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Fast reactors

The reactors qualified as fourth generation that are being investigated at IRESNE are referred to as “fast” reactors. The term “fast” concerns the neutrons. This reactor technology boasts three key advantages:

• A new fuel source: plutonium
  IRESNE is invested in studying this new technology that has the potential to operate with plutonium as the fuel. Though this element does not normally exist in nature, it is produced by the current fleet of pressurised nuclear reactors; part of the uranium-238 in the fuel is transformed into plutonium during fission. The resulting plutonium is separated from the spent fuel at the reprocessing plant in La Hague and then be used to make plutonium-based fuel called MOX, which powers EDF’s 900 MW nuclear reactors. Once MOX has been used in a 900 MW PWR, it is placed in disposal because the isotopic vector is too deteriorated to be reused in MOX form in another PWR. Thanks to their neutron spectrum characteristics, fast reactors can reuse the plutonium in the spent MOX, even recycling it multiple times. These reactors can operate in different modes, i.e. in self-sufficient or breeder conditions, which means that they can burn plutonium, stabilise the plutonium inventory, or produce plutonium from uranium-238. France is considering the deployment of this type of reactor in the second half of this century.
  

• Optimised use of uranium resources
  Pressurised water reactors (PWRs) only use uranium-235 for fuel, it being the only fissile isotope of natural uranium. Yet uranium-235 represents a mere 0.7% of all natural uranium, whereas uranium-238 represents the remaining 99.3%, which is a non-fissile element. Fast reactors generate plutonium-239 when uranium-238 captures neutrons. This is why fast reactors open the door to using fuels that exploit both plutonium and all of the uranium. Being able to use all of the uranium means that we can multiple the extractable energy from natural uranium by a factor of 100. Additionally, with all the depleted uranium (uranium-238) kept during the different uranium enrichment programmes in France, together with the plutonium extracted from spent fuel, these new fast reactors could generate electricity for several thousand years without needing to extract any more natural uranium.
  

• Reducing nuclear waste
  This new technology can also be used to transmute minor actinides into shorter-lived elements, thereby limiting the quantity of long-lived high-level waste generated during the operation of pressurised water reactors.
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Sodium-cooled fast reactors (SFR)

Using sodium as a coolant makes transmutation possible

The CEA is actively committed to research on these fourth generation reactors. At IRESNE, we have devoted part of our research to investigating this fast reactor technology, more specifically with liquid sodium as the coolant (S being the chemical symbol for sodium). There are also other fluid coolants that can replace sodium, such as gas, lead and molten salts, which are also studied by our IRESNE.

The advantages of sodium

Sodium offers real advantages as a fluid coolant which justify this choice and commitment from our researchers:

• Good thermal properties:
  Sodium has the advantage of high thermal inertia, which can be used to absorb and efficiently remove the heat generated in the reactor core. It boasts a high heat capacity and good thermal conductivity.
  

• Low viscosity:
  Sodium also boasts low viscosity, which means it does not require high pumping power.
  

• No need to be pressurised:
  Contrary to water, sodium does not need to be pressurised. This is because its boiling temperature is much higher than the reactor’s operating temperature.
  

• Low neutron activation:
  Sodium is only slightly activated by neutrons, which means that large quantities of radioactive waste will not be produced.
  

• Very good compatibility with materials:
  Sodium is not very corrosive, making it compatible with steels and avoiding the early deteriorating of reactor structures.
  

• Purification (filtering):
  The fluid coolant must not degrade under the effect of irradiation or become overly radioactive so reactor maintenance and eventually dismantling can be carried out with relative ease. More than often, activation is caused by impurities in the coolant. It is therefore important to be able to keep the sodium sufficiently clean during reactor operations.
  The CEA - more specifically thanks to its teams at IRESNE - has developed processes to purify sodium and control its impurity levels. The two main devices used in these processes are the cold trap and the plugging indicator. Some measuring instruments have also been developed to better understand the sources of pollution and to monitor the quality of sodium during reactor operation.
  

• Economical and available:
  Sodium has no availability issues and can be procured at low cost, which makes it a very interesting coolant.
  

  
  

Disadvantages of sodium as a coolant

Sodium offers many advantages when used as a coolant. However, it also has several disadvantages that must be managed:

• It is a highly reactive chemical product:
  Sodium has a very strong reaction with water and oxygen in the air. It can ignite spontaneously in air and reacts violently in contact with water, which produces explosive hydrogen gas. Very specific precautions are required to prevent sodium fires and sodium-water interactions from occurring.
  

• Opaque liquid:
  Liquid sodium is not transparent. The periodic inspection of surfaces inside the reactor vessel, the identification of components, and the search for foreign objects in a sodium environment are complex tasks to perform. Faced with these difficulties, IRESNE set about developing a set of innovative instruments capable of overcoming these issues.
  

The use of sodium calls for specific, in-depth research on the design and operation of reactors using this coolant.​
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