Until now, in the Energy section, we have discussed various sources of energy, but we have scarcely dealt with nuclear power. This is a controversial topic and it is technically difficult, but I believe it is important to form an opinion on this subject, particularly regarding the situation in our country. Our collaborator, Alexandra Dumitrescu, wrote last week an introductory article on nuclear energy in general. I propose that today we take a look at Cernavodă in particular.
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
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
