Another anniversary of the Chornobyl disaster, the largest man-made disaster of the twentieth century, is approaching. In addition to Ukraine, Belarus, and Russia, most European countries have experienced its consequences to varying degrees. In the years since then, the causes that led to the Chornobyl tragedy have been thoroughly studied by scientists and experts in the operation of nuclear power plants. Technologies to minimize the impact of nuclear facilities on the environment and human life were also actively developed. In other words, the disaster forced a significant revision of approaches to the organization of safety measures at nuclear power plants and their standards.

After the Chornobyl disaster and several decades later, Igor Sikorsky Kyiv Polytechnic Institute has been actively conducting research and education in the field of nuclear safety. Vadym Kondratiuk, the university's vice-rector for administrative work, is also engaged in this topic as a scientist. He has recently successfully defended his doctoral dissertation on "Improving Safety of Nuclear Power Plants by Qualification and Modernization of Accident Management Systems". On the eve of the next anniversary of Chornobyl, Vadym Anatoliiovych talked to a correspondent of Kyiv Polytechnic about today's safety problems at nuclear power plants.


- What has changed since Chornobyl in the field of organizing safety measures at nuclear power plants?

- Of course, after such a large-scale nuclear disaster, great efforts of scientists and scientific institutions around the world and, in particular, in Ukraine were aimed at solving the issues of predicting both design and beyond-design accidents at nuclear power plant sites and effectively counteracting them. Conclusions were drawn, relevant calculations were made, instructions were issued, certain experiments were conducted, data was collected, and new protection systems, both active and passive, were introduced. All research is aimed at improving safety, primarily by modernizing equipment, safety systems, and improving the design of reactor facilities. Such work is being carried out not only at RBMK reactors, but also at VVER reactors operated at the sites of 4 NPPs in Ukraine - Rivne, Khmelnytsky, Zaporizhzhya, and South Ukraine. Systems have been implemented to manage severe accidents involving radioactive releases to the environment. Calculations of probable accidents at the sites of all Ukrainian NPPs are being carried out. The IAEA has a database called the Evaluated Nuclear Data File, which receives experimental data and research results from all world NPPs to assess how power units behave in certain accidents and how a radioactive isotope behaves when it is released from a sealed environment into the environment. Today, a large number of research nuclear reactors have been built around the world to model initial events for accident processes that can be considered design basis for nuclear power plants. 

This work became more effective when experts began to use deterministic codes. This is a computer modeling that takes into account various systems of equations, initial data and allows for certain analyzes and qualifications of accident management systems. However, these codes have drawbacks that are explained by what experts call "code difference and user difference". Code difference is the use of different software products that contain different systems, mathematical models and differential equations, which, based on the results of calculations, can produce data that will have discrepancies. And the difference between users is a common human factor, i.e., the dependence on the level of knowledge of the user who uses the software product. For example, the user may not take into account a certain criterion or some parameters of the nuclear reactor core, and this will affect the final result and conclusions drawn from the calculations. The codes have proven themselves quite well, but, in my opinion, work should be done to prioritize them. There are universal codes that fully model the operation of the entire power unit, and there are codes that model the thermophysical processes and parameters created in the core of a nuclear reactor.          

The second surge in the intensity of work in the field of NPP safety measures occurred after the Fukushima disaster, where a severe nuclear accident with a release of radioactive material into the environment also occurred. And it was a projected event, because no one had calculated a 14-meter-high tsunami. 

- Were the lessons of Chornobyl already taken into account?

- Of course, the lessons of the Chornobyl disaster were taken into account, both in Ukraine and around the world. One of their main aspects is the human factor. According to many scientists, during the operation of ChNPP, insufficient attention was paid to retraining and professional development of the skills of the plant's personnel, as the power units had been operating in an accident-free mode for a long time. Therefore, the need for continuous training, preparation and retraining of personnel was confirmed. Taking into account the experience of accidents that occurred at nuclear power plants in Ukraine and around the world, the largest training center in Europe was built at the site of the Zaporizhzhya NPP, which is currently occupied by the aggressor. This center fully recreates one circulation loop in full-scale dimensions of a VVER reactor, which makes it possible to better train operating, maintenance and engineering personnel for NPP operation, reduces the time for repair, preventive and scheduled maintenance, and improves the skills of operators who control the nuclear facility at the control room.

- All the measures envisaged by the updated safety protocols were designed for peacetime, or for the fact that even in the event of military operations in some countries, no one would think of conducting them on the territory of nuclear power plants. As far as we know, this is also defined in international nuclear law. However, the war unleashed by Russia against Ukraine has clearly demonstrated that the aggressor state blatantly ignores all safety laws and regulations and, moreover, uses its presence at the seized Zaporizhzhya NPP as a tool of nuclear blackmail. What can this mean for Ukraine and Europe?

