Nuclear power plants have been around for over half a century, the first one that began to generate electricity for commercial use was built by Russia in 1954. The first plant that used enriched uranium in its reactor and pressurized water as a cooling agent was inaugurated in Shippingport, Pennsylvania, on December 2^nd 1957, and had an output of 60W.

On January 1^st 1988, there were 417 operational nuclear reactors in 26 countries, with a total installed capacity of 29700 Mwe, and another 120 reactors were under construction. In 1990, the power generated by nuclear plants amounted to 20% of the world energy production.

Photo 1. Cernavodă nuclear power plant

Photo 2. Fukushima nuclear power plant

How does it work?

A nuclear power plant generates electricity by using the fission of uranium atoms, which produces high temperatures that heat water which then turns to steam, and this steam is used to turn the blades of turbines that power electricity generators.

Atomic fission is the process through which the nucleus of a uranium atom is split in two (a krypton atom and a barium atom), releasing a huge amount of energy in the form of heat.

Uranium is the heaviest element found in nature. More that 99% of all uranium is U-238 (238 meaning it has 92 protons and 146 neutrons), and 0.7% is U-235.

One gram of U-235 uranium has 2.562.553.191.489.360.000.000 atoms. Therefore, the fission reaction must be controlled in order generate just the amount of heat desired. The control is achieved with the help of heavy water, which slows down the neutrons. Additionally, rods of barium or cadmium are inserted into the reactor container to absorb neutrons and to control their concentration, keeping the power produced by the reactor at a constant level over time. If the neutrons released during the fission reaction are slowed down, the probability of an atomic collision that generates heat increases. In this way, it is possible to maintain a fission chain reaction that multiplies the energy produced.

Opinions are divided over the issue of nuclear power stations. Some people claim that they are unreliable and affect the health of people, while others have opposing views. It is true that a nuclear power plant can generate more electricity than a hydroelectric facility, a thermal power station or a petrol-burning power generating plant (a single ton of uranium produces more energy than 12 million barrels of oil). But in the event of a malfunction (a fissure or an explosion at a reactor), the damage caused to the environment and to human health can be immense.

Examples of such accidents are the explosions at Cernobil (in northern Ukraine) and Fukushima (situated in eastern Japan), which have polluted the environment on a radius of thousands of kilometres, or even more, depending on the speed and direction of the winds. The Cernobil explosion (which occurred in 1986) had consequences that can still be seen today. Many people have died or will die from cancer caused by this disaster. The level of radioactivity around Cernobil is still very high.

To a certain degree, nuclear energy is clean and does not pollute the atmosphere, but this cleanliness concerns only the process of generating electricity. As a residue of this process, we are left with nuclear waste, which has to be stored in concrete or titanium structures, often for centuries, until it will become harmless.

After the Fukushima accident, caused by a tsunami wave in 2011, the European Council decided that ,,the nuclear safety of all nuclear power plants in the EU must be revised, on the basis of a transparent and broad system of risk assessment – the Stress Tests”.

According to the NGO Terra Mileniul III, the stress tests were carried out superficially, without a clear methodology and the recommendations issued by the European Commission after one year of analysis are not legally binding. This situation also applies for the specific structures of Romania.

The effects of radiations   

The interaction between radiation and living or non-living matter is basically a transfer of energy.

The use of radiation in controlled conditions can have positive effects on humans. Radiation is used in medical treatments (for destroying cancer cells), in diagnostics (X-rays), in the food industry (to preserve foodstuff), in the pharmaceutical industry (for sterilising medical instruments) and more.

On the other hand, radiations can cause significant cell damage.

The biological effects of radiations can be classified in the following manner:

• Somatic effects, that appear at cell level and impact on the physiology of the individual subjected to radiations, causing damage that can lead to death or a decrease of the life expectancy.
• Genetic effects appear in the germ cells, thus causing mutations in the descendants.

One solution to these problems would be to use renewable sources of energy, which do not pose a hazard to the environment or to human health and require significantly lower investments than nuclear energy.

What do you think about nuclear power plants? Are you for or against?

Article written by Alexandra Dumitrescu, Greenly collaborator, and translated by Mitoşeriu Mihail-Andreas.

