What classifies a Gen IV reactor
A Generation IV Nuclear Reactor is a reactor technology that is set for deployment between the years 2010 and 2030. Each reactor is designed to operate at temperatures higher than present reactors. Generation IV reactor designs represent advances in sustainability, economics, safety, reliability and proliferation-resistance. In 2002 the GIF (Generation IV International Forum) selected six reactor technologies. Their selection was based on what they believe will shape the future of nuclear energy. These reactors were selected for being a clean, cost-effective, and most importantly safe solution to the increase demands for energy. Another important characteristic of a Gen IV reactor is their resistance to terrorist attacks and weapons' proliferation. Closed fuel cycle systems are utilized in most of the purposed systems. Three of the six are fast reactors, one can be built as a fast reactor, one is described as epithermal, and only two operate with slow neutrons which are used in present systems. One is cooled by light water, two are helium-cooled and the others have lead-bismuth, sodium or fluoride salt coolant. The lead-bismuth, sodium, and fluoride salt systems operate at low pressures which provides a significant safety advantage. Operating temperatures range from 510°C to 1000°C compared to today's reactors which operate at around 330°C. Size ranges for Generation IV systems are from 150 to 1500 MWe. The lead-cooled system optionally available as a 50-150 MWe "battery" with long core life (15-20 years without refueling) as replaceable cassette or entire reactor module. At least four of the systems have significant operating experience already in most respects of their design, which may mean that they can be in commercial operation well before 2030.
Older Generation reactors
Why the time and effort?
Generation IV Reactors are systems aimed to:
- reduce capital cost
- enhance safety
- minimize nuclear waste generation
- help decrease chances of proliferation.
Examples of Gen IV Reactors
- Very-High-Temperature Reactor (VHTR): a graphite-moderated, helium-cooled reactor with a once-through uranium fuel cycle
- Supercritical-Water-Cooled Reactor (SCWR): a high-temperature, high-pressure water-cooled reactor that operates above the thermodynamic critical point of water
- Gas-Cooled Fast Reactor (GFR): features a fast-neutron-spectrum, helium-cooled reactor and closed fuel cycle
- Lead-Cooled Fast Reactor (LFR): features a fast-spectrum lead of lead/bismuth eutectic liquid metal-cooled reactor and a closed fuel cycle for efficient conversion of fertile uranium and management of actinides
- Sodium-Cooled Fast Reactor (SFR): features a fast-spectrum, sodium-cooled reactor and closed fuel cycle for efficient management of actinides and conversion of fertile uranium
- Molten Salt Reactor (MSR): produces fission power in a circulating molten salt fuel mixture with an epithermal-spectrum reactor and a full actinide recycle fuel cycle
- definitions from http://www.gen-4.org/Technology/systems/index.htm
Supercritical-Water-Cooled Reactor (SCWR)
Supercritical-Water-Cooled Reactor (SCWR) is a unique concept that operates above the critical point of water. The SWCR is a high-temperature and high-pressure water cooled reactor. This design provides the ability to achieve a thermal efficiency 1/3 higher than present light water reactors. There is no secondary steam system present in this design because the turbine is driven by the supercritical water coolant. The SCWR is rated at 1,500 MWe and operates at a pressure and temperature of 25 MPa and 510°C. Uranium Oxide is used to fuel this system. The primary concept of the SCWR is to produce electricity efficiently. Two core design concepts have been proposed. The first concept may have a fast-spectrum or thermal reactor. The second concept is a closed cycle fast-spectrum reactor. The closed cycle concept also recycles all actinide using advanced aqueous processing. Design and development of the SWCR has been mostly done in Japan.
Gas-Cooled Fast Reactor (GFR)
Gas-Cooled Fast Reactor (GFR) is a closed fuel cycle system that is cooled by helium and features a fast neutron spectrum. These features allow the GFR to perform 3 processes: power generation, hydrogen production, and process heat. The reactor is rated at 288 MWe and operates at a temperature of 850°C. This system utilizes a Brayton cycle gas turbine which provides a high thermal efficiency. To fuel the system, several possibilities are available. Some of the fuel choices include: composite ceramic fuel, advanced fuel particles, ceramic clad elements of actinide compounds and even depleted uranium. After the fuel life is completed, on site fuel reprocessing, treatment and refabrication is proposed. The core assembly concepts include: prismatic blocks, pin or plate based. The systems direct-cycle helium turbine is used to generate electricity but can also use its process heat for hydrogen production. The GFR also minimizes production of long-lived radioactive waste. This is possible not only because of the fast spectrum but also because of the recycling of all actinides. The GFR system was heavily pursued in the 70’s by companies like General Atomics however, no plants utilizing the GFR concept were ever built.
