El IFR se enfría con sodio líquido y emplea una aleación de uranio y plutonio como combustible. El combustible se encierra en vainas de acero con sodio líquido llenando el espacio entre el combustible y la vaina. Un espacio vacío sobre el combustible permite recolectar con seguridad helio y xenón radiactivo, sin aumentar significativamente la presión dentro del elemento combustible, y también permite que el combustible se expanda sin romper la vaina, lo que hace viable el uso de combustible metálico en lugar de óxido.

Las ventajas del sodio líquido como refrigerante, en comparación con el plomo, incluyen su menor densidad y viscosidad (reduciendo los costes de bombeo), su menor corrosividad hacia los aceros comunes, y la generación prácticamente nula de subproductos activados por neutrones. Sin embargo, el sodio es químicamente reactivo, especialmente con el agua o el aire. Alternativamente, puede utilizarse una aleación eutéctica de plomo y bismuto, como en los reactores de los submarinos soviéticos de clase Alfa.

Basic design decisions

Metallic fuel with a sodium-filled space within the cladding to allow for expansion was demonstrated in the EBR-II. This type of fuel favors the use of pyroprocessing as a reprocessing technology.

Its manufacture is simpler and cheaper than that of ceramic fuel (oxide), especially under remote manipulation conditions.[11]​.

In addition, metallic fuel has better thermal conductivity and lower heat capacity than oxide, which represents an advantage in terms of safety.[11]​.

The use of a liquid metallic coolant eliminates the need for a pressure vessel around the reactor. Sodium has excellent nuclear properties, a high heat capacity, low density, low viscosity, a reasonably low melting point, a high boiling point, and good compatibility with other structural and fuel materials. The high heat capacity of the coolant and the removal of water from the nuclear reactor core") increase the inherent safety of the core.[11]​.

Containing all of the primary coolant in a pool provides several advantages in terms of safety and reliability.[11]​.

Reprocessing is essential to obtain most of the benefits of a fast reactor, improving fuel utilization and reducing radioactive waste by several orders of magnitude.[11]​.

On-site processing is what makes IFR "comprehensive." This characteristic, together with the use of pyroprocessing, reduces the risk of proliferation.[11]​[12]​.

Pyroprocessing (using an electrorefiner) was demonstrated in the EBR-II as viable at the required scale. Compared to the aqueous PUREX process), it is more economical in capital cost and is inappropriate for the production of weapons material, unlike PUREX which was developed precisely for military purposes.

Pyroprocessing makes metallic fuel the preferred option. These two decisions are closely related.[11]​.

Breeder reactors like the IFR could, in principle, extract almost all of the energy contained in uranium or thorium, reducing fuel requirements by almost two orders of magnitude compared to traditional single-cycle reactors, which extract less than 0.65% of the energy from natural uranium, and less than 5% from the enriched uranium they are fed. This could significantly reduce concerns about fuel supply or energy consumption in uranium mining.

Most important today is why fast reactors are more efficient: because fast neutrons can fission or "burn" all components of transuranic waste. Transuranic waste is composed of actinides - reactor-grade plutonium - and minor actinides - many of which have half-lives of tens of thousands of years or more, and make the disposal of conventional nuclear waste so problematic. Most radioactive fission products generated by an IFR have much shorter half-lives: they are intensely radioactive in the short term, but decay quickly. Through multiple cycles, the IFR achieves that 99.9% of the uranium and transuranic elements undergo fission and generate energy; therefore, its only waste is the products of nuclear fission"), which, after 300 years, will have a lower radioactivity than that of the original uranium ore.[13]​[14]​[15]​.

The term "comprehensive" refers to on-site reprocessing using electrochemical pyroprocessing. This process separates spent fuel into three fractions: uranium, isotopes of plutonium and other transuranic elements, and fission products. The uranium and transuranics are recycled into new nuclear fuel rods), and the fission products are converted into blocks of glass or metal for safe disposal. Because transuranics and combined fission products are highly radioactive, fuel rod transfer and reprocessing operations require robotic or remotely controlled equipment.

An additional claimed benefit is that, because the fissile material never leaves the facility (and would be lethal to handle if it did), the potential for nuclear proliferation is greatly reduced.

