Composition of Irradiated Fuel

The composition of irradiated fuel to be fed to a reprocessing plant varies widely. It depends on the composition of the fresh fuel charged to the reactor, the neutron spectrum in which the fuel is irradiated, the specific power or rate of heat generation in the fuel, the duration of irradiation, and the length of time the fuel is cooled -the interval between end of irradiation and start of reprocessing. 

---

Figure 16.9 Changes in the chemical composition of irradiated fuel. (From Murray, 2003.)...

Symmetric fission, in which the or Pu nuclei disintegrate into two products of equal mass number, has a low probability. For this reason, their products do not play a significant role in the radionuclide composition of irradiated fuel. 

With regard to the radionuclide composition of irradiated fuel, there are also deviations from the simple relationship between fission product activity concentrations of longer-lived nuclides and fuel bumup. Similarly to the buildup of the mass concentrations, these deviations are due to the increasing contribution of plutonium fissions to radionuclide production as well as to consumption of long-lived radionuclides by neutron capture in extreme cases, such as with the short-lived... 

Selection of the radiolysis conditions is of primary importance. If studies carried out with a pure extractant enable the intrinsic stability of the molecules to be verified, radiolysis in solution and especially in a basic medium are indispensable to guarantee the approaches good representativity, as much from the point of view of species formation as from that of their distribution (potential elimination of the shortest degradation products, the most polar to the aqueous phase). The characteristics of the irradiation source (nature, dose rate, integrated dose) and also the temperature are essential parameters. Thus, the nature of the irradiation depends on the composition of the fuel, and the dose rate is dependent on the bum-up and cooling time of the fuel, while the exposure time of a solvent depends on the implementation conditions of the proposed process (flowsheet and nature of the contactors). 

Tracer techniques, for example, are used to obtain very small but representative and measurable samples of highly radioactive spent fuel solutions. One millilitre of the solution is then spiked with a known amount of uranium and plutonium tracer isotopes. A few microlitres of the spiked solution are dried and shipped to SAL. One to fifty nanograms of uranium or plutonium extracted from this tiny sample are sufficient for a complete analysis representing the composition of half a tonne of irradiated fuel with an accuracy of 0.3 to 0.5%. 

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. 

The CAPRA mixed oxide fuel has high Pu enrichment, up to 45%, and would be irradiated to hi bumup. Codes to calculate the performance of the fuel during irradiation have been adapted to take account of CAPRA conditions. The features of most concern are the migration of Pu and the chemical composition of the fuel material. The predictions have been compared with PIE data from a range of sources. The comparisons have been used to adjust the calculation parameters and validate the codes. 

At the Hanford plant, a variety of irradiated fuel elements require safe storage prior to reprocessing. Arrays of such subcritical units may be safely stored in water if a water gap between units, s ifficient to inhibit excess neutron interaction, is provided. The size of the gap is limited by the composition of the unit, the k-effec-tive of the unit and the maximum k-effective permitted for the array. In the past, a computer calculation has often been necessary to determine array safety but a simple relationship for determining safe unit spacing for less than complete isolation has been developed which can be used in many studies and which can reduce computer calculation. 

Schuck, E. A., H. W. Ford, and E. R. Stephens. Air Pollution Effects of Irradiated Automobile Exhaust as Related to Fuel Composition. Report No. 26. San Marino. Calif. Air Pollution Foundation, 1958. 91 pp. 

Kleykamp, H. 1990. Post irradiation examinations and composition of the residues from nitric acid dissolution experiments of high bum-up LWR fuel. Journal of Nuclear Materials, 171, 181-188. 

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).]...

No systematic investigations of SNF corrosion resistance in water solutions with different salt concentrations have been performed yet. So far only individual corrosion tests of non-irradiated fuel compositions and fuel element fragments have been conducted. 

Fully active laboratory scale experiments were started using firstly a Windscale HAW solution (5000 l/t) generated by the reprocessing of Magnox fuel elements with a burn-up value of 3500 MWd/t. The overall decay time was about 10 months and as the composition was not known, only relative activity measurements were performed. Other fully active HAW solutions were subsequently prepared in Ispra hot cells by dissolving U02 samples irradiated at 26 — 36,000 MWd/t and cooled for about 4 years. Successive TBP batch-extraction steps were carried out under the 1st extraction cycle conditions of the Purex process to remove the bulk of U and Pu. 

Figure 3.3 is an example of the change in composition of fuel in a PWR during irradiation, calculated by the computer code CELL [B2]. In this example fuel charged to the reactor contained 3.2 w/o in total uranium. The extent of irradiation, plotted along the x axis, is... 

The changes in fuel composition just described cause the reactivity of the fuel to decrease with increasing bumup. The reactivity is defined as the difference between the rate of neutron production by fuel and the rate of neutron consumption, divided by the rate of neutron production. If the reactivity is zero, the reactor will be just critical without insertion of control poisons if the reactivity is negative, the reactor power will die out if the reactivity is positive, the reactor can be brought to a steady power level by insertion of sufficient neutron-absorbing control poison to reduce its reactivity to zero. Figure 3.4 shows how the reactivity of a PWR whose fuel composition is spatially uniform decreases with bumup. Lines are plotted for four different initial fuel compositions 2.8, 3.2, 3.6, and 4.0 w/o To a rough approximation, reactivity decreases linearly with bumup and increases linearly with w/o in fuel at the start of irradiation. 

To point to the importance of using improved methods of fuel and poison management, we shall discuss qualitatively the multiple drawbacks of the simplest method, which is batch irradiation of fuel initially uniform in composition, with spatially uniform distribution of boron control poison and with complete replacement of fuel at the end of its operating life. An example of this would be a PWR charged with fuel of uniform enrichment containing 4 percent and 96 percent and controlled by adjusting the concentration of boric acid dissolved in the water coolant to keep the reactor just critical at the desired power level. When this reactor starts operation, the compositions of fuel and poison are uniform throughout the core, and the flux and power density distribution are very nonuniform. 

Equations (3.44) through (3.80) have been used to calculate the effect of irradiation in this PWR on the composition of fuel initially containing 3.2 w/o U, with results given in Table... 

Table 3.15 Effect of irradiation in a PWR on composition, bumup, and reactivity of fuel containing initially 3.2 w/o U...

---