Fresh nuclear fuel
Fresh fuel includes approximately 4% of uranium-235 isotope. As natural uranium contains only 0.7% of uranium-235, prior to the production of the fuel cells the so-called uranium enrichment must be carried out. In the course of operation the contents of uranium-235 is reduced by the fission process. The Dukovany Nuclear Power Plant used fuel that has been designed for three-years use in the reactor (the so-called three-year fuel campaign). Presently, there is also fuel designed for the four-year fuel campaign.
Spent nuclear fuel
Fuel assemblies containing spent nuclear fuel removed from the reactor look the same as the assemblies containing the fresh fuel. They are clean and undamaged. However, there is a significant difference in radioactivity of substances contained in the fuel assembly. In the course of operation radioactivity increases from almost zero with a successive increase of the quantity of fission products in nuclear fuel. It is caused mostly by the fact that fission of one uranium-235 atom results in the creation of two unstable atoms of various elements that continue in conversion. Therefore, even after removing the nuclear fuel from the reactor, the nuclear transmutation and release of gamma radiation, neutrons and heat that must be removed continues.
What is the function of nuclear fuel? |Nuclear fuel contains a minor quantity of uranium-235 isotope. If the uranium-235 atom meets a slow neutron, it is fissured into two atoms of the lighter elements, namely into two or three fast neutrons. Practically at the same time, energy in the form of gamma radiation and heat is released. To increase or reduce the reactor output use a lifting and lowering control rod assembly.
Control rod assemblies
The control rod assembly height is approximately doubled in comparison with the ordinary fuel assembly that form the reactor core located in the very centre of the reactor vessel. The lower half of the control rod assembly is the same as the fuel assembly; the upper half is made from materials that absorb neutrons. If the control rod assembly is lowered into its bottom position, the part that absorbs neutrons is placed in the reactor core. When lifting up the control rod assembly, its lower half containing the nuclear fuel is gradually inserted into the reactor core, and results in increasing the reactor output.
Designed fuel charge
The designed fuel charge assumes use of the nuclear fuel in the so-called three-year cycle, which means that every fuel assembly operates in the reactor for the period of three years and then is placed into the spent fuel pool and substituted by a fresh fuel assembly. Every year approximately one third of the fuel assemblies in the reactor are reolaced.
The basic scheme of the fuel reloading was placement of the fresh fuel assemblies on the side of the reactor core, and their relocation towards the centre of the reactor core during fuel assembly replacements in the individual years. In an economical point of view (use of fuel), this scheme was not an ideal one. In addition, the fresh fuel assemblies provide a higher performance in the reactor core, and their location on the side of the reactor core was not ideal even in viewpoint of the radiation load of the reactor vessel (high neutron flows contribute to degradation of the reactor vessel). Improved fuel parameters enabled transition from a three- to four-year fuel cycle in 1997, and since 2003 the five-year cycle has also been successively started up.
Regulation of the reactor performance
In the course of the reactor operation the nuclear fuel is burning up (uranium contents in the fuel is reduced), and this needs to be compensated by reducing the boric acid contents (the so-called absorbent – i.e. the substance that absorbs neutrons) in cooling water. This process is called the long-term changes of the reactor performance.
The short-term fast changes of the reactor performance are carried out by means of a group of seven control rod assemblies. The control rod assembly consists of the fuel part which is the same as the ordinary fuel assembly, and the absorbing part which is of the same shape but made of boron steel.
Each control rod assembly is linked, by means of the inserted rod, with the electric drive located on the cover of the reactor vessel.
When the control rod assemblies are inserted downwards, the fuel part is lifted out of the reactor core, and the absorbing part of the control rod assembly is inserted into its place. This results in the increased absorption of neutrons and the reactor performance is reduced. On the contrary, when the control rod assembly is moved upwards, the reactor performance is increased.
Fast shut-down of the reactor
The fast shut-down of the reactor, or scram, is a fast discontinuation of the fission reaction, and it is one of the essential demands made on nuclear energy. For this purpose, the reactor is equipped with a safety protection system. This system consists of 37 control rod assemblies with the appropriate electronic circuits that put the system automatically into operation in case of an inadmissible exceeding of the permissible parameters and technological condition of the primary or secondary circuit.
This system can also be put into operation upon operator intervention by pressing the push-button at the unit control room.
In case of meeting the conditions for activation of the safety protection system, the power supply for all electric drives that maintain the control rod assemblies in the upper positions is discontinued. After discontinuation of the electric drives power supply, all the control rod assemblies start moving downwards under their own mass into the reactor core, and the fission reaction is terminated within 12 seconds.
Protection against radiation
Protection against radiation in nuclear facilities situated in the territory of the Czech Republic is defined in Act No. 18/1997 Coll. on peaceful utilisation of nuclear energy and ionising radiation (Atomic Act) and Enforcing Regulation No. 184/1997 Coll. on the requirements for ensuring radiological protection. Both the Atomic Act and the Enforcing Regulation on radiological protection stipulate requirements for personal and environment protection system against undesired effects of ionising radiation. There are defined basic duties and conditions for realisation of activities related to use of nuclear energy and ionising radiation. Legislation of the Czech Republic in the field of radiation protection consistently comes out of the internationally respected principles of radiation protection, namely recommendations of the International Commission on Radiological Protection and the subsequent basic international standards on radiological protection. The law in the field of radiological protection in the Czech Republic has been harmonised with the appropriate guidelines of the European Union. Assessment of the influence of the gaseous and liquid discharges into the environment forms a part of the safety documentation of the power plant. It is carried out by measurements inside and outside the power plant, and by means of mathematic simulation. The result is compared with the strictly defined permissible (limiting) values. What is measured in the power plant surroundings? Radiation control laboratory workers systematically carry out monitoring of the environment and collect samples of air, soil, water, vegetation and agricultural products, and make expert analysis. Independent measurements are additionally carried out by state supervision bodies too.
Annual levels of discharge activities, released into the air and watercourses, represent only a negligible fraction of the permissible values in the entire course of the power plant operation. Comparison of this indicator with other power plants puts the Dukovany Nuclear Power Plant into the group of the best power plants in the world.