In traditional light water reactors (LWRs), the core must be kept at high pressure to keep liquid water at high temperatures. In contrast, the IFR is a liquid metal cooled reactor), so the core can operate at near atmospheric pressure, significantly reducing the risk of a coolant loss 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 the primary coolant very unlikely. The coolant loops are designed to allow cooling by natural convection, meaning that in the event of a power loss or reactor shutdown, the heat from the core would be enough to keep the coolant circulating even if the pumps failed.

The IFR also has passive nuclear safety advantages over the LWR. The fuel and the pods&action=edit&redlink=1 "Pods (fuel elements) (not yet written)") are designed so that, as they expand due to an increase in temperature, more neutrons escape from the nucleus, decreasing the fission rate. That is, an increase in temperature acts as a negative feedback mechanism, reducing the power of the core. This attribute is called negative reactivity coefficient. Most LWRs also have it, but in the IFR this effect is strong enough to prevent damage to the core without the need for external intervention from operators or safety systems.

This principle was demonstrated in a series of safety tests on the prototype. Pete Planchon, the engineer who led the tests, joked: "In 1986, we gave a small advanced fast reactor prototype a couple of chances to melt down. He politely refused both times.

Liquid sodium presents safety concerns as it ignites spontaneously upon contact with air and can cause explosions if it comes into contact with water. This occurred at the Monju reactor in an accident in 1995. To reduce the risk of explosions, the IFR design (like other sodium-cooled fast reactors) includes an intermediate loop of liquid metallic coolant between the reactor and the steam turbines. Its purpose is to ensure that any explosion caused by the accidental mixing of sodium and water from the turbines is limited to the secondary exchanger, without affecting the reactor. Other alternative designs use lead as the primary refrigerant, which, although it has greater density and viscosity (higher pumping costs), also generates radioactive activation products due to neutron absorption.

A eutectic alloy of lead and bismuth (as in Russian submarines) reduces viscosity, although radioactive activation products remain a problem.

The objectives of the IFR project were to increase uranium use efficiency by breeding plutonium and eliminate the need for transuranic isotopes to leave the site. The reactor has a moderatorless design, operating with fast neutrons, allowing it to consume any transuranic isotopes (and in some cases use them as fuel).

Compared to current light water reactors, with a single-pass fuel cycle that induces fission in less than 1% of natural uranium, a breeder reactor such as the IFR has a much more efficient fuel cycle (it is claimed that up to 99.5% of uranium can undergo fission).[14]

The basic scheme uses pyroelectric separation to extract transuranics and actinides from the waste and concentrate them. These concentrated fuels are reformed, in the same place, into new fuel elements.

The available metal fuel is never separated from plutonium or all fission products,[12]​, making it difficult to use for war purposes. Furthermore, since the plutonium never has to leave the site, the risk of unauthorized diversion is much lower.[17]​.

Another important benefit of extracting long-lived transuranics is that the remaining waste becomes a much shorter-term risk. Once the actinides (reprocessed uranium"), plutonium, and minor actinides are recycled, the remaining radioactive waste is fission products – with half-lives of 90 years (samarium-151") or less, or 211,000 years (technetium-99) or more – in addition to activation products of the reactor itself.

Nuclear waste

IFR residues have either short half-lives (they decay quickly) or long half-lives (they are barely radioactive). Neither form contains plutonium or other actinides, thanks to pyroprocessing.

The total volume of fission products from the IFR is 1/20 of the volume of spent fuel generated by a light water plant of equal power, a volume that is also generally considered non-reusable. 70% of fission products are stable or have half-lives of less than one year.

Technetium-99 and iodine-129, which make up 6% of the fission products, have long half-lives, but can be transmuted into short-lived isotopes by neutron absorption. For example, technetium can be transformed into an isotope with a half-life of 15.46 seconds, and iodine into another with a half-life of 12.36 hours.

Another 5% corresponds to zirconium-93"), which in principle can be recycled as a material for fuel pods, where its radioactivity is not problematic.

Excluding the contribution of transuranic waste (TRU), all high-activity waste" remaining after reprocessing has a lower radiotoxicity (in sieverts) than natural uranium, after between 200 and 400 years, and continues to decrease from that point.[19]​[15]​.

