Improving the sintering behaviour of thorium oxide

Wangle Tadeáš

Promoter

Vleugels Jozef, (KULeuven - MTM), Jozef.vleugels@mtm.kuleuven.be

SCK•CEN Mentor

Verwerft Marc
marc.verwerft@sckcen.be
+32 14 33 30 48

SCK•CEN Co-mentor

Cardinaels Thomas
thomas.cardinaels@sckcen.be
+32 14 33 32 00

Expert group

Radiochemistry

PhD started

2015-10-01

Short project description

Although uranium dioxide (UO2) is currently the most important nuclear fuel, thorium dioxide or thoria (ThO2) is gaining more and more interest. Resource and long-lived waste concerns are some of the driving forces to consider thorium as an alternative nuclear fuel. Thorium is a valuable resource due to its high abundance in the earth crust and in addition, the thorium fuel cycle generates less transuranium elements than its uranium analogue. However, natural thorium is quasi-monoisotopic (232Th) which is fertile (233U breeding occurs in a thermal or epithermal spectrum), but not fissile and can thus not directly be used in a nuclear reactor: initial “enrichment” with a fissile isotope is needed (239Pu, 241Pu 233U or 235U). Besides the above mentioned pros and cons for the thorium fuel cycle, another reason for the growing interest in the use of thorium as nuclear fuel is the existence of large stockpiles of thorium, generated in the past mainly via monazite ore processing (monazite is an important commercial source of rare earth elements, thorium and uranium). Without valorisation in nuclear fuel, these thorium stocks will have to be considered as radioactive waste, a much more costly alternative.

Already since more than 10 years, SCK•CEN performs research on thorium-plutonium fuels including design, neutronic and safety studies, manufacturing, behaviour under irradiation, post-irradiation examinations, behaviour of spent nuclear fuel etc. Until now, all manufacturing processes were following routes that only work well on lab-scale but cannot be applied on a larger scale. The lack of data on production methods that can potentially be scaled up was identified as the biggest challenge to achieve a breakthrough in this alternative fuel cycle.

Objective

This project focusses on the processing and sintering behaviour of binary thorium oxides. Sintering is an important step in the processing of ceramic nuclear fuel in order to create highly dense, solid fuel pellets out of a pressed powder compact. Sintering is performed at high temperatures so that material can diffuse over the grain boundaries resulting in a densification of the material. As a general rule, sintering needs to be carried out at around 2/3 of the melting temperature. Thoria has one of the highest melting points of all oxides (3300°C), which is about 500°C higher than that of urania. Therefore, sintering of thoria is not at all straightforward and much more challenging than sintering of urania. It is e.g. known from literature and confirmed by recent scoping studies that without modification, compacts of standard available thoria powders can be densified only up to around 90% of the theoretical density and that the sintered body strength is insufficient.

The aim of this project is to develop routes to improve the sinterability of binary thorium oxides. As thorium needs a fissile element to be used as nuclear fuel, we will study thorium-cerium oxide as simulant for thorium-plutonium oxide fuel. Also the thorium-uranium oxide system will be studied as well as thorium-plutonium oxide in the final stage. Several methods will be explored such as the addition of small amounts of dopants (e.g. the addition of pentavalent niobium oxide substantially increases the sinterability of pure thoria), wet route synthesis of binary thorium oxide precursor powders (via co-precipitation or sol gel synthesis) will be assessed, and advanced sintering routes such as flash sintering will be explored.

Analysis of the sintering process as well as the produced powders and fuel pellets will be performed via a wide range of characterisation tools available at SCK•CEN. Recently, a unique nuclear fuel laboratory was installed. The acquired high-resolution thermal analysis equipment (STA and dilatometer) will be particularly useful to examine the sintering mechanisms of the binary thorium oxides. In addition, the effect of introducing foreign elements in the thoria matrix was hardly investigated and is of great importance since thoria is envisaged as a new nuclear fuel. The effect on the microstructure of the fuel pellets and on the thoria crystal lattice will be thoroughly investigated by electron microscopy and diffraction techniques.