Production in Nuclear Reactors

The samples used for irradiation must withstand the temperature and the radiation in the reactor without decomposition. In general, they are encapsulated in aluminium cans or in quartz ampoules which are introduced in irradiation tubes or directly into special positions within the reactor. 

Some research reactors are equipped with a so-called thermal column of about 1 m X 1 m X 1 m, consisting of blocks of graphite and installed near the reactor core. Due to the moderator properties of graphite, only thermal neutrons are present in such a column. 

Fast pneumatic transport systems are needed for the investigation of short-lived radionuclides. They can be operated over distances of several metres and the samples can be brought, within 0.1 to 1 s, directly from the place of irradiation to the counter. 

A survey of neutron-induced reactions used for the production of radionuclides is given in Table 12.1. (n, 7) reactions are preferably triggered by thermal neutrons giving isotopic products of limited specific activity. 

As an example, irradiation of 1 g NaCl at a thermal neutron flux density 0 = 10 cm s may be considered  

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He is present in natural gases with a concentration of MO-7 of that of 4He and 1(T6 of the helium in the atmosphere. The separation is very expensive. Hence 3He is instead obtained as by-product of tritium production in nuclear reactors. Tritium in fact produces, by beta decay (the half life is 12.26 years), 3He the separation of 3He is obtained through a diffusion process. 

Because of their intimate link with energy production in nuclear reactors, fission products and their nuclear data have long occupied an important position in reactor technology. In recent years, interest in short-lived fission-product decay data has increased markedly, as their relevance to different areas of research and technology has become recognized. In addition to their importance for estimation of the fission-product decay-heat source term in nuclear reactors, the increasing attention being focused on the assessment of the hazards associated with the release, transport and... 

Although Tc is present in uranium ores in extremely small concentrations, the main importance of technetium is that of a man-made clement and technetium is counted as an artificial radioclement, due to its production in nuclear reactors and by nuclear explosions. 

The primary use for plutonium (Pu) is in nuclear power reactors, nuclear weapons, and radioisotopic thermoelectric generators (RTGs). Pu is formed as a by-product in nuclear reactors when uranium nuclei absorb neutrons. Most of this Pu is burned (fissioned) in place, but a significant fraction remains in the spent nuclear fuel. The primary plutonium isotope formed in reactors is the fissile Pu-239, which has a half-life of 24 400 years. In some nuclear programs (in Europe and Japan), Pu is recovered and blended with uranium (U) for reuse as a nuclear fuel. Since Pu and U are in oxide form, this blend is called mixed oxide or MOX fuel. Plutonium used in nuclear weapons ( weapons-grade ) is metallic in form and made up primarily (>92%) of fissile Pu-239. The alpha decay of Pu-238 (half-life = 86 years) provides a heat source in RTGs, which are long-lived batteries used in some spacecraft, cardiac pacemakers, and other applications. 

Using nuclear chemistry, scientists today can change one element into another and even produce elements artificially. How are elements made artificially Some are produced as by-products in nuclear reactors. However, most are made by bombarding nuclei with small particles that have been accelerated to high speeds. This is done mainly in three instruments, shown in Figure 21.19. 

The absorption of radiation leads to an increase in die tenqierature of the absorber. An exanqile of this is the absorption of the kinetic energy of fission products in nuclear reactor fuel elements which is a main source of the heat production in reactors. The absorption of decay energy of radioactive nuclides in appropriate absorbing material can be used in a similar - albeit more modest - way as an energy source. 

QF, is a powerful flucHinating agent for inorganic compounds. It forms volatile UF widi uraniiun which helps in separating the spent fuel and fissirm products in nuclear reactors. 

Coolant A liquid or gas used to reduce the heat generated by power production in nuclear reactors, electric generators, various industrial and mechanical processes, and automobile engines. 

Further, the existence of a number of stable radioisotopes of ruthenium holds promise for development of agents for organ imaging, and it should be remembered that ruthenium will possess some properties similar to those of technetium, whose complexes have been used for some considerable time for this purpose. In this respect, Clarke [28] has pointed out that considerable information on distribution of radioruthenium in animals has been accumulated because it is a major product in nuclear reactor waste. 