Carbon dioxide

Both IFRs and LWRs do not emit carbon dioxide (CO₂) during their operation. However, its construction and fuel processing do involve CO₂ emissions, especially if non-renewable energy sources or conventional cement are used during construction.

A 2012 Yale University review of the life cycle greenhouse gas (LCA) emissions of nuclear power concluded:[20]​.

The review primarily analyzed data from second-generation reactors, but also summarized estimates of developing technologies:

Proliferation

Both IFRs and LWRs produce reactor-grade plutonium), which even with high burnup remains potentially usable in nuclear weapons.[21]​.

However, the IFR fuel cycle presents several barriers to proliferation. Unlike PUREX reprocessing, IFR pyroprocessing does not separate pure plutonium. Plutonium remains mixed with minor actinides and fission products, making it unattractive for weapons.[12]​.

Additionally, by not transporting reprocessed fuel outside the plant, the risk of diversion is reduced. The material remains on site and is consumed in situ. Even so, it would be technically possible to separate the plutonium with chemical techniques, although more difficult than in PUREX or MOX.

In 1962, the United States detonated a nuclear device using reactor-grade plutonium, although it was later reclassified as “fuel” grade plutonium.[22]​[23]​.

Although the IFR can be configured as a burner reactor, it can also operate as a breeder reactor. If a blanket of natural uranium is used to produce plutonium, this plutonium may have a high content of Pu-239, useful for nuclear weapons.[24]​.

Metal Sodium Coolant

"Fast reactors" need coolants that do not moderate neutrons, as water does. Therefore, metallic sodium is suitable for its physical properties:

• - Low melting point.

• - High boiling temperature.

• - Excellent thermal conductivity.

• - Low viscosity.

• - Low density.

• - Thermal stability and low activation level.

However, sodium is extremely flammable in the presence of oxygen and reacts violently with water, releasing flammable hydrogen. The activated isotope sodium-24") is highly radioactive, although with a half-life of only 15 hours. To avoid direct contact with the vapor circuit, the IFR design includes a secondary sodium circuit, which adds costs but improves safety.[25]​.

• - Gas-cooled fast reactor").

• - IV generation reactor.

• - Lead-cooled fast reactor").

• - Molten salt reactor.

• - Traveling wave reactor").

• - This work contains a translation derived from «Integral fast reactor» from Wikipedia in English, specifically from this version of July 6, 2025, published by its editors under the GNU Free Documentation License and the Creative Commons Attribution-ShareAlike 4.0 International License.

• - The integral fast reactor at Argonne National Laboratory.

• - UC Berkeley IFR Site Archives:

• Index (archived)

• Introduction (archived)

• IFR reactor (archived)

*IFR Metal Fuel (archived)

• Security features (archived)

• Fuel Cycle Facility (archived)

*Fuel Manufacturing Facility (archived)

• IFR Vision (archived)

• The reactor burns waste as fuel (archived).

• - IFR: Secure, Abundant, and Clean Energy Source by George S. Stanford, Ph.D.

• - Interview with Dr. Charles Till – FRONTLINE.

• - IFR Q&A with Tom Blees and George Stanford.

• - IFR by Tom Blees, part 2 of 3 – Interview with author Tom Blees.

• - The role of the IFR in the face of climate change.

• - The Restoration of the Earth, Theodore B. Taylor and Charles C. Humpstone, 166 pages, Harper & Row (1973) ISBN 978-0060142315.

• - Sustainable energy – Without the Hot Air, David J.C. MacKay, 384 pages, ITU Cambridge (2009) ISBN 978-0954452933.

• - 2081: A Hopeful View of the Human Future, Gerard K. O'Neill, 284 pages, Simon & Schuster (1981) ISBN 978-0671242572.

• - The Second Nuclear Era: A New Start for Nuclear Power, Alvin M. Weinberg et al., 460 pages, Praeger Publishers (1985) ISBN 978-0275901837.

• - Thorium Fuel Cycle – Potential Benefits and Challenges, IAEA, 105 pages (2005) ISBN 978-9201034052.

• - The Nuclear Imperative: A Critical Look at the Approaching Energy Crisis, Jeff Eerkens, 212 pages, Springer (2010) ISBN 978-9048186662.

