Although all nuclear reactors operate on the same basic principles, various designs are used throughout the world. Most commercial reactor designs in use today were initially developed in the 1950s with many ongoing enhancements made to increase operational safety and efficiency.
Water Cooled Reactors
Pressurized Water Reactor (PWR)
Almost 63% of commercial reactors operating in today's global fleet are a type of PLWR design using plain water as a coolant and moderator with enriched UO2 as fuel. They are located in primarily the USA, Europe and eastern Asia.
Water is pressurized which keeps it from boiling, even at 300°C. The pressurized water is pumped through a closed system of pipes called the primary circuit. Heat from the primary circuit warms up water in a secondary circuit which is allowed to boil, turning into steam that spins turbines. This system completely isolates water that has come in contact with the radioactive core from the water vapour that is used to generate electricity.
Pressurized Heavy Water Reactor (PHWR)
About 11% of commercial operating reactors are PHWRs which use heavy water as coolant and moderator along with natural (un-enriched) UO2 as fuel. Most are operated in Canada and India with the entire Canadian fleet operating CANDU reactors.
Heavy water is a rare but natural form of water and an effective moderator for natural uranium reactors. The advantage of heavy water is that it permits the use of natural uranium as fuel. This means two less steps in the conversion process resulting in a more economical fuel source.
Fuel bundles are placed horizontally in a tank called a calandria. Heavy water coolant is pumped through tubes containing the fuel assemblies to absorb heat from the nuclear reaction. It's then circulated to the steam generator to produce the steam to drive turbines. As with other PWRs, the two water circuits are separate and transfer only heat, increasing safety.
Boiling Water Reactor (BWR)
Roughly 20% of the world's commercial fleet consists of BWR reactors that use ordinary water as coolant and moderator and enriched UO2 as fuel. They are located in primarily the USA, Europe and eastern Asia.
The BWR reactor design is more streamlined, and less expensive to build, than PWRs. Water circulating through the core, is allowed to boil and vaporize into steam which is then piped to the turbines outside of the reactor. Because the water is in direct contact with the core, it is contaminated with traces of short-lived radionuclides with very brief half-lives of several seconds. This means that the turbine units must also be shielded and radiological protection provided during maintenance. The costs of additional protective measures tend to balance the savings seen from the simplified design.
Light water graphite-moderated reactor (RBMK)
Operating solely in Russia, RBMK reactors represent a mere 3.4% of the world's fleet. The RBMK is very different from most other power reactor designs as it was derived from a design originally intended for plutonium production. It was used in Russia briefly to produce plutonium but is only used to produce electricity now. It is a pressurized water-cooled reactor with individual fuel channels, using graphite as its moderator. It employs long vertical pressure tubes running through the moderator, and is cooled by water. The water is allowed to boil in the core at 290°C, much like a BWR.
Non-Water Cooled Reactors
Though not as common, operating commercial reactors featuring non-water cooling systems include:
High temperature gas cooled reactors (HTGR)
HTGR reactors have had limited success operating in the competitive electricity generation market. Their inherent safety characteristics and innovative fuel designs are have sparked increased research attention, particularly in developing nations. Today, while none are operating commercially, one is currently under construction in China.
Gas cooled reactors use graphite as the moderator and an inert gas such as helium or carbon dioxide as the coolant. As the gaseous coolant comes in contact with the core, like water, it absorbs heat which is then either transferred to a water circuit to produce steam or drives turbines directly.
The uranium fuel for these reactors is highly specialized and consists of a "kernel" of enriched uranium which is coated in layers of carbon and silicon carbide. This fuel design provides containment of fission products and enables the reactor to safely operate at very high temperatures – up to 1600°C or higher.
Fast Breeder Reactor (FBR)
Initially developed in the 1950's, only a handful of FBR reactors have produced electricity on any commercial scale. FBRs can utilize uranium at least 60 times more efficiently than a normal reactor. However, they are very expensive to build, and current supply of moderately priced uranium does not make them economic for power generation at this point. Only one, located in Russia, is generating electricity commercially today.
FBRs 'breed' their own fuel, therefore the fuel cycle is closed. They have a core of plutonium surrounded by rods of non-fissile U238. The U238 nuclei absorb neutrons from the core and are transformed into plutonium (P239). For every four atoms of plutonium that are used up in the core of the breeder, five new plutonium atoms are made from the U238. They are not equipped with a moderator to slow down neutrons, and for this reason are called "fast" breeders.
Fast breeder reactors work at such a high temperature that they need a special coolant such as liquid sodium. Because they operate under such high temperatures, researchers are studying the potential use of FBRs as high-level radioactive waste burners.
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 1000 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.