The confinement of radioactive materials in nuclear power plants is one of the fundamental safety functions to be ensured, not only during normal operation, but also in every kind of accident that could possibly occur. This is obtained through a defence in depth approach, whose main goals are to protect the health and safety of the public and plant workers, besides protecting the environment and ensuring the operational readiness of the facility.
Defence in Depth is implemented through a number of measures, including robust physical barriers, redundant and diverse safety systems, strong physical security, and emergency response readiness.
The primary barriers are the fuel matrix itself and the cladding, which are designed to contain radioactive material under the extreme conditions inside the reactor core.
There are different types of nuclear fuel, either commercially available or under evaluation/research, and each one of them have different safety levels embedded already in the chemical structure of the respective material. The fuels can be of type:

  • Oxides: UO2, (U, Pu)O2
  • Uranium metal (used in old MAGNOX reactors)
  • Carbides: UC, (U, Pu)C
  • Nitrides: UN, (U, Pu)N
  • Metal Alloys: U-Pu-Zr-Mo
  • Inert matrix fuels: CERCER, CERMET
  • Others: UAlx, U3Si2, U/Zr hydride, UCO.

These chemical materials have specific properties which allow the resulted radionuclides to be safely confined in the respective pellet, under normal operation and, up to a certain point, also during severe events.
The cladding is the material encasing the fuel and has an important role in the well-functioning of a reactor and the inherent safety. Basically, besides the fact that it encases the nuclear fuel and confines the eventual fission products from spreading into the coolant, the cladding should be more or less “invisible” for neutrons, thus not hindering the nuclear reaction. There are several types of cladding materials, currently in use or under evaluation, characterized by very well-defined properties and specially designed for a

certain type of reactor (thermal or fast neutron spectrum), coolant and fuel:

  • Zirconium Alloys for LWRs
  • Stainless Steels
  • Aluminum Alloys for Research and Test Reactors
  • Refractory Alloys for High Temperature applications (W, Ta, Nb, Mo, V)
  • Ceramic matrix composites (SiC CMCs – ceramic matrix composite).

Fuel elements and assemblies are designed to withstand satisfactorily the irradiation and environmental conditions in the reactor core, taking into account also the processes of deterioration that can occur in normal operation (e.g. the irradiation of fuel, the change of temperature and pressure from changes of the power demand, chemical changes, …) and in anticipated operational occurrences.
The secondary barrier is the reactor vessel and the reactor coolant system pressure boundaries: the reactor vessel and the pressurizer as well as any welds are the main parts which can eventually fail.
The coolant, besides the fact that it serves a very important role in the reactor system as cooling media and sometimes also as moderator, is the next physical barrier for containment of fission products which might escape from the fuel pin (cladding). Generally, the coolant has to fulfill the following properties:

  • Efficient heat transfer
  • Fluid that fills the interstices of the core
  • Thermal and chemical compatibility with structural materials
  • Chemically stable at high temperatures
  • Non-corrosive and a poor neutron absorber
  • Cost-effectiveness

And, historically, the following coolants have been considered:

    • Liquid metals (Hg, Na, K, Li, NaK, Pb, Pb/Bi)
    • Molten salts
    • Water (light or heavy water)
    • Gases (He, CO2)
  • Organic compounds (polychlorinated biphenyl - PCB)

Nowadays, light and heavy water as well as sodium are used as coolant in the commercial reactor fleet, while in the future reactors cooled by He, supercritical water, molten salts and liquid metals (such as Pb or eutectic mixture Pb/Bi) will be developed. While fuel-coolant chemical interactions in the case of water and sodium are well known, the chemistry of fuel and metallic coolants such as those under consideration in GEN IV systems is currently under study.
The outer barrier is the containment building, which is designed to mitigate the release of radioactive material in the event that both the primary and secondary barriers are compromised. The primary containment is designed to withstand the most severe, credible event, either internal or external, for the location of the plant. The containment structure includes many systems with the functions of isolation, energy management, and control of radionuclides and combustible gases. These are called the containment systems.
In the case of release of fission materials which escaped from: fuel, cladding and coolant, the last barriers are the welds which are tightening the pipes in the reactor system and the reactor building, including the basement. The reactor building is a very strong structure, able to sustain earthquakes or other events, like a plane crash. The basement is reinforced and contains a series of redundant systems which are capable on catching and confining the possible core melt, thus even there the safety and the right materials are very important. It is clear that the right chemistry combination of the fuel, cladding, coolant and the surrounding materials, as well as the integrity of the buildings, are the base of the safe operation of a nuclear reactor, and paramount for preventing any escape or fission products. The mechanisms behind all the chemical processes happening during normal operation (including structural materials), as well as potential nuclear accident scenarios is ideal to know, thus mitigation measures can be taken in good time.
Even more, the multiple physical barriers approach is adopted for the next step, where nuclear fuel becomes nuclear waste and safe disposal is needed. The final storage of nuclear waste is a difficult and very long-time project, unless the nuclear fuel waste is reprocessed. Confinement of radionuclides have a series of physical barriers which have to withstand over 100 000 years, thus the overall final repository has to be carefully designed and executed.