Comparison of different reactors

Main features based on which various types of existing reactors differ:

  • Fuel the part of which is represented by fission material.
  • Moderator necessary for maintaining the fission reaction.
  • Coolant collecting the heat produced in the reactor and transferring it to a steam generator.

From the technological viewpoint, there are three main reactor types that have been used in commercial operation. Detailed description of each type will appear once clicking on it:

1. Gas cooled reactor

Gas Cooled Reactor (GCR) and Advanced Gas-cooled Reactor (AGR) use uranium as a fuel, graphite as a moderator and compressed carbon dioxide as a medium for a heat transferring liquid. These types were developed for industries in Great Britain and France. There are 15 GCR/AGR reactors operated today.

The RBMK type uses uranium and graphite like GCR, but differs by the heat transferring liquid – its uses water instead of gas. This type was developed in the former Russian Federation, and there are still 15 reactors operating at the moment.

2. Pressurized Heavy Water Reactor

Pressurized Heavy Water Reactor (PHWR) uses natural uranium as a fuel, heavy water (i.e. water having deuterium isotope instead of hydrogen) as a moderator, and compressed heavy water as a coolant.The industrial reactor of this type is the Canadian Deuterium Uranium (CANDU), and its derivative works projects of similar type. Currently, there are 49 operating reactors in the world, beside Canada mainly in Asia.

3. Light Water Reactor

Light Water Reactor (LWR) has clearly experienced the biggest success of all reactor types. It uses normal water as a moderator and also as a heat transferring liquid and enriched uranium as a fuel. The LWR type is divided to two main types:

  • Boiling Water Reactor (BWR),
  • Pressurized Water Reactor (PWR).

BWR is a reactor with direct cycle with heat and steam generation directly in the reactor pressure vessel. It does not have isolated circuits (primary and secondary) but only one where the same water is used as moderator and as coolant changing to high-pressure steam, which drives a turbine. If compared with the competitive pressurized water reactor, the advantage is higher simplicity of the system but the disadvantage is more complicated design of the reactor pressure vessel and more complicated control system. There are 78 BWR type power plants operated worldwide at present.

PWR is a reactor with indirect cycle, i.e. primary circuit pressurised water as a moderator and a coolant does not rotate turbines but transfers its thermal energy to the water of the secondary circuit in the so-called steam generator, where high-pressure steam used for electricity generation is produced. The advantage of separation of these two circuits is that the steam in the turbine will never come into contact with the nuclear fuel so it does not contain any fission products. PWRs are the most used reactors: there are 279 operated today.

The PWR type encompasses also the Russian Water-Water Energy Reactor (VVER). Four such reactors are operated by Enel in Slovakia at Bohunice and Mochovce power plants. It differs from the Western reactors only by a different engineering design: the primary circuit consists of 6 loops, i.e. six primary pipes with six separate steam generators arranged horizontally around the reactor.

Except for the three already mentioned types, there were other two developed as well:using high-temperature gas with several prototypes built in the USA, Germany, Great Britain and Japan, and Fast Breeder Reactor (FBR) with ten prototypes constructed (USA, France, Great Britain, Germany, Japan and former USSR), and one large power plant Superfenix with the capacity 1,200 MW put into operation in France in 1986 and definitively closed in 1996. No fast breeder reactor is operated today in the European Union.

Reactor Evolution

Almost all commercially operating reactors are based on one of the basic water-cooled designs above. However, Advanced or Generation III+ reactors are seen in the new reactor builds and expansion plans today. Most Generation III+ reactors are larger than their predecessors and are aimed at producing in well excess of 1,000 MW of electricity.

Their designs have been built upon the half-century of knowledge and operational experience from preceding nuclear systems to:

  • increase safety, fuel efficiency, operating lifecycles and output capacities,
  • decrease capital and operating costs and,
  • reduce radiological waste products.

At the same time, parallel development in small, modular reactors (SMRs), better suited for lower demands in more isolated locations and modest budgets, has also occurred. Small reactors are designed to produce up to 300 MW and are very diverse in their technology. The most advanced are under development by companies in the USA, Russia, China, France, Argentina and South Korea.

Even as new Generation III+ reactors begin to slowly come online, nuclear scientists from around the globe are working collaboratively on Generation IV reactors designs. Six conceptual reactor designs are being researched. It will be several decades before even the most promising design(s) would be developed to testing stages.