Fission products, buildup decay

Hermann, O.W. and Westfall, R.M. (1995) ORIGEN-S Scale System Module to Calculate Fuel Depletion, Actinide Transmutation, Fission Product Buildup and Decay, and Association Source Terms,... 

O. W. Hermann and R. M. Westfall, ORIGEN-S A SCALE System Module to Calculate Fuel-Depletion, Actinide Transmutation, Fission Product Buildup and Decay, and Associated Radiation Source Terms, Sect. F7 of SCALE A Modular Code System for Performing Standardized Computer Analyses for Licensing Evaluation, NUREG/CR-0200, Rev. 5 (ORNL/NUREG/CSD-2/R5), Vols. 1, 2 and 3 (draft November 1993). Available from Radiation Shielding Information Center as CCC-545. 

Another case of practical interest in nuclear engineering is the buildup and decay of fission products formed in a nuclear reactor operating at a steady fission rate for a time T and that have been removed from the reactor and aUowed to undergo radioactive decay for an additional time. The schematic diagram for continuous production of the first member of the drain at rate P is... 

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... 

Despite this favorable record, the further development of nuclear power is greatly handicapped in many countries because of public concern over the radioactive products arising in the course of plant operation and the consequences of their possible release to the environment. Energy generation from the neutron-induced fission of heavy atoms is inevitably accompanied by the formation of radioactive nuclides. This is, first of all, the direct consequence of nuclear fission, which leads initially to fission products that are unstable due to an excess of neutrons in the newly formed nuclei. These products are transformed by a sequence of p decays (mainly with associated y emission) to stable end products. Moreover, neutron capture in the heavy atoms of the fuel results in the buildup of nuclei which are heavier than those of the starting element (uranium, plutonium) and which mostly decay — in part, with very long halflives — by a emission. Finally, from elements present in structural and cladding materials, as well as in the coolant, its additives and impurities, additional radionuclides are formed, induced by neutron capture reactions which take place in the intense neutron field inside the reactor pressure vessel. 

In principle, two fundamentally different methods can be applied to solve this task. The first one is determination of the residual concentrations of the fissile nuclides after irradiation and calculation of the burnup from the difference between final and initial values. For this purpose, the uranium and plutonium fraction has to be separated from the fission and activation products and from each other (e. g. by extraction chromatography) subsequently, the concentrations of the individual isotopes, in particular of the fissile isotopes, are analyzed by mass spectrometry. Well-established analytical techniques for performing such analyses are available, so that only small error margins are to be expected in the determination of the concentrations of the isotopes under consideration. However, there are two problems that can potentially cause systematic errors. The first one is the well-known question of the accuracy of results which have been obtained as a difference between two numbers, which limits the accuracy at lower burnup values in particular. The second problem is that the fissile nuclides are not only consumed by nuclear fission but by neutron capture as well in order to avoid systematic errors here, the capture-to-fission ratio valid for the particular irradiation conditions has to be taken into account in the calculation of depletion during irradiation. If one recalls the complicated buildup and decay mechanisms of actinide nuclides during reactor irradiation (see Fig. 3.5.), it is obvious that such correction requires complex calculations. On the other hand, the direct determination of the residual concentration of fissile nuclides is not influenced by errors due to inaccuracies in the fission yields of fission products to be measured nor by migration-induced inho-mogenities in the fuel. 

Each fission of a fuel nucleus in the reactor leads to the production of two nuclei of intermediate mass, known as the fission products. Not only is there a large number of isotopes produced directly by the fission process, but radioactive decay of these products leads to the formation of further species. Some of the fission products or their daughters have large cross sections for the capture of thermal neutrons, and consequently their presence reduces the reactivity of the core. The control system has to be able not only to compensate for the long-term effect of the buildup of stable isotopes but also to deal with major fluctuations of the concentrations of the radioactive ones, of which the most important is xenon-135. The fission products of large cross section are listed in Table 3.6, along with their half-lives and relative fission yields. The latter are total cumulative yields, including not only direct production in fission, but also subsequent formation as a result of decay of a radioactive parent. 

As it appears from Fig. 57.4, there are numerous other buildup reactions. Among those are the very important formation of Pu from by neutron capture and two successive P decays. O Figure 57.4 also indicates that a number of the products built up are undergoing fission after neutron capture. 

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