Integral fast reactor

thumb|upright=1.5|[[Experimental Breeder Reactor II, which served as the prototype for the integral fast reactor (IFR)]]

The integral fast reactor (IFR), originally the advanced liquid-metal reactor (ALMR), is a design for a nuclear reactor using fast neutrons and no neutron moderator (a "fast" reactor). IFRs can breed more fuel and are distinguished by a nuclear fuel cycle that uses reprocessing via electrorefining at the reactor site. The IFR was a sodium-cooled fast reactor (SFR) is its closest surviving fast breeder reactor, a type of Generation IV reactor.

The U.S. Department of Energy (DOE) began designing an IFR in 1984 and built a prototype, the Experimental Breeder Reactor II. On April 3, 1986, two tests demonstrated the safety of the IFR concept. These tests simulated accidents involving loss of coolant flow. Even with its normal shutdown devices disabled, the reactor shut itself down safely without overheating anywhere in the system. The IFR project was canceled by the US Congress in 1994, three years before completion.

S-PRISM (from SuperPRISM), also called PRISM (power reactor innovative small module), is the name of a nuclear power plant design by GE Vernova Hitachi Nuclear Energy based on the IFR. In 2022, GE Vernova Hitachi Nuclear Energy and TerraPower began exploring locating five Natrium SFR-based nuclear power plants in Kemmerer, Wyoming; the design incorporates a PRISM reactor plus TerraPower's Traveling Wave design with a molten salt storage system.

History

Research on IFR reactors began in 1984 at Argonne National Laboratory in Argonne, Illinois, as a part of the U.S. Department of Energy's national laboratory system, and currently operated on a contract by the University of Chicago.

thumb|upright=1.5|The [[Experimental Breeder Reactor II (EBR II)]]

Argonne previously had a branch campus named "Argonne West" in Idaho Falls, Idaho, that is now part of the Idaho National Laboratory. In the past, at the branch campus, physicists from Argonne West built what was known as the Experimental Breeder Reactor II (EBR-II). In the meantime, physicists at Argonne designed the IFR concept, and it was decided that the EBR-II would be converted to an IFR. Charles Till, a Canadian physicist from Argonne, was the head of the IFR project, and Yoon Chang was the deputy head. Till was positioned in Idaho, while Chang was in Illinois. Chang later served as acting director of Argonne.

The IFR concept is described in detail in Plentiful Energy: The Story of the Integral Fast Reactor.

Cancellation

With the election of President Bill Clinton in 1992, and the appointment of Hazel O'Leary as the Secretary of Energy, there was pressure from the top to cancel the IFR. Senator John Kerry (D-MA) and O'Leary led the opposition to the reactor, arguing that it would be a threat to non-proliferation efforts, and that it was a continuation of the Clinch River Breeder Reactor Project that had been canceled by Congress. Charles Till related that when he told Frank N. von Hippel, a science advisor to President Clinton, that it would cost more to terminate the research program and destroy the reactor than to finish the program and mothball the reactor, von Hippel replied "I know; it's a symbol. It has to go."

Simultaneously, in 1994 Energy Secretary O'Leary awarded the lead IFR scientist with $10,000 and a gold medal, with the citation stating his work to develop IFR technology provided "improved safety, more efficient use of fuel and less radioactive waste".

IFR opponents also presented a report by the DOE's Office of Nuclear Safety regarding a former Argonne employee's allegations that Argonne had retaliated against him for raising concerns about safety, as well as about the quality of research done on the IFR program. The report received international attention, with a notable difference in the coverage it received from major scientific publications. The British journal Nature entitled its article "Report backs whistleblower", and also noted conflicts of interest on the part of a DOE panel that assessed IFR research. In contrast, the article that appeared in Science was entitled "Was Argonne Whistleblower Really Blowing Smoke?".

Since 2000

In 2001, as part of the Generation IV roadmap, the DOE tasked a 242-person team of scientists from DOE, UC Berkeley, Massachusetts Institute of Technology (MIT), Stanford, ANL, Lawrence Livermore National Laboratory, Toshiba, Westinghouse, Duke, EPRI, and other institutions to evaluate 19 of the best reactor designs on 27 different criteria. The IFR ranked #1 in their study which was released April 9, 2002.

At present, there are no integral fast reactors in commercial operation. However, the BN-800 reactor, a very similar fast reactor operated as a burner of plutonium stockpiles, became commercially operational in 2016.