Sources:

http://www.scientia.ro/tehnologie/39-cum-functioneaza-lucrurile/745-cum-functioneaza-o-centrala-nucleara.html

http://www.ecomagazin.ro/centralele-nucleare-un-pericol-real/

http://www.ecomagazin.ro/reactoarele-de-la-cernavoda-controlate-de-mantuiala/

http://www.et.upt.ro/admin/tmpfile/fileH1330943017file4f549429e2463.pdf

http://www.nuclearelectrica.ro/user/content/12f011_radiatiile_si_centrale.pdf

Sources photo:

http://www.dcnews.ro/2011/03/wikileaks-centrala-nucleara-de-la-cernavoda-nu-este-foarte-sigura/

http://www.filtersfast.com/blog/wp-content/uploads/2011/12/Fukushima.jpg

Let’s begin with the easy part:

History of CNE (Centrala NuclearElectrică – Nuclear Power Station) Cernavodă  

-          1979 – Construction works begin

-          1996 – The first reactor becomes operational

-          2007 – The second reactor becomes operational

According to www.cne.ro, each reactor at Cernavodă nuclear power station provides 706, 5/Mwe to the National Energy System, thus covering about 18% of the present national energy requirement.

CNE Cernavoda. Sursă foto: www.nuclearelectrica.ro

How it works

The Cernavodă Power station uses Canadian technology, which is based on the CANDU (CANadian Deuterium Uranium). As the name suggests, such a reactor operates with natural uranium and heavy water (a chemical compound of deuterium) as a moderator and cooling agent.

Deuterium is a stable isotope of hydrogen, with a mass number of 2 and is represented by the chemical symbol D or ²H. Heavy water (D₂O or ²H₂O) differs from ordinary water by the fact that it has in its composition the isotope deuterium in a greater proportion.

Operating a nuclear power plant is based on initiating and maintaining a controlled nuclear fission chain reaction, a process which can be achieved in a nuclear reactor. The core of the reactor contains the fuel and has the shape of a cylindrical vessel called a Calandria vessel. The nuclear fuel used is made up of synthesized nuclear dioxide pellets, which are inserted into rods made of zircaloy. Thirty-seven such rods form a fuel bundle. The fuel bundles are, in turn, inserted into pressure channels (there are 380 channels with 12 bundles each). In total, the core of the reactor holds about 90 tons of nuclear fuel.

Combustibil nuclear. Sursă: raport de mediu 2010, nuclearelectrica.ro

In brief – the Calandria vessel is composed of 380 pressure channels, each one holding 12 fuel bundles, and each fuel bundle being made of 37 fuel rods of synthesized uranium dioxide pellets, which totals 90 tons of nuclear fuel.

The fission reaction taking place inside the pressure channels generates heat, which is absorbed by the heavy water running through the fuel channels. The heavy water then transfers the heat to the steam generators, which, as their name suggests, create water vapors (from ordinary water) that expand and drive the turbine and create electricity.

When it exits the turbine, the steam is condensed inside the condenser which is cooled with water from the Danube – Black SeaCanal. The water used to cool the condenser is then evacuated back into the Danube.

Safety  

In case there is a technical malfunction, the reactor can be quickly shut down by means of two safety stopping mechanisms which operate independently (using different functioning principles). Additionally, the parts of the reactor where the fission reaction and the generation of radioactive materials occur are situated inside a sealed structure made of prestressed concrete called the containment building.

Another safety feature is the fact that every control element of the power station has a backup system which can take over its function in case of a breakdown.

Environmental protection 

The main effects on the environment are created by the evacuation of the water used for cooling (which has a high temperature that can harm the ecosystem of the Danube) and the radioactive waste. The radiological impact (measured in terms of dosage received by the population) proved to be negligible (a supplementary dose of 10 μSv/year – five times less than the dose received after a taking an X-ray exam, according to the 2011 Environmental Report).

