How St. Petersburg scientists eliminated radioactive soil contamination on Vasilievsky Island
March marked the tenth anniversary of the accident at Japan’s Fukushima Daiichi nuclear power plant. Its consequences turned out to be not as catastrophic as might have been expected immediately after the accident. However, the area around the plant is still uninhabitable, and the radioactive waste collected in the contamination area is stored in temporary warehouses, and no one knows what to do with it.
In the 1990s, merchants who bought a plot of land on Vasilyevsky Island for construction had to solve a similar, though more localized, problem. In the middle of the last century, there was a military-industrial complex enterprise where spent nuclear waste was stored in special mines. The waste was utilized back in the Soviet times, but the concrete used for the walls of the wells was not a sufficient barrier for radioactive elements - strontium-90 and cesium-137. They seeped into the soil and made the area unsuitable for any construction.
Such a change of events turned out to be an unpleasant surprise for businessmen. After all, at that time, contamination was fought mostly by creating concrete barriers that prevented the migration of radionuclides. But in this case, the construction of buildings would have to be forgotten. The only available way not to get into trouble was to “take the soil out of economic circulation,” that is, remove it and replace it with new soil, but even that was not possible.
Entrepreneurs were told that there were no free burial grounds of that size, and in case they suddenly appeared, such an application would have almost no chance to be approved. The operation of removing a colossal amount of contaminated soil from the city center is a troublesome business (the WHSD was built much later), it will at least attract unnecessary attention and cause an unhealthy public outcry. Let everything remain as it is, all the more so, as this place is deserted, and the radioactive background will normalize sooner or later.
What is to be done? After all, considerable sums were invested in the purchase of the valuable, as it seemed, at first sight, asset. With this question, the builders turned to St. Petersburg Mining University, whose scientists were engaged in specialized research. In particular, they studied the possibility of cleaning the soil to permissible standards by means of ion-exchange displacement of radioactive isotopes. The innovation had already been partially tested on samples taken from Chernobyl-affected areas, but there was still a long way to go before the research was completed.
"The diffusion coefficient of strontium in the soil on the territory of the former industrial zone was two orders of magnitude higher than that of cesium. Therefore, while the first element was fixed in the upper layer 20-30 centimeters thick, the second one penetrated to the depth of several meters. As a result, the volume of soil to be decontaminated was estimated at at least five thousand cubic meters. This is about two and a half thousand tons. Besides, the area was littered with construction debris, which was also contaminated," recalls Olga Cheremisina, head of the Department of Physical Chemistry at St. Petersburg Mining University (in the 1990s, the department was headed by Dmitry Chirkst).
The technology developed by the scientists resembles heap leaching, which is used to extract gold from poor ores. Its essence is that the rock is placed on a slightly sloping concrete pad and poured with an acid solution, which after a certain time flows into a special tray, along with the precious metal. In this case, strontium and cesium were to play the role of the precious metal.
The main problem facing the researchers was the choice of the so-called eluent, that is, the basis for a liquid capable of displacing radioactive isotopes from the soil. If acid is used in the heap leaching process, then a fundamentally different, much gentler solution for the soil was required here. Otherwise, its entire organic layer would be scorched, and no trees, bushes, or even grass could grow next to the building.
In the end, based on the results of scientific experiments, the scientists relied on a solution of iron chloride with the addition of ammonium chloride to stabilize the acidity. Testing of the method of ion-exchange displacement of radionuclides in the contamination area began in 1994. And three years later the chemists began practical cleaning of the area.
“We placed samples of the most contaminated soil, weighing 80-100 kilograms each, on a constructed tray made of plasticate on a concrete base (a simple polyethylene roof was mounted over it to protect it from rain). These piles were evenly wetted with elution solutions from the irrigation system. Washing water slowly seeped through them and flowed through holes with drain tubes at the bottom of the tray into special receivers together with strontium and cesium isotopes. Using the developed technology of ion-exchange leaching of soil with salt solutions made it possible to bring it to the required degree of purification, corresponding to the standards of construction of buildings and structures," says Olga Cheremisina.
The work began in August and was not interrupted until November when the temperature had already dropped to sub-zero levels. The scientists had to hurry and work overtime because the frozen soil would not let the solvent through, and the work would have been postponed until spring. However, they had time - the soil was cleaned up to normal values and mixed with new soil in order to exclude local activation centers, after which investors were finally able to proceed with the construction of the unfortunate site.
The technology of ion-exchange displacement of radioactive isotopes proved its effectiveness. Immediately after the “battle test”, it was patented and subsequently used repeatedly in the elimination of local radioactive contamination.