5. September 2013

Well-rounded concept for less nuclear waste

Energy and Environment Nuclear Power Plant Safety

The idea of producing fuel for nuclear power stations in form of a package of spheres instead of today’s customary pellets was already born back in the 1960s. There was promise of a subsequent simplification of fuel production and a considerable reduction in the amount of radioactive waste both in the production of the fuel itself and after its use in a nuclear power station. However, the spherical fuel was never implemented as the fast reactors for which it was conceived were never built at a large scale. The Paul Scherrer Institute (PSI) has also been involved in the research on spherical fuel in the past. Now several projects partly funded by the EU are currently underway at the PSI again to refine the production of fuel spheres further. This form of fuel could either be used in special plants to reduce waste or in fast generation IV reactors, which in a closed cycle also produce less long-lived waste.

How nuclear waste is produced

In the most common reactor type, the light water reactor, the atomic nuclei (uranium 235) contained in the fuel are split by bombarding them with relatively slow (thermal) neutrons. This nuclear fission releases energy that is used to generate steam. The steam drives a generator, which in turn produces electric power. During nuclear fission, free neutrons also emerge and split other nuclei, making the reaction self-sustaining – this is what specialists call a controlled chain reaction. Some of the uranium nuclei in the fuel are not split, however, but rather converted into plutonium and subsequently into even heavier elements through neutron capture. These transuranic elements stay in the spent fuel from a light water reactor and remain highly radioactive for hundreds of thousands of years. This waste thus needs to be disposed of safely.

Breaking up tough waste products

Atom nuclei that cannot be split in a light water reactor can be, however, broken down further in a fast reactor of the fourth generation or in an accelerator driven facility (ADS) for nuclear waste transmutation. Both systems don’t exist yet anywhere in the world. At present, they are in fact concepts being worked on in research and development programmes.

In an ADS facility protons are first brought to high energies (speeds) in an accelerator and then directed at a target of heavy metals. The fast protons collide with the atomic nuclei of the heavy metals causing fast neutrons to evaporate out of these nuclei - a process that specialists call spallation. The spent nuclear fuel is then bombarded with these high energy neutrons originiating from the heavy metal nuclei. In contrast to the few relatively slow (thermal) neutrons in a light water reactor, the large number of high-energy neutrons is more likely to induce the fission of the transuranic elements. One thereby converts the transuranic elements into more stable less long-lived nuclei. As a result, the time period over which the waste remains highly radioactive is reduced to about one hundredth.

Before being placed in an ADS system, the spent nuclear fuel needs to be reprocessed. For this purpose the spent fuel has to be dissolved and the uranium to be separated. The treatment has essentially the goal of producing a nuclear fuel enriched in transuranic elements (especially plutonium and minor actinides). This is necessary so that the transuranic elements are converted into more stable isotopes, without causing the conversion of other isotopes into unwanted transuranic elements by neutron capture. After processing the liquid solution containing the transuranic elements must be again formed into solid fuel elements.

However, with the conventional methods, a nuclear fuel with a high share of transuranic elements cannot be produced safely and inexpensively enough. It is right here that the spherical fuels currently under investigation at PSI enter the game.
Simulation of gelation in the microwave cavity. The falling droplets solidify in the bright disk-shaped zones, in which the energy of the microwaves is concentrated. Source: Paul Scherrer Institute.
Problem of dust formation

For the industry the problem is thus: in order to produce the conventional fuel from uranium oxide, the base material needs to be ground, pressed and polished before it attains the desired pellet shape. These process steps also cause the formation of dust, which is not problematic in the case of uranium oxide as fresh uranium oxide fuel is less radioactive and can be handled easily . However, it is a different story after the fuel has been exposed to neutron bombardment in a nuclear reactor. Then the fuel also contains highly radioactive elements and the further processing of this fuel for use in a fast reactor has to take place in special protected cells, known as hot cells, which enable the radioactive materials to be manipulated by remote control. Another round of grinding, pressing and polishing, however, would contaminate the hot cell through the accumulation of dust. While the decontamination of these chambers is feasible but complicated and expensive, what if the fuel could be produced or recycled without any grinding, pressing and polishing in the first place?

Little spheres instead of pellets

The spherical fuel offers this possibility. During its production, the fuel material remains in liquid form until the formation of the desired spheres, which already excludes the accumulation of any dust. The starting point for fuel production is a liquid mixture that contains the fuel itself and substances that act as a matrix for it. The formation of a gel from the liquid mixture is induced by heating and the associated shift in the acidity level. The gel is subsequently sintered in such a way as to give the fuel its final ceramic consistency. These little spheres can then fill a cylindrical fuel element shell like today’s conventional fuel. The formation of dust is kept to a minimum throughout these process steps and numerous radiation tests in several countries have revealed that the behaviour of the fuel produced in this way meets the safety standards in a nuclear power station.

Microwaves instead of silicone oil

However, there is one catch in producing fuel spheres, at least the way it has been conceived thus far: the conventional concept involves heating the base liquid to form the gel in a hot bath of silicone oil. This oil would become contaminated through the contact with the radioactive fuel and take a considerable amount of effort to dispose of it. In other words, the formation of radioactive dust would merely be replaced by the creation of radioactive oil instead.

One solution to this dilemma would be to dispense with the oil as a heat transfer medium. And that is precisely what the PSI researchers would like to attempt by using microwaves as the energy source to enable the temperature of the base liquid to be increased to such an extent as to induce gelation. Physicist Manuel Pouchon, a researcher at the Laboratory of Nuclear Materials at the PSI, explains the approach: “The falling liquid droplets would be heated up so quickly and precisely in a microwave cavity that they would condense into little balls of gel in the space of a few hundredths to tenths of a second, without the heat being so intense as to destroy the spheres.” Pouchon and his team have already tested the idea successfully, albeit not with real fuel, which would be highly radioactive and could only be handled in glove boxes or even hot cells. Instead, they studied cerium as a substitute substance, the chemical properties of which exhibit many similarities with transuranic elements.

The next steps

So far, the studies conducted by Pouchon and his team have demonstrated the reliable and precise function of the specially developed microwave cavity. The reproducible gelation into little spheres of the desired size has already been successfully demonstrated. Now, according to Pouchon, the aim is to test the gelation on a small scale with real, radioactive fuel instead of cerium, which will require the fine-tuning of the chemical processes. Moreover, the team still need to make sure that the microwaves for gelation can be incorporated into a glove box or later a hot cell efficiently. The PSI researchers are currently pursuing these partly fundamental, partly technical issues within the scope of three different research projects, one of which is funded by the ETH Domain’s Competence Centre for Energy and Mobility, and the other two by the European Union. Should their demonstrations be successful at laboratory scale, it would be up to industry to scale up the process.

Text: Leonid Leiva
Dr. Manuel Pouchon, Laboratory of nuclear materials, Paul Scherrer Institute,
Telephone: +41 56 310 22 45, E-Mail: manuel.pouchon@psi.ch