In a Fast Neutron Reactor, there is no moderator. Neutron speed is allowed to remain at 49 million miles per hour. Because the neutrons are traveling so fast, the likelihood of them interacting with a Uranium-235 nucleus and causing a fission is very much reduced. To counter this, the enrichment of the fuel must be increased, typically to 20-30% U-235.
Fast Neutron Reactors have a couple of advantages over Thermal Neutron Reactors:
- In a thermal neutron environment, neutrons less energetic (speed) and therefore are very easily absorbed by fissile atoms. Most of these atoms will fission, but others simply absorb the neutron and become radioactive. After decay, these atoms will become Neptunium, Plutonium, and Americium, which are heavier than Uranium, and are called "Transuranic" elements. These transuranic byproducts account for most of the very long-lived radioactive waste that is in spent nuclear fuel. Fast Neutrons are not absorbed as frequently to create transuranic elements. Furthermore a Fast Neutron has enough energy to fission a transuranic nucleus in the event one is created. Thus we get extra power from what was previously radioactive transuranic waste.
- Because the transuranic waste is burned up, the problem of nuclear waste becomes a much shorter term issue. Only the fission products (split nuclei with shorter half-lives) are of concern.
- Atoms split by fast neutrons produce more neutrons than atoms split by thermal neutrons. These excess neutrons can be put to use by allowing them to escape the core and be moderated, where they can create more fuel, such as Plutonium-239, or Thorium-233. This design is known as a "Fast Breeder Reactor".
- 20-30% enriched U-235 is called "highly enriched", and is actually weapon-grade material. This is one reason why Fast Neutron Reactors are uncommon. There are concerns about the diversion of fuel (either by large-scale theft, or in trace quantities) into someone's nuclear weapon program. The fact that it's pretty easy to make a Uranium weapon makes using highly enriched Uranium fuel even less desirable.
- Enriching Uranium to 20-30% U-235 is hugely expensive. Thousands of cascading gaseous distillation loops have to be run with compressors for long periods of time to enrich Uranium.
- Because the Fast Neutron Reactor cannot have a moderator present in the core, water cannot be used for cooling. These reactors all therefore circulate liquid metals for cooling, which brings about some other advantages, as well as difficulties
- Liquid metal cooling: Advantages and Disadvantages
- Advantage: Liquid metal does not boil at the temperatures useful for generating steam for electric power, so the primary coolant loop doesn't have to be very robust at all. It can be run at atmospheric pressure.
- Advantage: Liquid metal can be pumped with an electromagnetic pump. Wrap a wire around a pipe, energize it with DC power, and the liquid metal inside behaves like a solenoid, being forced out of the way by elecromotive force. This is called an "inductive pump", and it has no moving parts.
- Advantage: Liquid metals can carry a great deal more heat from the reactor core than water can, so a given size core can generate a great deal more power than a water-cooled one. (side note: The fastest submarine ever built used a liquid metal-cooled reactor)
- Disadvantage: Liquid metal in the reactor will absorb neutrons and become quite radioactive, compared to water.
- Disadvantage: Some of the more desirable liquid metals (lead/bismuth, sodium) will solidify if not kept hot. If circulation is ever lost, it's very possible that entire cooling loops will solidify.
- Disadvantage: Because liquid metal coolants absorb some neutrons, they are a mild poison for Fast Neutrons: If boiling occurs in the core, more of this neutron absorption mechanism is gone and more Fast neutrons suddenly become available for fission. So this reactor has a "positive void coefficient of reactivity", and thus a runaway thermal/nuclear reaction can occur.
- Disadvantage: Liquid Sodium is one preferred coolant, and at these temperatures, ignites on contact with air in the event of a leak.
So now that we (kinda) understand the nuclear characteristics of a Fast Neutron Reactor, let's look at a few examples. The very first Fast Neutron Reactor was built in 1946 at Los Alamos as an experment called "Clementine". It was cooled by liquid Mercury and fueled with Plutonium. It produced 25 KW of heat, no electric power. This is the only photo I could find of Clementine, which pretty much only shows shield doors.
EBR-1 was a very significant reactor, because it proved Enrico Fermi's theory that a reactor could create more fuel than was used in the fission process. EBR is short-hand for "Experimental Breeder Reactor". It is notable for another reason. This reactor was the first one ever connected to a steam plant to generate elecricity. As such, it was also the first nuclear power plant! It generated 200KW of electricity from a 1.4 MW heat source in 1951.
EBR-1 suffered a partial meltdown in 1955 while running at reduced coolant flow. The coolant flow was reduced in an effort to understand some irregularities in the reactor power. Boiling occured, and a power excursion happened. Remember the positive void coefficient?
The mosaic below shows the reactor shielding structure on the left. The small cylinders are to insert breedable materials, to test if breeding more fuel was feasible. The top right shows the first four light bulbs ever lit with electricity produced from nuclear power. The bottom right shows the small turbo-generator that eventually produced 200 KW of in-house power.
