Fission product decay

Short-Lived Fission-Product Decay Data... 

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... 

In addition to the difficulties in producing good samples, the study of short-lived fission-product decay schemes presents special problems. 

Many of the applications, particularly the more challenging ones, of fission-product decay data have involved the fission products as a group. Because of their large number (>700) and the fact that almost no data have been available for many of them, it has always been necessary for such applications to utilize calculated values for the unmeasured quantities. To do this, a number of different approaches have been taken to describe the p-decay properties (p-strength functions) of these nuclides [TAK72,... 

A further possible reason for separating plutonium from uranium and the fission products relates to the extreme toxicity of Pu. Plutonium(IV) mimics iron(lll) (the aqueous E° and charge-to-radius ratios of the two ions are very similar), so that cancers are likely to result from the absorption of even microgram amounts of ingested radioactive Pu into organs of the human body (bone marrow, spleen, liver) that store iron(III). It may therefore be considered desirable to remove Pu, a long-lived health hazard, from spent nuclear fuels before disposal of the latter in repositories that may not remain inviolate for thousands of years into the uncertain future (most of the fission products decay away to negligible levels of activity in an acceptable time). 

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 decay-heat rate F(T, t) per unit fission rate for finite irradiation time T can be synthesized from... 

The parameter fp is the total number of fissions after irradiation time T per initial fissile atom, calculated by techniques described in Chap. 3. Equation (2.94) applies for operating times < 1.2614 X 10 s (4 years), shutdown times <10 s, and <3.0. A more detailed technique for calculating fission-product decay-heat power from an arbitrary time-dependent fission power, including contributions from the fission of U, U, and Pu, is given in the ANS Standard [A2]. 

Estimated maximum values of the ratio G of fission-product decay-heat rate, with neutron absorption in fission products considered, to the decay-heat rate in the absence of neutron absorption in fission products are given in Table 2.13 [A2]. The data are calculated for U- U fuel irradiated for 4 years in a light-water reactor. For cooling times of <10 s, the... 

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. 

E i) rate of heat release from fission-product decay per fission event... 

F ratio of heat-generation rate from fission-product decay to fission rate, (MeV/s)/... 

SI. Siure, K. Fission Product Decay Energy, Report WAPD-BT-24, Westinghouse Atomic Power Division, 1961. 

The toxicity of the high-level wastes from a uranium-thorium HTGR fuel cycle is initially smaller, after the fission-product decay period of 600 years, because of the relatively small quantities of americium, curium, Pu, and Pu formed in this thorium fuel cycle. However, after about 100,000 years of isolation the theoretical ingestion toxicity of the wastes is governed by Ra, formed by... 

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. 

Delayed energy from fission product decay... 

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... 

In the equations above, 6 is the mean lifetime for the pronq>t neutrons. As discussed in 14.7.1, some fission products decay by leading to an excited state which emits a neutron, -delayed neutron emission. For example Kr decays partly by -delayed neutron emission it is a daughter of Br with a half-life as long as 56 s. A large number of such neutron emitting fission products have been discovered, all with shorter half-lives, see Table 19.5. In reactors where the moderator contains D or Be atoms, y,n reactions with energetic y-rays from shortlived fission products and from activation products is also a source of delayed neutrons. The delayed neutrons have lower kinetic raergies ( 0.5 MeV) than the prompt ones and amount to < 1 % of the total number of fission neutrons emitted the fraction of delayed neutrons, /3, is 0.27 % for 0.65 % for 0.21 % for Pu, and 0.52% for Wh the delayed neutrons are taken into account, the ective... 

In the shutdown mode, the reactor vessel is fully pressurized or, at different times, in various stages of depressurization. Afterheat from fission product decay is generated at rates of up to about 7 percent of the core power level prior to shutdown, depending on the time interval since shutdown. The core decay heat is removed by the HTS. When the HTS is not available, the heat is removed by the Shutdown Cooling System (SCS). The outer control rods are normally fully Inserted during shutdown, and meet the required shutdown margin, with due allowances for uncertainties, even if the maximvim reactivity worth rod remains fully withdrawn. For cold shutdown, the control rods in the inner reflector are also Inserted and for this case, the maximum reactivity worth control rod is in the inner reflector. The neutron flux level is continuously monitored by the source range detectors. 

The accuracy of decay heat calculations depends on the individual heat generation rate from fission product decay nuclides and actinides, and the burnup calculation for its production and transmutation. To obtain experimental data and to improve the accuracy of related calculations, the decay heat of MK-II spent fuel subassemblies was measured at the JOYO spent fuel storage pond [7], The fuel burnup was approximately 66 GWd/t and the cooling time was between 40 and 385 days. The measured decay heat is shown in Fig. 9. 

Cannon, et al.. Direct Calorimetric Study of Fission Product Decay in... 

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