El IFR se enfría con sodio líquido y emplea una aleación de uranio y plutonio como combustible. El combustible se encierra en vainas de acero con sodio líquido llenando el espacio entre el combustible y la vaina. Un espacio vacío sobre el combustible permite recolectar con seguridad helio y xenón radiactivo, sin aumentar significativamente la presión dentro del elemento combustible, y también permite que el combustible se expanda sin romper la vaina, lo que hace viable el uso de combustible metálico en lugar de óxido.

Las ventajas del sodio líquido como refrigerante, en comparación con el plomo, incluyen su menor densidad y viscosidad (reduciendo los costes de bombeo), su menor corrosividad hacia los aceros comunes, y la generación prácticamente nula de subproductos activados por neutrones. Sin embargo, el sodio es químicamente reactivo, especialmente con el agua o el aire. Alternativamente, puede utilizarse una aleación eutéctica de plomo y bismuto, como en los reactores de los submarinos soviéticos de clase Alfa.

Basic design decisions

Metallic fuel with a sodium-filled space within the cladding to allow for expansion was demonstrated in the EBR-II. This type of fuel favors the use of pyroprocessing as a reprocessing technology.

Its manufacture is simpler and cheaper than that of ceramic fuel (oxide), especially under remote manipulation conditions.[11]​.

In addition, metallic fuel has better thermal conductivity and lower heat capacity than oxide, which represents an advantage in terms of safety.[11]​.

The use of a liquid metallic coolant eliminates the need for a pressure vessel around the reactor. Sodium has excellent nuclear properties, a high heat capacity, low density, low viscosity, a reasonably low melting point, a high boiling point, and good compatibility with other structural and fuel materials. The high heat capacity of the coolant and the removal of water from the nuclear reactor core") increase the inherent safety of the core.[11]​.

Containing all of the primary coolant in a pool provides several advantages in terms of safety and reliability.[11]​.

Reprocessing is essential to obtain most of the benefits of a fast reactor, improving fuel utilization and reducing radioactive waste by several orders of magnitude.[11]​.

On-site processing is what makes IFR "comprehensive." This characteristic, together with the use of pyroprocessing, reduces the risk of proliferation.[11]​[12]​.

Pyroprocessing (using an electrorefiner) was demonstrated in the EBR-II as viable at the required scale. Compared to the aqueous PUREX process), it is more economical in capital cost and is inappropriate for the production of weapons material, unlike PUREX which was developed precisely for military purposes.

Pyroprocessing makes metallic fuel the preferred option. These two decisions are closely related.[11]​.

Breeder reactors like the IFR could, in principle, extract almost all of the energy contained in uranium or thorium, reducing fuel requirements by almost two orders of magnitude compared to traditional single-cycle reactors, which extract less than 0.65% of the energy from natural uranium, and less than 5% from the enriched uranium they are fed. This could significantly reduce concerns about fuel supply or energy consumption in uranium mining.

Most important today is why fast reactors are more efficient: because fast neutrons can fission or "burn" all components of transuranic waste. Transuranic waste is composed of actinides - reactor-grade plutonium - and minor actinides - many of which have half-lives of tens of thousands of years or more, and make the disposal of conventional nuclear waste so problematic. Most radioactive fission products generated by an IFR have much shorter half-lives: they are intensely radioactive in the short term, but decay quickly. Through multiple cycles, the IFR achieves that 99.9% of the uranium and transuranic elements undergo fission and generate energy; therefore, its only waste is the products of nuclear fission"), which, after 300 years, will have a lower radioactivity than that of the original uranium ore.[13]​[14]​[15]​.

The term "comprehensive" refers to on-site reprocessing using electrochemical pyroprocessing. This process separates spent fuel into three fractions: uranium, isotopes of plutonium and other transuranic elements, and fission products. The uranium and transuranics are recycled into new nuclear fuel rods), and the fission products are converted into blocks of glass or metal for safe disposal. Because transuranics and combined fission products are highly radioactive, fuel rod transfer and reprocessing operations require robotic or remotely controlled equipment.

An additional claimed benefit is that, because the fissile material never leaves the facility (and would be lethal to handle if it did), the potential for nuclear proliferation is greatly reduced.

