Keeping bubbles out: nuclear waste production can be cut in three times25 августа 2020 года
The generation of nuclear waste can be cut three times. Scientists of the Ural Federal University came to this conclusion as a result of model experiments, proposing a modification of the fuel composition for nuclear reactors. Their work is based on the study of the generation of xenon bubbles, which accumulate in uranium and lead to its degradation, which reduces the useful life of this radioactive substance. Changes in the fuel composition will also help increase the safety of nuclear power plants, experts say.
Spent nuclear fuel (SNF) is one of the most hazardous types of waste. Its disposal requires the construction of expensive storage facilities, which are replenished as SNF is depleted in operating reactors. Now this process is going on at a rather fast pace, given the relatively low service life of fuel elements (TVEL) – hermetically welded zirconium alloy tubes, with the help of which uranium fuel pellets are delivered to the reactor core.
The way to significantly slow down the accumulation of nuclear waste is proposed by Russian scientists who have created an accurate model of the processes occurring in the core.
– During a nuclear reaction, fissile uranium decays into xenon and other inert gases, which gradually appear in the fuel in the form of small bubbles (with a diameter of 1 to 10 nanometers), ” said Dastan Seitov, a researcher at the Department of Technical Physics of the Ural Federal University (UrFU). – Because of this, the operational characteristics of the fuel (in particular, thermal conductivity) are reduced, which ultimately leads to the need to replace the spent fuel element with a new one". According to the scientist, their research was aimed at finding the conditions necessary for the collapse of gas bubbles under external influence in order to slow down the degradation of the metal. As a result, it was found that the energy of collision of atoms (so-called cascades), which arises during a chain reaction, can act as an effective solution here. “We made these conclusions by conducting an experiment with the help of a mathematical model built by us,” explained Dastan Seitov. – Previously, it included already known data obtained from previous practical experiments, which allows us to talk about the reliability of the results.
The information obtained by the scientists will make it possible to modify the fuel in such a way that the effect of the cascades on the bubbles is maximal and the process of gas accumulation slows down significantly. For example, this can be done by introducing the necessary additives into uranium tablets. It is interesting that the remaining bubbles will begin to shift from the center of the element to the periphery, evenly dispersed in the substance. This will prevent their possible combination, which threatens a significant deterioration in thermal conductivity and subsequent failure of the fuel cell.
"Nuclear fuel in standard reactors is used at best by 7%, and its period of operation in standard reactors is about three years, after which the fuel assembly is taken out of the reactor and prepared for disposal", – said Kirill Nekrasov, associate professor of the Department of Physics of the Ural Federal University. – "The use of the data obtained in the creation of a new generation of fuel rods can increase the fuel burnup up to 20%. At the same time, the total life of its operation will increase by about three times". According to the expert, this will directly affect the rate of generation of nuclear waste, reducing the amount of which is one of the main goals of researchers.
“Storage and transportation of large volumes of spent nuclear fuel is expensive and potentially dangerous, and reliable technologies do not yet exist for their disposal on a permanent basis,” says Alexander Nikitin, director of the Bellona Environmental Legal Center. “That is why the task of reducing the amount of spent fuel should be a priority". According to the expert, the presented method may well become one of the solutions to the problem, along with the construction of more efficient fast neutron reactors – in them, the accumulation of gases in the fuel practically does not hinder its operation.
Additional protection of fuel from xenon will also affect the safety of using reactors, Kirill Nekrasov believes. "In particular, it will not allow the gas to accumulate under the TVEL cladding, which can cause overheating and the subsequent emergency situation", – the scientist specified.
In addition, reducing the number of gas bubbles will reduce the risks of destruction of fuel cells, which become brittle when the concentration of xenon is too high and can break down directly during their removal from the reactor.
Specialists from the Higher School of Atomic and Thermal Power Engineering of St. Petersburg Polytechnic University declared the value of the scientific work carried out. “Uranium fuel already has low thermal conductivity, and the presence of inert gas bubbles reduces them even more,” says Maxim Konyushin, senior lecturer at the Higher School of Atomic and Thermal Power Engineering of SPbPU (the university is a participant of the 5-100 Education Competitiveness Project). "Understanding how these bubbles move and collapse will indeed open up opportunities for deeper exploitation of the potential of fuel cells". According to the expert, the research results can be applied both when working with promising reactors and in relation to existing installations, which will improve their operational characteristics.
However, there are also more restrained assessments of the new work.
"Academician Igor Kurchatov also wrote that nuclear fuel pellets “swell” and burst the nanobubbles of isotopes of inert gases," said Alexander Gromov, professor at NUST MISIS. – "The presented scientific work, although it has value due to the proposed approaches to improving nuclear fuel, is still theoretical. To confirm it, additional practical experiments are required".
It is expected that the work on the modification of nuclear fuel, taking into account the data obtained by scientists, could last about 10 years. Their research received a grant from the Russian Foundation for Basic Research.