Improving the sintering of thorium oxide

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Abstract

Globally, a great majority of nuclear fuel is composed of a fissile component within a 238UO2 matrix. This can take the form of natural or enriched UO2 or of a mixture of PuO2 with 238UO2. Such fuel is generally well described and has a high reliability. Nonetheless, there are several advantages to supplementing the fuel with rods on the basis of a ThO2 matrix. Thorium dioxide presents several advantages over uranium dioxide as a matrix. These include the higher melting point and chemical stability of thorium dioxide. The 232Th-233U breeding cycle is also superior to the 238U-239Pu cycle in the thermal spectrum. Together these allow the fuel to be kept in the reactor longer under safe conditions. The better chemical stability of ThO2 allows the spent fuel to be stored more safely both in the short and the long term. Yet despite these advantages, thorium dioxide is unlikely to be used as a nuclear fuel matrix in the short term. One of the barriers to commercial adoption is the lack of supply of thorium dioxide pellets which is both reliable and competitive with uranium dioxide on price. Here the same properties which are a boon during operation present barriers during manufacture. ThO2 presents additional costs over UO2 both in the powder precipitation and pellet sintering stage. The higher thermal stability of ThO2, which increases operation safety margins, introduces costs during sintering. It often pushes the sintering temperature needed to achieve the operator-specified density beyond the range of commercial furnaces. Though 232Th breeds better than 238U, its decay chain is less favorable, as it introduces costs during powder precipitation. The half-lives of the decay products are such that significant activity will build up during storage. This can lead to significant waste disposal costs if not adequately addressed. Of the decay products, radium (as 228Ra and 224Ra) is the one which must be dealt with. The other decay products are sufficiently short lived and can be simply allowed to decay away. Ideally radium would be removed alongside thorium into the product, where it would present no additional waste disposal costs. Of the two costs introducing effects, the poor sintering is much more limiting and needs to be addressed first. The common oxalate precipitation route was turned to first to help improve the sintering of ThO2 without introducing milling, which is an undesirable source of radioactive dust. Precipitation at reduced temperatures produced thorium oxalate platelets with a wide size distribution. When calcined, these produced a well-sintering ThO2 powder. If calcined at < /800 °C, the powder also pressed to form stable green pellets. These two factors together make ThO2 from oxalate precipitation viable for the commercial nuclear fuel production. Besides the calcination temperature effects, changing the order of addition during precipitation also changed things. Adding oxalate to thorium produced oxalate platelets stacked into cubes which produced fragile green pellets even when calcined at low temperatures. In contrast, adding thorium to oxalate produced thinner platelets with a larger face. Green pellets from the calcined oxide formed stable green pellets at low calcination temperatures. To further expand on the effect of platelet shapes, several different oxalate shapes were produced: both large and small platelets in the shape of squares, rectangles and octagons which were intact or contained holes. The outer shape was shown to have little effect on the sintering. Instead, the sintering was influenced by the size of platelets and the presence or absence of holes. Small platelets of oxalates and/or oxalates containing holes sintered to much higher density, but produced considerably smaller sintered grain sizes. Despite the interesting sintering effects, oxalate precipitation did not precipitate radium sufficiently to remove radioactive waste disposal costs. Because of this, an alternate precipitation strategy needs to be attempted to reduce manufacturing costs further. The thermal decomposition of urea offered good control for homogeneously precipitating Th4+ with ammonia and Ra2+ with carbonate. The process produced very uniform, nanometric ThO2 and had good agglomeration control. It did not, however, present a significant improvement over oxalate precipitation in terms of radium removal. Because the radium removal of urea-based precipitation was insufficient, a move was made instead to a heterogeneous two-step precipitation process on the same ammonia and carbonate basis. The slight sacrifices in terms of uniformity is more than compensated by the good properties this confers. By splitting the ammonia and carbonate steps, the precipitation could take advantage of the good parts of each step. With good pH control, the ammonia step can confer good morphology and filterability on a nanometric ThO2 precipitate. Good filterability allows the product to be easily separated from the liquid phase by a simple, low investment-cost method. After the ThO2 precipitate is fully formed, adding carbonate binds radium to the precipitate very efficiently. The produced fresh product is a nanometric ThO2, with large quantities of weakly bound water and carbon dioxide. These beneficial processing characteristics of good filtering and efficient radium removal make the process good for a powder manufacturer. Upon calcination, the ThO2 coarsens gradually but retains some porosity in the agglomerates produced. This internal porosity gives the powder some softness, which allows it to press into stable green pellets. The coarsening reduces sinterability slightly, but the green pellets still sinter to closed porosity. Because it forms stable green pellets and sinters to closed porosity, ThO2 precipitated in two-steps with ammonia and carbonate is also suitable for the fuel manufacturer. On the basis of these findings, the two-step ammonia-carbonate precipitation process might outperform the simpler one-step oxalate precipitation route by reducing or avoiding radium contaminated liquid effluents. Further studies about the scalability of the two-step process will be needed to confirm its industrial viability.

Details

Original languageEnglish
QualificationDoctor of Science
Awarding Institution
  • KUL - Katholieke Universiteit Leuven
Supervisors/Advisors
Award date20 Jan 2020
Publisher
  • KUL - Katholieke Universiteit Leuven
Publication statusPublished - 20 Jan 2020

Keywords

  • Sintering, Thorium oxide, Thoria fuel, Precipitation, ThO2

ID: 7113254