Mobile application: Data analysis on radioactive sites in the Chernobyl Exclusion Zone

The R&D team SoftChapter is currently working on the application – analyzing and displaying data on the radioactive sites in the Chernobyl Exclusion Zone (hereinafter referred to as the CEZ). According to the data we collected and on the basis of mathematical calculations, the application shows information about a specific location in the Zone, which includes the following aspects:

Background

I have long wanted to visit the CEZ (Sergii Trizna, co-founder of the company). My father was a liquidator. A few days after the disaster, he was sent there to deliver cement to the destroyed reactor. I often asked him about it when he was alive, and since then my interest in this issue has grown. “Why not make an application that will display data in a clear form about the radiation pollution of the Chernobyl Zone?”, I thought.

People visit these places, but some neglect their own safety without knowing what radiation is. In addition, it is an excellent challenge to explore something new related to physics. Our team of 11 people went to the CEZ on March 30, 2019. It was an expedition for several days.

We had a unique route. Apart of the main interesting places: Pripyat, the nuclear power plant, the Jupiter Factory and the Red Forest; we visited the village of Staroselie, Krasno, Zimovishche, Mashevo (northern radiation trail, the border of Belarus). We collected information on the most contaminated by radionuclides sites in the 30 Kilometer Zone that we managed to visit (using GPS coordinates and dosimeter data). So far there are several dozens of places where we carried out measurements. This data will be sufficient for the first version of the application. Based on the data collected, a great deal of information we read, and consultations with physicists, we decided to make the first prototype.

We would like to share with you the information about radiation that we studied in the context of our research. After the disaster at the Chernobyl nuclear power plant (April 26, 1986), the territory around hundreds of square kilometers became contaminated with radionuclides.

What are radionuclides? These are atoms whose nuclei are unstable and undergo radioactive decay – a spontaneous change in the structure of the nucleus. As a result of the spontaneous fission, various isotopes emit three types of ionizing radiation:

Alpha radiation. Particles flow is located not far from the source of radiation. In case of transmission by air, it is from 2 to 11 centimeters (depending on the energy of the particles). Alpha particles penetrating power is weak, so a hazmat suit can protect against this kind of radiation, even ordinary clothing is sufficient. However, in close contact with the human body, the particles penetrate into the body to a depth of about 1 cm. Alpha decay is typical for heavy elements from the periodic table (with a mass > 200), such as plutonium-239 and uranium-235.

Gamma radiation. Gamma particles are a stream of high-energy, non-charged photons. Unlike alpha and beta rays, gamma particles have a greater penetrating power, so it is much more difficult to protect against them. This type is similar to X-radiation. The list of the main radionuclides formed after the disaster at the nuclear power plant is as follows:

Strontium-89, 90
Ruthenium-103, 106
Plutonium-238, 239, 240, 241, 242
Iodine-131, 133
Caesium-134, 136, 137
Americium-241
Zirconium-95
Molybdenum-99
Tellurium-132
Barium-140
Cerium-141, 144
Neptunium-239
Curium-242

Each of these isotopes, during the decay, emits ionizing radiation of one of the types described above. Each element has a half-life, the time during which half the radioactive nuclei of the original substance decay.

Some isotopes have a relatively short half-life. For instance, yodine-133 has a half-life of 20 hours, yodine-131 – 8 days (in 8 days half of the original substance decays). Therefore, some radionuclides decayed long ago and were dangerous only in the early days after the accident. As a rule, radionuclides with a short half-life have very high activity. Caesium-137 radioactivity may not be as high initially, but it decreases much more slowly (its half-life is 30 years).

Effects of radiation exposure on the human body

Iodine-131 (or radioiodine) easily enters the human body with air, food and water, also can pass through the skin. It is mainly absorbed by the thyroid gland, which does not distinguish stable iodine from its radioactive isotopes. That is why the intake of Potassium iodide supplements was crucial for the residents of the Chernobyl-affected areas. These supplements contain stable iodine, which temporarily saturates the thyroid gland, thereby preventing the absorption of radioactive Iodine. The more energy ionizing radiation transfers to the tissues of a living organism, the greater the damage is. The amount of this energy is called the dose, by analogy with any substance entering the body. The organism can receive the radiation dose regardless of whether the radionuclide is outside the body or inside it.

The quantity of radiation energy absorbed by irradiated body tissues, per unit of mass, is called the absorbed dose. The SI unit of measure is the gray (Gy). However, this value does not take into account the fact that with the same absorbed dose, alpha radiation is 20 times more dangerous than beta or gamma radiation (as has more devastating consequences for the body). The dose recalculated this way is called the equivalent dose, and is measured in units called the Sievert (Sv). The process of radiation exposure on the body is called irradiation. This is an extremely destructive force that transforms cells, disrupting biological processes in them. Different isotopes and types of radiation affect different human organs. For example, strontium-90 is accumulated mainly in bone tissue. It should also be considered, that some body parts are more sensitive to radiation than others. For instance, with the same equivalent dose of radiation, cancer is more likely to occur in the lungs than in the thyroid gland. Many isotopes affect the bone marrow, disrupt the formation of new blood cells.

