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Other Designs of Nuclear Power Stations

1. MAGNOX Design
2. Pressurised Water Reactor (PWR) Design
3. Boiling Water Reactor (BWR) Design
4. Boiling Water Graphite Moderator Reactor Design (RBMK)
5. Pressurised Heavy Water Reactor (CANDU) Design
6. Fast Breeder Reactor (FBR)

 
1. MAGNOX Design
 

Magnox is a generation I Gas Cooled type reactor (on which the AGR design was based) that utilises a graphite moderator core and carbon dioxide as a coolant, and is a now obsolete type of nuclear power reactor which was designed and used in the United Kingdom, and exported to other countries, both as a power plant as a producer of plutonium for nuclear weapons when operating accordingly. The name Magnox comes from the alloy (Magnesium Non-Oxidising: mainly of magnesium with small amounts of aluminium and other metals) used to clad the fuel rods inside the reactor. Due to the low neutron absorption rate of the fuel cladding the fuel itself is un-enriched Uranium, eliminating expensive costs of Uranium enrichment.

 

In all, 11 power stations totalling 26 units were built in the UK the first of which was Calder Hall: the world's first commercial nuclear power station officially opened by Queen Elizabeth II on 17 October 1956. The Nuclear Decommissioning Authority (NDA) is considering whether to preserve Calder Hall Reactor 1 as a museum site.

 

The main advantage of Magnox over the first Pressurised Water Reactors (PWRS) was that on-load refuelling was included as part of the design to maximise power station availability by eliminating refuelling downtime. This was particularly important for Magnox as the unenriched fuel had a low burn-up, requiring more frequent changes of fuel than most enriched uranium reactors. In terms of limitations the Magnox fuel design itself had a limited thermal efficiency.

 

The AGR’s main enhancement over the earlier Magnox is the ability to operate at a higher gas temperature of 640°C for improved thermal efficiency (approximately 41%). In addition the design of the AGR was such that the final steam conditions were identical to that of conventional power stations allowing use of the same plant for electricity generation.

 

2. Pressurised Water Reactor (PWR) Design

 

Pressurized Water Reactors (PWRs) are also generation II nuclear power reactors using ordinary water under high pressure as both the coolant and moderator. PWRs are the most common type of power producing nuclear reactor (over 230 stations existing worldwide), though were originally designed for use as a nuclear submarine power plant in the USA.

 

The use of water as a moderator represents the main safety benefit of PWRs. Any increase in temperature causes the water to expand and become less dense; thereby reducing its moderating properties and hence reducing the reactivity in the reactor. Therefore, if reactor activity increases beyond normal, the reduced moderation of neutrons will cause the chain reaction to slow down, producing less heat.

 

However coolant water must be highly pressurized to remain liquid at high temperatures: this requires high strength piping and a heavy pressure vessel and, more critically, can increase the consequences of a Loss of Coolant Accident. Other disadvantages include that, like AGR reactors, enriched Uranium is required, and most pressurized water reactors cannot be refuelled while operating.

 

 

 

Pressurised Water Reactor
Boiling Water Reactor

 

 

3. Boiling Water Reactor (BWR) Design

 

In contrast to the above PWRs, Boiling Water Reactors (BWR) use ordinary water as both the moderator, coolant and as the primary loop for electricity generation. About 10% of the water is converted to steam and passed to steam turbines. After condensing it returns to the pressure vessel to complete the circuit.

 

The fuel is similar to that of a PWR, but the power density (power produced per unit volume of core) is about half, with lower pressures and temperatures. The cost advantage of not having steam generators is offset by the disadvantages of a single cooling system, which can potentially cause contamination throughout the steam plant if fuel can failures occur. A BWR’s thermal efficiency is about 32%.

 

4. Boiling Water Graphite Moderator Reactor Design (RBMK)

 

The Soviet designed RBMK is a hybrid graphite-moderated and water-cooled reactor only found in the former Soviet Union. The core is an assembly of graphite blocks, not unlike the core of a Magnox reactor, through which run pressure tubes containing the fuel. Water is pumped through the pressure tubes, where it boils to steam and is piped to the steam turbines. The fuel is uranium dioxide, enriched to about 2%, contained in Zircaloy tubes. The reactors are physically very large with high electrical outputs of up to 1500MW. The physics of RBMK reactors are complex because the graphite, water and steam are all used to moderate the neutrons in the core. No more RBMK type reactors are due to be built, and Russia is under international pressure the close the ones that it still operates.

