Działanie elektrowni jądrowej

Before you start, do the following.

• Remind what a chain reaction is all about.

The first nuclear power plants were built in the 1940s.

Nuclear power plants belong to the same group of thermal power plants as conventional thermal power plants. The operation of thermal power plants has a common feature - in both cases the working circuit is a steam‑water circuit. The difference is in steamsteamsteam generating sources - in coal‑fired power plants there is coal, in nuclear - a nuclear reactornuclear reactornuclear reactor filled with nuclear fuel.

Nuclear power plants have much more power than conventional ones. The problem is side production of radioactive waste. Waste fuel from reactors is exchanged every three years on average. They are stored in specially adapted places.

Structure of a nuclear power plant

Each thermal power plant is a system composed of two main parts:

• producing hot, compressed water vapour (nuclear part), 

• generating electricity thanks to turbines driven by expanding water vapour (conventional part).

The main element of the nuclear part is a reactor in which the fission reaction produces energy for heating the water flowing through the reactor corecorecore and converting it to water vapour.

The most widespread types of reactors are the pressurized water reactor (PWR) and the boiling water reactor (BWR).

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Na rysunku przedstawiony jest schemat działania elektrowni na paliwa stałe. Z lewej strony rysunku widoczna jest hałda węgla opisana coal, poniżej płynie rzeka opisana river, z której czerpana jest woda do chłodzenia elektrowni. Widać komin odprowadzający spaliny oraz schematycznie jest pokazana budowa wewnętrzna elektrowni. Wyróżniony jest kocioł - Boiler (furnace), turbiny napędzane parą - Turbine, generator - Generator, transformator - Transformer, linie przesyłowe - Transmission Lines. Z rzeki woda do chłodzenia przesyłana jest rurami - Condenser Cooling Water.

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Na rysunku pokazany jest schemat elektrowni jądrowej. Lewa część rysunku to schemat rdzenia elektrowni - Containment structure oraz urządzeń doprowadzających i odprowadzających wodę - Control Rods, Reactor, Pomp, Steam Generator, Steam Line. Prawa część przedstawia turbiny - Turbine, generator - Generator, układ chłodzący - Water, Condenser, Cooling Water, Reservoir, Cooling Towers. Zaznaczone są wychodzące linie przesyłowe.

Generation of electricity

In the nuclear part of the power plant there is a reactor, circulation pumps and a steam generatorgeneratorgenerator. Together they form a primary water circuit. It is a closed circuit in which the thermal energy from the reactor produced by the fission reaction is transported by means of water to the steamsteamsteam generator. Changes in water volume in the primary circuit, caused by temperature changes, are controlled by means of a pressure regulator.

The element connecting the two circuits in the power plant is the steamsteamsteam generator. Water supplied to it by a secondary circuit from a conventional part of the power plant receives heat from the primary circuit, resulting in water vapour. It flows through a high pressure pipeline from the steam generator to the steam turbine. The steam expands, sets the turbine in motion, causing the generatorgeneratorgenerator to move and generate electricity.

These processes take place in a pressurized water reactor (PWR). In a boiling water reactor (BWR), the water vapour exchange takes place in the reactor corecorecore.

Structure of the reactor

In the nuclear reactornuclear reactornuclear reactor, the process of initiating, controlling and maintaining the chain reaction takes place. As a result of fission, gamma radiation and neutrons and a significant amount of heat energy arise. Other nuclides are also formed. The failure‑free operation of the reactor depends on proper control and maintenance of the chain reaction.

The most important element of the nuclear reactor is the corecorecore in which there are fuel elements and a moderatormoderatormoderator. Moderator is a substance that slows down neutrons, e.g. graphite, heavy water.

Nuclear fuel is made usually from substances containing fissile isotopes whose nuclei easily undergo nuclear fission by bombarding with neutrons of low energy, e.g. Indeks górny 235²³⁵U, Indeks górny 233²³³U, Indeks górny 239²³⁹Pu, Indeks górny 241²⁴¹Pu.

Nuclear fuel is enclosed within fuel elements. They have the form of rods, for example in the shape of a cylinder or cuboid.

This solution avoids unwanted release of fission products outside the reactor.

A coolant flows through the reactor, which is intended to discharge the generated heat. It must be a substance that weakly absorbs neutrons. The most commonly used are ordinary or heavy water, liquid sodium, helium or carbon dioxide. Its flow is forced by means of pumps.

Control and maintenance of the chain reaction

In most nuclear power plants, fuel is enriched uranium, i.e. uranium Indeks górny 238²³⁸U, which contains more uranium Indeks górny 235²³⁵U than it is in natural uranium (over 0,72%). In the corecorecore of the nuclear reactornuclear reactornuclear reactor the Indeks górny 235²³⁵U uranium fission reaction takes place. As a result of this reaction, neutrons, lighter nuclei and energy are generated. These neutrons are responsible for causing further fission. In this way, a chain reaction is created.

When the number of neutrons produced is equal to the number of neutrons that are lost as a result of absorption and escape, we deal with the so‑called critical condition of the reactor. Despite its name, this is a normal state of the reactor. Controlling the reactor is based on keeping it in a critical condition. Materials strongly absorbing neutrons are used for this. Control rodscontrol rodsControl rods, made of such materials, are appropriately set, which affects the number of available neutrons, thereby weakening or strengthening the course of the reaction. The change in the neutron flux allows adjusting the corecorecore power.

The control rods also play an important role as so‑called safety rods - their task is to interrupt the course of the reaction. When the reaction gets out of control, they are shot into the core - then the reactor's operation is interrupted.

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Na rysunku przedstawiony są dwa schematy rdzenia reaktora. Pręty sterujące zaznaczone są na czerwono. Pręty sterujące spełniają ważną rolę w zakresie bezpieczeństwa, ich zadaniem jest przerwanie przebiegu reakcji. Gdy reakcja wymyka się spod kontroli, są one wstrzeliwane do rdzenia, wtedy praca reaktora zostaje przerwana. Po lewej stronie rysunku pręty sterujące są wsunięte do rdzenia, po prawej wysunięte.

Applications of reactors

Reactors are used for:

• electricity generation,

• production of nuclear fuel (in the form of a Indeks górny 239²³⁹Pu) for military purposes,

• production of radioactive isotopes (e.g. those applicable in medicine),

• scientific experiments in physics, biology, etc.

Remember

• A nuclear reactornuclear reactornuclear reactor is the only source of heat in a nuclear power plant and is equivalent to a steamsteamsteam boiler found in a classic coal‑fired power plant.

• As a result of proper control of the reactor operation, thermal energy is released in a controlled manner.

Exercises

Glossary

Keywords

control rodscontrol rodscontrol rods

corecorecore

moderatormoderatormoderator

nuclear plantnuclear plantnuclear plant

nuclear reactornuclear reactornuclear reactor