The unique composition of graphite make it a perfect choice for nuclear reactors.
In this guide, we will explore all possible applications of graphite material in the nuclear reactor industry. Besides, you will also learn how graphite in nuclear industry works.
Lastly, we will answer common questions people have been asking us about graphite for nuclear reactor.
Let’s dive right in;
Why is Graphite used in Nuclear Reactors?
Many a times, graphite’s use in nuclear reactors is as a moderator. In this role, it helps slow down neutrons that are fast-moving to effectively accommodate nuclear chain reaction.
Here are some of the reasons why graphite finds use:
- Graphite is readily available in large quantities and highly affordable.
- Graphite absorbs very few neutrons ensuring they are retained within the system enhancing chain reaction.
- The stability of graphite at very high temperatures is excellent, conducive in the high-temperature environment of a nuclear reactor.
- The strong covalent bonds within the graphite structure offer good mechanical strength. This allows utilization within the reactor core as a structural component.
How does Graphite Work in Nuclear Reactor
Graphite in a nuclear reactor performs the function of neutron moderation. In this role, it slows down the movement of neutrons produced during fission. This way, it makes the nuclear reaction more effective.
During the fission process in a nuclear reactor, fissile material such as uranium-235 releases energy alongside fast neutrons. The movement of these neutrons is at very high speeds unlikely to initiate further splitting on collision. Thus, the need to slow them down to thermal neutrons.
The effectiveness of graphite in this role is down to its low neutron absorption capacity.
As a result, while it slows down the movement of neutrons, it doesn’t absorb much of them. Consequently, more neutrons are available to sustain the chain reaction.
During the moderation process, the work of nuclear graphite can be broken down as follows:
- The neutrons collide with the graphite’s carbon atoms in a manner defined as elastic. Here, some of the neutrons’ kinetic energy is transferred to the carbon atoms without being absorbed.
- There are multiple collisions of the neutrons with graphite’s carbon atoms occurring simultaneously with the former losing speed in the process.
- Several collisions result with the slowing down of neutrons to thermal energies (low speeds). In this state, they can induce further fission reactions in the fissile material to sustain chain reaction.
Types of Graphite-moderated Reactor Core
Graphite-moderated reactors utilize graphite in slowing down neutrons for sustained nuclear chain reaction. You can implement different cooling mechanisms for reactor cores utilizing graphite for moderation.
Here are some types of graphite-moderated reactor cores:
· Air-cooled Plutonium Production Graphite Piles
Originally designed during the Second World War for plutonium production utilized in nuclear weapons, rather than power generation. This graphite-moderated reactor core featured large graphite blocks in which uranium fuel rods were inserted with air as the cooling medium.
· Light Water Cooled Graphite-moderated Pipes
Designed to support both power generation and plutonium production, these reactor cores utilized ordinary water as the coolant. It featured fuel rods with uranium placed inside graphite blocks. The water when circulated transferred the collected heat to a steam turbine for power generation.
· Carbon Dioxide Cooled Reactors
Known as Gas-Cooled Reactors (GCRs), this type was made purely for power generation with carbon (IV) oxide gas as the coolant. CO₂ gas was pumped through the reactor transferring heat from the nuclear core to a heat exchanger. From the exchanger, water was heated to generate steam to drive turbines.
· High Temperature Helium Cooled Reactors
The unique design of this rector core was purposed for highly efficient generation of electricity. They were also utilized in specialized industrial applications like production of hydrogen. The fuel containing uranium was in the form of pebbles and surrounded by graphite.
Also broadly classified as High-Temperature Gas-Cooled Reactors (HTGRs), these used helium gas as coolant. Since helium is an inert gas it displays good heat-transfer properties allowing high operating temperatures. The result is highly improved thermal efficiency in the generation of electricity.
· Sodium Cooled Graphite Moderated Reactors
Liquid sodium finds use as the coolant in these reactors designed for power generation as well as research into fast breeder reactors. The liquid sodium is circulated through the reactor core, transferring the generated heat to a power generation unit.
Sodium use is due to its excellent thermal properties and thus highly efficient heat transfer. However, its high reactivity with water and air poses a safety challenge.
· Pulse Reactor IGR in Semipalatinsk
The Impulse Graphite Reactor (IGR) is a facility for research and experimentation. This reactor core in Semipalatinsk, Kazakhstan was developed to understand nuclear reactor safety and behavior under extreme conditions. It has no active cooling system, instead utilized in a pulsed mode to support brief experiments of high-intensity.
Type of Graphite used in Nuclear Reactor
Graphite for nuclear reactors, also nuclear-grade graphite, is made to meet the rigorous requirements of nuclear environments. When used in reactor cores, graphite primarily functions as a neutron moderator and also as part of the core’s structural material.