 
Boiling Water, Graphite Moderated Reactor
Pressurised Heavy Water Reactor
 

5. Pressurised Heavy Water Reactor (CANDU) Design

 

The Canadian Deuterium Uranium, or CANDU, reactor is so called because it is a Canadian design using Deuterium oxide (heavy water) as both coolant and moderator and natural Uranium fuel. Heavy water is an oxide of heavy hydrogen or deuterium (D2O), rather than regular hydrogen (H2O). Pressurised heavy water is pumped through horizontal fuel tubes and heated by the nuclear reaction before being passed to a steam generator as in a PWR, but without the need for a pressure vessel as the tubes provide the containment. The fuel is natural uranium oxide contained in Zircaloy tubes. The average power density is about one-tenth that of a PWR or four times that of an AGR. The output of CANDU reactors ranges up to 930MW with a thermal efficiency of about 30%.

 

6. Fast Breeder Reactor (FBR)

 

A breeder reactor is a nuclear reactor that consumes fissile and fertile material at the same time as it creates new fissile material. While fast neutrons are less likely to be absorbed by uranium-235 or plutonium-239 than thermal neutrons, the highly enriched fuel used in fast breeder reactors allows for a self-sustaining nuclear chain reaction. For this reason, no moderator is required to thermalize the fast neutrons.

 

These reactors were initially (1950's and 1960's) considered appealing due to their superior fuel economy; a normal reactor can consume less than 1% of the natural Uranium that begins the fuel cycle, whereas a breeder can use much more with a once-through cycle and nearly all of it with reprocessing. Renewed interest is also due to the dramatic reduction in waste they produce and especially long-lived radioactive waste components.

 

All current fast reactor designs use liquid metal as the primary coolant, to transfer heat from the core to steam used to power the electricity generating turbines. As of 2006, all large-scale FBR power stations have had reactors cooled by liquid sodium.

 

The table below provides a useful summary of the types of reactors and some of their properties.

 

 

Reactor Design

Fuel

Moderator

Coolant

Thermal Efficiency

Typical Pressure

Typical Output

Typical Outlet Temp.

Main Economic and Safety Benefits

Magnox

Natural uranium (0.7% U235)

Graphite

Gas (CO2)

31%

300 PSIA

250MW

350°C

Safety benefit that coolant cannot undergo a change of phase. Also ability to refuel give potential for high availability.

AGR

Enriched uranium (2.3% U235)

Graphite

Gas (CO2)

42%

600 PSIA

660 MW

650°C

Same operational and safety advantages as Magnox but with higher operating temperatures and pressures, leading to reduced capital costs and higher steam cycle efficiency

PWR

Enriched uranium (3.2% U235)

Light Water

Light Water

32%

2235 PSIA

1000 MW

320°C

Low construction costs resulting from design being amenable to fabrication in factory built sub-assemblies. Wealth of operating experience now accumulated world wide. Off load refuelling necessary

BWR

Enriched uranium

(2.4% U235)

Light Water

Light Water

35%

1050 PSIA

600 MW

285°C

Similar construction costs and advantages to PWR enhanced by design by design not requiring a heat exchanger, but offset by need for some shielding of steam circuit and turbine. Off load refuelling necessary.

RMBK

Enriched uranium (1.8% U235)

Graphite

Light Water

31%

1000 PSIA

1500 MW

285°C

Believed by the West to be inherently less safe. Operated in considerable numbers in the former USSR.

CANDU

Natural uranium

(0.7% U235)

Heavy Water

Heavy Water

30%

1285 PSIA

1000 MW

305°C

Good operational record but requires infrastructure to provide significant quantities  of heavy water at reasonable costs.

 

 

Table 4: Summary of main thermal reactor types and corresponding data. Please note that thermal efficiencies, typical pressures, output and temperature are indicative only.