Table 12.4. Characteristics of the Most Important Gaseous or Volatile Fission or Activation Products in Nuclear Reactors"...

Neptunium-237 is a long-lived isotope of element 93 that is produced in kilogram amounts. It is formed as a by-product in nuclear reactors when neutrons produced in the fission of uranium-235 react with uranium-238 ... 

Plutonium is an important activation product in nuclear reactors. Like neptunium, it has also been identified to occur naturally in deposits like that at Oklo in Gabon. It has similar chemistry to both uranium and neptunium and can form trivalent, tetravalent, pentavalent and hexavalent oxidation states. Moreover, it is expected that the stability and solubility of the hydroxide and oxide phases and hydrolysis species of these oxidation states will be similar in magnitude to those of uranium and neptunium. 

Tritium is readily produced in nuclear reactors and is used in the production of the hydrogen bomb. 

Deuterium is used as a moderator to slow down neutrons. Tritium atoms are also present but in much smaller proportions. Tritium is readily produced in nuclear reactors and is used in the production of the hydrogen (fusion) bomb. It is also used as a radioactive agent in making luminous paints, and as a tracer. 

Kilogram amounts of neptunium ( Np) have been isolated as a by-product of the large-scale synthesis of plutonium in nuclear reactors that utilise 235u and 238u as fuel. The following transmutations occur ... 

Hafnium is obtained as a by-product of the production of hafnium-free nuclear-grade 2irconium (see Nuclear reactors Zirconiumand zirconium compounds). Hafnium s primary use is as a minor strengthening agent in high temperature nickel-base superakoys. Additionally, hafnium is used as a neutron-absorber material, primarily in the form of control rods in nuclear reactors. 

Unstable niobium isotopes that are produced in nuclear reactors or similar fission reactions have typical radiation hazards (see Radioisotopes). The metastable Nb, = 14 yr, decays by 0.03 MeV gamma emission to stable Nb Nb, = 35 d, a fission product of decays to stable Mo by... 

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. 

Economic Aspects. The principal market for deuterium has been as a moderator for nuclear fission reactors fueled by unenriched uranium. The decline in nuclear reactor constmetion has sharply reduced the demand for heavy water. The United States has stopped large-scale production of D2O, and Canada is the only suppHer of heavy water at this time. Heavy water is priced as a fine chemical, and its price is not subject to market forces. 

All the techniques discussed here involve the atomic nucleus. Three use neutrons, generated either in nuclear reactors or very high energy proton ajccelerators (spallation sources), as the probe beam. They are Neutron Diffraction, Neutron Reflectivity, NR, and Neutron Activation Analysis, NAA. The fourth. Nuclear Reaction Analysis, NRA, uses charged particles from an ion accelerator to produce nuclear reactions. The nature and energy of the resulting products identify the atoms present. Since NRA is performed in RBS apparatus, it could have been included in Chapter 9. We include it here instead because nuclear reactions are involved. 

Nuclear fuel reprocessing was first undertaken with the sole purpose of recovering plutonium, for weapons use, from uranium irradiated in nuclear reactors. These reactors, called the production reactors, were dedicated to transmuting as much of the uranium as possible to plutonium. From its original scope of recovering exclusively plutonium, with no attempts to either recover or recycle uranium, nuclear fuel reprocessing has since grown into a much more sophisticated and complex operation with expanded scope. It is now called upon to separate uranium and plutonium from the fission products, and to purify these elements to levels at which these fissile materials can be conveniently recycled for reuse. The present scope also extends to fission products separation and concentration. 

UIC. 1997. Most smoke detectors contain an artificially produced radioisotope americium-241. Americium-241 is made in nuclear reactors, and is a decay product of plutonium-241. Uranium Information Center. Nuclear Issues Briefing Paper 35. http //www.uic.com.au/nip35.htm. January 27, 2000. 

Technetium then became available in a weighable quantity because of uranium nuclear fission leading to the production of "Tc in nuclear reactors. The total amount of "Tc in the world at the end of 1993 is estimated to be 78 tons, more abundant than rhenium on the earth. 

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