Amazonian pianhas at the Semipalatinsk Nuclear Test Site: The Story of a forgotten laboratory at lake Chagan

In 1965, a nuclear explosion at the Semipalatinsk Nuclear Test Site created a lake, after which Amazonian piranhas and hundreds of other species of living organisms were introduced into it. El.kz reports on how this unusual experiment was conducted and what researchers discovered decades later.

A Lake Created by a Nuclear Explosion

On January, 15, 1965, a 140-kiloton underground nuclear explosive device was detonated in the steppe near the confluence of the Shagan and Ashchisu rivers. The experiment was named Chagan. The device was emplaced at a depth of 178 metres in borehole No. 1004. Its purpose was to demonstrate that nuclear explosions could be used for earthmoving operations: instead of relying on excavators and years of construction work, a large reservoir for floodwater could be created through a single powerful explosion.

According to the National Nuclear Center of the Republic of Kazakhstan, the explosion ejected approximately 10.3 mln tonnes of soil and rock. The resulting crater reached 430 metres in diameter and over 100 metres in depth. A rim of fragmented rock, up to 35 metres high, formed around it. In the spring of 1965, a channel was constructed to divert meltwater from the Shagan River valley into the crater, which subsequently filled with water. This marked the creation of Atomic Lake, also known as Chagan or Atom-Kol.

In Soviet technical documentation, the objective was formulated in entirely pragmatic terms: reservoirs were needed in arid regions where water was available primarily during seasonal floods. Nuclear explosions were considered a means of rapidly displacing enormous volumes of rock and soil. However, the cost of this engineering solution was determined not only by the volume of the resulting reservoir. The displaced material contained nuclear explosion products, and their behaviour in water, air and living organisms would require decades of scientific investigation.

Why the Idea Was Considered Promising

The Chagan experiment was the first undertaking under the Soviet programme of peaceful nuclear explosions. The term peaceful referred to the intended economic applications of the technology, including mineral extraction, the construction of reservoirs and canals, and the creation of underground cavities. In physical terms, however, the device remained a nuclear explosive. Unlike nuclear tests designed to contain the explosion deep underground, this project envisaged the ejection of material onto the surface as an integral part of the operation.

The concept was not exclusively Soviet. In 1962, the United States conducted the underground Sedan nuclear test in Nevada, one of the best-known experiments under Project Plowshare, which explored the potential civilian applications of nuclear explosions. The Soviet Chagan experiment investigated a similar possibility, but under the specific conditions of the Kazakh steppe and river valley. Such projects were initially assessed from an engineering perspective. However, radioactive releases and long-term environmental contamination ultimately became among the principal constraints on the use of this technology.

The mere existence of a crater lake did not establish that a reservoir suitable for agricultural use had been created. Its practical application required data on water contamination, the leaching of radionuclides from surrounding rock and soil, and the impact on the surrounding ecosystem. Chagan therefore gave rise to a separate research question: was it possible to establish sustainable biological activity in an artificial environment created by a nuclear explosion?

Piranhas, Molluscs and an Unusual Living Laboratory

In the late 1960s, an experimental biological research station began operating near the lake. According to the National Nuclear Center, over a period of several years, the water body was stocked with 36 fish species, 27 mollusc species, 32 amphibian species, 11 reptile species, eight mammal species and 42 invertebrate species. In addition to animals, the experiment involved nearly 150 species of aquatic plants. The fish species introduced into the lake did indeed include Amazonian piranhas. This detail is confirmed by an official publication issued by the Center on November,27, 2020.

The available publication by the National Nuclear Center does not provide a detailed account of how the piranhas were introduced, including their place of origin, the number of specimens released or how long they survived. Nor does it identify the precise species of these South American fish. Consequently, claims that an independent population of predatory piranhas became established in the Kazakh lake or that monstrous mutants emerged there are not substantiated by this source.

For biologists, the appeal of introducing such a diverse range of organisms lay in the opportunity to compare the responses of different biological groups. Fish are in continuous contact with water and bottom sediments; molluscs can accumulate certain substances; and plants absorb elements from water and soil. However, field experiments involving the introduction of organisms into an unfamiliar environment are more complex than laboratory trials, as survival is influenced by numerous factors simultaneously.

What Does the Mortality Rate of 90% Mean?

The National Nuclear Center reports that approximately 90% of the introduced organisms died after some time. This is a striking finding, but it must be interpreted with caution. The same statement emphasises that virtually all the species introduced into the lake were non-native to the local fauna. For Amazonian fish, the Kazakh winter, seasonal temperature fluctuations and an entirely different food supply could have proved fatal even in the absence of radioactive contamination.

To establish what proportion of the mortality was specifically attributable to radiation exposure, researchers would have needed comparable control groups, exposure dose data, water quality measurements and information on the conditions under which each species was maintained. The historical account cited above does not contain such findings. Accordingly, the statement that “radiation killed 90% of the animals” might sound sensational, but it would not be scientifically accurate. A more defensible conclusion is that the large-scale introduction of predominantly non-native species was unsuccessful and that multiple factors may have contributed to the outcome.

The experimental station was closed in 1974. The National Nuclear Center’s publication does not provide a documented explanation for its closure; therefore, it would be inappropriate to attribute the decision solely to radiation hazards or animal mortality. Nevertheless, the station’s history illustrates how quickly an engineering experiment evolved into an environmental research challenge: once the crater had been created, scientists had to investigate the behaviour of water, living organisms and residual explosion products within the newly formed environment.

What Researchers Have Been Studying Decades Later

Since the late 1990s, the National Nuclear Center has conducted radioecological research in the area surrounding the lake. In 2018, the Institute of Radiation Safety and Ecology announced an international research project examining the consequences of the Chagan excavation explosion, in cooperation with the Fukushima Environmental Safety Center of the Japan Atomic Energy Agency. The researchers planned to investigate contamination of water, bottom sediments and vegetation; the wind-driven transport of radioactive dust; seasonal variations in environmental parameters; and the spatial distribution of radionuclides.

This type of research goes beyond simply measuring ambient radiation levels along the shoreline. Water may contain certain concentrations of radionuclides, soil and sediment others, and fish from different parts of the connected aquatic system still others. The research requires measurements of specific isotopes in specific samples, as well as an understanding of the pathways through which they migrate. Even if a dosimeter records a relatively low background radiation level at a particular location, this does not provide a comprehensive assessment of contamination throughout the entire water body or of the risks associated with contact with bottom sediment.

A study published in Scientific Reports in December 2025 examined radioactive isotopes in cyprinid fish — common carp and roach — collected from Atomic Lake, Lake Shagan, the Shagan River and another nuclear explosion crater, No. 101. The research team was led by Andrey Panitsky. The analysis covered caesium-137, strontium-90, plutonium isotopes, americium-241 and tritium. This research is no longer concerned with the search for unusual exotic species, but rather with determining which fish organs accumulate different radionuclides and under what environmental conditions.

The researchers found that isotope distribution depends on the chemical properties of each radionuclide. For example, strontium is predominantly associated with bone tissue, whereas caesium is detected in muscle and other examined tissues. Within the study sample, the highest levels of several radionuclides were recorded not at Chagan, but in Crater No. 101. At the same time, the absence of a statistically significant difference in one particular indicator between fish from certain water bodies does not mean that the entire shoreline can be considered safe for recreational use. The authors emphasise the need for continued monitoring.