Nuclear reactor fuel, isotopic composition

Once the radioactive fission products are isolated by one of the separation processes, the major problem in the nuclear chemical industry must be faced since radioactivity cannot be immediately destroyed (see Fig. 10-7c for curie level of fission-product isotopes versus elapsed time after removal from the neutron source). This source of radiation energy can be employed in the food-processing industries for sterilization and in the chemical industries for such processes as hydrogenation, chlorination, isomerization, and polymerization. Design of radiation facilities to economically employ spent reactor fuel elements, composite or individually isolated fission products such as cesium 137, is one of the problems facing the design engineer in the nuclear field. 

The recognition in 1940 that deuterium as heavy water [7789-20-0] has nuclear properties that make it a highly desirable moderator and coolant for nuclear reactors (qv) (8,9) fueled by uranium (qv) of natural isotopic composition stimulated the development of industrial processes for the manufacture of heavy water. Between 1940 and 1945 four heavy water production plants were operated by the United States Government, one in Canada at Trail,... 

The relatively long-lived isotopes of Pu suitable for chemistry and metallurgy are those of masses 238, 239, 240, 241, 242, and 244. Plutonium formed in nuclear reactors occurs as a mixture of isotopes. A typical isotopic composition of Pu in spent fuel containing 10.4 kg of Pu/ton of fuel is given in Table VIII. 

The process route of uranium as nuclear fuel is shown in Fig. 11.7. It begins with processing of uranium ores, from which pure uranium compounds are produced. These may be used in the natural isotopic composition or transformed into other compounds suitable for isotope separation. The next step is the production of fuel elements for the special requirements of reactor operation. Solutions of uranium compounds are applied only in homogeneous reactors. 

Table 9.13 lists the isotopes of plutonium important in nuclear technology and some of their important nuclear properties. Plutonium isotopes are produced in reactors by the nuclide chains shown in Fig. 8.5. Typical quantities and isotopic compositions of plutonium in various reactor fuel cycles are listed in Tables 8.4, 8.5,8.6, and 8.7. In reactors fueled with uranium and plutonium, Pu is the principal isotopic constituent, but Pu contributes the greatest amount of alpha activity. With U-thorium fueling, Pu is the principal isotopic constituent. 

Hafnium-like boron is known to be a neutron absorber or neutron moderator element, and, therefore, composites of boron carbide, B4C, and hafnium diboride, HfB2, can be considered as nuclear materials. These boron compounds after sintering and °B/"B isotopic ratio adapting are found to be heterogeneous polyphone cermets useful for nuclear applications (Beauvy et al. 1999). Boron acid obtained from the °B enriched boron trifluoride also was used in nuclear reactors (Shalamberidze et al. 2005). Amorphous boron powders enriched both in °B and "B, boron carbide, and zirconium diboride (ZrB2) powders and pallets labeled with °B isotope And applications in nuclear engineering too. The °B enriched Fe-B and Ni-B alloys are useful for the production of casks for spent nuclear fuel transfer and storage. 

The reactor components of greatest interest to the forensic radiochemist are the clean fissionable material that constitutes fresh nuclear fuel, and the neutron-irradiated fuel that is the result of nuclear reactor operation. The design of the reactor determines both the ingoing fuel composition and the outgoing fuel isotopic content (Glasstone and Sesonske 1967). 

The spent fuel from a nuclear reactor is a mixture of nuclides in which the percentage of each is a function of the isotopic content of the starting material, the temperature of the core and moderator, the period of irradiation, the energy distribution of the incident neutrons, and the radioactive half-lives of the nuclides produced. Fuel efficiency in production reactors is less important than is the isotopic composition of the plutonium product. Particularly important is the minimization of the amount of Pu produced relative to Pu Pu has undesirable nuclear properties for use in nuclear explosives. The operation of a plutonium production reactor is often incompatible with the economical production of power, unless the reactor design permits refueling during power production. 

