Fission products and actinides

Due to classification issues, complete descriptions of the PWR nuclear SGIs are not yet available. Thus, even though the fuel for the submarine PW and the icebreaker were described as a U-Al alloy and UO2, respectively, the model for the icebreaker SGI was assumed for the PWR SGIs. Fission product activities for the nuclear submarine PWRs are based on the quotient of the nuclear submarine and icebreaker bumups, and are given by the following equation  

Actinide activities for the nuclear submarine PWRs are based on the product of the quotients of the icebreaker and nuclear submarine enrichments and the nuclear submarine and icebreaker bumups, and are given by the following equation  

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Hamilton, J. G. (1948a). The metabolic properties of the fission products and actinide elements, Rev. Mod. Phys. 10, 718. 

In the year 2000, 15% of the world s electric power was produced by 433 nuclear power reactors 169 located in Europe, 120 in the United States, and 90 in the Far East. These reactors consumed 6,400 tons of fresh enriched uranium that was obtained through the production of 35,000 tons of pure natural uranium in 23 different nations the main purification step was solvent extraction. In the reactors, the nuclear transmutation process yielded fission products and actinides (about 1000 tons of Pu) equivalent to the amount of uranium consumed, and heat that powered steam-driven turbines to produce 2,400 TWh of electricity in 2000. 

The Oklo natural reactors in Gabon are the best natural analogues for assessing the geological behaviour of fission products and actinides (see also Gauthier-Lafaye et al. 2004). Elements that were compatible with the U ore structure were retained, whereas elements that... 

Retention of fission products and actinides in the Gabon reactors... 

The most efficient matrix for retention of actinides and fission products is the uraninite mineral. However, it has been shown that other matricies such as apatite, clay minerals, zirconium silicates, and oxides (Fe, Mn) may also be important in the retention of fission products and actinides. For example, Pu was stored in apatite (Bros et al. 1996) and chlorite (Bros et al. 1993) in the core of the reactor 10. In the core of the reactors, between uraninite grains, 20-200 (j.m-sized metallic aggregates containing fissiogenic Ru, Rh, and Te associated with As, Pb, and S were found. These aggregates also exist in spent fuels of water-pressured type reactor plants, suggesting their analogy with spent fuels. 

Fig. 3. Retention of the various fission products and actinides by different mineral phases.

The quasi-simultaneous separation and determination of lanthanides and actinides by ion chromatography inductively coupled plasma mass spectrometry combined with the isotope dilution technique and the further use of ion chromatography for the determination of fission products and actinides in nuclear applications are described by Betti et ul.10 48 68... 

Fission products and actinides in spent nuclear fuels have also been analyzed using cation-exchange LC [98]. Chromatography was essential in order to separate fission Cs from Ba for the analysis of the lanthanides and to eliminate isobaric interferences in the separation of the actinides. Separation of fission... 

In the reverse TALSPEAK process, the An(III) + Ln(III) fraction is first coextracted from a feed, the acidity of which has to be reduced to 0.1 M by denitration or nitric acid extraction. An(III) are then selectively stripped using DTPA in citric acid (1 M) at pH 3 (hence the name reverse TALSPEAK process), and the Ln(III) are finally stripped by 6 M HN03. Attempts to apply this TALSPEAK variant to the treatment of actual UREX + raffinates are reported in the literature, but they involve several steps. The problematic Zr and Mo elements are first removed by direct extraction with HDEHP (0.8 M in di-iso-propylbenzene) from the high-acidity raffinate stream arising from the UREX + co-decontamination process (238). The remaining fission products and actinides can then be concentrated by acid evaporation and denitration processes. This concentrate is further diluted to a lower acidity (e.g., [HN03] = 0.03 M) to allow the coextraction of An(III) and Ln(III) by the TALSPEAK solvent. 

Garcia Alonso, J. I., Sena, R., Arbore, P., Betti, M., and Koch, L., Determination of fission products and actinides in spent nuclear fuels by isotope dilution ion chromatography inductively coupled plasma mass spectrometry,./. Anal. At. Spectrom., 10,381-393, 1995. 

