Reactors of the Future: The Integral Fast Reactor (Peyton Williams)

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Reactors of the Future: The Integral Fast Reactor by Peyton C. Williams North Carolina State University, 1st Year Engineering September, 2014 Former U.S. President Ronald Reagan once said, “there are no constraints on the human mind, no walls around the human spirit, no barriers to our progress except those we ourselves erect.” The tremendous growth and achievement of the Integral Fast Reactor’s developmental period and its unfortunate demise at the hands of the Clinton administration prove the accuracy of Reagan’s statement. Despite the wildly ambitious goals of the IFR program—a reactor that is capable of moderating itself even in the absence of human operators and safety systems; greatly reduced waste volume and toxicity; and the ability to recycle spent fuel, resulting in fuel efficiency two orders of magnitude greater than the most common reactors utilized today—the project was a monumental success. During the ten-year period that the majority of IFR development occurred in, there was not a single technical issue that could not be overcome by the great minds at Argonne National Laboratory. As it stands today, the IFR is a fully-fledged reactor design with the potential to change the nuclear power industry for the better. It is neither theoretical nor wishful thinking, but a revolutionary project that was cancelled before it had the chance to shine. The technology of the Integral Fast Reactor is clean, safe, efficient and, most importantly, a huge potential provider of energy in a world limited by dwindling supplies of fossil fuels.

The Integral Fast Reactor project, brainchild of the highly respected research facility Argonne National Laboratory, began in 1984 and development lasted ten years, ending after a very heated congressional battle over funding to advanced reactor development. The Integral Fast Reactor project built off of a previous period of development that decided the fundamentals of the IFR, including material (fuel and coolant) and design (reactor configuration) choices. The earlier stages of development provided the concept, and the 1984-1994 period of development ironed out the flaws and initiated testing of the IFR design. EBR-II or Experimental Breeder Reactor-II, an IFR prototype, was constructed in 1964 and operated until the termination of the IFR program in 1994—its thirty year period of operation provided crucial testing for the IFR concept, proving the reliability of the IFR’s unique material choices, fuel recycling process, and inherent safety features.

The IFR initiative began out of concern for uranium scarcity. Researchers in the early years of nuclear power development anticipated that the world’s uranium supply, which at the time was greatly underestimated, would need to be conserved if nuclear power was to ever amount to anything. They began to look into reactor designs that could “breed” new fissile material, converting unusable, non-fissile material into usable fissile material and immensely extending the lifetime of fuel supplies. Understanding the concept of fuel breeding is crucial to

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understanding the IFR, as the IFR’s ability to breed fissile material is arguably its most important trait.

Breeding works because of the nature of uranium’s isotopes, specifically the two most common isotopes, uranium-235 and uranium-238. Uranium-235 is fissile, meaning it will sustain a chain reaction. Uranium-238, on the other hand, will not sustain a chain reaction because too often it will absorb incoming neutrons without releasing any back out. This is not entirely bad news, however, as occasionally when uranium-238 absorbs a neutron it will decay into plutonium-239, which is fissile and can sustain a chain reaction. As the fuel’s in-reactor lifespan progresses, the non-fissile uranium-238 is slowly but surely converted into fissile plutonium-239.

Uranium-238 and other isotopes that are capable of becoming fissile through neutron absorption and subsequent conversion are referred to as fertile isotopes.

It should be noted that this conversion of fertile uranium into fissile plutonium occurs to some degree in every reactor that utilizes uranium fuel. The most common reactors of today, Light Water Reactors (LWRs), convert enough plutonium that about half of their energy output is the result of plutonium fissioning. The IFR differs from these reactors in that more fissile material is bred over the lifetime of the fuel than is consumed. Reactors that are capable of this are termed breeder reactors, and have a unique mix of design choices that enable them to efficiently breed fissile material.

