| INTRODUCTION | | | | Generation IV designs are still on the drawing board |
| The principles for using nuclear power to produce | | | | and will not be operational before 2020 at the |
| electricity are the same for most types of reactor. | | | | earliest, probably later. They will tend to have closed |
| The energy released from continuous fission of the | | | | fuel cycles and burn the long-lived actinides now |
| atoms of the fuel is harnessed as heat in either a gas | | | | forming part of spent fuel, so that fission products |
| or water, and is used to produce steam. The steam | | | | are the only high-level waste. Many will be fast |
| is used to drive the turbines which produce electricity | | | | neutron reactors. |
| (as in most fossil fuel plants). If graphite or heavy | | | | More than a dozen (Generation III) designs are in |
| water is used as moderator, it is possible to run a | | | | various stages of development. Some are |
| power reactor on natural instead of enriched uranium. | | | | evolutionary from the PWR, BWR and CANDU |
| Natural uranium has the same elemental composition | | | | designs above, some are more radical departures. |
| as when it was mined (0.7% U-235, over 99.2% | | | | The former include the Advanced Boiling Water |
| U-238), enriched uranium has had the proportion of | | | | Reactor, a few of which are now operating with |
| the fissile isotope (U-235) increased by a process | | | | others under construction. The best-known radical |
| called enrichment, commonly to 3.5 - 5.0%. In this | | | | new design is the Pebble Bed Modular Reactor, using |
| case the moderator can be ordinary water, and such | | | | helium as coolant, at very high temperature, to drive |
| reactors are collectively called light water reactors. | | | | a turbine directly. |
| Because the light water absorbs neutrons as well as | | | | Considering the closed fuel cycle, Generation 1-3 |
| slowing them, it is less efficient as a moderator than | | | | reactors recycle plutonium (and possibly uranium), |
| heavy water or graphite. | | | | while Generation IV are expected to have full actinide |
| Practically all fuel is ceramic uranium oxide (UO2 with | | | | recycle. |
| a melting point of 2800°C) and most is enriched. | | | | Fast neutron reactors |
| The fuel pellets (usually about 1 cm diameter and 1.5 | | | | Some reactors (only one in commercial service) do |
| cm long) are typically arranged in a long zirconium | | | | not have a moderator and utilise fast neutrons, |
| alloy (zircaloy) tube to form a fuel rod, the zirconium | | | | generating power from plutonium while making more |
| being hard, corrosion-resistant and permeable to | | | | of it from the U-238 isotope in or around the fuel. |
| neutrons.* Numerous rods form a fuel assembly, | | | | While they get more than 60 times as much energy |
| which is an open lattice and can be lifted into and out | | | | from the original uranium compared with the normal |
| of the reactor core. In the most common reactors | | | | reactors, they are expensive to build and await |
| these are about 3.5 to 4 metres long. | | | | resource scarcity to come into their own. |
| Zirconium is an important mineral for nuclear power, | | | | Lifetime of nuclear reactors. |
| where it finds its main use. It is therefore subject to | | | | Most of today's nuclear plants which were originally |
| controls on trading. It is normally contaminated with | | | | designed for 30 or 40-year operating lives. |
| hafnium, a neutron absorber, so very pure 'nuclear | | | | However, with major investments in systems, |
| grade' Zr is used to make the zircaloy, which is about | | | | structures and components lives can be extended, |
| 98% Zr plus tin, iron, chromium and sometimes nickel | | | | and in several countries there are active programs to |
| to enhance its strength. | | | | extend operating lives. In the USA most of the |
| Burnable poisons are often used (especially in BWR) | | | | more than one hundred reactors are expected to be |
| in fuel or coolant to even out the performance of | | | | granted licence extensions from 40 to 60 years. |
| the reactor over time from fresh fuel being loaded to | | | | This justifies significant capital expenditure in |
| refuelling. These are neutron absorbers which decay | | | | upgrading systems and components, including building |
| under neutron exposure, compensating for the | | | | in extra performance margins. |
| progressive build up of neutron absorbers in the fuel | | | | Some components simply wear out, corrode or |