In traditional light water reactors (LWRs), the core must be kept at high pressure to keep liquid water at high temperatures. In contrast, the IFR is a liquid metal cooled reactor), so the core can operate at near atmospheric pressure, significantly reducing the risk of a coolant loss 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 the primary coolant very unlikely. The coolant loops are designed to allow cooling by natural convection, meaning that in the event of a power loss or reactor shutdown, the heat from the core would be enough to keep the coolant circulating even if the pumps failed.

The IFR also has passive nuclear safety advantages over the LWR. The fuel and the pods&action=edit&redlink=1 "Pods (fuel elements) (not yet written)") are designed so that, as they expand due to an increase in temperature, more neutrons escape from the nucleus, decreasing the fission rate. That is, an increase in temperature acts as a negative feedback mechanism, reducing the power of the core. This attribute is called negative reactivity coefficient. Most LWRs also have it, but in the IFR this effect is strong enough to prevent damage to the core without the need for external intervention from operators or safety systems.

This principle was demonstrated in a series of safety tests on the prototype. Pete Planchon, the engineer who led the tests, joked: "In 1986, we gave a small advanced fast reactor prototype a couple of chances to melt down. He politely refused both times.

Liquid sodium presents safety concerns as it ignites spontaneously upon contact with air and can cause explosions if it comes into contact with water. This occurred at the Monju reactor in an accident in 1995. To reduce the risk of explosions, the IFR design (like other sodium-cooled fast reactors) includes an intermediate loop of liquid metallic coolant between the reactor and the steam turbines. Its purpose is to ensure that any explosion caused by the accidental mixing of sodium and water from the turbines is limited to the secondary exchanger, without affecting the reactor. Other alternative designs use lead as the primary refrigerant, which, although it has greater density and viscosity (higher pumping costs), also generates radioactive activation products due to neutron absorption.

A eutectic alloy of lead and bismuth (as in Russian submarines) reduces viscosity, although radioactive activation products remain a problem.

The objectives of the IFR project were to increase uranium use efficiency by breeding plutonium and eliminate the need for transuranic isotopes to leave the site. The reactor has a moderatorless design, operating with fast neutrons, allowing it to consume any transuranic isotopes (and in some cases use them as fuel).

Compared to current light water reactors, with a single-pass fuel cycle that induces fission in less than 1% of natural uranium, a breeder reactor such as the IFR has a much more efficient fuel cycle (it is claimed that up to 99.5% of uranium can undergo fission).[14]

The basic scheme uses pyroelectric separation to extract transuranics and actinides from the waste and concentrate them. These concentrated fuels are reformed, in the same place, into new fuel elements.

The available metal fuel is never separated from plutonium or all fission products,[12]​, making it difficult to use for war purposes. Furthermore, since the plutonium never has to leave the site, the risk of unauthorized diversion is much lower.[17]​.

Another important benefit of extracting long-lived transuranics is that the remaining waste becomes a much shorter-term risk. Once the actinides (reprocessed uranium"), plutonium, and minor actinides are recycled, the remaining radioactive waste is fission products – with half-lives of 90 years (samarium-151") or less, or 211,000 years (technetium-99) or more – in addition to activation products of the reactor itself.

Nuclear waste

IFR residues have either short half-lives (they decay quickly) or long half-lives (they are barely radioactive). Neither form contains plutonium or other actinides, thanks to pyroprocessing.

The total volume of fission products from the IFR is 1/20 of the volume of spent fuel generated by a light water plant of equal power, a volume that is also generally considered non-reusable. 70% of fission products are stable or have half-lives of less than one year.

Technetium-99 and iodine-129, which make up 6% of the fission products, have long half-lives, but can be transmuted into short-lived isotopes by neutron absorption. For example, technetium can be transformed into an isotope with a half-life of 15.46 seconds, and iodine into another with a half-life of 12.36 hours.

Another 5% corresponds to zirconium-93"), which in principle can be recycled as a material for fuel pods, where its radioactivity is not problematic.

Excluding the contribution of transuranic waste (TRU), all high-activity waste" remaining after reprocessing has a lower radiotoxicity (in sieverts) than natural uranium, after between 200 and 400 years, and continues to decrease from that point.[19]​[15]​.

