scale Processing. The tritium produced by neutron irradiation of Li must be recovered and purified after target elements are discharged from nuclear reactors. The targets contain tritium and He as direct products of the nuclear reaction, a small amount of He from decay of the tritium and a small amount of other hydrogen isotopes present as surface or metal contaminants. 

Size requirements are limited by packaging considerations for neutron irradiation. Typically, polyethylene or quartz containers are used to contain the sample in the reactor core. For example. Si wafers are cleaved into smaller pieces and dame sealed... 

NAA cannot be used for some important elements, such as aluminum (in a Si or Si02 matrix) and boron. The radioactivity produced from silicon directly interferes with that ftom aluminum, while boron does not produce any radioisotope following neutron irradiation. (However, an in-beam neutron method known as neutron depth profiling C3J be used to obtain boron depth profiles in thin films. ) Another limitation of NAA is the long turn-around time necessary to complete the experiment. A typical survey measurement of all impurities in a sample may take 2-4 weeks. 

Fig. 5. Neutron irradiation induced dimensional changes in graphite composites.

Table 2. Effect of neutron irradiation on some graphite or CFC materials studied for fusion applications [12]...

The third term in Eq. 7, K, is the contribution to the basal plane thermal resistance due to defect scattering. Neutron irradiation causes various types of defects to be produced depending on the irradiation temperature. These defects are very effective in scattering phonons, even at flux levels which would be considered modest for most nuclear applications, and quickly dominate the other terms in Eq. 7. Several types of in-adiation-induced defects have been identified in graphite. For irradiation temperatures lower than 650°C, simple point defects in the form of vacancies or interstitials, along with small interstitial clusters, are the predominant defects. Moreover, at an irradiation temperatui-e near 150°C [17] the defect which dominates the thermal resistance is the lattice vacancy. 

Fig. 6. Normalized thermal eonduetivity of neutron irradiated pile grade A graphite...

Fig. 8. Calculated thermal conductivity of neutron irradiated MKC-1 PH composite.

J. Price, Thermal conductivity of neutron-irradiated reactor graphites, 1974, General Atomics. 

The neutron dose to graphite due to irradiation is commonly reported as a time integrated flux of neutrons per unit area (or fluence) referenced to a particular neutron energy. Neutron energies greater that 50 keV, 0.1 MeV, 0.18 MeV, and 1 MeV were adopted in the past and can be readily foimd in the literature. In the U.K., irradiation data are frequently reported in fluences referenced to a standard flux spectrum at a particular point in the DIDO reactor, for which the displacement rate was measured by the nickel activation [ Ni(np) t o] reaction [equivalent DIDO nickel (EDN)]. Early on, neutron irradiation doses to the graphite moderator were reported in terms of the bum-up (energy extracted) from imit mass of the adjacent nuclear fuel, i.e., MW days per adjacent tonne of fuel, or MWd/Ate. 

Fig. 7. High-temperature neutron irradiation a-axis shrinkage behavior of pyrolytic graphite showing the effects of graphitization temperature on the magnitude of the dimensional changes [60].

Fig. 8. Neutron irradiation induced dimensional changes for GraphNOL N3M graphite irradiated a 600 or 875 °C [61]. Note that the radial dimensional changes exceed the axial changes due to textural effects.

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