TECHNOLOGY

How does the reactor work?

Mainly all power plants, with steam generation circuit, works on the principle for a high power steam engine. In fossil fuel power plant heat is produced in boiler, as a consequence of fuel burning. Next, the heat is used to turn water into steam. Being under high pressure, it drives the turbine. In principle, a nuclear power plant does the same, a difference is in heat source. Instead of boiler, nuclear reactor is installed [14].

Figure 11. Energy conversion in nuclear and fossil fuel power plant.
Source: Own development based on G.J e z i e r s k i "Elektrownia jadrowa a konwencjonalna", ", "Energetyka cieplna i zawodowa" , 10/2009 [15]

The reactor is a construction, where energy is released from fission reaction. To ensure continuous and controllable process reactor consists of a several, essential elements. The reactor's core contains the fuel rods. Sort of used fuel depends on reactor's type, but the uranium (natural or enriched) is most common in mature nuclear technologies. In these, the fission is caused by thermal neutrons (with energy around 0,025 eV), striking a uranium nucleus. To sustain reaction at least one free thermal neutron per one neutron's generation is needed. Unfortunately, each reaction produces 2 MeV neutrons, which fission cross-section is almost 1000 times smaller than 0,025 neutrons. It ensure continuous process with significantly smaller probability. Thus a moderator is used. It is a material, with a big dissipation cross-section to slow down the neutrons, emitted during fission that they can start next fission. Most common material for moderator is light or heavy water and graphite. Control rods are to make possible to adjust reactor power in limited range. They are a rods, with a big absorption cross-section. They pick up neutrons that they do not cause next fission. They can be inserted to core or withdrawn from there. It is impossible to completely stop heat generation. Coolant - liquid or gas, with continuous circulation - it captures huge amount of energy from the core and transports it to next medium and produces steam that can be directly used to drive a turbine. Coolant is kept under specific pressure to avoid sudden state of aggregation change and critical decrease of cooling ability in special pressure vessel or tubes. Steam parameters in nuclear power plants are different from fossil fuel power plants. Typical constructions allow for obtaining not superheated but only saturated steam. Attempts for using superheaters in nuclear power plants was failed. [13] Containment is a external element of the reactor. It is usual the construction made from reinforced concrete, and it has to protect everything outside from the reactor radiation. [16],[17],[18],[19]

For the purpose of rector's type description it is necessary to divide them into specific groups. It is not obvious and depends on the criterion assumed. First, can be a number of separate circuits in thermal part of power plant (without coolant's circuit). Schemes of basis thermal cycles in nuclear power plants are shown in Fig. 12. But such a division does not provide any other information, similar to distribution by kind of fuel. Quite useful can be sorting out by neutrons energy: fast, thermal, epithermal. Although many divisions exist, they can not sort reactors in detail. They create only a few groups, while constructions differ significantly. Factors that had found acceptance, was coolant or moderator type. Important is also the reactor construction. There are two basic solutions: pressure tube reactor and pressure vessel reactor. Tabel 1 shows types of reactors operating at present. From the listed reactors LWGR(RMBK), HWLWR,PHWR(Candu) are pressure tube reactors.

Figure 12. Basis thermal cycles, installed in nuclear power stations
Source: Own development based on P r a c a z b i o r o w a "Poradnik Inzyniera Elektryka" tom 3, Warszawa, WNT 2005 [20]

Basis types of operating reactors are presented in Table 1.

Table 1. Basis types of operating reactors
Source: Own development based on G.J e z i e r s k i "Energia jadrowa wczoraj i dzis", Warszawa, WNT 2006 [21]

AGR - Advanced Gas Reactor
BWR - Boiling Water Reactor
CANDU - CANadian Deuterium Uranium reactor - Canadian type of PHWR
FBR - Fast Breeder
GCR - Gas Cooled Reactor
HTR - High-Temperature Reactor
HWLWR - Heavy Water Light Water Reactor
HWR - Heavy Water Reactor
LWGR - Light Water Graphite Reactor
LWR - Light Water Reactor
PHWR - Pressurized Heavy Water Reactor
PWR - Pressurized Water Reactor
RBMK - Reaktor Bolszoj Moszcznosti Kanalnyj - Russian type of LWGR
WWER - Wodo Wodjanoj Energeticzeskij Reaktor - Russian type of PWR

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