Fission product decay chains

Use of these equations is illustrated for the fission-product decay chain of mass number 92 considered in Sec. 3.2. Assume production of Sr, the first nuclide of the chain, at a constant rate P = 1/h for a period of 3 h (T = 3 h), followed by several hours of radioactive decay with P = 0. The amounts of Sr and Y, calculated by applying Eqs. (2.37) and (2.38), respectively, are shown in Fig. 2.9. The amount of stable Zr during the period of F=0 is obtained from the material-balance equation ... 

Figure 2.14 Fission-product decay chain for mass 90. 58...

The fission product Xe has the largest absorption cross section of all the nuclides in a thermal-neutron flux, and its buildup is especially important in affecting the neutron balance in a thermal reactor. The fission-product decay chain involving the production and decay of Xe is... 

Because the half-life of Te is so short compared to the half-lives of the other members of the chain, Te buildup may be ignored in calculating time variations in the amount of Xe, and the chain is assumed to originate with I, such that yi = 0.0609. The production rate Pj of 1, which is now the first member of a fission-product decay chain, is... 

The equations of Sec. 6.2 give the number of atoms of each fission product after a reactor has been run at stated conditions for a specified time. If the reactor is then shut down, the fission products build up and decay in accordance with the laws of simple radioactive decay, which were outlined in Sec. 3. If the nuclides in the decay chain are removed orJy by radioactive decay during reactor operations, the equations of Sec. 3 describe the changes with time of the number of atoms of any nuclide in the decay chain. If a member of a fission-product decay chain or its precursors in the decay chain are removed by neutron absorption, equations for the amount of each nuclide present at time t after shutdown may be obtained by applying the equations of radioactive decay to the amount present at shutdown. 

Assume that a reactor is operated for a very long time so that all fission products reach saturation activity. On the average there are three radioactive decays in each fission-product decay chain. Calculate the fission-product radioactivity at saturation in units of curies per watt of thermal power from fission. 

FIG. 14.10. Fission product decay chain of. 4=93. The independent yields in the upper row refer to nuclides believed to be formed directly in fission of by thermal neutrons. 

Case (i) Successive radioactive decay. This occurs after formation of the first radioactive (parent) member Xj. Thus we only consider the left vertical chain, and assume a2, etc. = 0. In this A + I chain the second index is constant, and we omit it for case (i). This case is valid for all natural decay chains and for fission product decay chains. Each member of the decay chain can be described by the differential equation... 

Figure 3.4. Fission product decay chains (schematic)...

Baumgartner and Reichold prepared carrier-free Mo(CO)g in high yield by neutron irradiation of powdered mixtures of UjOg and Cr(CO)g. As with their preparation of ° RuCp2, the Cr(CO)g acted only as a catcher for fission-product molybdenum (and for its precursors niobium and zirconium). The yield of 60% found for Mo(CO)6 is higher than the fractional chain yield of Mo in fission, so that the reaction must be partly thermal, starting with molecular fragments which survive j8 decay. 

In the new designs, if coolant were lost, the nuclear chain reaction would be terminated by the reactor s negative temperature coefficient after a modest temperature rise. Core diameter of the modular units would be limited so that decay heat could be conducted and radiated to the environment without overheating the fuel to the point where fission products might escape. Thus, inherent safety would be realized without operator or mechanical device intervention. 

Given that the implanted nuclei are radioactive, the positions measured for the implantation and all subsequent decay processes are the same. This is the case because the recoil effects are small compared with the range of implanted nuclei, emitted a particles or fission products, and detector resolution. Recording the data event by event allows for the analysis of delayed coincidences with variable position and time windows for the identification of the decay chains [14],... 

The last decade was marked with the discovery of five new members of the Periodic Table The heaviest elements of the last transition element series 110 through 112 were identified in the Gesellschaft fiir Schwerionenforschung (GSI), Darmstadt [1-3] and some decay chains and fission products associated with production of even more heavy elements 116 and 114 were recently reported by the Joint Institute for Nuclear Research (JINR), Dubna [4], This period of time was also very fruitful with studying chemical properties of the very heavy elements [5-9],... 

The uranium-graphite nuclear reactor (or nuclear pile ) was important not merely because it proved the feasibility of a self-sustaining fission chain. It could be used, with minor modification, for neutron irradiation of a sample by placing the sample in the interior of the reactor. Also, the system could be used as a source for the easily fissionable Pu239. This isotope (half-life 24,100 years) is a product in the decay chain from U239, which in turn results from the (n,y) reaction on U238 ... 

Fission of heavy nuclei always results in a high neutron excess of the hssion products, because the neutron-to-proton ratio in heavy nuclides is much larger than in stable nuclides of about half the atomic number, as already explained for spontaneous hssion (Fig. 5.15). The primary fission products formed in about 10 " s by fission and emission of prompt neutrons and y rays decay by a series of successive / transmutations into isobars of increasing atomic number Z. The final products of these decay chains are stable nuclides. 

The mass distribution curves in Figs. 8.13 to 8.15 give the total yields of the decay chains of mass numbers A. The independent yields of members of the decay chains, i.e. the yields due to direct formation by the fission process, are more diflicult to determine, because the nuclides must be rapidly separated from their precursors. Only a few so-called shielded nuclides (shielded from production via decay by a stable isobar one unit lower in Z) are unambiguously formed directly as primary... 

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