Lead-Cooled Fast Reactor (LFR)
Lead-Cooled Fast Reactor (LFR) is a system that utilized to efficiently convert fertile uranium and also actinide management. The LFR is a closed fuel cycle that features liquid-metal cooling for the reactor. This fast spectrum system is cooled by lead or lead-bismuth. The LFR has three plant options: 50-150 MWe battery option that results in long refueling intervals, a 300-400 MWe modular system option, and a 1200 MWe large monolithic plant option. This system is cooled by natural convection and operates at 550°C up to 800°C with some advanced materials. The advantage to such a high temperature is the ability to produce hydrogen. To fuel the LFR, depleted Uranium metal or nitride is used. This system also utilizes full actinide recycling. This concept is similar to Russia’s BREST fast reactor technology and LFR designs have been composed by the US and Japan.
Sodium-Cooled Fast Reactor (SFR)
Sodium-Cooled Fast Reactor (SFR) is a unique system that uses sodium to cool the reactor as the name states. The SFR is similar to the LFR because it efficiently converts fertile uranium and also manages actinides. This closed fuel cycle system is also a fast-spectrum reactor. The SFR fuel cycle operates a full actinide recycle that offers two options: a uranium-plutonium-minor-actinide-zirconium metal alloy fuel based reactor rated from 150 to 600 MWe and a mixed uranium-plutonium oxide fuel based reactor rated from 500 to 1,500 MWe. The first option is supported by pyrometallurgical processing integrated with the reactor and the second option is supported by advanced aqueous processing serving multiple reactors. The operating temperature for both SFR options is approximately 550°C which generates electricity from the secondary sodium circuit. The fast spectrum of the SFR allows fissile and fertile materials to be used more efficiently than once-through fuel cycles. No real design or development of Sodium-Cooled Fast Reactors has been explored.
Molten Salt Reactor (MSR)
Molten Salt Reactor (MSR) is a system that circulates a molten salt fuel mixture through core channels and produces an epithermal spectrum. The generated heat is then transferred by a heat exchanger to a secondary coolant system which is finally transferred to the power conversion system by another heat exchanger. Fuel for a MSR is a circulating liquid mixture of sodium, zirconium, and uranium fluorides. One unique feature of this system is that fission products are continuously removed and actinides are fully recycled. The MSR is rated at 1,000 MWe and has an operating temperature of 700°C up to 800°C which ultimately improves thermal efficiency. The MSR was first developed by the US in the 60’s as a back-up option for conventional fast breeder reactors and concept operated as a small prototype model at one point.
Very-High-Temperature Reactor (VHTR)
This reactor has received the highest attention from the US Government. The VHTR, like the rest of the Gen IV reactors, is designed with high efficiency in mind. The design was adapted from the High-Temperature Gas Reactor (HTGR), with the intent to supply electricity and process heat for many applications. The reactor core is comprised of either a prismatic block core or a pebble-bed core. Fuel elements are coated with successive high temperature resistant material layers, then either embedded in graphite block for the prismatic block-type core reactor, or formed into graphite coated pebbles. This structure of layering the fuel gives it the name "TRISO," or tri-structural-isotropic fuel. Because of the multi layers, the pellets are extremely heat resistant, able to reach temperatures around 1600°C. In addition, due to the high durability of the pellets, it is difficult to remove the trapped fission products and ultimately discourages proliferation. Waste disposal also becomes easier as the fuel is concentrated.
Since helium is used as the coolant, this allows for much higher operating temperatures. The core is designed to reach 1000 °C which will increase overall plant efficiency and provide process heat for hydrogen production or the petrochemical industry. Helium is also a very good choice for coolant as it nearly incombustible, it has a relatively high specific heat, and it is an inert gas which means it will not become radioactive or react with other materials inside the reactor.
Fuel and Coolant Information
|neutron spectrum||coolant||temperature (°C)||pressure||fuel||fuel cycle||size(s) (MWe)||uses|
|Gas-cooled fast reactors||fast||helium||850||high||U-238||closed, on site||288||electricity & hydrogen|
|Lead-cooled fast reactors||fast||Pb-Bi||550-800||low||U-238||closed, regional||50-150, 300-400, 1200||electricity & hydrogen|
|Molten salt reactors||epithermal||fluoride salts||700-800||low||UF in salt||closed||1000||electricity & hydrogen|
|Sodium-cooled fast reactors||fast||sodium||550||low||U-238 & MOX||closed||150-500, 500-1500||electricity|
|Supercritical water-cooled reactors||thermal or fast||water||510-550||very high||UO2||open (thermal), closed (fast)||1500||electricity|
|Very high temperature gas reactors||thermal||helium||1000||high||UO2 prism or pebbles||open||250||hydrogen & electricity|
-table reproduced from http://www.usnuclearenergy.org/GEN%20IV%20Reactors.htm