- Indeed, no one expected that there could be any unforeseen events related to the conduct of war at the NPP. And yet, while working on my dissertation, I found out that there were several scientific studies in the world that considered the operation of nuclear power plants during the war. And today's war has prompted me to think that the deliberate destruction of a nuclear power plant site and equipment should also be taken into account. Therefore, in one of the chapters of my dissertation, I considered the issue of operation of nuclear power plants in extreme conditions, i.e. during hostilities. I considered several scenarios.  

The first one is deliberate destruction of NPP protection barriers by the aggressor. These are the barriers that prevent the leakage of radioactive products into the environment beyond the NPP site. The second factor is no less important - the destruction of critical infrastructure that ensures stable operation of the nuclear reactor around the power unit site. This refers to diesel generators and distribution transformer substations that supply the power unit. The third factor is related to the fact that as a result of the explosion of the Kakhovka HPP and the possible explosion of the Dnipro HPP cascade, water may rise to such a level that the Zaporizhzhya NPP industrial site will be flooded. Given that this plant houses a spent nuclear fuel storage facility, there is a possibility of radioactive fission products leaking into the liquid environment. And then the water will go into the soil, rivers, lakes - the impact on the environment and living organisms will spread. And the last factor considered is the ingress of warheads into the cooling pond, where water is taken to cool the heat exchange equipment of the power unit. This could lead to a change in the energy balance, overheating of the nuclear reactor and core, and, unfortunately, quite possibly to a nuclear disaster.  

For most of us, the consequences of such a development are clear, because we have a bitter example of the Chornobyl disaster, which resulted in a large number of people receiving an excessive dose of radiation, which had a very negative impact on their health. Moreover, no one in the world is immune from the consequences of the disaster: radioactive dust that is released into the air moves with air masses and moves with the wind. And anywhere in the world, at any given time, this dust settles on the ground along with precipitation. It is impossible to predict and model anything because it all depends on the time of year, wind speed and intensity, and other factors. But the consequences for humanity can be large-scale and extremely negative. The half-life of heavy radioactive elements is decades. After the accident, the industrial site around the nuclear power plant and the surrounding 30-kilometer zone, as we can see from the example of Chornobyl, become an exclusion zone that is uninhabitable and turns into an abandoned territory.

- Is there an understanding of what should be done to prevent another nuclear disaster? What should be done not by politicians or international organizations, but by nuclear experts?

- In my opinion, first of all, work should be done to improve and modernize in-reactor monitoring and diagnostics of the nuclear reactor itself. During the war and during the destruction of protection barriers, the main task of the operating personnel is to bring the nuclear reactor to a controlled state, i.e. to ensure the removal of heat generated in the core of the nuclear reactor. The second thing is, of course, to qualify safety systems for beyond design basis accidents. Accordingly, they should be divided into systems that can operate during design basis accidents and that can be used to bring the reactor to a controlled state and stabilize its operation, and systems related to the restoration of power supply. This can be an emergency supply from diesel generators or from special batteries, capacitive batteries, which can also operate for a certain period. In other words, it is necessary to qualify and modernize the emergency management systems at NPPs.

Of course, it is also necessary to improve the safety systems involved in managing severe accidents. Appropriate symptom-oriented instructions for managing power unit accidents associated with complete and prolonged blackout should be developed. It is also necessary to prioritize the deterministic codes that are used to calculate accidents that may occur at NPPs and their consequences.

- Judging by the fact that you prepared your thesis at Igor Sikorsky Kyiv Polytechnic Institute, our university also conducts research in this area. What can you tell us about them?

- In order to conduct research and train personnel to work at NPPs, the first in Ukraine Scientific and Training Center "NPP Reliability and Safety" was established at the Igor Sikorsky Kyiv Polytechnic Institute with the support of the US Department of Energy and the US Argonne National Laboratory. 

The analytical simulator installed at the Center is used to conduct research related to the safe operation of nuclear power plants in Ukraine, to model various design-basis accidents that may occur at a nuclear power plant during operation, and to calculate the performance of the first and second circuit systems of a nuclear power plant to improve reliability in emergency and extreme operating conditions. The analytical simulator even makes it possible to make certain predictions regarding the operator's actions to prevent steam and gas explosions, return the nuclear reactor to a controlled state, restore power supply, etc. It also allows to make predictions on what measures should be taken to improve NPP safety. 

All this is called modernization and qualification of accident management systems under extreme operating conditions. "Modernization" means, of course, improvement, replacement, and equipment. And "qualification", as defined by the IAEA, is a calculated or experimental confirmation of the performance of a system important for safety in severe accidents under various operating conditions. 

Therefore, Igor Sikorsky Kyiv Polytechnic Institute is actively working in this area, their results are published in articles, research materials are transferred for further implementation directly to NPPs, to the Energoatom Company. For the first time, such work was carried out for Zaporizhzhya NPP, and now the calculations are being carried out for all Ukrainian NPPs, because all of them are currently at risk due to missile attacks. We see how the enemy is trying to destroy critical energy infrastructure, and in order to prevent radioactive releases, we need to make informed decisions.

Volodymyr Shkolny said

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