Figure 1- Simplifed sketch of a typical thermal nuclear reactor. (Image courtesy of WNA)
Now that we know about the principles of nuclear fission power, we need to know how it all fits together and it does so in the reactor. Figure 1 shows a sketch of a pressurised water reactor (PWR), the most common type of reactor design throughout the world. The specifics will be discussed later, but for the moment, it illustrates some of the fundamental properties of your average nuclear power reactor. Of course, it is a simplified diagram. Most nuclear reactors are built with rulers.
At the heart of the reactor is the core. The core is always under high pressure and temperature so it is contained within a metal pressure vessel. It is here where the fuel assemblies are located. Different reactors types have fuel manufactured into different forms, but they generally take the form of long rods. In the confines of the reactor core, they are busy fissioning away, releasing large amounts of energy as neutrons fly around. The core is surrounded by a biological shield, which absorbs any neutrons trying to escape.
Control rods, shown here inserted from the top, are made of materials like cadmium, which are thick neutron absorbers. By varying how far into the reactor they sit, they determine the level of absorption and so can make the reactor power up or down. By having them inserted from the top, it means that in the event of a total power failure, they will simply fall all the way into the core, shutting down the reactor.
Amidst the fuel assemblies is the moderator, which slows down the neutrons released from the fissioning of the fuel to thermal energies. It is clearly indicated on the diagram, because in the case of the PWR, the moderator is in fact also the coolant.
The coolant is some fluid that carries heat away from the fissioning fuel. This is obviously very useful because without coolant the fuel would heat up and perhaps melt. In the diagram, the super hot water is the coolant as well as the moderator, which cycles through the core and the primary coolant loop to the steam generator. At the steam generator, the primary coolant transfer heat to the water in the secondary coolant loop. This heat boils the water, turning it to high pressure steam.
At this point, the reactor becomes essentially the same a coal-fired power station. The high pressure steam in the secondary coolant loop drives the turbines to generate electricity. From there, any heat remaining is taken away by a condenser, to return the steam to liquid, where it flows back into the steam generator. The cold water in the condenser that takes away this heat may either dissipate heat in cooling towers or may simply use water taken from a river or the sea.
All materials relating to the core are encased inside a thick concrete containment structure, the most hardened structures you will ever find in a civil building. These are designed to contain any hazardous material regardless of what goes wrong inside as well as protect the core from outside threats up to and including impacts from large airliners. Because the primary coolant is considered at risk of contamination with radioactive materials, it too is kept entirely inside the containment. Only after heat has been transferred to the secondary coolant loop, which is safe from contamination by radioactive material, is it carried outside of this protection.
The many generations of the nuclear reactor
All nuclear reactors, from the old and decommissioned to the conceptual, are grouped into one of four generations. Sure this implies quantised development where none exists, but that is the convention. The first generation was built in the fifties and early sixties. They were the early prototype reactors, including some experimental fast reactors, which were used to generate electricity. Naturally, being the first attempt, they were less economical, produced more waste, and were not generally as resilient as later designs. The only ones still operating around the world are the Magnox reactors in Britain.
The second generation saw nuclear technology applied to large scale commercial use. This generation is considered to cover the sixties through to the early nineties, which covers the bulk of the reactors in service in the world. They are the ones we all know and love, the light water reactors, the CANDU, the AGR, and those we do not love as much, the VVER and the infamous RBMK. But despite following the basic outline, Generation II reactors were generally custom designed from scratch each time. While these reactors proved that nuclear power could be used on a commercial scale, it is, of course, the case that there was much improvement to be made.
The third generation started in the late nineties and features evolutionary reactor designs, building on Generation II but improving them in all respects by reducing lead times, improving capacity factors, improving efficiency, reducing waste volume, reducing costs, improving safety systems, including the introduction of passive safety systems. The other major improvement for generation III is the standardisation of designs, which reduces costs and lead times by not requiring a blank slate design process each time an operator wants to build a new reactor, by not requiring individual certification for the reactor itself, only site issues, and allowing mass production of parts. Generation III designs that are still in the development stage are generally termed Generation III+ and make further improvement through evolution.
The fourth generation, expected to mostly arrive by 2030, features revolutionary designs, rather than evolutionary. An international forum dedicated to the development of Generation IV has selected six basic concepts, many of which are fast, all of which are noticeably different to the concepts applied in Generation II and III.
Old favourites
The oldest commercial reactors operating today are the British Magnox reactors, the only remnants left of Generation I. They are graphite moderated and carbon dioxide cooled. The graphite allows these designs to use natural uranium, but, as is the case with graphite moderator, it makes the reactors much larger in size for a given power, and hence less economical. The magnesium oxide cladding of the fuel (hence the name Magnox), is chemically reactive and called for immediate reprocessing of the spent fuel, rather than allowing it to remain in cooling pools after removal from the reactor. This makes spent fuel handling more expensive and troublesome because it is far preferable to leave it in the cooling pools for the highly radioactive isotopes to decay away, after which, it will be easier to handle.
Descended directly from the Magnox is the advanced gas cooled reactor (AGR), still uniquely British. These offered a number of improvements in all areas, particularly thermal efficiency, which is still the highest of any reactor in service. The uranium this time was low enriched to around 2%, allowing AGRs to be more powerful than Magnox. But, still being graphite moderated, they are larger for their size than light water reactors. The fuel this time is clad in stainless steel, which means that spent fuel is easier to handle.