| as it is burned. The best known is gadolinium, which is | | | | degrade to a low level of efficiency. These need to |
| a vital ingredient of fuel in naval reactors where | | | | be replaced. Steam generators are the most |
| installing fresh fuel is very inconvenient, so reactors | | | | prominent and expensive of these, and many have |
| are designed to run more than a decade between | | | | been replaced after about 30 years where the |
| refuellings. | | | | reactor otherwise has the prospect of running for 60 |
| Pressurised Water Reactor (PWR) | | | | years. This is essentially an economic decision. |
| This is the most common type, with over 230 in use | | | | Lesser components are more straightforward to |
| for power generation and a further several hundred | | | | replace as they age. In Candu reactors, pressure |
| in naval propulsion. The design originated as a | | | | tube replacement has been undertaken on some |
| submarine power plant. It uses ordinary water as | | | | plants after about 30 years operation. |
| both coolant and moderator. The design is | | | | A second issue is that of obsolescence. For |
| distinguished by having a primary cooling circuit which | | | | instance, older reactors have analogue instrument and |
| flows through the core of the reactor under very | | | | control systems. Thirdly, the properties of materials |
| high pressure, and a secondary circuit in which steam | | | | may degrade with age, particularly with heat and |
| is generated to drive the turbine. | | | | neutron irradiation. In respect to all these aspects, |
| A PWR has fuel assemblies of 200-300 rods each, | | | | investment is needed to maintain reliability and |
| arranged vertically in the core, and a large reactor | | | | safety. Also, periodic safety reviews are |
| would have about 150-250 fuel assemblies with | | | | undertaken on older plants in line with international |
| 80-100 tonnes of uranium. | | | | safety conventions and principles to ensure that |
| Water in the reactor core reaches about 325°C, | | | | safety margins are maintained. |
| hence it must be kept under about 150 times | | | | Floating nuclear power plants |
| atmospheric pressure to prevent it boiling. Pressure is | | | | Apart from over 200 nuclear reactors powering |
| maintained by steam in a pressuriser (see diagram). In | | | | various kinds of ships, Rosatom in Russia has set up |
| the primary cooling circuit the water is also the | | | | a subsidiary to supply floating nuclear power plants |
| moderator, and if any of it turned to steam the | | | | ranging in size from 70 to 600 MWe. These will be |
| fission reaction would slow down. This negative | | | | mounted in pairson a large barge, which will be |
| feedback effect is one of the safety features of the | | | | permanently moored where it is needed to supply |
| type. The secondary shutdown system involves | | | | power and possibly some desalination to a shore |
| adding boron to the primary circuit. | | | | settlement or industrial complex. The first will have |
| The secondary circuit is under less pressure and the | | | | two 40 MWe reactors based on those in icebreakers |
| water here boils in the heat exchangers which are | | | | and will operate at Severodvinsk, in the Archangel |
| thus steam generators. The steam drives the turbine | | | | region. Five of the next seven will be used by |
| to produce electricity, and is then condensed and | | | | Gazprom for offshore oil and gas field development |
| returned to the heat exchangers in contact with the | | | | and for operations on the Kola and Yamal peninsulas. |
| primary circuit. | | | | One is for Pevek on the Chukotka peninsula, another |
| Boiling Water Reactor (BWR) | | | | for Kamchatka region, both in the far east of the |
| This design (diagram next page) has many similarities | | | | country. Further far east sites being considered are |
| to the PWR, except that there is only a single circuit | | | | Yakutia and Taimyr. Electricity cost is expected to be |
| in which the water is at lower pressure (about 75 | | | | much lower than from present alternatives. |
| times atmospheric pressure) so that it boils in the | | | | The Russian KLT-40S is a reactor well proven in |
| core at about 285°C. The reactor is designed to | | | | icebreakers and now proposed for wider use in |
| operate with 12-15% of the water in the top part of | | | | desalination and, on barges, for remote area power |
| the core as steam, and hence with less moderating | | | | supply. Here a 150 MWt unit produces 35 MWe |
| effect and thus efficiency there. | | | | (gross) as well as up to 35 MW of heat for |
