When someone talk about Nuclear technology for sure the topic of radioactive waste comes out. But what is radioactivity? how dangerous is it and how we deal with radioactive stuff? In this page i will try to answer these questions.
Contents:
what is radioactivity?
Some nuclei are unstable and eventually they will decay to a stable state emitting some kind of particles. Main examples of radioactive decays are: \alpha , \beta \gamma . In order these decay emits \alpha particle (Helium nucleus), \beta particle (electron or proton) and \gamma photon. Typically the radioactivity of a material is measured through it’s Activity, that is the number of decay per second (Becquerel Bq).
These radiations can induce some damage to living organisms, for this reason radioactive sources must be handled with care. There are two main aspect that guarantee the safety, that are distance and shielding.
Higher is the distance between you and the radioactive source, lower is the amount of radioactivity that will hit you.
More obstacles are present between you and the source higher is the attenuation.
\alpha particles for example cannot travel long distances in air and are easly stopped by matter, for this reason this kind of radiation is less dangerous than the others.
To evaluate the damage induced to organims we cannot rely on Activity, because different radiations have different effects. So Scientists introduced the Dose (Sievert Sy) which is a quantity what take into account both the activity and the damage on the organism. Higher is the Dose higher is the possible damage.
Now let’s answer the question: what is radioactive?
Answer: Almost everything. From rocks to bananas, every material contains a certain amount of radioactive nuclei, in bananas for example we find a lot of potassium, and within the normal potassium there is a certain amount of potassium – 40 which is a radioactive isotope. Potassium is present also in people, this mean that also your friends are radioactive. Now you have another excuse to avoid social life, you can thank me later.
Fun fact: we can use bananas as unit of measure for radioactivity adopting the banana equivalent dose.
Radioactive exposure near plants
As we said before, almost everything is radioactive. Depending on where you live you are subjected to a certain amount of environmental radiation exposure. The average dose is 2.6 mSv/y (milli Sievert per year). To understand how much is this value we can compare it with some sources of radiation:
- Eating a banana: 0.1 \mu Sv (1 micro Sievert = 1/1000 milli Sievert)
- Plante flight from New York to LA: 40 \mu Sv
- Chest X-ray: 0.2 m Sv
Now that you have an idea of radioactivity sources is time to answer the question: what is the radiation exposure near a nuclear power plant?
Answer: On average, living for an year within 80 km from a nuclear power plant will bring to you a dose equivalent to eating a banana. In other words the effect of radiation emitted by a NPP is 10000 smaller than the environmental one.
It’s interesting to compare this value with the dose exposure near a coal plant. It might surprise you that near these plants the dose is 100 times higher than close to NPP, this because in coal is present a certain amount of Uranium and Thorium which are released in atmosphere after combustion.
Ah! I almost forgot: if you are an average (cigaretts) smoker you are exposed to a dose in between 150 and 400 \mu Sv every year! so stop Smoking (we could say you are more radioactive than a nuclear power plant).
waste treatment
During operation, fuel and materials inside a nuclear reactor undergoes activation, aka becomes radioactive. Waste is classified accorting to it’s radioactive intensity:
- Low Level Waste LLW
- Intermediate Level Waste ILW
- High Level Waste HLW
From a Fission reactor, about 97% of radioactive waste is LLW and ILW, this mean that the “most dangerous” part (HLW) is just the 3% of total.
Since this waste remain radioactive for long time we have two options.
Option one: Waste deposit
Radioactive stuff is compressed and solified in inert impermeable matrix, which take the name of manufact. Some manufacts are placed in an appropriate barrel and drowned in impermeable concrete. Multiple barrels are placed in a cell and drowned again with concrete. The philosophy is to put multiple impermeable barriers in order to avoid liquid infiltrations which can trasport radioactive materials.
In the picture below you can see a scheme of a classical near surface deposit:
Near surface deposits are adopted only for LLW and ILW, instead for HLW are displaced in geological sites deep ungerground since their radioactivity last for thousands of years.
Note: Radioactive waste deposits are a MUST not only for power plant wastes, but also for Industry, medicine and research activities and renewables.
Option two: Reprocessing
Instead of treat the spent fuel as a waste is possible to use it once again in so called fast reactors which are able to consume most of the radioactive part of the fuel. Doing so, we are able to reduce the amount of HLW and the time required for the storage, from thousands to hundreds of years.
conclusions
If after reading this page you turn to nuclear side congratulations, Enrico Fermi is proud of you, if not hold on, i have just few more words to say. When we analyze something we have to compare it with other options, so if you don’t like how radioactive waste is treated you should ask yourself: “how other technologies deal with waste?” because also wind and solar field produces toxic waste and quality standards are usually worse than in nuclear field, now you know…