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| According to Oleg Tashlykov, the container protects from radiation in all directions. Photo Credit: Anastasia Farafontova |
Ural Federal University scientists designed a container to store solidified liquid radioactive waste containing "long-lived" cesium-137 and cobalt-60, the most potentially dangerous of all radioactive waste. Due to their innovative design and filling, the simulated containers are capable of reducing radiation from radioactive waste to safe levels. One such container could replace five or six of the standard type. An article about the scientific work was published in the journal Progress in Nuclear Energy.
The modeled container consists of three main layers: a stainless steel inner capsule, halloysite clay filler, and an outer cementation concrete layer. The stainless steel capsule holds more than 450,000 cm3 of radioactive waste. Radionuclides are concentrated in a special sorbent, which is used in ion-selective purification and is placed inside the capsule. Stainless steel was chosen because, unlike carbon steel, it is more resistant to corrosion and does not require shielding.
"As a rule, such containers consist of two layers: outer cementation concrete and an inner metallic hosting capsule with a radioactive sorbent (or a sorbent in a cement matrix is placed inside the container). The main disadvantage of such a container arrangement is that their shielding, i.e. protective, capacity is limited. We suggest a three-layer container - with an additional layer between the inner metal capsule and the outer shell. The material that fills this space must be inexpensive and still effectively reduce the gamma radiation emitted by the radioisotopes inside the radioactive waste container. In this case, we investigated the protective properties of the intermediate layer consisting of halloysite - a fine-dispersed nanoscale white clay with a chemical composition rich in aluminum and silicon," says Oleg Tashlykov, Associate Professor at the Department of Nuclear Power Plants and Renewable Energy Sources at UrFU, Head of Research and one of the authors of the article.
The results of simulation of the absorbed dose and equivalent dose rate in the detectors located behind the container walls showed that increasing the inner capsule thickness to 3 cm leads to absorption of almost 83% of gamma-photons emitted by the radioactive waste. The use of 17 cm thick halloysite filler reduces the equivalent dose rate by about 15 % more. Taking into account the 15 cm concrete wall, the container design reduces the equivalent dose rate to safe values.
"The absorbed dose decreases as the wall thickness of the radioactive waste capsule and the halloysite filler layer increase, because the distance that gamma photons travel in effectively shielding materials increases. The number of their interactions with surrounding atoms increases. Colliding with atoms, they lose most of their energy. At the same time, the container provides protection from radiation in all directions from the container and meets the radiation safety requirements for their storage," Tashlykov explains.
It is calculated that for processing the same amount of liquid radioactive waste using the proposed model, five to six times fewer containers will be required than in the traditional method, when the sorbent containing radioactive isotopes is mixed with cement mortar and placed inside the container. Therefore, the developed container is promising for storage of solidified liquid radioactive waste with provision of necessary radiation protection.
Reference
Liquid radioactive waste is generated during the operation and decommissioning of nuclear power plants, such as the removal of radioactive isotopes from the water used to cool the reactor cores or the decontamination of nuclear power plant equipment. Containers are needed for transporting and long-term placement of waste at storage facilities while ensuring the safety of workers and the environment.
Earlier, Ural Federal University scientists modeled the efficiency of such containers where some cheap rocks (limestone, rhyolite, etc.) served as a filler. It turned out that basalt has the greatest ability to reduce gamma radiation from radioactive waste inside the container. The use of a steel capsule with solidified liquid radioactive waste, a layer of basalt, and concrete container walls reduces the gamma radiation level of the waste to a safe level. Basalt is common in Egypt, where the El Dabaa Nuclear Power Plant is currently under construction by Rosatom. This explains the attention to the protective properties of basalt by Oleg Tashlykov's PhD students and co-authors from Egypt.
Ural Federal University scientists continue searching for the most optimal composition of protective containers for economical and maximally safe transportation and storage of liquid radioactive waste. The next works will be devoted to investigation of protective properties of low-active crushed metal elements, which are formed during repair and dismantling of radioactive equipment at nuclear power plants. According to the researchers' forecasts, the use of these materials will make it possible to solve several problems at once: to create additional shielding of containers, simultaneously increasing their capacity of radioactivity to be placed and at the same time to utilize low-active metal wastes, reducing the cost of their recycling.
Source/Credit: Ural Federal University
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