Sursă: raport de mediu 2011, www.cne.ro

It is important to mention that the Environmental Management System of the Cernavodă Nuclear Power Plant has received the ISO 14001 Certification in 2004, and was recertified in 2010, after the addition of Reactor number 2. This means that the measures used for protecting the environment are well prepared.

a) The cooling water

The use of cooling water is the main difference between a CANDU – type uranium based reactor and a thorium – based nuclear reactor (which is, in many aspects, much more efficient). But the differences between the two types will be discussed another day. Here, you can read more about thorium – based reactors.

Cernavodă is authorized to use for cooling the waters of the Danube, collected through the Danube – Black SeaCanal, and released back into the river after having cooled down the reactors. The volume of water taken from the river and the temperature of the hot water dumped back into the Danube are the two main factors that can impact the aquatic environment. This is why these two parameters must be closely monitored and the conditions required for this activity are laid down in the ,,Agreement regarding the methodology employed for monitoring the usage of the water resources and the returning of the used waters into the water resources” concluded between the Water Administration Agency – Dobrogea-Litoral Constanta and CNE Cernavodă (which is an integral part of the water management authorization). According to the 2010 Environmental report, there were no violations of the conditions imposed by the environmental authorization.

b) Chemical and radioactive emissions in the water

Concerning these aspects, the evacuation of water into the river can be stopped by a special mechanism, in case certain parameters are exceeded, thus avoiding to surpass the maximum limits allowed for radioactive and chemical emissions. In addition, 1400 water samples are analyzed each year by the Dosimetry Laboratory in order to determine the amount of radioactivity evacuated. The 2010 Environmental Report states that the maximum limits for emissions have never been exceeded during the entire period the power plant has been active. Also, neither the emissions of non-radioactive substances have ever risen above the maximum allowed concentrations.

c) Radioactive waste  

These waste materials are the result of the daily operation of the plant, such as maintenance or repair works or interruptions. Such substances are dangerous and require special handling. They present themselves as solids (filters, plastics, and glass), organic liquids (oils, solvents) or liquid-solid flammable mixtures. Liquid waste is solidified to reduce the risk of a fire and some solids are compacted so that they take up less space. All waste materials are sorted following certain criteria, stored in stainless steel containers and then shipped to an intermediary storage facility. Often, less radioactive materials are transported (in exchange for a fee) to countries that possess technologies to treat the waste (such as Sweden – for cremation – but this process only reduces the volume of the waste materials, and the remains are then sent back for final storage). In the final storage phase, the waste is placed in compact molds that guarantee safe storage for 300 years. An important institution involved in the final storage phase (but also with important attributions in the field of nuclear energy) is Agenţia Nucleară şi pentru Deşeuri Radioactive – ANDRAD (The Nuclear and Nuclear Waste Agency – www.andrad.ro).

Another interesting problem is the intermediary storage of the spent fuel. This is done for a period of at least 50 years in a special deposit (Depozitul Intermediar pentru Combustibil Ars – DICA – The Intermediary Deposit for Spent Fuel). Here, several physical processes occur which help keep the bundles of nuclear fuel in safety (that is without any gaseous or liquid leaks).

DICA- Depozit Intermediar pentru Combustibil Ars. Sursă: nuclearelectrica.ro

d) Atmospheric protection

For all nuclear power plants, there are no CO₂ emissions. Replacing the amount of energy produced at Cernavodă with energy obtained from fossil fuels would increase the annual national CO₂ output by 10 million tons (according to www.cne.ro)

Emergency situations                

Canadian – designed nuclear power stations are known for their safety, and have the technology necessary to reduce the risk of accidents. Still, periodic drills are conducted in order to prepare the workers, and also the population, for an emergency situation.

Conclusion  

Judging from the data presented by NuclearElectrica on official websites and in the pages of environmental reports, one can affirm that Cernavodă is an important energy source that at the moment does not present any cause for concern. We can only hope that mother–nature will be kind to us, sparing the region of earthquakes and other unpredictable events which could pose a nuclear threat for the life of the population.

Bibliography  

www.cne.ro

www.nuclearelectrica.ro

www.andrad.ro

Environmental reports http://www.cne.ro/m/aspx?id=86&it=2    

Article written by the editor Ioana Stoicescu and translated by the associated editor of Greenly Magazine, Mihail Andreas Mitoseriu