At the heart of the IFR project is its ability to recycle the spent fuel of itself and other reactors, greatly extending available uranium supplies and reducing immensely the volume, longevity, and toxicity of its nuclear waste. The key to economical fuel recycling is breeding more fissile material than is consumed during the fuel’s in-reactor lifespan. Because most of today’s reactors (including LWRs) do not breed sufficient amounts of fissile fuel to make recycling economical, they must discard their used-up fuel pins after three or so years. In doing so, they waste an incredible amount of uranium. LWRs and other non-breeder reactors fission at most a few percent of their fuel before the amount of fissile material is too low to sustain a chain reaction. After the amount of fissile material reaches low enough levels, the fuel pins are discarded with ~95% of the fertile uranium content remaining untouched. Thus, the vast majority of LWR fuel goes to waste. The IFR’s ability to breed large amounts of fissile material makes recycling economical, giving it an incredible advantage efficiency-wise over LWRs and other thermal reactors.

This recycling is done through pyroprocessing, a process that removes the unusable fission products and recovers for later use the fissile material bred by the reactor. EBR-II, the IFR’s prototype, proved economical viability of pyroprocessing at least on a reduced scale, and preliminary analysis has indicated that scale-ups of fuel recycling through the pyroprocess are very much economically viable.

Pyroprocessing makes use of an electrorefiner—a device that separates fuel constituents based on electric charge—to separate spent fuel into three groups: uranium, all elements heavier than uranium (called actinides in this context), and fission byproducts. Of these three groups, the uranium and actinides are recycled as fuel for the reactor, while the various fission

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byproducts can be further processed to harvest useful radioisotopes. The electrorefiner separates these three groups by submerging the chopped-up spent fuel in a molten salt electrolyte, then applying a voltage across the setup. As the voltage is applied, the fuel dissolves into the electrolyte and each desired fuel constituent gathers separately. The uranium of the spent fuel attracts to and gathers on a steel rod cathode, while the actinides gather on a different cathode—a layer of liquid cadmium—leaving the fission products dissolved in the electrolyte. Through this process, the reactor’s spent fuel is cleansed of the broken-down fission products while the stillusable uranium and actinides are gathered for another go through the reactor.

One of the reasons for the IFR project’s untimely cancellation was fear that its fuel recycling process would enable countries and groups to separate out and utilize the plutonium bred by the reactor for production of nuclear weapons. Such fear was gravely misconceived. The prospect of obtaining plutonium for destructive use is slim as a result of many complications inherent in the IFR recycling process. It is the nature of the electrorefiner that plutonium cannot be purely separated—it must always be gathered with its fellow actinides as a group. In addition, spent fuel reprocessing occurs on-site, minimizing the risk of spent fuel being diverted by hostile groups. All this, in combination with the fact that any plutonium-actinide material separated from the spent fuel remains dangerously radioactive, makes IFR plutonium highly undesirable for nuclear weapons fabrication.

The fact that IFR-produced plutonium remains dangerously radioactive complicates not only fabrication of the nuclear weapon, but actual usage of the weapon. As a result of ongoing radioactive decay, IFR spent-fuel-derived plutonium outputs a large amount of heat (fifty times more than acceptable weapons-grade fuel) that could cause a multitude of problems inside a nuclear weapon. Compounding this problem is the tendency of IFR plutonium to emit neutrons spontaneously (up to 5,000 times more than is desired in weapons-grade fuel), an issue that could result in premature firing of the nuclear weapon.

Increased nuclear proliferation as a result of IFR technology is a low risk, as the IFR will not be looked upon as a desirable source of weapon-grade fuel. The many complications inherent in usage of IFR-derived plutonium for the construction and usage of nuclear weapons make other sources of plutonium much more desirable. The pyroprocess is an economical, well-studied, and proliferation-resistant design. It is unique to the IFR, and is made possible through the reactor’s material choices—especially its choice of metal fuel over metal oxide fuel—which allow for the necessary amounts of fissile fuel breeding to make recycling of spent fuel a logical and economical choice.

Reactors are classified in a plethora of ways: coolant type, fuel type, reactor configuration, and moderator type/neutron energy. The IFR is a sodium-cooled, metal-fuelled, pool-configured reactor that utilizes a fast neutron spectrum without use of moderators. The inherent safety of the IFR and the advantages of its design are made possible as a result of the unique materials and configuration design chosen for the reactor. Each of these aforementioned design choices will be discussed in order to better convey the nature of the IFR.