Fermi I was a larger scale prototype Fast Breeder Reactor, and was actually operated by an electric utility, Detroit Edison. Its nameplate power was 94 MWe. It attained first criticality in August 1963. Lengthy tests at low power were conducted before bringing it to higher power levels. High coolant temperatures were noted on a couple of the in-core thermocouples. Fuel was shuffled around to see if the issue was the thermocouples or if the temperatures were real. At just 13% power, the reactor experienced a partial meltdown due to blockage in just two of the coolant channels.
Because Fast Neutron Reactors contain such highly enriched fuel, there is the potential that following a meltdown, the molten fuel could drop to the bottom of the reactor vessel and become critical again, burning its way out the bottom of the reactor vessel, and continue downward into the earth, still critical and lava-hot. This is where the term "The China Syndrome" came from: The notion that a fast nuclear reactor meltdown would burn through the earth 'all the way to China'.
The irony at Fermi I is that what caused the coolant blockage and meltdown was a loose piece of something called a "fuel diverter". The fuel diverter was installed to separate a melted-down reactor core into several smaller, sub-critical units at the bottom of the reactor vessel, and yet this damaged device actually caused a meltdown.
Below, Fermi I in its heyday. The reactor containment dome is to the left.
Below, a diagram of the Fermi Fast Breeder coolant system. Note the pump is a mechanical type, not an induction type. Also note that there are TWO sodium coolant loops. One is radioactive and cools the reactor. The item right of the core labeled "IHX" is an intermediate heat exchanger. It transfers heat from the primary (radioactive) sodium coolant into the secondary (non-radioactive) sodium loop. This takes the heat and transfers it ouside the containment, where it is used to generate steam.
The diagram below shows how a more modern "Pool-type" Fast Neutron Reactor would function, and gives a clearer indication of how heat was removed from the EBR-2 reactor.
There have been a number of Fast Neutron Reactors Built, although nowhere near the number of thermal reactors built. Some of the notable ones (from Wiki):
- CLEMENTINE, the first fast reactor, built in 1946 at Los Alamos National Laboratory. Plutonium metal fuel, mercury coolant, power 25 kW thermal, used for research, especially as a fast neutron source.
- EBR-I at Idaho Falls, which in 1951 became the first reactor to generate significant amounts of electrical power. Decommissioned 1964.
- Fermi 1 near Detroit was a prototype fast breeder reactor that began operating in 1957 and shut down in 1972.
- EBR-II Prototype for the Integral Fast Reactor, 1965–1995?.
- SEFOR in Arkansas, a 20 MWt research reactor which operated from 1969 to 1972.
- Fast Flux Test Facility, 400MWt, Operated flawlessly from 1982 to 1992, at Hanford Washington, now deactivated, liquid sodium is drained with argon backfill under care and maintenance.
- DFR (Dounreay Fast Reactor, 1959–1977, 14MWe) and PFR (Prototype Fast Reactor, 1974–1994, 250MWe), in Caithness, in the Highland area of Scotland.
- Rhapsodie in Cadarache, France, (20 then 40 MW) between 1967 and 1982.
- Superphénix, in France, 1200MWe, closed in 1997 due to a political decision and very high costs of operation.
- Phénix, 1973, France, 233 MWe, restarted 2003 at 140 MWe for experiments on transmutation of nuclear waste for six years, ceased power generation in March 2009, though it will continue in test operation and to continue research programs by CEA until the end of 2009. Stopped in 2010.
- KNK-II, Germany
- Small lead-cooled fast reactors used for naval propulsion, particularly by the Soviet Navy.
- BR-5 - research fast neutron reactor at the Institute of Physics and Energy in Obninsk. Years of operation 1959-2002.
- BOR-60 - sodium-cooled reactor at the Research Institute of Atomic Reactors in Dmitrovgrad. In operation since 1980.
- BN-350, constructed by the Soviet Union in Shevchenko (today's Aqtau) on the Caspian Sea, 130MWe plus 80,000 tons of fresh water per day.
- BN-600 - sodium-cooled reactor at the Beloyarsk Nuclear Power Station in operation since 1980.
- BN-800 - sodium-cooled reactor at the Beloyarsk Nuclear Power Station under construction.
- BN-1200 - in design study at the Beloyarsk Nuclear Power Station. Build will be started in 2015.
- BN-K - project.
- IBR-2 - research fast neutron reactor at the Joint Institute of Nuclear Research in Dubna (near Moscow).
- Clinch River Breeder Reactor, USA
- Integral Fast Reactor, USA. Design emphasized fuel cycle based on on-site electrolytic reprocessing. Cancelled 1994 without construction.
- SNR-300, Germany
- Monju reactor, 300MWe, in Japan. was closed in 1995 following a serious sodium leak and fire. It was restarted May 6, 2010 and in August 2010 another accident, involving dropped machinery, shut down the reactor again. As of June 2011, the reactor has only generated electricity for one hour since its first testing two decades prior.
- Jōyō (常陽), 1977–1997 and 2003–, Japan
- BN-600, 1981, Russia, 600 MWe, scheduled end of life 2010 but still in operation.
- FBTR, 1985, India, 10.5 MWt
- China Experimental Fast Reactor, 65 MWt, planned 2009, critical 2010