| The steam passes through drier plates (steam | | | | desalination or district heating. These are designed to |
| separators) above the core and then directly to the | | | | run 3-4 years between refuelling and it is envisaged |
| turbines, which are thus part of the reactor circuit. | | | | that they will be operated in pairs to allow for |
| Since the water around the core of a reactor is | | | | outages, with on-board refuelling capability and used |
| always contaminated with traces of radio nuclides, it | | | | fuel storage. At the end of a 12-year operating cycle |
| means that the turbine must be shielded and | | | | the whole plant is taken to a central facility for |
| radiological protection provided during maintenance. | | | | overhaul and removal of used fuel. Two units will be |
| The cost of this tends to balance the savings due to | | | | mounted on a 20,000 tonne barge. A larger Russian |
| the simpler design. Most of the radioactivity in the | | | | factory-built and barge-mounted reactor is the |
| water is very short-lived*, so the turbine hall can be | | | | VBER-150, of 350 MW thermal, 110 MWe. The larger |
| entered soon after the reactor is shut down. | | | | VBER-300 PWR is a 325 MWe unit, originally |
| mostly N-16, with a 7 second half-life | | | | envisaged in pairs as a floating nuclear power plant, |
| A BWR fuel assembly comprises 90-100 fuel rods, | | | | displacing 49,000 tonnes. As a cogeneration plant it is |
| and there are up to 750 assemblies in a reactor core, | | | | rated at 200 MWe and 1900 GJ/hr. |
| holding up to 140 tonnes of uranium. The secondary | | | | Primary coolants |
| control system involves restricting water flow | | | | The advent of some of the designs mentioned |
| through the core so that steam in the top part | | | | above provides opportunity to review the various |
| means moderation is reduced. | | | | primary coolants used in nuclear reactors: |
| Pressurized Heavy Water Reactor (PHWR or | | | | Water or heavy water must be maintained at very |
| CANDU) | | | | high pressure (1000-2200 psi, 7-15 MPa) to enable it |
| The CANDU reactor design has been developed since | | | | to function above 100°C, as in present reactors. |
| the 1950s in Canada. It uses natural uranium (0.7% | | | | This has a major influence on reactor engineering. |
| U-235) oxide as fuel, hence needs a more efficient | | | | However, supercritical water around 25 MPa can give |
| moderator, in this case heavy water (D2O).** | | | | 45% thermal efficiency - as at some fossil-fuel |
| with the CANDU system, the moderator is enriched | | | | power plants today with outlet temperatures of |
| (ie water) rather than the fuel, - a cost trade-off. | | | | 600°C, and at ultra supercritical levels (30+ MPa) |
| The moderator is in a large tank called a calandria, | | | | 50% may be attained. |
| penetrated by several hundred horizontal pressure | | | | Helium must be used at similar pressure (1000-2000 |
| tubes which form channels for the fuel, cooled by a | | | | psi, 7-14 MPa) to maintain sufficient density for |
| flow of heavy water under high pressure in the | | | | efficient operation. Again, there are engineering |
| primary cooling circuit, reaching 290°C. As in the | | | | implications, but it can be used in the Brayton cycle |
| PWR, the primary coolant generates steam in a | | | | to drive a turbine directly. |
| secondary circuit to drive the turbines. The pressure | | | | Carbon dioxide was used in early British reactors and |
| tube design means that the reactor can be refuelled | | | | their AGRs. It is denser than helium and thus likely to |
| progressively without shutting down, by isolating | | | | give better thermal conversion efficiency. There is |
| individual pressure tubes from the cooling circuit. | | | | now interest in supercritical CO2 for the Brayton |
| A CANDU fuel assembly consists of a bundle of 37 | | | | cycle. |
| half metre long fuel rods (ceramic fuel pellets in | | | | Sodium, as normally used in fast neutron reactors, |
| zircaloy tubes) plus a support structure, with 12 | | | | melts at 98°C and boils at 883°C at atmospheric |
| bundles lying end to end in a fuel channel. Control | | | | pressure, so despite the need to keep it dry the |
| rods penetrate the calandria vertically, and a | | | | engineering required to contain it is relatively modest. |
| secondary shutdown system involves adding | | | | However, normally water/steam is used in the |
| gadolinium to the moderator. The heavy water | | | | secondary circuit to drive a turbine (Rankine cycle) at |