The third fact is that spent nuclear fuel is not waste. Spent nuclear fuel contains 2% to 3% waste, but is about 97% recoverable uranium and plutonium. Each bundle has the potential electric energy equivalent of more than 10 million barrels of oil. High-level nuclear wastes consist of fission products and actinides that are extracted from spent fuel, but not saved for commercial use or research. Spent fuel may be temporarily stored until it is reprocessed to separate the waste from the valuable plutonium and uranium. The remaining glassified waste will then be permanently entombed. 

Although ICP-MS has been used for analysis of nuclear materials, often the entire instrument must be in an enclosed hot enclosure [350]. Sample preparation equipment, inlets to sample introduction systems, vacuum pump exhaust, and instrument ventilation must be properly isolated. Many of the materials used in the nuclear industry must be of very high purity, so the low detection limits provided by ICP-MS are essential. The fission products and actinide elements have been measured by using isotope dilution ICP-MS [351]. Because isotope ratios are not predictable, isobaric and molecular oxide ion spectral overlaps cannot be corrected mathematically, so chemical separation is required. 

One of the key processes here is the dissolution of the spent nuclear fuel matrix in groundwater liberating radioactive fission products and actinides. Without this process no radioactivity will be released to the biosphere. 

World-wide the production of energy by nuclear power amounts to about 470 GWe (1995) and increases by about 4% per year, although the problems with respect to the storage of the radioactive waste (fission products and actinides) are not yet solved in a satisfactory way. 

The result of the separation of U and Pu from the fission products and other actinides is characterized by the decontamination factor, given by the ratio of the activity of the fission products and actinides in the fuel to that in uranium and plutonium after separation. The decontamination factors should be in the order of 10 to 10, and the recoveries of U and Pu should be near to 100%. These requirements are best met by solvent extraction procedures. With respect to the high activity of the fuel, remote control of all operations is necessary. 

Reprocessing of nuclear fuel by the Purex process leads to the following amounts of waste per ton of U 1 m HLW (fission products and actinides in HNO3 solution), 3 m MLW as organic solution, 17m MLW as aqueous solution, 90m LLW (aqueous solution). By further processing a volume reduction is achieved 0.1m HLW, 0.2m MLW (organic), 8m MLW (aqueous), 3m LLW (aqueous). 

Analysis of the natural reactors at Oklo gives valuable information about the migration behaviour of fission products and actinides in the geosphere. Uranium and the lanthanides have been redistributed locally. Plutonium produced in the Oklo reactors did not move during its lifetime from the site of its formation 85-100% of the lanthanides, 75-90% of the Ru and 60-85% of the Tc were retained within the reactor zones. Small amounts of U, lanthanides, Ru and Tc moved with the water over distances of up to 20-50 m. 

Radioactive fall-out Radionuclides, in particular fission products and actinides, deposited from the air... 

In this section, results of studies of the geochemistry of fission products and actinides are summarized. The chemistry of the fission products is described as a group first because their behavior is relatively simple compared to the actinides. Next, general trends and then site-specific environmental chemistry of the actinides are summarized. 

Haug, H. 0., "Production, Disposal, and Relative Toxicity of Long-Lived Fission Products and Actinides in the Radioactive Wastes from Nuclear Fuel Cycles," (in German), KFK-2022, translated as 0RNL-tr-it302, Oak Ridge National Laboratory,... 

To a first approximation, and with a few obvious exceptions, the fossil remains now present in the reactor zones contain the residual fission products and actinides, or their decay products. This has been demonstrated by detailed analyses, isotopic and elemental, of a substantial number of samples from Zone 2, as well as some from Zone 1. These findings are summarized in Table I. Before considering Table I further it is appropriate to review the details of reactor operation, as well as the related uncertainties, which form the basis for the conclusions in Table I. 

