Pressure tube reactor

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A pressure tube reactor is a special type of nuclear reactor in which the fuel elements are not located in a large common reactor pressure vessel , but individually in pressure-tight tubes. The coolant flows through the pressure tubes. The coolant is not, however, also a moderator , but rather the moderator (in solid or liquid form) surrounds the pressure tubes on the outside. Pressure tube reactors for use in nuclear power plants can work according to the boiling water principle - the steam for the turbine is generated directly in the pressure tubes, i.e. in the reactor core - or according to the pressure water principle with a steam generator and a separate water-steam cycle.

The best-known types of pressure tube reactors for nuclear power plants are the Russian RBMK reactor and the Canadian CANDU reactor . In the boiling water reactor RBMK, light water is used as the coolant and graphite as a moderator. In the pressurized water reactor CANDU, heavy water serves both as a moderator and as a coolant. However, the cooling circuit is under high pressure and is separate from the moderator tank. Less well known is the MKER , the successor to the RBMK with improved safety devices. Another type had been tested in the German nuclear power plant Niederaichbach . Here, gaseous carbon dioxide (CO 2 ) served as a coolant and heavy water as a moderator.

The military ADE reactors used in the Soviet Union to produce weapons plutonium were also pressure tube reactors.

Fuel assemblies

Fuel elements of a CANDU reactor

In the pressure tube reactor, the fuel elements are bundles of parallel fuel rods . Corresponding to the shape of the pressure tube, however, the cross section of the fuel assembly is circular. Special design features of the CANDU fuel assembly result from the fact that it is not used hanging vertically, but in a horizontal position.

advantages

Pressure tube reactors offer several technical and economic advantages:

  • Individual pressure tubes are easier to manufacture than a large pressure vessel.
  • Reactors can be built more easily with different output sizes, since the number of tubes (and thus the output) can be adapted to the respective requirements without great technical effort.
  • Individual fuel assemblies can be changed during ongoing power operation (power generation). Regular longer downtimes to change fuel, such as with normal pressure and boiling water reactors, are no longer necessary. The reactor therefore does not need to be loaded with a large excess of fuel; this improves the security against reactivity accidents .
  • By connecting or disconnecting groups of pressure tubes (with RBMK and MKER also individual pressure tubes), the reactor output can be set to various values.

Disadvantages and Risks

The disadvantages from a safety point of view include:

  • Operating parameters have to be read out and checked for hundreds of pressure tubes. The control and monitoring of the reactor is therefore more complex and more susceptible to failure. This must be compensated for by the corresponding effort in the control technology.
  • In the event of a loss of coolant accident , the moderator does not automatically drop out, so that the reactivity does not necessarily decrease. In the case of light water as a coolant, as in the case of the RBMK, it even increases, since the neutron-absorbing effect of the coolant is missing; the coolant loss coefficient is therefore positive. This can cause a rapid increase in performance. In the Chernobyl accident , this property of the RBMK reactor contributed significantly to the occurrence of immediate overcriticality with ignition of the graphite and the further catastrophic consequences.

Proliferation risk

The possibility of replacing individual fuel assemblies while the power is in operation makes it possible to continuously extract weapons plutonium , i.e. relatively pure plutonium-239 with only a small proportion of higher Pu isotopes, which is suitable for military-grade nuclear weapons , at the same time as electricity is being generated . Exporting such reactors therefore poses a greater risk with regard to nuclear proliferation than reactors with large pressure vessels, which have to be shut down and opened as a whole for each fuel element change.

literature

  • KH Grote, J. Feldhusen (Hrsg.): Dubbel - pocket book for mechanical engineering. 23rd edition. Springer, 2011, ISBN 978-3-642-17305-9 .

Individual evidence

  1. ^ Dieter Smidt: Reactor technology . Volume 2, G. Braun, Karlsruhe 1971, ISBN 3-7650-2004-4 , pp. 142-143.