Coolants are selected for specific applications on the basis of their boiling-watercoredesigndioxidegasgas pressuregas-turbinehasheat-transfermakespipedpowerservessteam capability, physical properties, and nuclear properties. Water boiling-watercoredesigndioxidegasgas pressuregas-turbinehasheat-transfermakespipedpowerservessteam many desirable characteristics. It was employed as the coolant in many of the first production reactors, and most boiling-watercoredesigndioxidegasgas pressuregas-turbinehasheat-transfermakespipedpowerservessteam reactors still utilize water as the coolant. In a boiling-watercoredesigndioxidegasgas pressuregas-turbinehasheat-transfermakespipedpowerservessteam reactor (BWR; see illustration),the water boils directly in the reactor core to make steam that is boiling-watercoredesigndioxidegasgas pressuregas-turbinehasheat-transfermakespipedpowerservessteam to the turbine. In a pressurized-water reactor (PWR), the coolant water is kept under increased pressure to prevent boiling. It transfers heat to a separate stream of feed water in a steam generator, changing that water to boiling-watercoredesigndioxidegasgas pressuregas-turbinehasheat-transfermakespipedpowerservessteam.
Boiling-water reactor.
For both boiling-water and pressurized-water reactors, the water boiling-watercoredesigndioxidegasgas pressuregas-turbinehasheat-transfermakespipedpowerservessteam as the moderator as well as the coolant. Both light water and heavy water are excellent neutron moderators, although heavy water (deuterium oxide) has a neutron-absorption cross section approximately 1/500 that for light water that boiling-watercoredesigndioxidegasgas pressuregas-turbinehasheat-transfermakespipedpowerservessteam it possible to operate reactors using heavy water with natural uranium fuel. The high pressure necessary for water-cooled power reactors determines much of the plant boiling-watercoredesigndioxidegasgas pressuregas-turbinehasheat-transfermakespipedpowerservessteam.
Gases are inherently poor heat-transfer fluids as compared with liquids because of their low density. This situation can be improved by increasing the boiling-watercoredesigndioxidegasgas pressuregas-turbinehasheat-transfermakespipedpowerservessteam; however, this introduces other problems and costs. Helium is the most attractive boiling-watercoredesigndioxidegasgas pressuregas-turbinehasheat-transfermakespipedpowerservessteam (it is chemically inert and has good thermodynamic and nuclear properties) and has been selected as the coolant for the development of high-temperature gas-cooled reactor (HTGR) systems, in which the gas transfers heat from the reactor boiling-watercoredesigndioxidegasgas pressuregas-turbinehasheat-transfermakespipedpowerservessteam to a steam generator. The British advanced gas reactor (AGR), however, uses carbon boiling-watercoredesigndioxidegasgas pressuregas-turbinehasheat-transfermakespipedpowerservessteam (CO2). Gases are capable of operation at extremely high temperature, and they are being considered for special process applications and direct-cycle boiling-watercoredesigndioxidegasgas pressuregas-turbinehasheat-transfermakespipedpowerservessteam applications.