Abstract
Neutron absorbers are elements with a high neutron capture cross section that are employed at nuclear reactors to control excess fuel reactivity. If these absorbers are converted into materials of relatively low absorption cross section as the result of neutron absorption, they consume during the reactor core life and so are called burnable. These elements can be distributed inside an oxide ceramic that is stable under irradiation and thus called inert. Cubic zirconium oxide is one of the preferred materials to be used as inert matrix. It is stable under irradiation, experiments very low swelling, and is isomorphic to uranium oxide. The cubic phase is stabilized by adding small amounts of dopants like Dy2O3 and Gd2O3. As both dysprosium and gadolinium have a high neutron cross section, they are good candidates to prepare burnable neutron absorbers. Pyrochlores, like Gd2Zr2O7 and Dy2Ti2O7, allow the solid solution of a large quantity of elements besides being stable under irradiation. These characteristics make them also useful for safe storage of nuclear wastes. We present a preliminary study of the thermal analysis of different compositions in the systems Gd2O3-ZrO2 and Dy2O3-TiO2, investigating the feasibility to obtain oxide ceramics useful for the nuclear industry.
Highlights
At the beginning of a reactor cycle, the fuel presents an excess reactivity to compensate future fuel depletion and fission products buildup
Its phase stability is basically determined by the A and B cationic radius ratio, as similar cationic radii are more likely to form as disordered fluorites than ordered pyrochlores
We focus on the feasibility to obtain cubic phases that could be employed as burnable neutron absorbers, studying composition and suitable sintering temperatures
Summary
At the beginning of a reactor cycle, the fuel presents an excess reactivity to compensate future fuel depletion and fission products buildup. This is controlled by using burnable neutron absorbers or neutron poisons, that is, by inclusion into the fuel assembly materials with a large neutron cross section. When the burnable neutron absorbers are dispersed into an inert matrix, they are called discrete or inhomogeneous and they are distributed among the fuel pellets. Zirconium oxide is a good candidate to be used as inert matrix mainly because of its stability under irradiation and its compatibility with reactor materials [3] It accommodates both fission materials and neutron absorbers. CANDU reactor has among the 43 bars that compose the fuel element a central bar that includes a neutron absorber in zirconium oxide inert matrix [4]
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