Power Reactors: Classification and Characteristics

Power reactors can be heterogeneous or homogeneous. A heterogeneous power reactor has a large number of fuel rods with the coolant circulating around them and carrying away the heat released by nuclear fission. In a homogeneous reactor, the fuel and moderator are mixed, e.g. a fissionable salt of uranium like uranium sulfate or uranium nitrate dissolved in the moderator like water (H2O) or heavy water (D2O). The solution is critical in the core. Due to difficulties in component maintenance, induced radioactivity, erosion and corrosion, homogeneous reactors are not common. Present day nuclear reactors are of the heterogeneous class. These power reactors are again classified on the criteria fuel used, the neutron flux spectrum, the coolant and the moderator, if used.

Table 1: Reactor classification

Neutron flux spectrum
Moderator
Coolant
Fuel material 
Thermal Light water Light water Enriched uranium
Heavy water Heavy water Natural uranium
Graphite Gas (CO2) Natural or enriched uranium
Fast Nil Liquid metal (Na, K) Plutonium, thorium

Light water-cooled and moderated reactors (LWR) using slightly enriched uranium fuel are the type most commonly used for power production These reactors are further divided into:

  1. Pressurized water reactor (PWR)
  2. Boiling water reactor (BWR).

High temperature gas-cooled reactors (HTGR) have been used in European countries like the United Kingdom, France and Germany. Fast reactors which use high energy neutrons for fission and require no moderator utilize a liquid metal as a coolant with either plutonium or a plutonium-uranium mixture of fuel. Liquid metal fast breeder reactors (LMFBR) are likely to be source of electric power for the future. A breeder reactor produces more fissionable isotope then what it consumes. The schematic diagrams for nuclear steam supply systems (NSSS) of five different types of reactors are shown in the below figure.
Schematics diagram of different nuclear power reactors

The characteristics of different reactor systems are provided in the below table 2.

Table 2: Characteristics of typical power reactors

Character/property
PWR
BWR
LMFBR
HTGR
Electric power (MWe) 1300 1050 1000 330
Thermal power (MW th) 3800 3000 2750 842
Specific power (kW th/kg) 33 26 575 50
Power density (kW th/m3) 100 60 300 10
   Core height (m) 4.25 3.75 1.50 5.0
   Core diameter (m) 3.50 4.90 3.25 5.9
   Coolant H2O H2O Liq.Na He
   Pressure (MPa) 15.5 7.2 0.8 4.8
Inlet temperature (°C) 280 275 330 400
Outlet Temperature (°C) 310 285 500 770
Coolant Flow rate (Mg/s) 20 12 11 0.45
Average linear heat rate (kW/m) 22.5 20 30

Where,
PWR: Pressurized Water Reactor,
BWR: Boiling Water Reactor,
LMFBR: Liquid Metal Fast Breeder Reactor,
HTGR: High Temperature Gas-cooled Reactor.

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