Fission products chemical composition

The chemical composition of fission products in discharge fuel is controlled by the long-lived and stable species. The amounts of most of the fission-product chemical elements change but little for thousands of years after discharge. Those elements that do change significantly in amount over long decay periods include ... 

Touring the formation of radioactive fallout particles, one of the most important processes is the uptake, in the cooling nuclear fireball, of the vaporized radioactive fission products by particles of molten soil or other environmental materials. Owing to the differences in the chemical nature of the various radioactive elements, their rates of uptake vary, depending upon temperature, pressure, and substrate and vapor-phase composition. These varying rates of uptake, combined with different residence times of the substrate particles in the fireball, result in radiochemical fractionation of the fallout. This fractionation has a considerable effect on the final partition of radioactivity, exposure rate, and radionuclides between the ground surface and the atmosphere. 

In the chemistry of the fuel cycle and reactor operations, one must deal with the chemical properties of the actinide elements, particularly uranium and plutonium and those of the fission products. In this section, we focus on the fission products and then chemistry. In Figures 16.2 and 16.3, we show the chemical composition and associated fission product activities in irradiated fuel. The fission products include the elements from zinc to dysprosium, with all periodic table groups being represented. 

Figure 16.2 Chemical composition of the fission products in irradiated fuel as a function of decay time after a 2-month irradiation. [From J. Prawitz and J. Rydberg, Acta. Chem. ScantL 12, 393 (1958).]...

Fission Products in Equimolar Sodium-Potassium Nitrate. Fission-product behavior in molten alkali metal nitrates is largely unknown. Information on the behavior of various elements and their compounds is incomplete. In addition, the complexity of the composition and chemical nature of irradiated fuel material makes prediction of individual fission-product behavior even more difficult. 

Figure 8.2 Chemical composition of fission products (for uranium-fueled PWR 150 days after discharge).

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. 

Summary. Phosphate and silicate apatite offer a number of advantages as nuclear waste forms (1) a high capacity for the incorporation of actinide elements, as well as selected fission products such as °Sr (2) a reasonable chemical durability depending on the geochemical environment and (3) a propensity for rapid annealing of radiation damage for the phosphate compositions. Considerable work remains to be done, mainly systematic studies, under relevant repository conditions, of the effects of composition on chemical durability. 

Once the radioactive fission products are isolated by one of the separation processes, the major problem in the nuclear chemical industry must be faced since radioactivity cannot be immediately destroyed (see Fig. 10-7c for curie level of fission-product isotopes versus elapsed time after removal from the neutron source). This source of radiation energy can be employed in the food-processing industries for sterilization and in the chemical industries for such processes as hydrogenation, chlorination, isomerization, and polymerization. Design of radiation facilities to economically employ spent reactor fuel elements, composite or individually isolated fission products such as cesium 137, is one of the problems facing the design engineer in the nuclear field. 

The loop consists of an in-pile section with the fuel element, deposition section (heat exchanger), filters for collecting condensible Fission Products (FP) during depressurization tests and an out-of-pile section devoted to chemical composition control of the gas and online analysis of gaseous FP. 

Highlights This section did not deal directly with the analytical chanistry or characterization of uranium, but is highly relevant to ensure the safety of the main commercial application of uranium—production of electric power in nuclear power plauts. Therefore, the analytical procedures were ouly briefly described. The main safety function of the fuel elements is to prevent the escape of fission products (mainly gaseous elements or volatile compounds) into the coolant and atmosphere. Therefore, very rigorous quality control measures are employed to ensure the physical integrity and chemical composition of these elements. 

The chemical state is characterized by the existing valency state of the relevant fission product atom chemical compounds with a well-deflned chemical composition and an own habitus cannot be defined, with the exception of fuel inclusions. 

The isotopic composition of fission product iodine present in the BWR reactor water in the case of failed fuel rods in the reactor core is quite similar to that in the PWR primary coolant. Since the iodine purification factor of the reactor water cleanup system is on the order of 100, i. e. virtually identical to that of the PWR primary coolant purification system, this similarity in isotopic composition demonstrates that the release mechanisms of iodine isotopes from the failed fuel rods to the water phase are virtually identical under both PWR and BWR operating conditions. On the other hand, the resulting chemical state of fission product iodine in the BWR reactor water is quite different from that in the PWR primary coolant. The BWR reactor water usually does not contain chemical additives (with the possible exception of a hydrogen addition, see below) as a result of water radioly-... 

The effectiveness of a spray system in removing fission product iodine from the containment atmosphere depends on several parameters, as is known from early investigations, among others those conducted by Row et al. (1969), by Patterson and Humphries (1969) and by Row (1971). Other experiments have demonstrated that spray solutions buffered at pH 9.5 are more effective in iodine removal by a factor of 3 than pure water, probably because acidification of the outer droplet layer is prevented which, in pure water, may be caused by the I2 hydrolysis and disproportionation reactions (Hyder, 1991). Other parameters controlling iodine removal by sprays are specific for the individual system under consideration (internal geometry, spray rate etc.). Spraying experiments performed in the context of the CSE tests (see Section 7.3.3.3.8.) showed a reduction of the airborne iodine concentration of more than one order of magnitude within a spray time of about 10 minutes subsequent spray periods were much less effective (Hilliard and Postma, 1981). In the case of airborne particulate iodide, the chemical composition of the spray solution (water, boric acid, alkaline borate solution) does not affect... 

The quantity, time-dependence and composition of the volatilized substances may significantly influence the further behavior of the radionuclides as well as the progress and the products of chemical reactions occurring in the primary system and in the reactor containment. On the other hand, the volatilization of the fission products from the reactor core is able to affect the further progress of core heatup and degradation, since in the first hour after reactor shutdown the gaseous and volatile fission products contribute approximately 30% of the total decay power of the core. 

The gas-steam mixture escaping from the degrading core shows a complex chemical composition besides H2O and hydrogen in varying proportions (see below), a number of chemical elements - vaporized fission products as well as constituents of structural and control rod materials - will be present. The situation is even more complicated by the fact that a part of these elements is in aerosol form while the other part is in vapor form. 

Besides the gas composition, the temperature and the temperature gradient within the primary system are important parameters for establishing the chemical species and physical forms of the fission products and other materials released from the overheated core. The temperatures prevailing on the way through the pipes to the break are a function of the location of the leak, but they depend also on the gas-flow rate and on the composition of the gas mixture therefore, they are strongly influenced by the details of the particular accident sequence. Very roughly spoken, values in the range of 1000 to 1200 (1300 to 1500 K) are to be assumed... 

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