| moderator circulating through the body of the | | | | lower thermal efficiency than the Brayton cycle. |
| calandria vessel also yields some heat (though this | | | | Lead or lead-bismuth are capable of higher |
| circuit is not shown on the diagram above). | | | | temperature operation. They are transparent to |
| Advanced Gas-cooled Reactor (AGR) | | | | neutrons, aiding efficiency, and do not react with |
| These are the second generation of British | | | | water. However, they are corrosive of fuel cladding |
| gas-cooled reactors, using graphite moderator and | | | | and steels, and Pb-Bi yields Po activation products. |
| carbon dioxide as coolant. The fuel is uranium oxide | | | | Pb-Bi melts at 125°C and boils at 1670°C, Pb melts |
| pellets, enriched to 2.5-3.5%, in stainless steel tubes. | | | | at 327°C and boils at 1737°C. In 1998 Russia |
| The carbon dioxide circulates through the core, | | | | declassified a lot of research information derived |
| reaching 650°C and then past steam generator | | | | from its experience with submarine reactors, and US |
| tubes outside it, but still inside the concrete and steel | | | | interest in using Pb/Pb-Bi for small reactors has |
| pressure vessel. Control rods penetrate the | | | | increased subsequently. |
| moderator and a secondary shutdown system | | | | Molten fluoride salt boils at 1400°C at atmospheric |
| involves injecting nitrogen to the coolant. | | | | pressure, so allows several options for use of the |
| The AGR was developed from the Magnox reactor, | | | | heat, including using helium in a secondary Brayton |
| also graphite moderated and CO2 cooled, and a | | | | cycle with thermal efficiencies of 48% at 750°C to |
| number of these are still operating in UK. They use | | | | 59% at 1000°C, or manufacture of hydrogen. |
| natural uranium fuel in metal form. | | | | Low-pressure liquid coolants allow all their heat to be |
| Light water graphite-moderated reactor | | | | delivered at high temperatures, since the |
| This is a Soviet design, developed from plutonium | | | | temperature drop in heat exchangers is less than |
| production reactors. It employs long (7 metre) | | | | with gas coolants. Also, with a good margin between |
| vertical pressure tubes running through graphite | | | | operating and boiling temperatures, passive cooling |
| moderator, and is cooled by water, which is allowed | | | | for decay heat is readily achieved. |
| to boil in the core at 290°C, much as in a BWR. Fuel | | | | The removal of passive decay heat is a vital feature |
| is low-enriched uranium oxide made up into fuel | | | | of primary cooling systems, beyond heat transfer to |
| assemblies 3.5 metres long. With moderation largely | | | | do work. When the fission process stops, fission |
| due to the fixed graphite, excess boiling simply | | | | product decay continues and a substantial amount of |
| reduces the cooling and neutron absorbtion without | | | | heat is added to the core. At the moment of |
| inhibiting the fission reaction, and a positive feedback | | | | shutdown, this is about 6% of the full power level, |
| problem can arise. | | | | but it quickly drops to about 1% as the short-lived |
| Advanced reactors | | | | fission products decay. This heat could melt the |
| Several generations of reactors are commonly | | | | core of a light water reactor unless it is reliably |
| distinguished. Generation I reactors were developed in | | | | dissipated. Typically some kind of convection flow is |
| 1950-60s and very few are still running today. They | | | | relied upon. |
| mostly used natural uranium fuel and used graphite as | | | | During this long reaction period about 5.4 tonnes of |
| moderator. Generation II reactors are typified by the | | | | fission products as well as 1.5 tonnes of plutonium |
| present US fleet and most in operation elsewhere. | | | | together with other transuranic elements were |
| They typically use enriched uranium fuel and are | | | | generated in the ore body. The initial radioactive |
| mostly cooled and moderated by water. Generation | | | | products have long since decayed into stable |
| III are the Advanced Reactors, the first few of | | | | elements but close study of the amount and location |
| which are in operation in Japan and others are under | | | | of these has shown that there was little movement |
| construction and ready to be ordered. They are | | | | of radioactive wastes during and after the nuclear |
| developments of the second generation with | | | | reactions. Plutonium and the other transuranics |
| enhanced safety. | | | | remained immobile. |