Radioactivity in Chernobyl Zone

A short overview

Probably, you will agree that nearly the first thing one remembers in relation to Chernobyl Zone is radioactivity and radioactive contamination. It won't be an overestimation to call this place the world's biggest technogenic radioactive source, and same time, the most complex source. So when you go here, it is the best practice to have basic understanding of how it all works.

Let's start from the fundamental things to refresh what we all know from the school.

The phenomenon of radioactivity — a process of decay of nuclear materials followed by particle/wave emission — surrounds us every day. Mostly it is gamma-emission. Switch on the Geiger counter in your home town and you will see, that it will show you not zero (probably it will be something like 0.09 uSv/h). Why? There was no nuclear accidents here! So why it is not zero? The answer is that it is so called natural ambient radiation — it is normal and caused by many factors, such as natural presence of specific geological substances in the soil. The natural radiation varies from place to place, but generally it does not exceed 0.30 uSv/h for most places, though sometimes in some places it can be more than 0.50...1 uSv/h (we personally experienced this few times in mountains).

If you take plane, and fly on the height of 10 kilometers, the Geiger counter will show you something like 1.20...2.75 uSv/h. That's because you are being exposed to cosmic rays — which are also a type of radioactive emission that comes naturally from the space. Not even talking about technogenic emission — for example, when you take the CT or X-Ray check.

Basically, the way how the gamma-radioactivity levels measured, can be explained with the metaphora of speedmeter in your car. If you drive with a speed of 100 km/h, in one hour you will pass approximately 100 kilometers. Same with the Geiger counter — it displays you a power of a doze per hour. So if you stay on a place where it shows you 1.00 uSv/h, in one hour you get approximately 1.00 uSv of a doze of radiation.

In every country there are state standards on radiation safety. Here, in Ukraine, it is called NRBU-97 and it according to this document, safe level is 0.30 uSv/h. The annual doze for regular people must not exceed 1 mSv (1000 uSv), and for those, who work with radioactive materials, it is 20 mSv per year.

As you, as the visitor to Chernobyl Zone, from the point of safety regulations belong to population, your doze must not exceed mentioned 1 mSv per year even if you visit contaminated areas. So all routes for visitors are built in such a way to fit in this requitements. Typically, even during full-day tour to Pripyat, which is quite contaminate area, your doze per day will be approximately 1..2 uSv, which is pretty the same as 1 hour of flight.

Now let's shortly look to physics.

As we said above, in normal life you deal mostly with gamma-radioactivity (γ) — emission of electromagnetic waves with wave length less then atom size. As these are waves with no mass, they can travel much very far, losing half its energy for approximately every 150 meters. The most present gamma-source in Chernobyl Zone is 137Cs (Cesium-137).

Types of Radioactivity in Chernobyl

There are more types of emission — and many of them can be found across Chernobyl Exclusion Zone. Different elements can emit different types of radiation.

Alpha-radioactivity (α) is the emission of heavy particles, consisting of two protons and two neutrons. Due to their energy and mass, these particles can be stopped with any matter, and only travel a few centimeters in air. Alpha particles are unable to pass through the outer layer of dead skin cells, but are capable to cause cell damage if they get inside a body with food or air. For measuring alpha-particles, you need a special detector, and a unit is different — number of particles per minute per square cantimiter (particle * cm2 * min).
As for Chernobyl Zone, alpha-sources are quite rare.

Beta-radioactivity (β) is the emission of an electron. These particles have less mass than alpha, so they can travel further in the air, but can be significantly stopped by layer of matter (for example, thick layer of plastic, clothes, etc). The unit for measuring density of beta-flow is the same as for alpha one. In the Zone, the beta-emitting elements, such as 90Sr (Strontium-90) are widely present, therefore in the scope of safety it is required to have clothes that covers the body maximally.

Neuron radioactivity is the emission of neutron from the nucleus of atom. They can travel very far, and this type in fact is the only that can make any material radioactive by exposure. In the Zone this type of radioactivity, along with X-Ray emission is extremely rare and mostly can be found only inside the nuclear reactors.

Understanding the radioactive spots of Chernobyl

As we said above, radioactivity always has some physical source that emit waves or particles. In the scope of Chernobyl Zone, we deal with contamination with long-term living elements, that was caused by initial fallout from exploded Unit 4 of Chernobyl Nuclear Power Plant. It was spread by winds forming 3 major tracks — northern, towards villages of Krasno and Mashevo, western, towards Poliske town, and south (less powerful), towards the Dityatky village. These tracks are collection of spots of radioactive materials with different nature, density and power of emission.

Altough all the territory of the Zone is contaminated, it does not mean that straight behind the fence of the Exclusion Zone starts the high level of radiation. In fact, in most cases, unless you come into the inner, so-called 10-km subzone, the Geiger counter will show you level of radiation within acceptable levels — even in Chernobyl town it is mostly like somewhere in Kyiv, Vienna or New York. This caused by natural drainage of nuclear materials into the soil with rains and snow. So the picture changes if you take a spectrometer — a device that is capable to identify the precense of nuclear matherials. 

The overall configuration can be illustrated, for example, by 137Cs diagram — as darker, as higher the concentration in the soil:

Cesium 137 in Chernobyl Zone

From the practical point of view, the difference between spots' power and concentration can be very different sometimes on a distance of a meter. A quick example of such diverse hotspot is one of the yellow cabins of the 'famous' Ferris wheel in Pripyat. Being one of the common attractions, this thing actually is one of the most polluted things in the city, as it got a lot of dust from helicopters that were landing nearby during liquidation process.

Normally, the key elements for overall control of radiological conditions are 137Cs, 90Sr and trans-uranium heavy elements, such as 239Pu.

During our Chernobyl tour we explain how it works straight on location using survey Geiger-counters, that allow to display a proximity to particular hot sports.

A word on half-decay

It is often mentioned that half-decay of e.g. 137Cs is around 30 years — which means that after 30 years a half of initial amount of Cesium will remain on site. This sometimes leads to conclusion, that in 30 years more there will be no of this element, hence the land will become clean.

First part is right, second is not.

The problem is, that during decay, in most cases an element turns to another radioactive element, and the chain of these mutations can be quite long, before it will become a stable one — typically lead (Pb). For example, this how it looks with 137Te and 90Kr:

137Te  4s   137I  24,5s   137Xe  3,82s   137Cs  30,2y   137Ba (stable)

90Kr  32s   90Rb  24,5s   90Sr  28y   90Y  64h   90Zr (stable)

Not that bad, eh? Now let's look on 238U:

238U  4.468×109y   234Th  24,1d   234Pa  1.2min   234U  2.5×105y   230Th  7.5×104y   226Ra  1.602y   222Rn  3.8d   218Po  3.05min   214Pb  26.8min   214Bi
 19.7min   214Po  0.16ms   210Pb  22.3y   210Bi  5d   210Po  138d   206Pb (stable)

So as you can see, the first stage of decay of Uranium is being followed by more than a dozen of others, making overall transition to stable lead very long. Therefore, in the scope of scale of average human life duration, it is possible to say that major part of lands of the Zone won't be normally populated ever again.

Headline picture of our Geiger counter — courtesy of Pavel Floresku, 2017.

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