Nuclear Power Plant Types Advantages and Disadvantages

Sunday, 29 September 2013

Nuclear Power Plant Types: Advantages and Disadvantages 



 Figure-1 :Typical Diagram of a MAGNOX Reactor

Figure 1 shows,
MAGNOX- Magnesium Alloy Graphite Moderated Gas Cooled Uranium Metal Reactor
For magnox reactors, uranium ore is chemically processed to yield uranium metal that is formed into fuel rods which are inserted into cans made of an alloy consisting mostly of magnesium with small amounts of aluminium and other metals. Heat generated due to fission is extracted by CO2 gas that is circulated from the reactor core to the steam generators. Graphite which is the moderator is, in the form of rectangular blocks, placed in a steel pressure vessel. Fuel rods are placed inside the graphite blocks such that coolant can flow through the annular space between the fuel rod and the moderator. Control rods made of Boron are inserted from the top. The whole assembly is housed inside a concrete containment vessel. The steam generator is placed outside the containment. Gas circulator, turbo alternator and the feed pump are also shown. Stand pipes, which are used for fuel loading and unloading, are inserted from the top. Fueling machine can operate from the top when the reactor is on power.  

  • Natural uranium is used as fuel. So no enrichment is necessary.
  • Fuel loading and unloading including small repair is possible when the reactor is power. Such       feature allows high level of availability.
  • Track record of safety is excellent.
 Heavy water is water in which the hydrogen is replaced by its heavier isotope, deuterium, 2H.
The term light water designates water containing the protium isotope (contains no neutrons)


Figure-2Typical Diagram of a Pressurized Water Reactor (PWR)

Figure-2 shows a Pressurized Water Reactor (PWR). The containment structure includes Reactor Pressure Vessel, Steam Generator, Pressurizer and the Primary Pump. Steam turbine, generator and condenser are situated just like any other conventional thermal power station. It may be seen that the control rods are inserted from the top. Fuels which are slightly enriched ( 3% to 4%)  uranium dioxide are in the form of fuel bundles canned in Zircalloy. Light water is moderator as well as coolant. High pressure is maintained inside the reactor vessel so that water does not boil in the core. This is known as primary circuit. Steam is generated at a separate steam generator which is known as secondary circuit. Incidentally, Three Mile Island Reactor where accident occurred in 1979 was of this type.

  • Water from the reactor and the water in the steam generator that is turned into steam never mix. In this way, most of the radioactivity stays in the reactor area.
  • In case of any accident radioactive fall out is contained inside the containment, it cannot spread to surrounding area. It was amply proved in case of Three Mile Island Accident in 1979. 
  •  Loading and unloading cannot be done when the reactor is on power. Reactor has to be shut down and then the same is affected. 
  • Since enriched fuel is used, enrichment services are necessary.

Figure-4         Figure 3:  Typical Diagram of a Boiling Water Reactor (BWR)

Figure-3 shows a Boiling Water Reactor (BWR). Unlike the PWR it does not have a separate steam generator or in other words, the Primary and the Secondary circuit is same. Water is boiled inside the pressure vessel. Hence, steam generated is radioactive. Fuel elements, canning material, moderator and coolant are exactly same as those of PWR. The distinguishing feature of BWR is that the control rods are, on control point of view, inserted from the bottom. Loading and unloading is exactly similar to PWR.

  • Radioactive fall out from the pressure vessel is contained in the containment.

  • Loading and unloading cannot be done when the reactor is on power. Reactor has to be shut down  and then the same is affected.
  • Turbine hall is also radioactive zone.
  • Since enriched fuel is used, enrichment services are necessary.
CANDU Reactor: which is heavy water moderated and cooled reactor. The acronym "CANDU", a registered trademark of Atomic Energy of Canada Limited, stands for "CANada Deuterium Uranium". This is a reference to its deuterium-oxide (heavy water) moderator and its use of uranium fuel (originally, natural uranium). All current power reactors in Canada are of the CANDU type. Canada markets this power reactor abroad .The reactors are used in nuclear power plants to produce nuclear power from nuclear fuel.
The primary loop is in yellow and orange, the secondary in blue and red. The cool heavy water in the calandria can be seen in pink, along with partially-inserted shutoff rods.
The CANDU reactor is conceptually similar to most light water reactors, although it differs in the details.


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