Carbon dioxide

Both IFRs and LWRs do not emit carbon dioxide (CO₂) during their operation. However, its construction and fuel processing do involve CO₂ emissions, especially if non-renewable energy sources or conventional cement are used during construction.

A 2012 Yale University review of the life cycle greenhouse gas (LCA) emissions of nuclear power concluded:[20]​.

The review primarily analyzed data from second-generation reactors, but also summarized estimates of developing technologies:

Proliferation

Both IFRs and LWRs produce reactor-grade plutonium), which even with high burnup remains potentially usable in nuclear weapons.[21]​.

However, the IFR fuel cycle presents several barriers to proliferation. Unlike PUREX reprocessing, IFR pyroprocessing does not separate pure plutonium. Plutonium remains mixed with minor actinides and fission products, making it unattractive for weapons.[12]​.

Additionally, by not transporting reprocessed fuel outside the plant, the risk of diversion is reduced. The material remains on site and is consumed in situ. Even so, it would be technically possible to separate the plutonium with chemical techniques, although more difficult than in PUREX or MOX.

In 1962, the United States detonated a nuclear device using reactor-grade plutonium, although it was later reclassified as “fuel” grade plutonium.[22]​[23]​.

Although the IFR can be configured as a burner reactor, it can also operate as a breeder reactor. If a blanket of natural uranium is used to produce plutonium, this plutonium may have a high content of Pu-239, useful for nuclear weapons.[24]​.

Metal Sodium Coolant

"Fast reactors" need coolants that do not moderate neutrons, as water does. Therefore, metallic sodium is suitable for its physical properties:

• - Low melting point.

• - High boiling temperature.

• - Excellent thermal conductivity.

• - Low viscosity.

• - Low density.

• - Thermal stability and low activation level.

However, sodium is extremely flammable in the presence of oxygen and reacts violently with water, releasing flammable hydrogen. The activated isotope sodium-24") is highly radioactive, although with a half-life of only 15 hours. To avoid direct contact with the vapor circuit, the IFR design includes a secondary sodium circuit, which adds costs but improves safety.[25]​.

• - Gas-cooled fast reactor").

• - IV generation reactor.

• - Lead-cooled fast reactor").

• - Molten salt reactor.

• - Traveling wave reactor").

• - This work contains a translation derived from «Integral fast reactor» from Wikipedia in English, specifically from this version of July 6, 2025, published by its editors under the GNU Free Documentation License and the Creative Commons Attribution-ShareAlike 4.0 International License.

• - The integral fast reactor at Argonne National Laboratory.

• - UC Berkeley IFR Site Archives:

• Index (archived)

• Introduction (archived)

• IFR reactor (archived)

*IFR Metal Fuel (archived)

• Security features (archived)

• Fuel Cycle Facility (archived)

*Fuel Manufacturing Facility (archived)

• IFR Vision (archived)

• The reactor burns waste as fuel (archived).

• - IFR: Secure, Abundant, and Clean Energy Source by George S. Stanford, Ph.D.

• - Interview with Dr. Charles Till – FRONTLINE.

• - IFR Q&A with Tom Blees and George Stanford.

• - IFR by Tom Blees, part 2 of 3 – Interview with author Tom Blees.

• - The role of the IFR in the face of climate change.

• - The Restoration of the Earth, Theodore B. Taylor and Charles C. Humpstone, 166 pages, Harper & Row (1973) ISBN 978-0060142315.

• - Sustainable energy – Without the Hot Air, David J.C. MacKay, 384 pages, ITU Cambridge (2009) ISBN 978-0954452933.

• - 2081: A Hopeful View of the Human Future, Gerard K. O'Neill, 284 pages, Simon & Schuster (1981) ISBN 978-0671242572.

• - The Second Nuclear Era: A New Start for Nuclear Power, Alvin M. Weinberg et al., 460 pages, Praeger Publishers (1985) ISBN 978-0275901837.

• - Thorium Fuel Cycle – Potential Benefits and Challenges, IAEA, 105 pages (2005) ISBN 978-9201034052.

• - The Nuclear Imperative: A Critical Look at the Approaching Energy Crisis, Jeff Eerkens, 212 pages, Springer (2010) ISBN 978-9048186662.