A particular point of departure between nuclear-grade graphite and industrial graphite is in its density, structural properties and purity. Nuclear-grade graphite is used for its thermal stability and ability to slow down neutrons with little absorption.
Some key characteristics of nuclear graphite include:
- High Purity: Nuclear-grade graphite contains very little impurities. Impurities interfere with the moderation process and can contribute to neutron absorption e.g. boron, reducing process efficiency. Ideally, purity levels for nuclear graphite are in excess of 99.9% carbon.
- Isotropy: Graphite for nuclear reactors should be isotropic. This means their behavior and response should be similar in all directions. This is especially when it comes to neutron moderation and thermal expansion. As a result, it retains its structural integrity under intense radiation and heat.
- High Density: Controlling the density of nuclear-grade graphite is necessary for optimal performance. Graphite with a higher density is favored owing to its reduced porosity. Consequently, it minimizes damage from radiation while enhancing mechanical properties.
Uses of Graphite in Nuclear Reactor
Graphite reactors implement graphite in various components thanks to it mechanical properties, thermal stability and of course neutron moderation. Some of these uses are captured below:
· Graphite Block Nuclear Reactor
Graphite blocks have a dual purpose use in many graphite-moderated reactors, as moderator and fuel’s structural support. These blocks reduce the kinetic energy of fast-moving neutrons resulting from the nuclear fission process for sustained chain reaction.
In Advanced Gas-Cooled Reactors and Magnox reactors, graphite blocks form the bulk of the reactor core’s structure. Here, they find use housing coolant channels and fuel rods. Furthermore, their excellent thermal conductivity is useful in heat distribution throughout the reactor.
· Graphite Core in Nuclear Reactor
You can find an entire reactor core built from graphite components to execute functionality as a support structure and neutron moderation. For instance, old RBMK reactors utilized graphite core surrounding fuel rods operating as central moderator.
Additionally, using graphite cores in Advanced Gas-Cooled Reactor (AGR) allows for high-temperature operation stemming from the superior thermal efficiency.
· Graphite Rods in Nuclear Reactor
When using graphite rods, it is typically as control or moderator rods in certain nuclear reactors where they oversee neutron moderation. In such reactors, you find the graphite rods inserted into the core. This helps in efforts to optimize the fission process by slowing down neutrons further.
Other designs, feature graphite rods capable of in and out movement in the core. This adjusts the fission reaction rate allowing fine neutron flux control.
This way, it is more effective in regulating the power levels of the reactor. The good thermal conductivity of graphite rods is also useful in heat distribution.
· Graphite Ball Nuclear Reactor
Some graphite reactors utilize graphite as fuel elements in spherical or ball shape. This is common in high-temperature gas-cooled reactors referred to as pebble bed reactors.
Additionally, the nuclear fuel is embedded in the graphite balls. Still, the graphite is able to moderate the neutrons during nuclear fission.
This unique design allows for continuous refueling seeing as the graphite balls move unrestricted within the reactor core. Once a ball is spent, it is removed while fresh balls are introduced without needing to shut down the reactor.
· Graphite Bars in Nuclear Reactor
Use of graphite bars in nuclear reactors can be as structural components or as reflectors. As reflectors, graphite bars reflect back escaping neutrons into the core improving neutron economy to sustain a chain reaction. Its use as a structural component helps maintain the core’s mechanical structure.
· Graphite Containers for Nuclear Reactor
Graphite containers are employed in housing materials such as nuclear fuel elements in the reactor. These containers aid in dissipating heat while also offering protection from mechanical damage. When used in fuel encapsulation, graphite containers ensure the fuel is retained during reactor operation.
Since graphite can withstand high temperatures, these containers protect the nuclear fuel from cases of overheating evenly distributing heat across the core. Additionally, the contribution of graphite containers to neutron moderation cannot be understated.
FAQs
1. Why did Chernobyl use Graphite?
The reactor at Chernobyl was an RBMK (Reaktor BolshoyMoshchnostiKanalny) reactor, designed to use graphite for neutron moderation. As such, it relied on large graphite blocks in slowing down neutrons to sustain the nuclear fission process.
The RBMK design also combined the graphite moderator with water as a coolant but with physical separation of functions. This way, it could operate using low-enriched uranium fuel unlike other reactors that required highly enriched uranium. This was economical as it meant a reduced fuel cycle cost.
2. Is Graphite Still used in Nuclear Reactor?
Yes.
While its role in the Chernobyl disaster negatively affected its reliance in reactors, graphite still finds limited use. Its advantages as a neutron moderator and structural integrity cannot be overlooked.