Technical overview

The IFR is cooled by liquid sodium and fueled by an alloy of uranium and plutonium. The fuel is contained in steel cladding with liquid sodium filling in the space between the fuel and the cladding. A void above the fuel allows helium and radioactive xenon to be collected safely without significantly increasing pressure inside the fuel element, and also allows the fuel to expand without breaching the cladding, making metal rather than oxide fuel practical. The advantages of liquid sodium coolant, as opposed to liquid metal lead, are that liquid sodium is far less dense and far less viscous (reduced pumping costs), is not corrosive (via dissolution) to common steels, and creates essentially no radioactive neutron activation byproducts. The disadvantage of sodium coolant, as opposed to lead coolant, is that sodium is chemically reactive, especially with water or air. Lead may be substituted for the eutectic alloy of lead and bismuth, as used as reactor coolant in Soviet Alfa-class submarines.

Basic design decisions

Metallic fuel

Metal fuel with a sodium-filled void inside the cladding to allow fuel expansion has been demonstrated in EBR-II. Pyroprocessing is primarily used for the reprocessing of metallic fuel.

Fabrication of metallic fuel is easier and cheaper than ceramic (oxide) fuel, especially under remote handling conditions. The high heat capacity of the coolant and the elimination of water from the reactor core increase the inherent safety of the core.

Pyroprocessing (using an electrorefiner) has been demonstrated at EBR-II as practical on the scale required. Compared to the PUREX aqueous process, it is economical in capital cost, and is unsuitable for the production of weapons material, again unlike PUREX which was developed for weapons programs.

Pyroprocessing makes metallic fuel the fuel of choice. The two decisions are complementary.

Advantages

Breeder reactors (such as the IFR) could in principle extract almost all of the energy contained in uranium or thorium, decreasing fuel requirements by nearly two orders of magnitude compared to traditional once-through reactors, which extract less than 0.65% of the energy in mined uranium, and less than 5% of the enriched uranium with which they are fueled. This could greatly dampen concern about fuel supply or energy used in mining.

What is more important today is why fast reactors are fuel-efficient: because fast neutrons can fission or "burn out" all the transuranic waste components. Transuranic waste consists of actinides – reactor-grade plutonium and minor actinides – many of which last tens of thousands of years or longer and make conventional nuclear waste disposal so problematic. Most of the radioactive fission products produced by an IFR have much shorter half-lives: they are intensely radioactive in the short term but decay quickly. Through many cycles, the IFR ultimately causes 99.9% of the uranium and transuranium elements to undergo fission and produce power; so, its only waste is the nuclear fission products. These have much shorter half-lives; in 300 years, their radioactivity will fall below that of the original uranium ore. The fact that 4th generation reactors are being designed to use the waste from 3rd generation plants could change the nuclear story fundamentally—potentially making the combination of 3rd and 4th generation plants a more attractive energy option than 3rd generation by itself would have been, both from the perspective of waste management and energy security.

"Integral" refers to on-site reprocessing by electrochemical pyroprocessing. This process separates spent fuel into 3 fractions: uranium, plutonium isotopes and other transuranium elements, and nuclear fission products. The uranium and transuranium elements are recycled into new fuel rods, and the fission products are eventually converted to glass and metal blocks for safer disposal. Because the combined transuranium elements and the fission products are highly radioactive, fuel-rod transfer and reprocessing operations use robotic or remote-controlled equipment. An additional claimed benefit of this is that since fissile material never leaves the facility (and would be lethal to handle if it did), this greatly reduces the proliferation potential of possible diversion of fissile material.

Safety

In traditional light-water reactors (LWRs) the core must be maintained at a high pressure to keep the water liquid at high temperatures. In contrast, since the IFR is a liquid metal cooled reactor, the core could operate at close to ambient pressure, dramatically reducing the danger of a loss-of-coolant accident. The entire reactor core, heat exchangers, and primary cooling pumps are immersed in a pool of liquid sodium or lead, making a loss of primary coolant extremely unlikely. The coolant loops are designed to allow for cooling through natural convection, meaning that in the case of a power loss or unexpected reactor shutdown, the heat from the reactor core would be sufficient to keep the coolant circulating even if the primary cooling pumps were to fail.

