Question

Describe the role of ceramic materials in nuclear power generation systems with emphasis on nuclear fuel and nuclear waste chemistry. Provide examples of reactor fuel materials, fuel fabrication processes, and reaction schemes during the power generation mode. Discuss materials handling and waste management approaches currently being used.

Answer 1

Ceramics have played a crucial role in the development of fission based nuclear power, in glass & glass composite high level wasteforms, in composite cements to encapsulate intermediate level wastes (ILW) and also for oxide nuclear fuels based on UO2 and PuO2/UO2 mixed oxides. They are also used as porous filters with the ability to absorb radionuclides (RN) from air and liquids and are playing a key role in the cleanup at Fukushima. Non‐oxides also find current fission applications including in graphite moderators and B4C control rods. Ceramics will continue to be significant in the near‐term expansion of nuclear power via next‐step developments of fuels with inert matrices or based on thoria and in wasteforms using alternative composite cements or single or multiphase ceramics that can host Pu & other difficult RN. Longer term advances for Generation IV reactors, which will operate at higher temperatures & with higher fuel burn‐up require innovative fuel developments potentially via carbides & nitrides or composite fuel systems. Novel non‐thermal (cement‐like) and thermal techniques are currently being developed to treat some of the difficult legacy wastes. Non‐thermally derived wasteforms developed from geopolymers, composite cements, hydroceramics, and phosphate‐bonded ceramics and thermally derived wasteforms made by Hot Isostatic Pressing and fluidized bed steam reforming (FBSR) as well as vitrification techniques based on cold crucible melting (CCM), Joule‐heater in‐container melting and plasma melting (PM) are described. Future developments in waste treatment will be based on separation technologies for partitioning individual RN along with design & construction of RN‐containing ceramic targets for inducing transmutation reactions. Near demonstration actinide‐hosting ceramic wasteforms including multiphase Synroc systems are described. Opportunities also exist for ceramics in structural applications in Generation IV reactors such as composite SiC/SiC and C/C for fuel cladding and control rods and MAX phases and ultrahigh‐temperature ceramics (UHTCs) may find near core fuel coating and cladding applications. Uses of ceramics in fusion reactor systems will be both functional (ceramic superconductors in magnet systems for plasma control and in Li silicate breeder blankets in tokamaks) and structural including as sapphire diagnostic windows, graphite diverters, and plasma facing C and UHTCs.

In all these cases, performance is limited by poorly understood radiation damage and interface controlled processes, which demands a combined modeling/experimental approach. In the nuclear-related functions, ceramics are of major importance. Since the beginning of nuclear power generation, oxide ceramics, based on the fissionable metals uranium and plutonium, have been made into highly reliable fuel pellets for both water-cooled and liquid-metal-cooled reactors. Ceramics also can be employed to immobilize and store nuclear wastes. Although vitrification (glass formation) is a favoured approach for waste disposal, wastes can be processed with other ceramics into a synthetic rock, or synroc, or they can be mixed with cement powder to make hardened cements. All these nuclear applications are extremely demanding. In addition to severe thermal and chemical driving forces, nuclear ceramics are continuously subjected to high radiation doses.

Nuclear energy holds the promise to provide vast amounts of reliable baseline electricity at commercially competitive costs with modest environmental impact. However, the future of nuclear energy lies beyond the current generation of light-water reactors. Future reactors will be expected to provide additional improvements in safety, maintain high reliability, use uranium resources more efficiently, and produce lower volumes of less toxic solid wastes. Several advanced reactor concepts are under development to meet these demands. In most cases, these designs translate into higher operating temperatures and longer lifetimes, more corrosive environments, and higher radiation fields in which materials must reliably perform. The safe and economical operation of any nuclear power system relies to a great extent on the success of the fuel and the materials of construction. Materials used for fission and fusion-based nuclear engineering mainly include fuels, materials for fuel cladding, moderators and control rods, first-wall materials, materials for pressure vessels and heat exchangers. During the lifetime of a nuclear power system, the materials are subject to high temperature, corrosive environment, and damage from high-energy particles released during fission. The fuel which provides the power for the reactor has a much shorter life but is subject to the same types of harsh environments. This chapter will review and update nuclear energy reactors and the materials challenges that will determine the feasibility of these advanced concepts and define the long-term future of nuclear power.

Ceramics hold a unique position in nuclear fission reactors since they are used for fuel, the coating for fuel particles and pressure or reactor vessels, and as the materials of moderator and reflector, control and shielding. Ceramics also play important roles in fusion reactors as the materials for first walls, breeding, electric insulator and shielding. Uranium dioxide as the important nuclear fuel for LWRs, silicon carbide as the cladding material for nuclear fuel, graphite as moderator and reflector, boron carbide as control material, europium oxide for use as the control element in FBRs, europium hexaboride as a substitute for boron carbide, and europium nitride pellets as the material for control element in FBRs. Graphite, glass and even cement are included in ceramics, accordingly, the definition of ceramics has become much broader.

In a nuclear power plant the fuel rods are used to generate electricity. The fissile material in the fuel is depleted over time, typically in ≈ 10 years, after which the rods cannot be used further. Such fuel is called spent fuel. Its activity, however, is still sufficiently high to initiate runaway reactions if it is stored in uncontrolled conditions. It remains harmful to the environment for millions of years because of the long-lived radioactive isotopes it contains. Because it radiates neutrons that trigger nuclear reactions, it needs to be stored in water pools for at least a decade so that the neutrons are absorbed by water till the neutron intensity is depleted sufficiently and it can then be stored in alternative dry storage casks and transferred to a permanent repository. Temporary storage in water pools produces debris of the spent fuel at the bottom of the pool. The water is now contaminated and becomes radioactive sludge waste that needs to be immobilized for proper disposal to prevent it from entering groundwater, surrounding soil or air.