Uranium induced fission

The nucleus """U (uranium) does not fission spontaneously, but It can be induced to fission through interaction with a neutron. Pictorially, a typical neutron-induced fission of " U producing two nuclei and three neutrons is depicted in Figure 2. 

Induced nuclear fission is fission caused by bombarding a heavy nucleus with neutrons (Fig. 17.23). The nucleus breaks into two fragments when struck by a projectile. Nuclei that can undergo induced fission are called fissionable. For most nuclei, fission takes place only if the impinging neutrons travel so rapidly that they can smash into the nucleus and drive it apart with the shock of impact uranium-238 undergoes fission in this way. Fissile nuclei, however, are nuclei that can be nudged into breaking apart even by slow neutrons. They include uranium-235, uranium-233, and plutonium-239—the fuels of nuclear power plants. 

Fission of the nucleus, whereby it splits into two roughly equal halves, is accompanied by a huge release of energy. It was first observed by Hahn and Strassman (1939), who were bombarding uranium with neutrons. Many heavy elements are susceptible to induced fission, but spontaneous fission can occur in some of the heaviest elements, and is thought to be the principal mode of decay for the transuranic elements. 

Among the long-lived isotopes of technetium, only Tc can be obtained in weigh-able amounts. It may be produced by either neutron irradiation of highly purified molybdenum or neutron-induced fission of uraniimi-235. The nuclides Tc and Tc are exclusively produced in traces by nuclear reations. Because of the high fission yield of more than 6%, appreciable quantities of technetimn-99 are isolated from uranium fission product mixtures. Nuclear reactors with a power of 100 MW produce about 2.5 g of Tc per day . 

Only a small fraction of Bk-249 is obtained by the above reaction because neutrons also induce fission. Alternatively, uranium—238 may be converted to Bk-249 by very short but intense neutron bombardment followed by five successive beta decays. 

Another application of track detectors is dosimetry of a particles and neutrons. For neutron dosimetry the track detectors may be covered with uranium foils in which the neutrons induce fission. Alternatively, the detectors may be covered with a foil containing B or Li, and the a particles produced by (n, a) reactions are recorded. 

The discovery of technetium (Z = 43) in 1937 and of promethium (Z = 61) in 1947 filled the two gaps in the Periodic Table of the elements. These gaps had been the reason for many investigations. Application of Mattauch s rule (section 2.3) leads to the conclusion that stable isotopes of element 43 cannot exist. Neighbouring stable isotopes could only be expected for mass numbers A 93, A < 91, A = 103 and A > 105. However, these nuclides are relatively far away from the line of jd stability. The report by Noddak and Tacke concerning the discovery of the elements rhenium and masurium (1925) was only correct with respect to Re (Z = 75). The concentration of element 43 (Tc) in nature due to spontaneous or neutron-induced fission of uranium is several orders of magnitude too low to be detectable by emission of characteristic X rays of element 43, as had been claimed in the report. 

When a uranium-235 atom undergoes neutron-induced fission two atoms are produced, one of mass about 95, the other about 140, together with neutrons. 

Fig. 1.3 Setup for first chemical experiments with element 104 - now Rf Dubna, the mid-1960s [10]. The broken frames outline the placement of resistive heaters, paraffin and cadmium shielded the detectors from neutrons to prevent induced fission of uranium impurities in mica. Thermal decomposition of NaNbClg was the source of NbC E vapor. A Faraday cup was placed inside the target chamber (not shown).

Some billion years ago natural nuclear reactors must have operated and generated Tc as a high yield fission product by induced fission of with slow neutrons. The relics of a natural reactor were discovered in 1972 at the Oklo uranium mines in the Republic of Gabon, Africa. lire Oklo phenomenon occurred 1.72 billion years ago and produced a greater amount of Tc than detected in other uranium ores [20. Ruf-fenach et al. [21] reported values of integrated flux of thermal neutrons for the Oklo uranium ores of up to 1. . 2 10 n cm and a atomic ratio down to 0.00410,... 

Among the long-lived technetium isotopes only the p -emitter Tc with a half-life of 2.13-10 a is obtained in vveighablc amounts, either by neutron irradiation of highly purified natural molybdenum or by induced fission of with thermal neutrons. Because of the high fission yield of 6.13 atom%, appreciable quantities of Te ean be isolated from uranium fission product mixtures. Nuelear reactors with a power of 3500 MWth produce about 100 g of Tc per day or 6 TBq ( 10 kg) c/GWn, per year. 

We discussed the sources of artificial occurrence of " Tc at the beginning of this chapter and demonstrated that the nuclear fuel cycle is the predominant source of Tc in the environment. Other, much less important, sources are the fallout from nuclear weapons testing, the Chernobyl accident, nuclear power production and the radiopharmaceutical use of the metastablc "Tc decaying to ground state Tc. The natural occurrence of Tc formed in the earth s crust by spontaneous fission of and neutron-induced fission of in uranium ores are negligible. 

Determination of a fission track age requires several further experimental steps to measure the uranium concentration. The uranium concentration is not measured directly, but a second set of fission tracks is created artificially in the sample by a thermal neutron irradiation. This irradiation induces fission in a tiny fraction of the atoms, which are present in a constant ratio to U in natural uranium. Knowing the total neutron fluence received during irradiation, the number of induced tracks provides a measure of the uranium concentration of the grain. Because the induced tracks are derived from a different isotope of uranium than the spontaneous tracks an important consideration in fission track dating is the assumption that the isotopic ratio of the two major isotopes of uranium, and is constant in nature. With the notable exception of the unique natural nuclear reactors of Oklo in Gabon (Bros et al. 1998), where this isotopic ratio is disturbed, this is a very safe assumption. Numerous measurements have shown that and are always present in their natural abundances of 0.73% and 99.27%, respectively. 

The uranium is stored inside the reactor core in alloy tubes called fuel rods. A minimum mass of is needed in the reactor in order to ensure that there are enough nuclei to absorb the neutrons emitted during the induced fission, so that further fission may continue. This critical mass of depends upon the shape of the uranium fuel. It is usually several kilograms. 

Radon ( Rn) is the dominant source of human exposure to ionizing radiation in every country of the world. It is dominant in most circumstances at home and at work, for individual persons and for whole populations. The worst characteristic of radon, apart from its carcinogenicity, is its ubiquity. Before radon became a matter of concern for human exposure, it was studied and measured for many purposes. It was an inert tracer for air masses, it was a geological indicator for radium and uranium, and it was a shortlived source of y-radiation for cancer treatment. Radon plus beryllium was used as neutron source by Fermi for the discovery of neutron-induced fission reactions. 

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