When discussing the isotope composition of uranium, the abundance is usually given in units of atom percent (i.e., the relative number of atoms of each isotope in a given mass of uranium) rather than in terms of weight percent (i.e., the relative weight of each isotope in the uranium mass). Thus, the weight percent of each isotope in natural uranium differs slightly from the atom percent as shown in parenthesis in Table 1.2. is the only fissile isotope of natural uranium, but the artificial isotope (produced in thorium-fueled nuclear reactors) is also fissile. All uranium isotopes are radioactive and the half-lives of the isotopes of importance are shown in Table 1.2. Due to the important role of radioactivity in the uranium industry, a short discussion is presented in Frame 1.1. 

Evidently, throughout all these processes the isotopic composition of uranium remains in its natural abundance form, that is, containing 0.72% of U. Uranium of this composition is suitable for use as nuclear fuel in reactors that operate with heavy water (DjO) such as CANDU reactors or graphite (such as the old Magnox reactors) as the moderator for slowing neutrons. In this case, the UO2 is ground and sintered to form pellets that will be placed in fuel elements (see Chapter 2 for analytical procedures to characterize these pellets). As an example. Frame 1.4 describes the process used in India for production of UO2 powder, pellets, and fuel elements as an example of the processes in the UCF. 

In natural uranium ores, the fraction of the atoms of the fissile isotope is about 0.72%. For many commercial applications, like production of fuel for light water reactors or several types of research reactors and other nuclear functions, its fraction must be increased, that is, isotope enrichment is carried ont. The main isotope separation methods, or isotope enrichment processes, ntilize the small differences in between the mass of U-235 and U-238. The two major commercial methods that have supplied most of the enriched uranium to date, gaseous diffusion and gas centrifuges, use the only gaseous compound of nraninm, nranium hexafluoride (UFg), as the feed material. Both methods utilize the difference between the mass of UFg (349 Da) and UFg (352 Da) where the mass ratio difference that is 0.86%. The product and tails of the enrichment process are also with the same chemical form, but the isotope composition of the material is altered in the enrichment process. Schematic diagrams of the principle of operation of these methods can be found on the web and in many textbooks, so will not be shown here. 

Highlights The characterization of UO2 powders and pellets to examine compliance with specifications involves the use of several physical and analytical test methods. The chemical analyses include determination of the uranium content and isotopic composition, the 0 U ratio, and the measurement of the content of several elemental impurities. Of special importance are elements that may affect the neutron absorption properties of the fuel pellets. Each of these elements is determined and the total neutron absorption of all these impurities is summed up as EEC. Modern nuclear fuel may include burnable neutron poisons that are used to increase the operational lifespan of the fuel. The intentional addition of these poisons, like gadolinia, must be carefully controlled to avoid fluctuations of the neutron density in the reactor. Basically, after dissolution of the nranium oxide samples the analytical methods that... 

Anthropogenic effects Via a variety of processes, human-made changes in the isotopic composition of an element can be accomplished by enhancing the fractionation beyond those in normal reactions, or by producing specific isotopes. Production of enriched U for fueling nuclear reactors and production of enriched isotopic tracers for tracer experiments or for isotope dilution MS are examples of the effects of human intervention on isotopic compositions. 

A recent study by Isnard et al. [12] compared the use of TIMS and MC-ICP-MS for the determination of Cs isotope ratios in spent nuclear fuels. Cs isotopic information is relevant in investigations of reactor processes and in the assessment of bum-up of the fuel - slightly different Cs isotopic compositions are anticipated for the fission of versus Pu or Pu. In this isotopic analysis application, Cs has four relevant isotopes Cs (stable), Cs (T = 2.07 years),... 

The existence of some of these databases is acknowledged publicly. For example, as mentioned earlier, LLNL has a database that includes 1800 samples of yellow cake (Kristo and Dirnet 2013), and the Nuclear Forensics Analysis Center (NFAC) in Savannah River National Laboratory (SRNL) provides support for the FBFs Radiological Evidence Examination Facility (REEF) (Nichols 2011). The latter contains a database of spent nuclear fuel from several reactors in the United States and other countries. An example of the processing of interdicted nuclear material at REEF uses traditional forensics combined with nuclear forensics to determine the origin and make attribution. The results of the isotopic measurements are compared to known compositions in the database based on reactor physics models (see flowchart in Figure 5.19). 

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