The IFR also has passive safety advantages as compared with conventional LWRs. The fuel and cladding are designed such that when they expand due to increased temperatures, more neutrons would be able to escape the core, thus reducing the rate of the fission chain reaction. In other words, an increase in the core temperature acts as a feedback mechanism that decreases the core power. This attribute is known as a negative temperature coefficient of reactivity. Most LWRs also have negative reactivity coefficients; however, in an IFR, this effect is strong enough to stop the reactor from reaching core damage without external action from operators or safety systems. This was demonstrated in a series of safety tests on the prototype. Pete Planchon, the engineer who conducted the tests for an international audience, quipped "Back in 1986, we actually gave a small [20 MWe] prototype advanced fast reactor a couple of chances to melt down. It politely refused both times."

Liquid sodium presents safety problems because it ignites spontaneously on contact with air and can cause explosions on contact with water. This was the case at the Monju Nuclear Power Plant in a 1995 accident and fire. To reduce the risk of explosions following a leak of water from the steam turbines, the IFR design (as with other SFRs) includes an intermediate liquid-metal coolant loop between the reactor and the steam turbines. The purpose of this loop is to ensure that any explosion following the accidental mixing of sodium and turbine water would be limited to the secondary heat exchanger and not pose a risk to the reactor itself. Alternative designs use lead instead of sodium as the primary coolant. The disadvantages of lead are its higher density and viscosity, which increases pumping costs, and radioactive activation products resulting from neutron absorption. A lead-bismuth eutectate, as used in some Russian submarine reactors, has lower viscosity and density, but the same activation product problems can occur.

Efficiency and fuel cycle

The goals of the IFR project were to increase the efficiency of uranium usage by breeding plutonium and to eliminate the need for transuranic isotopes to ever leave the site. The reactor was an unmoderated design running on fast neutrons, designed to allow any transuranic isotope to be consumed (and in some cases used as fuel).

Compared to current light-water reactors with a once-through fuel cycle that induces fission (and derives energy) from less than 1% of the uranium found in nature, a breeder reactor like the IFR has a very efficient fuel cycle (99.5% of uranium undergoes fission). The basic scheme uses pyroelectric separation, a common method in other metallurgical processes, to remove transuranics and actinides from the wastes and concentrate them. These concentrated fuels are then reformed, on-site, into new fuel elements.

The available fuel metals are never separated from the plutonium isotopes nor from all the fission products,

Another important benefit of removing the long-half-life transuranics from the waste cycle is that the remaining waste becomes a much shorter-term hazard. After the actinides (reprocessed uranium, plutonium, and minor actinides) are recycled, the remaining radioactive waste isotopes are fission products – with half-lives of 90 years (Sm-151) and less, or 211,100 years (Tc-99) and more – plus any activation products from the non-fuel reactor components.

Comparisons to light-water reactors

thumb|Transmutation flow between 238[[Plutonium-238|Pu and 244Cm in a LWR. Current thermal-neutron fission reactors cannot fission actinide nuclides that have an even number of neutrons. Thus, these build up and are generally treated as transuranic waste after conventional reprocessing. An argument for fast reactors is that they can fission all actinides.|360x360px]]

Nuclear waste

The waste products of IFR reactors either have a short half-life, which means that they decay quickly and become relatively safe, or a long half-life, which means that they are only slightly radioactive. Neither of the two forms of IFR waste produced contain plutonium or other actinides. Due to pyroprocessing, the total volume of true waste/fission products is 1/20th the volume of spent fuel produced by a light-water plant of the same power output, and is often considered to be all unusable waste. 70% of fission products are either stable or have half-lives under one year. Technetium-99 and iodine-129, which constitute 6% of fission products, have very long half-lives but can be transmuted to isotopes with very short half-lives (15.46 seconds and 12.36 hours) by neutron absorption within a reactor, effectively destroying them (see more: long-lived fission products). Zirconium-93, another 5% of fission products, could in principle be recycled into fuel-pin cladding, where it does not matter that it is radioactive. Excluding the contribution from transuranic waste (TRU) – which are isotopes produced when uranium-238 captures a slow thermal neutron in an LWR but does not fission – all high level waste/fission products remaining after reprocessing the TRU fuel is less radiotoxic (in sieverts) than natural uranium (in a gram-to-gram comparison) within 200–400 years, and continues to decline afterward.-->

The on-site reprocessing of fuel means that the volume of high-level nuclear waste leaving the plant is tiny compared to LWR spent fuel.