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The most common reactors of today, Light Water Reactors, utilize metal oxide fuel (i.e., uranium dioxide) which consists mostly of the non-fissile U-238 isotope with a few percent of the uranium atoms being U-235, the fissile isotope that enables chain reactions. The IFR runs on fuel composed (approximately, and by weight) of 70% uranium, with 20% being plutonium and 10% being zirconium. The IFR fuel is not oxidized, but is instead just pure metal—this is a key difference between the fuel of the Integral Fast Reactor and that of LWRs. In the beginnings of the nuclear power era, metal oxide fuel appeared to triumph over metal fuel as a result of its higher melting point and the unfortunate tendency of metal fuel to swell out of its cladding, which complicated its use in reactors. It was not until more research was conducted that metal fuel was discovered to have a huge advantage over metal oxide fuel: its superior performance in breeding fissile material within a fast neutron spectrum. The earlier problems found with metal fuel—the swelling and the relatively low melting point—were solved during the lifetime of EBR-II, making metal fuel the clearly superior choice when considering the breeding goals of the IFR. Adopting metal fuel rather than the metal oxide fuel used in LWRs grants the IFR superior breeding performance in a fast neutron (non-moderated) spectrum.

Neutron moderation is the process of lowering a neutron’s kinetic energy so that it fissions atoms more readily. A ‘thermal neutron’ is a neutron that has collided enough with a moderator—typically water or graphite—such that its energy is comparable to the thermal energies of its surroundings. This differs from a ‘fast neutron’, the type utilized in the IFR, which has not been moderated and maintains a respectable amount of kinetic energy. In non-breeder reactors, thermal neutrons are preferred for their superior performance in fissioning atoms.

However, in reactors whose aim is high amounts of fissile fuel breeding, fast neutrons are preferred for their greater performance in breeding fissile atoms. Neither fast nor thermal neutrons have an advantage safety-wise over the other; the choice between fast and thermal neutrons is merely a matter of what the reactor aims to achieve with its free neutrons.

One of the many benefits to adopting a fast neutron spectrum is the added ability to utilize actinides as fuel. Reactors that do not use a fast neutron spectrum cannot fission most actinides, and therefore must discard them as part of their nuclear waste. This is very unfortunate, as actinides are the reason nuclear fuel remains dangerously radioactive for so long; without actinides in nuclear waste, the only major concern is short-lived (but very radioactive) isotopes, which die off relatively quickly and leave the nuclear waste in a fairly harmless state.

Since the IFR’s fast neutron spectrum enables it to burn actinides just as it would any other uranium fuel, its waste products need not contain the long-lasting actinide radioactivity that haunts today’s nuclear waste. This greatly reduces the time it takes for IFR spent fuel to reach a safe level of radioactivity from millions of years to on the order of a few hundred years.

Taking advantage of the benefits of a fast neutron spectrum also means forgoing the typical materials used in thermal reactors. Choosing metal fuel over metal oxide fuel is one such example; utilizing liquid sodium over light water as coolant is another. Because water is an effective neutron moderator and the IFR’s goal is to utilize fast neutrons rather than thermal neutrons, the reactor requires a coolant that performs well without significantly lowering the

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energy of free neutrons. Liquid sodium fits the bill perfectly, bringing with it numerous advantages that enable a great deal of inherent safety in the IFR design. Sodium’s inertness in the IFR environment, excellent thermal conductivity, reasonable specific heat, ability to remain liquid without pressurization, and high boiling point (881°C) all make it the clearly superior choice, enabling the IFR to function safely without hindering its ability to breed fissile material.

The coolant choice is an essential decision for nuclear reactors, however the method by which the coolant is circulated through the reactor is just as essential of a decision. By carrying excess heat away from the core, nuclear reactors are able to drive steam turbines and generate electricity. This is typically done in one of two ways—namely, the ‘loop configuration’ or the ‘pool configuration’. Of the coolant choices available to nuclear reactors, each lends themselves best to a particular configuration. LWRs are more or less forced into adopting the loop configuration as a result of their choice of coolant, light water, which must be highly pressurized (and thus confined to smaller diameter pipes) to prevent boiling when exposed to the heat of the reactor. The Integral Fast Reactor, on the other hand, is capable of utilizing the pool configuration because its sodium coolant has a much higher boiling point and therefore does not require pressurization to maintain its liquid phase.

