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Welding the components of nuclear power plants

Nuclear power engineering holds an important place in the production and distribution of electricity in the Czech Republic. The technological diagram (pic. 1) of the power plant matches the latest and most modern world parameters. In this article we will focus on the description of technological sections and the welding of individual components.


Rubrika: Nuclear Power Plants
Publishet: 25. 10. 2007, Read: 99785 x, Print: Print, Send to: Send to

The entire primary circuit of the unit with the nuclear reactor, four steam-generators, circulation pumps, etc. is located in fully pressurized reinforced concrete containment – a hermetically enclosed protection envelope. The secondary circuit of the unit contains a turbo-generator with a 1,000 MW output.

Pic. 1 – Diagram of the Temelin nuclear power plant
Pic. 1 – Diagram of the Temelin nuclear power plant

The reactor core contains 163 fuel assemblies, each with 312 fuel rods, and 61 regulating rods. Each steam-generator generates 1,470 tons of steam per hour with an output pressure of 6.3 MPa and temperature of 278.5° C. The turbine works at 3,000 revolutions per minute.
Our nuclear power plants with pressurized-water reactors are fuelled by UO2, uranium dioxide, with a uranium enrichment of an average of 3.5 % with the fi ssion isotope U235. The fuel for the Temelín Nuclear Power Plant is supplied by the American company Westinghouse, which also supplies the new control system.
The thermal energy released by the controlled fi ssion of uranium U235 nuclei is removed from the reactor core by the demineralised water of the primary circuit and conducted to four heat exchangers – steam-generators. Heat generation in the reactor can be regulated by the control rods and by changing the boron concentration in the coolant. The cooling water, sealed under high pressure in the primary reactor circuit, circulates by means of four tubular loops with steam-generators and pumps. In the steam-generators, the water of the closed primary circuit transfers its heat to the steam turbine circuit, i.e. the secondary circuit.
The secondary circuit is also closed and fi lled with demineralised water. The secondary circuit water boils in the steam-generators and the resulting steam is fed to the turbine. There are two turbines in the power station, each powering a 1,000 MW alternator which generates electricity with 24 kV voltage. The entire turbine-generator units for the Temelín Nuclear Power Plant were made by Škoda Plzeň. Downstream from the turbine, the steam condenses back to water on the cool surface of titanium pipes in three condensers. Condensation heat is then removed from the steam by the cool water of the tertiary coolant circuit fl owing through the condenser pipes. The circuit is then led out into four cooling towers where the tertiary circuit water is again cooled by natural air fl ow. What remains, and is released in the atmosphere, is pure water vapour.
The high professional level of the staff plays an important role in ensuring the safe operation of the plant. A full-range simulator of a unit’s control room was built in the Temelín Nuclear Power Plant complex for training the staff . It is an exact mathematical model of the behaviour of the reactor and the primary and secondary circuit. It is situated in a precise copy of the control room of a reactor unit.
After 1990, many alterations to the original design of the Temelín Nuclear Power Plant were made with the objective of improving reliability and safety to Western levels. Since 1991, 21 inspections were carried out by International Atomic Energy Agency inspectors in the power plant. Their recommendations, focusing on improving the reliability and safety of the power plant, were implemented in the course of the plant’s construction and entry into service. Trial operations of the fi rst unit were commenced on 10 June 2002. The second unit began trial operations on 18 April 2003.
In 2003, the Temelín Nuclear Power Plant produced 12.11 TWh of electricity and 13.4 TWh in 2004. The entry of both Temelín units into trial operations, together with the Dukovany Nuclear Power Plant, increased the share of the ČEZ Joint-Stock Company‘s production of nuclear sources of electricity to 42.5 % in 2003.

TEMELÍN NUCLEAR POWER PLANT PRIMARY CIRCUIT
The primary circuit contains the fundamental parts of the system – the reactor and steamgenerator (see pic. 2 and 3). Water in the reactor is heated to a temperature of 290° C to 320° C under a pressure of 15.7 MPa. The steam-generator’s function is to transfer the thermal energy
from the coolant in the primary circuit to the water of the secondary circuit. The water in the secondary circuit, so-called feed water, in the steam-generator occupies the area between heat-exchange pipes through which fl ows the coolant of the primary circuit.

Pic. 2 – Reactor vessel during installation
Pic. 2 – Reactor vessel during installation

Pic. 3 – Steam generator
Pic. 3 – Steam generator

TEMELÍN NUCLEAR POWER PLANT SECONDARY CIRCUIT
The fundamental part of the secondary circuit of the power plant is the turbine, which via the collection piping feeds steam from the steam-generator to the fi rst high-pressure component and subsequently (from the superheater) to the low-pressure components (see pic. 5).

Pic. 4 – Fuel element charging device with the spent fuel tank
Pic. 4 – Fuel element charging device with the spent fuel tank

Pic. 5 – A turbine at the Temelin nuclear power plant 1,000 MW
Pic. 5 – A turbine at the Temelin nuclear power plant 1,000 MW

WELDING TECHNOLOGY AT THE TEMELÍN NUCLEAR POWER PLANT
The process of welding at nuclear power plants consists of an irreplaceable technology during the building, operation and repair of individual components. The technology of welding components, whether it is the welding of components in the primary or secondary circuits of the nuclear power plant, is subject to the strict regulations laid down by the State Offi ce for Nuclear Safety, which state clear and unambiguous procedures for making and approving welded joints pursuant to European standards (WPS and WPQR). One of the organizations that ensures welding approval procedures is the Czech Welding Institute at the Mining University-Technical University of Ostrava (pic. 6). The welding technology itself is proposed with regard to the basic material, thermal and pressure stress on the welded joints, and last but not least, the repair possibilities on the equipment itself. Testing the welded joints is carried out by an independent testing centre, the Vítkovice Testing Center.

Pic. 6 – Inspection certifi cate showing the approved welding procedure
Pic. 6 – Inspection certifi cate showing the approved welding procedure

The basic materials that are welded can be divided into several groups: high-alloy austenitic materials and high-temperature materials mostly utilized in the primary section of the power plant, and high-temperature materials and structural steels utilized in the secondary section of the power plant.
Fundamental types of welded joints can be found in both sections of the power plant, along with special types of welded joints that are utilized, for example, during the repair of parts or during the replacement of individual components during operation. Pic. 7 depicts the welding of a socket joint on the primary side of high-pressure piping. In order to ensure the repair of components of nuclear power plants, special welding procedures were created that are applicable to specifi c types of faults. Examples that can be given are the splicing of leaking water pipes of exchangers (pic. 8) or the technology of the assembly welds of capillaries (tubes with a diameter of 6.2 mm) for reading the physical quantities of the primary circuit of the power plant.
Welding in nuclear power engineering is an extensive fi eld that has a fundamental impact on nuclear safety during the operations of power plants.

Pic. 7 – Sockets on the high-pressure pipeline in the primary section of the power plant
Pic. 7 – Sockets on the high-pressure pipeline in the primary section of the power plant

Pic. 8 – The method of sealing leaking pipes in the exchanger
Pic. 8 – The method of sealing leaking pipes in the exchanger

Pic. 9 – Performing assembly welds on the capillaries (pipe diameter 6.2 mm)
Pic. 9 – Performing assembly welds on the capillaries (pipe diameter 6.2 mm)

LITERATURE

  • Company literature of ČEZ, Temelín Nuclear Power Plant, www.cez.cz
  • Company literature of Czech Welding Institute, Ostrava, www.csuostrava.eu
Kolektiv autor:

  - Hlavaty Ivo
  - Krejci Lucie

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