Figure 8.1 Radioactivity of fission products and actinides in high-level wastes produced in 1 year of operation of a uranium-fueled 1000-MWe PWR.

Table 11.4 Radioactivities of fission products and actinides in the waste from 1 MT LWR uranium fuel (30,000 MWd/MT bumup) at discharge from reprocessing (ISO days cooled fuel elements) and 6 years after discharge (cont utions of more than 0.1 percent only), according to ORIGEN...

Cladding hulls. Cladding hulls as collected from the chop-leach head end are radioactive due to activation products in the zircaloy and to fission products and actinides from (U,Pu)02 adsorbed at the inside of the hulls. The principal activation products are Co, Sb/ Te, and Ni. Their total activity is on the order of 10 /rCi/g zircaloy after 6 years and about 100 times lower after 100 yrars. The fission-product activity after 6 years is of the same order of magnitude and is dominated by Cs, Sr, and tritium. The residual (U,Pu)02 after leaching is estimated to be of the order of 0.1 percent of the charge. 

Calcines are products obtained by removing the volatile components of the waste, i.e., water and nitrate, at temperatures between 400 and 900° C. The result is a mixture of oxides of fission products, actinides, and corrosion products in particulate form with a specific surface of 0.1 to 5 ra /g. The plain calcine is not very stable chemically because of its large surface area and the chemical properties of some of the oxides, and it is highly friable. To improve the properties of calcines, advanced forms are developed. One such product is the so-called multibarrier waste form, a composite consisting of calcine particles with inert coatings, such as pyrocarbon, silicon carbide, or aluminum, embedded in a metal matrix. Another advanced calcine is the so-called supercalcine. This is essentially a ceramic obtained by adding appropriate chemicals to the HLW to form refractory compounds of fission products and actinides when fired at 1200°C. Supercalcine requires consolidation by embedding in a matrix but does not need to be coated, as the material is supposed to have inherent chemical stability. 

Irradiation stability. Any solidified HLW will be exposed to energetic radiation from radioactive decay of fission products and actinides. Part of the radiation energy is dissipated in elastic collisions with atoms from the solid material, thereby displacing them and causing radiation damage. This may affect macroscopic properties such as mechanical or chemical ones, and it may cause storage of energy. 

The used fiiel elemmts may later be reprocessed to recover the remaining amount of fissile material as well as any fertile material or regarded as waste fertile atoms are those which can be transformed into fissile ones, i.e. " Th and U, which through neutron capture and jS-decays form fissile and Pu, respectively. The chemical reprocessing removes the fission products and actinides other than U and Pu. Some of the removed elements might be valuable enough to be isolated although this is seldom done. The mixed fission products and waste actinides are stored as radioactive waste. The recovered fissile materials may be refabricated (the U may require re-enrichment) into new elements for reuse. This "back-end" of the nuclear fuel cycle is discussed in Chapter 21. 

In the "ideal" nuclear reactor all fission products and actinides produced are contained in the fuel elements. In all practical reactors there are four processes through which radioactivity leaves the reactor vessel in all cases the carrier of activity is the coolant ... 

Table 1.1 lists some historical "firsts" with regard to nuclear weapons. The extensive tests in the atmosphere up to 1963 lead to a large global q>read of tritium, fission products and actinides. Scimtists have used this to learn more about global wind and water currents. Radiochemists have studied the migration of deposited radionuclides, as discussed in Chapter 22, radioecologists the uptake of radioactive elements by plants and animals, as described in Chapter 18, etc. 

Corrosion and fission products appear in dissolved ionic form and "precipitated" in the crud, d nding on the chemistry and water conditions. Most of the corrosion products giving rise to induced activity enter with the feed water. The dominating activated corrosion products are Cr, Mn, Fe, Co, Co, Zn, and Sb, and the dominating fission products are H, Cs, and Cs. Other fission products and actinides are released... 

TABLE 21.5. Decay heat from unpartitionedJuel, fission products and actinides (Basic data as in Table 21.2)... 

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