Some nuclear reactors still utilizing graphite include Advanced Gas-Cooled Reactors (AGR), and High-Temperature Gas-Cooled Reactors (HTGR). Nonetheless, modern reactors have inherently replaced graphite with water as primary moderator.
3. Do American Nuclear Reactors use Graphite?
No.
Most American nuclear reactors are light water reactors (LWRs) utilizing light water in the functions of moderation and cooling. Since graphite was primarily used for moderation of the fission process, using water instead makes it redundant. Nonetheless, graphite still finds some use in specialized reactors for researching advanced reactor concepts.
4. Do Russian Nuclear Reactor Rods still have Graphite Tips?
No.
Following the disaster that was Chernobyl, modern Russian nuclear reactors eliminated graphite tips from their designs. Unique to RBMK reactors, graphite-tipped control rods were a design flaw pointed out to largely aggravating the Chernobyl accident.
The undoing of graphite-tipped rods was their increasing of reactivity albeit momentarily when inserted into the core instead of desired decrease. Consequently, graphite tips were eliminated from control rods in the remaining operational RBMK reactors as well as new reactor designs.
5. Has Boron Replaced Graphite in Nuclear Reactors?
Not really.
Use of graphite in nuclear reactors was primarily as a moderator. While boron has found increasing use in nuclear reactors, it is not as a direct replacement for graphite in moderation. Instead, boron serves a different but complementary role, primarily as a neutron absorber, where it controls the reaction rate of nuclear fission.
Boron is typically used in the form of boric acid or boron carbide (B4C) in control rods. In some designs, it is dissolved in the reactor coolant. Control rods are central in the regulation or shutting down of the fission reaction when inserted into the reactor core.
6. Is Graphite used in Nuclear Reactor as a Lubricant?
No.
Graphite is recognized for its lubricating properties stemming from its ability to reduce friction between moving parts. While this has allowed its use in many industrial applications, it has found no such use in nuclear reactors. Its use in reactors is limited to slowing down neutrons in a moderation role.
7. Can you use Graphite from Nuclear Reactor?
Graphite from a nuclear reactor cannot be reused or repurposed in any way. Since graphite is used as a neutron moderator, it is exposed to high level neutron bombardment. This causes it to be radioactive over time depending on the reactor conditions and exposure duration. Transmutation of carbon atoms in the graphite into radioactive isotopes is also possible further contributing to its radioactivity.
Owing to this, irradiated graphite poses serious safety hazards in terms of handling and reprocessing such as extensive shielding. Additionally, continued exposure to heat and high radiation can degrade the material’s structural properties. Thus irradiated graphite is considered radioactive waste requiring careful management and disposal in strict accordance to regulations.
8. What is Wigner Effect in Graphite Moderated Nuclear Reactor?
The Wigner effect occurs when the graphite used in neutron moderation becomes damaged owing to extended exposure to neutron radiation. A select number of neutrons have very high kinetic energy capable of displacing some atoms in the lattice structure of graphite.
Vacancies are created by the displaced atoms in the graphite structure as well as interstitials distorting the material. On the other hand, displaced atoms store potential energy which increases as more atoms are displaced.
This energy can remain trapped for a long time but can be released under certain conditions. A particular instance is when the heat conditions reach graphite’s annealing temperature. At this point, the displaced atoms return to original positions.
When this happens, there is a sudden release of the stored energy as heat. Combined with the surrounding high temperature conditions, the resultant effect is a rapid temperature rise in the core. If uncontrolled, this results in localized overheating and in extreme cases reactor damage.
9. What is the Lifetime of Graphite Moderated Nuclear Reactors?
Several factors influence the lifetime of graphite-moderated nuclear reactors. These include: operational conditions, degradation of the graphite moderator and regulatory decisions. Graphite-moderated reactors were originally designed to operate for around 30-40 years.
With proper maintenance and improvements some of these reactors have achieved life extensions allowing operation totaling up to 50-60 years. Nonetheless, some graphite-moderated reactors have been shut down prematurely or decommissioned for various reasons prime of which is safety.
10. How much does Graphite in Nuclear Reactor Cost?
Factors such as purity and graphite volume required for specific reactor design influence the cost of graphite used in nuclear reactors. Owing to the highly specialized nature of nuclear-grade graphite, it costs more than standard industrial graphite. Nuclear graphite can cost anywhere between $10K to $20K dollars per metric ton. This variance can be attributed to market forces of demand and supply.
Conclusion
Graphite still finds use in select nuclear reactors such as those intended for research, as well as high-temperature and advanced gas-cooled reactors. However, many nuclear reactors constructed in recent times implement a design devoid of graphite with water replacing it as the primary moderator.
