Nuclear spent fuel pools: beware of evaporation !

Spent fuel from nuclear power plants spends a certain amount of time in storage pools. Fukushima’s pools contained almost four times as much radioactive material as did the six reactors on site. During the accident, the pools came close to releasing the material into the air. Since then, spent fuel storage pools have attracted more attention. Prof. Miltiadis Papalexandris’s team studies what happens in them.

Spent fuel (see box) from a nuclear power plant is stored in pools near the reactors. The spent fuel is radioactive and gives off heat. Water stops the radiation and absorbs the heat. Storage pools are also called deactivation pools, because the highly radioactive elements in the fuel can quickly lose much of their radioactivity and disappear. The pools are usually about 12 metres deep and have grids into which fuel rods (enriched uranium pellets stacked in zirconium alloy cladding) are inserted. The rods are typically four metres tall, leaving a comfortable eight metres of water above them.

‘Until about ten years ago,’ explains Prof. Papalexandris of the Institute of Mechanics, Materials and Civil Engineering and Louvain School of Engineering, ‘there was little interest in storage pools. It was the Fukushima accident that drew attention to them.’ As heat is released continuously, the water has to be pumped out and replaced by colder water. What if this mechanism breaks down? If the water level gradually drops until the fuel rods are exposed, several dangerous phenomena occur. Firstly, the gamma rays escaping from the fuel are no longer stopped by the water and spread into the air, endangering anyone in contact with it. Secondly, as they are no longer cooled by water, the fuel rods heat up and, on contact with the air, the zirconium in the cladding oxidises and may catch fire under the effect of the heat, with the risk of releasing radioactive elements. This was the risk in Fukushima in March 2011 in two pools close to reactors three and four: as the power plant was without electricity and had been destroyed by the tsunami, there was no way to pump out the hot water and replace it with cold water. As the water level dropped, the radiation increased, making it impossible to approach the facility (even helicopter pilots couldn’t come close). The pools had to be filled by water projected by powerful fire hoses. The situation was brought under control after a few days, and videos taken in the following weeks showed that the fuel rods stored in the deactivation pools had not degraded, thus radioactive elements were not released.

‘The fact remains,’ Prof. Papalexandris says, ‘that people have become aware of the risk of storage pools, whereas attention was previously focused on the reactors. How can we better deal with this type of accident?’ This is an important question because, as we often ignore, there is more radioactive material in the pools than in the reactor cores: at Fukushima, they contained almost four times more radioactive material than did the six reactors!

Hence the Belgian Federal Ministry of Energy and Sustainable Development has financed, via the Energy Transition Fund, a research programme which began in November 2020. Vincent Deledicque, a former PhD student under Prof. Papalexandris and now branch manager at Bel V, the technical subsidiary of the Federal Agency for Nuclear Control (FANC), which is in charge of regulatory controls and safety assessments in Belgian nuclear installations, turned to UCLouvain to tackle this problem, in collaboration with the French Institute for Radiation Protection and Nuclear Safety (IRSN). Prof. Papalexandris says, ‘There are many physical phenomena that occur simultaneously in the pools, particularly when the water is no longer cooled. We don’t know all of them but acquiring that knowledge is one of our objectives. In particular, we’re studying what happens at the air-water boundary and the turbulent thermal convection phenomena that occur during evaporation. Our aim is to provide estimates of evaporation times for various scenarios.’ Because even if the power plants shut down, the pools will last for several years thereafter!

Henri Dupuis

About the author
Miltiadis Papalexandris earned a master’s degree in naval architecture and marine engineering from the National Technical University of Athens in 1991. He then moved to the United States to attend the California Institute of Technology (Caltech), where in 1993 he earned a master’s degree in aeronautics and in 1997 a PhD in aeronautics and applied mathematics. From 1998 to 2002, he worked at NASA’s Jet Propulsion Laboratory in the field of space telescopes. In 2002 he joined UCLouvain, where he has been a full professor since 2018, teaching fluid mechanics and thermodynamics at the Louvain School of Engineering. He also supervises his research team at the Institute of Mechanics, Materials and Civil Engineering.

Spent fuel

Even if it depends on the initial quantity of fissile fuel (uranium, plutonium, etc.) and the duration of use, the composition of spent nuclear fuel can be schematically described. Let’s take the example of one tonne of uranium enriched to 3.5% (a common case). This means that the fuel consists of 965 kg of uranium-238 (238U, the most common non-fissile uranium isotope) and 35 kg of 235U (the fissile isotope). After three years of electricity production, 941 kg of 238U but only about 10 kg of 235U will remain. In addition, there are new elements produced by the irradiation of the fuel: about 30 kg of fission products, mainly plutonium and actinides, i.e. nuclei heavier than uranium, mainly plutonium but also neptunium, etc., and activation products (irradiated materials).

As a reactor’s load lasts about three years, it is renewed in thirds each year. The whole operation is carried out under water. Thus the fuel rods pass from the core to the reactor pool and then to a storage pool (this article’s subject), where they remain for at least a year. The water’s function is to stop radiation (thus protecting operators) and cool the fuel.

The fuel’s fate differs according to its use. Either it’s reprocessed (separation of plutonium and uranium) to be used as fuel again – in which case it remains in the pool for about a year and is then transported to a reprocessing plant – or it’s not reprocessed and can remain in the pool for several decades, pending a decision on how to store it permanently.


Published on May 06, 2021