The most common type of nuclear reactor around the water is the pressurised water reactor (PWR). It is considered to be the more economical compromise of various factors in reactor designs. It is light water moderated, which means it cannot use natural uranium, but the necessity of enrichment is covered by the benefits of having a more compact core than a graphite moderator reactor. PWR fuel is generally 3-4% enriched. The light water is also the coolant, which is very handy because it gives rise to a negative void coefficient, which is a property of a reactor where an increase in temperature leads to a decrease in reactivity, making the reactor inherently stable. The reactor is described as pressurised because the water in primary coolant loop is kept under high pressure to keep it from boiling, even in excess of 300°. Because light water is the moderator and coolant, the PWR is a type of light water reactor (LWR). Russian designs of PWR, the early ones being fundamentally flawed, were called VVER.
Another type of LWR, but less common is the boiling water reactor (BWR). It is similar to the PWR, except the primary coolant is not kept under the same pressure (although it is still much higher than atmospheric pressure) and the water is allowed to boil. This is helpful because changing water, or any liquid for that matter, from liquid to gas allows it to absorb energy without increasing temperature, thereby increasing the heat holding capability of the coolant. Also, it allows the steam from the core to drive the turbines directly, eliminating the need for a secondary coolant. Of course, this means the turbines are also shielded for radiological protection. While the simplicity of only having a single coolant loop from core to turbines reduces costs, the extra costs of shielded tends to balance this out. Further, because control rods are only effective below the water level, they are inserted from the base of the pressure vessel, which requires increasing redundancy to maintain the safety of the reactor.
Canada's contribution is the unique Canadian-Deuterium-Uranium series (CANDU). These reactors have several unique qualities. They are heavy water moderated and heavy water cooled (hence called a heavy water reactor (HWR), although the moderator and the coolant are in separate channels. Heavy water is a better moderator than light water and so the CANDU uses natural uranium, convenient for Canada because they have so much of it. Heavy water is more expensive than light water of course, but this is covered by the lack of need for enrichment. CANDUs also possess a special design of core, called a calandria, which, rather than simply being a large pressure vessel with the fuel sitting in a common pool of coolant, features separate channels for each fuel bundle, which can be individually isolated. This allows the reactor to be refuelled while still active, simply by shutting down the fuel channel of the bundle needing replacement. This helps to increase the capacity factor at the expensive of more complicated construction.
The new kids
Generation III+ is an evolutionary step from the reactors of Generation II. They improve the overall picture of nuclear energy in a variety of ways.
- Standardisation of design means common certification for all reactors of the type reducing capital costs and lead times for a construction project. It also prevents delays in the construction because there will not be the same ad hoc revisions to the design mid-construction that so plagued Generation II construction.
- Simplification of design means less raw materials required, not only further reducing capital costs and lead times, but also reducing decommissioning wastes at the end of the reactor's life. A simplified design also reduces operating costs by being easier to maintain and improving reliability. Simplification is achieved through greater reliance on passive safety rather than engineered components and advances in technology allowing fewer parts and less piping.
- Longer plant life improves the financial and environmental situation by allowing the capital and eventually decommissioning costs to be amortised over a longer period. It gives a better economic and environmental return on investment.
- Burnable poisons incorporated into the fuel allow for higher burn-ups and reduce the volume of high level waste. As neutron absorbing fission products build up to higher levels than usual, the neutron absorbing poisons disintegrate, keeping the level of poison in the fuel constant. By allowing fuel to remain in the reactor with a higher content of fission products, its radioactivity contributes more to the useful energy output of the reactor through the decay heat, than it would if the fuel was removed early and decay heat dissipated as waste heat in cooling pools.
- Higher enrichment of fuel to around 5% also helps to reduce HLW by making more efficient use of the fuel put into the reactor. With the help of those burnable poisons, the extra uranium-235 can be burnt properly meaning less fresh fuel, which will come out of the reactor as HLW, is needed. It also improves capacity factors by keeping fuel useable for longer and so reducing refuelling outages.
- Costs and waste are reduced further by increases in thermal efficiency, meaning more heat released from the fuel is turned into useful electricity. This reduces fuel requirements and therefore HLW produced at the other end.
- Increased fuel flexibility allows operators to more easily make use of MOX, reducing the cost of disposing of plutonium in a useful way. Some are even being designed to handle thorium.
Some notable new designs include the Westinghouse Advanced Passive 600MWe PWR AP-600 and its Generation III+ 1100MWe brother the AP-1000. The GE Economic and Simplified Boiling Water Reactor, the direct descendent of the Generation III Advanced Boiling Water Reactor, depends purely on convection for heat transfer rather than pumps, rendering it failsafe. Areva has developed the very large 1600MWe PWR, the European Pressurised Water Reactor, the first of which being built in Finland, and many more to replace the Generation II reactors throughout France.
Atomic Energy of Canada Ltd and very busy with multiple CANDU descendents for Generation III and Generation III+. The CANDU-9 offers excellent fuel flexibility, being able to take natural uranium, LEU, MOX, thorium among other alternatives. It would probably be able to burn steel if it didn't violate the laws of physics. Beyond that, AECL are developing the Advanced CANDU Reactor (ACR) series, which incorporated some features of LWRs, such as light water cooling giving rise to a negative void coefficient, and using higher temperature and pressure for increased thermal efficiency. It is more compact and has short construction times due to the use of prefabricated components. The ACR-700 is the 750MWe version, soon to be followed up by the 1200MWe ACR-1000.