This is beneficial for two major reasons: firstly, the pool configuration, which holds the reactor core and coolant piping inside a large ‘pool’ of coolant within a double-walled tank, can contain all radioactive material in one secure location. The only sodium coolant that leaves the main shielding in this configuration is non-radioactive, and has never been exposed directly to the core (always protected within piping). The loop configuration, on the other hand, does not contain all radioactive coolant in one secure location, as some amount of radioactive coolant is always being circulated outside of the main reactor shielding in order to transfer heat for power generation. The IFR forgoes the necessity of circulating radioactive coolant outside the main shielding by adopting a secondary, non-radioactive coolant loop that transfers heat from the primary loop to the steam turbines for power generation.

What this essentially amounts to is a better outlook in regards to leaks for the pool configuration than for the loop configuration. Any leaks that occur in the primary sodium (radioactive coolant) piping will simply leak back into the pool of already radioactive coolant.

Any leaks that occur in the secondary sodium (non-radioactive coolant) piping will have no possibility of releasing radiation into the environment. The loop configuration does not have this luxury, and must therefore deal with the possibility of a radioactive coolant leak outside of the main shielding.

Secondly, the pool configuration allows for excellent performance in heat regulation, giving the reactor a chance to slow down its fission activity in the event of an operator or system error. The large volume of sodium present in the ‘pool’, in combination with sodium’s exceptional thermal conductivity and reasonable specific heat, grants the IFR significant leeway in the event of an accident, in addition to giving the reactor the ability to self-regulate its power levels. In a nutshell, fuel expansion as a result of temperature spikes increases the amount of neutrons that escape the fuel without inducing fission—this in turn reduces the power output of

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the fuel, lowering the heat output and gradually bringing the reactor back to a stable condition.

Thus, in the event that the reactor gets out of hand and the fuel carries on fissioning without control, the rising temperature will gradually cause the fuel to reduce its rate of fission and bring itself back under control. This passive safety feature (called natural reactivity feedback) is not a possibility in loop-configured Light Water Reactors; without the pool configuration’s sizable pool of coolant to absorb the heat of the nuclear reactions, the reactor core would not have time to gradually bring itself back to stable power levels before fuel melting occurs.

The Integral Fast Reactor’s coolant system is what enables its ability to self-moderate even when safety systems and operators have failed—an ability called passive safety. Passively safe reactors are neither the result of advanced engineered safety systems nor the result of rigorous training of operators and reactor technicians—they are the result of intelligent material and configuration choices working harmoniously together. The combination and interaction of the IFR’s design choices enable a great deal of flexibility in handling less than ideal situations.

Accidents of the type that occurred at Fukushima, Three Mile Island, and Chernobyl are not a possibility in the IFR design as a result of its unique coolant choice and pool configuration.

The Fukushima and Three Mile Island plants both suffered from one of the most serious problems that can befall a reactor: loss of heat sink. In nuclear power plants, not all of the heat generated by the core can be used to generate electricity; leftover heat is released either through cooling towers or large bodies of water. Without this outlet for unused heat, reactors can quickly overheat and melt down. The IFR’s cooling system, utilizing a combination of the aforementioned ‘natural reactivity feedback’ effect and the pool’s ability to absorb large amounts of heat, provides sturdy safeguards against a loss-of-heat-sink type accident.

The IFR’s ability to deal with loss of heat sink was successfully tested in its prototype reactor, EBR-II. In a test of the passive safety concept and with the reactor’s core at full power, the secondary pumps were purposefully shut down. No heat was able to escape the pool system, however in the absence of personnel and without any control or safety systems operating, the sodium pool was able to absorb all excess decay heat until the natural reactivity feedback of the metallic fuel brought the reactor to a stable condition. As mentioned before, this passive safety feature is not a possibility in LWRs, and is unique to advanced reactor designs such as the IFR. The EBR-II test run proved the reliability of the IFR’s natural reactivity feedback in protecting against a loss of heat sink type accident.

