Short-lived 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. 

Most wastes that emerge from these processes are acidic, but are neutralized with sodium hydroxide before transfer to mild steel storage tanks. The waste is first transferred to cooled tanks, where it remains for about two years until most of the short-lived fission products have decayed. The waste is then transferred to uncooled tanks. 

In Section 2.5, we discussed the production of energy by nuclear fission, and the reprocessing of nuclear fuels. We described how short-lived radioactive products decay during pond storage, and how uranium is converted into [U02][N03]2 and, finally, UFg. One of the complicating factors in this process is that the fuel to be reprocessed contains plutonium and fission products in addition to uranium. Two dilferent solvent extraction processes are needed to elfect separation. 

The operations and facilities include ore exploration (not included in NFCIS list), mining, ore processing, uranium recovery, chemical conversion to UO2, UO3, UF4, UFg, and uranium metal, isotope enrichment, reconversion of UF to UO2 (after enrichment), and fuel fabrication and assembly that are all part of the front end of the NFC. The central part of the NFC is the production of electric power in the nuclear reactor (fuel irradiation). The back end of the NFC includes facilities to deal with the spent nuclear fuel (SNF) after irradiation in a reactor and the disposal of the spent fuel (SF). The spent fuel first has to be stored for some period to allow decay of the short-lived fission products and activation products and then disposed at waste management facilities without, or after, reprocessing to separate the fission products from the useful actinides (uranium and plutonium). Note the relatively large number of facilities in Table 2.1 dedicated to dealing with the spent fuel. Also listed in Table 2.1 are related industrial activities that do not involve uranium, like heavy water (D2O) production, zirconium alloy manufacturing, and fabrication of fuel assembly components. 

Although decommissioning could commence on cessation of power generation, some benefit may be sought dela dng some activities to allow short live fission products to decay. Current estimates are that Stage 1 activities would take approximately 2 years to complete. 

Substituting the bromine-67 half live of 55.7E seconds yields a limiting negative period, T = -80 seconds. No matter how much negative reactivity is added to a reactor, after the initial reduction in prompt neutrons and the decay of short lived fission products, core neutron population will decrease with a -80 second period. A typical power history following a scram is illustrated in Figure 4.3,... 

Theory is also used to predict the half-lives and the mean energies of the beta and gamma rays emitted in the beta decay of short-lived fission products (with half-lives of the order of 1 sec.). These are required for decay heat predictions. However, measured data form the major part of the decay data even at short decay times. 

The radioactivity of the SNF evolves over time as the various elements decay. The same is true of its radiotoxicity (expressed in Sv/metric tonne ) versus natural uranium that of fission products decreases very rapidly in a few hundred years and then persists at low level for millions of years, because of the presence of long-lived fission products. In fact, after approximately 600,000 years, the radioactivity of untreated spent fuel comes down to that of the natural uranium Irom which the fuel was made. The radioactivity of the SNF in fact decays by a factor of 65 within 1 year after unloading from the reactor core as a result of the decay of relatively short-lived fission products and actinides. The radioactivity of plutonium (mostly of half-lives 24,000, 6500, and 87 years) represents less than 10% of the total toxicity of the spent fuel when it comes out of the reactor. With the passage of time and the disappearance of the short-lived products, this proportion increases. In addition, MAs (Np, Am, Cm) significantly contribute to the radiotoxicity balance during a few thousand years. After several thousands of years, plutonium dominates and represents nearly 90% of the radiotoxicity. 

The total dose rates for three different fuel exposures are shown in Figure B-2. At short decay times, the dose rates are dominated by short-lived fission products and are comparable because they are primarily a function of the specific power at which the exposure was accumulated. At longer decay times, the dose rates are dominated by long-lived fission products and scale approximately with the total accumulated exposure. 

Fissile Elements. Reactor activation of fissile elements and counting of the delayed neutrons emitted in the decay of short-lived fission products provides a very specific method of analysis. Unfortunately, in order to provide discrimination between the different fissile isotopes (or elements) it is necessary to irradiate the samples in two different flux spectra thus utilizing the differing rates of reaction of fast and thermal fission. Decay curve analysis is impractical because of the similarity of the half-lives of delayed neutron groups from all the fissile isotopes. 

While a reactor is working the uranium-235 or plutonium-239 is slowly converted into fission products. The materials are intensely radioactive. Some of them capture thermal neutrons and thus diminish the efficiency of the pile. It is therefore necessary to take out the fuel rods from time to time and remove the fission products. For this purpose they are kept about 100 days to allow the short-lived radioelements to decay and then dissolved in nitric acid. Nitrous acid ensures that all the plutonium is in the Pu form. The reactions, with 11+ written for H3O+ (p. 194), arc... 

In addition to fissionable isotopes ( U, or plutonium) and fertile isotopes ( U or thorium), spent fuel from a reactor contains a large number of fission product isotopes, in which all elements of the periodic table from zinc to gadolinium are represented. Some of these fission product isotopes are short-lived and decay rapidly, but a dozen or more need to be considered when designing processes for separation of reactor products. The most important neutron-absorbing and long-lived fission products in irradiated uranium are listed in Table 1.4. 

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... 

Eventually, the fuel in a nuclear reactor becomes spent, and, rather than being disposed of, it is reprocessed. This both recovers uranium and separates from the fission products. Short-lived radioactive products are initially allowed to decay while the spent fuel is retained in pond storage. After this period, the uranium is converted into the soluble salt [U02][N03)2 (see Box 7.3). In the series of reactions 3.18 3.21, the nitrate is converted into UF. ... 

These transport processes require a certain time during which short-lived radionuclides will decay to a certain extent, so that the fission product mixture entering... 

Eventually, the 92 U fuel is spent, and requires reprocessing. This recovers uranium and also separates from the fission products. First, the spent fuel is kept in pond storage to allow short-lived radioactive products to decay. The uranium is then converted into the soluble salt [U02][N03]2 and finally into UFs ... 

There is a limit to the negative period that can be developed in a reactor by negative reactivity additions. Soon after the insertion of a large amount of negative reactivity such as a scram, the prompt neutron population decreases to a low level. Neutron population is predominantly the result of delayed neutrons which are produced by fission product decay. Within a short time, 2-3 minutes, all of the short lived delayed neutron precursors have decayed away. At this point, and from this point on, the core neutron population is sustained by decay of the longest lived fission product precursor, bromine-87, with a half life of = 55.72 seconds. Since the rate at which core neutron population decreases is determined by radioactive decay of bromine-87, an effective reactor period can be calculated by setting equations (2,9) and (4.7) equal. Neutron population, N will be used to replace activity, A, and power, P, respectively in the two equations. 

The primary gas envelope can also be considered a barrier against radionuclide release. However, for the short-lived fission gases, the dominant removal mechanism is radioactive decay. For the condensable fission products, the dominant removal mechanism is deposition or plate-out on the various helium wetted surfaces in the primary circuit. The primary pressure boundary, consisting of conventional steel pressure vessels, is designed to ASME Section HI. Through-wall cracks are considered unlikely. The chemically inert helium coolant minimizes corrosion and eliminates the complications associated with internal cladding, and only materials for which extensive data exist are to be used in the construction of the vessels. 

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