Why do we often refer to the risk of cancer after receiving a dose of radiation? What is cancer? In simple terms, this is an “error” occurring as a result of cell division. The higher the rate of cell division in a particular place is, the higher the risk of cell division error is, subsequently the higher the risk of cancer in that place is (refers to chronic inflammations that take a long time). Since radiation exposure disrupts the process of cell division, the risk of developing cancer increases. Doctors believe that small doses of radiation activate the system of biological protection of the body. Though the big ones destroy and kill. Radiation sickness is diagnosed at doses above 250 millisievert. Doses of 3 and 4 Sv are called the “half-deadly dose”. If left untreated, half of those irradiated die having received such a dose. Let’s take a look at how the application works using a real case example. The place that we are going to explore is the medical unit №126, Pripyat. The item we are examining is a radionuclide-contaminated object – the fire hood of a firefighter who extinguished the exploded reactor.

Location coordinates – 51°24’22.1″N 30°03’55.5″E

“Caution! Radioactive material from the hospital basement in Pripyat. Danger to life!”

What dose can be received near the radiation object?

Our measurements showed that, in close contact, the object emits beta radiation at 80,000 micro-roentgens per hour, which is 1600 times higher than the acceptable threshold of the radiation rate. 80000 microroentgen per hour = 0.8 millisievert per hour. Radiation sickness will be obtained if staying more than 13 days near the object. For instance, if the source emits ionizing radiation as well as beta and gamma radiation, the equivalent dose will be recalculated. Caesium-137 decomposes to barium-137, which, in turn, emits gamma radiation. This element is stable and has a half-life of 2.5 minutes but is toxic to the body. The intensity of gamma rays determines the concentration of caesium-137 nuclei. I have often seen guides show tourists the level of radiation at such places. They keep the dosimeter next to the source for several minutes. Go ahead and calculate how much time per year you spend next to such an object (we will add the calculation of the dose received to the application).

Safe distance to the source of radiation

The range of beta particles in the air can be calculated, thereby determining the safe distance. It is known that air density ρ = 0.00129 g/cm3 (depending on pressure, temperature and humidity) and the maximum energy of caesium-137 beta particles is 0.51 MeV (megaelectronvolts) at these values, the range of particles in the air will be up to 1500 cm. The safe distance to this object of radiation > 1.5 m. However, it should be borne in mind that strontium undergoes decay into yttrium-90 (that is even more dangerous than its parent isotope). It has decay energy of 2.28 MeV and the range of particles in the air can reach a distance of 7-8 m. Barium-137 emits gamma radiation, which travels at a higher distance than beta (but decays meeting a concrete wall), which means that in this room there will be decay particles everywhere. Given the activity, energy and decay time of these elements, it can be said that a relatively safe distance to this radiation object is > 7m.

As the distance to the object of radiation increases, the surface emission in the air decreases. Hence dosimeters may not detect particles, but this does not mean that you are at a safe distance. According to the particle energy calculation formulas (depends on the reference data for some materials or is determined by the time that a particle spends on flying a certain fixed distance in a particular environment), we can say how deep a particle penetrates a living organism (for example, through the skin) which can lead to tissue burns or cataracts.

How long will it take until this place becomes safe for humans?

Taking into account the fact that this is β-decay, one can say with great confidence that these are isotopes of caesium-137 (half-life is 30.1 years), or strontium-90 (half-life – 28.7 years) that affect the area the most. At the time of the disaster, there was formed about 8 times more caesium than strontium. As a result of calculations of this and other data (mass of the object, etc.), it is possible to calculate that in some number of years its radioactivity reaches a certain background level and this object will no longer pose a threat to life of living organisms. If the initial mass of the cesium-137 object is 50 g, in 30 years 50% will decay and 25 g will remain. In another half-life (i.e. 30 years), 12.5 g of the starting material will remain, and so forth. Here we need to understand that the decreasing exponent tends to zero asymptotically and one should not do the calculation for a zero value. Though it is enough to calculate how long it will take for the isotopes activity to fall, for example, to the level of granite radioactivity. Omitting the formula for calculating the decay time of a substance, we get the result of 490 years. This place will be safe in the year 2510.

Our goal is to analyze more accurate data

With this application, we want to show an idea, that will help perform data analysis and have a clear picture of pollution in the CEZ. Why conduct the analysis based on assumptions? The margin of error will be high, such isotopes as americium-241 are very difficult to detect without special equipment and laboratories. Its half-life is 432 years, i.e. this isotope will remain in the Zone for tens thousands of years. In collaboration with the Nuclear and Radiation Safety Inspectorate, we would able to collect more accurate data on all isotopes. We are also planning to add to the application the display of real-time data via Bluetooth with a dosimeter, and recalculation of values, record of visited sites and estimation of the dose received in the zone. These features might be included in the future release of the application. Our team is open to suggestions, so feel free to reach out to us with your ideas of what you would like to be implemented in the app.

We are thankful to those who sacrificed their health and lives to eliminate the consequences of the disaster

When my father came home after a two-week mission to Chernobyl, he wasn’t even told to throw away the work wear. Since the beginning of May, he had been wearing it for 2 weeks, brought it home, later it was washed with other clothes – mother says. The first equipment that participated in the liquidation was allowed to leave the Zone for the former work station. Only 2 or 3 weeks after arriving home, father was informed that the cement truck should be disposed of. Though the truck stayed in the storage shed for several weeks and he continued working on it.

P.S. I would like to thank the directors and producers of HBO for having done such impressive work and filmed the Chernobyl series. Much of what is shown in the film reflects the stories my father told me.