EBR-II also underwent testing on a loss-of-flow type accident, similar to the type of accident experienced by the Chernobyl plant. Similar to loss of heat sink, this type of accident is caused by a loss of power to coolant pumps—however, during a loss-of-flow type accident, it is both the primary and secondary coolant circuits rather than just the secondary coolant circuit that loses power. In the IFR, this means that the coolant immediately surrounding the core is not being circulated throughout the pool of surrounding sodium coolant (from which there is no coolant leaving). The immediate danger of a loss-of-flow type accident is coolant boiling within the local core area. This danger is mitigated by the primary pump’s designed-in flow-halving time, which continues moving coolant at a substantial rate due to the fluid’s inertia at the

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moment of power loss. The tens of seconds of minor cooling provided by the dying-down of coolant flow curtails local boiling of sodium; it also allows the reactor’s natural reactivity feedback to gradually reduce the fuel’s power output so that the temperature of the coolant stops rising, bringing the reactor under control. A loss-of-flow type accident was tested with success during the lifespan of EBR-II: as with the loss of heat sink test, the reactor was able to successfully moderate itself in the absence of any personnel or safety/control systems assistance.

The material and design choices of the IFR make it exceptionally safe in circumstances where the Light Water Reactor’s ability to safely cope is questionable. Despite this, the IFR’s use of sodium coolant brings up an issue that water-cooled reactors do not have to deal with.

Sodium reacts violently with water, and in a molten form is susceptible to react with the atmosphere, producing a dense white smoke. Although the resulting flame is lower both in heat and height than that of a traditional fire, it cannot be put out with traditional firefighting agents and requires an inorganic powder-extinguishing agent. Sodium-water reactions can become especially violent—explosions are not atypical. Sodium leak prevention is thus of the utmost concern in reactors utilizing sodium coolant.

The IFR takes numerous countermeasures to prevent any possibility of exposing sodium to a non-inert environment. In areas within close proximity of water, pipes are double-walled with an interference fit that safeguards against leaks. In the event that a leak does occur and sodium-water reactions begin to take place, multiple safeguards are in place to control the system and prevent explosions. One such example involves a system of blowout diaphragms designed to rupture at 100 psi that channels the reaction into pressure relief tanks. Issues arising from any potential sodium-water or sodium-air reactions have no possibility of releasing radioactivity into the environment, as only the non-radioactive secondary coolant system comes within proximity of the steam system. The primary tank itself is protected within a second, larger guard tank.

Filling the space between the two guard tanks is an inert atmosphere along with various sodium leak detection methods. Although sodium’s propensity to react violently with water and the atmosphere is a concern, the coolant has proven to be highly useful, as the many safety advantages it provides in the IFR design outweigh its single, easily controllable disadvantage.

The Integral Fast Reactor’s potential to greatly extend uranium supplies through the breeding of fissile fuel and subsequent collection/reprocessing of said fuel enables it to provide a clean, safe, and efficient outlook for the nuclear power industry, all while minimizing its impact on nuclear proliferation. By utilizing unique material and design choices, the IFR is capable of burning actinides as fuel rather than discarding them as waste, an advantage that enables it to reduce the toxicity and lifetime of its own waste in addition to giving it the ability to utilize the spent fuel of LWRs as its own fuel. These unique material and configuration choices also grant the IFR significant passive safety characteristics, making it one of the safest reactors possible, moderating itself even in the absence of operators and control systems. The combination of all these traits in one reactor design seems surreal, however the technology exists, has been extensively tested, and is capable of ushering in a new era of nuclear reactors. With the technology of the Integral Fast Reactor rests hope for a brighter future for mankind.

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Bibliography Till, Charles E., and Yoon Il. Chang. Plentiful Energy: The Story of the Integral Fast Reactor.

United States: Charles E. Till and Yoon Il Chang, 2011. Print.