Lecture 4: Classes of Materials in Nuclear Power Plants PDF

Lecture 4: Classes of materials in nuclear power plants Structural materials must be able to operate under demanding exposure conditions. For nuclear plants these are temperature, radiation and corrosive media. In principle there is no specific class of nuclear materials and the materials...

Lecture 4: Classes of materials in nuclear power plants Structural materials must be able to operate under demanding exposure conditions. For nuclear plants these are temperature, radiation and corrosive media. In principle there is no specific class of nuclear materials and the materials are essentially the same as the ones used also for other applications. The classification of the materials will be made according to their resistance to high temperatures. Starting with carbon steels and low alloy steels, stainless steels, and superalloys will be introduced. Intermetallics and nano-featured alloys with different matrices are considered as candidates for advanced applications. For very high temperatures and for some core internals and linings also ceramics are introduced. Concrete is used for the containment building and other structures. 1 The choice of materials of construction of a nuclear reactor, while important in terms of plant capital cost, is crucial to the safe and economic operation of the unit throughout its design lifetime; it also affects decisions about plant life extension. For structural nuclear applications basically the following classes of materials are considered: Metals and alloys Intermetallics Ceramics (bulk and fiber reinforced) Layered structures Concrete 2  Materials of construction for nuclear applications must be strong, ductile and capable of withstanding the harsh environment. For materials used in the core of a nuclear reactor, it is important that they have specific properties such as low neutron absorption and high resistance to radiation-induced creep, hardening and the associated loss of ductility. Nuclear materials must be selected or specified based upon their strength and interactions with the environment (including the effects of radiation), all of which are dependent upon the metallurgy of the material. 3 Expected radiation damage and operation temperatures of different advanced nuclear plants 4 The different steps of materials implementation 5 6 Nuclear Power Plants Materials Map 7  Metallurgy and Irradiation Effects Crystal Structure and Grain Boundaries All metals used in power plant construction are crystalline in nature, meaning that they have a defined and consistent crystal structure. Three of the basic and most observed crystal structures: Unit cell configurations of some common metallic crystal structures 8 Background: Grains 9  Grain structure, size and orientation may play significant roles in the specific materials properties such as strength, conductivity and creep resistance Grain structure for low-alloy ferritic steel (UNS G10800), austenitic (316 SS) steels and Zircaloy 4 10 Background: Phases 11 12 Background: Phase Transitions 13 Steel Heat Treatment and Microstructure 14 Heat Treatment 15 Predicting Change: TTT 16 Welding and the HAZ 17 18 Young's Modulus (E) 19 Grain Size vs. Strength 20 Fracture Toughness 21 Fracture Toughness (KIc) Fracture Toughness (KIc) 22 Toughness (Gc) 23 Hardness 24  Irradiation effects on materials The effects of irradiation on materials may be manifest in several ways, the most dramatic being the dislodging or dislocation of a random atom in the crystal lattice to a new location During neutron bombardment, the energy dissipated by the neutron upon collision with a metal atom can create new defects in the crystal structure, typically “interstitial and “vacancy” sites. The pair (interstitial/vacancy) is termed a Frenkel pair and is a key phenomenon associated with radiation damage in polycrystalline materials. The damage induced in the material is additive and substantial, considering that in a typical reactor the overall neutron flux may be 2x1017 neutrons/m2.s The creation of vacancies within the material leads to swelling, elongation and growth whereas production of interstitials can cause the material to shrink. The energy dissipated by radiation may induce phase changes or promote segregation of alloying constituents, affecting the material’s strength and ductility. Overall, the mechanical properties of a material under irradiation will change and the effect ultimately leads to challenges for the integrity of the components. 25  Irradiation Hardening When a material is irradiated by particles (neutrons) producing Frenkel pairs, the effect of the interstitital/vacancy sites is to provide barriers to the normal slip planes within the material. This effectively increases the yield strength, which will increase continuously in proportion to the increasing radiation dose. Effect of neutron irradiation on the stress/strain properties of materials with cubic crystal structures 26 27  Irradiation Creep and Growth All metals will exhibit creep over time when exposed to high temperatures and under operational stress. Thermal creep is a diffusion-based migration of atoms and vacancy sites within the metal’s lattice, which typically requires a minimum operating temperature before its effects are observed – generally the operating temperature must be greater than 0.3 of the melting temperature of the metal. Since diffusion is the primary mechanism for thermal creep, it typically increases exponentially with increasing temperature following an Arrhenius law. Irradiation of a metal creates interstitials and vacancies within the metal’s lattice, effectively mimicking the effects of thermal creep of the metal only occurring at much lower temperatures than those required for significant thermal creep alone. 28 Irradiation Embrittlement As a metal gains strength upon irradiation, it will also tend to lose its ductility, becoming hard and brittle with increasing damage and radiation dose. The material’s yield strength increases through build-up of dislocations along barriers to atom migration, but other effects such as the precipitation of secondary and tertiary phases within the material can change the mechanical properties dramatically. Materials that are typically ductile and not susceptible to brittle fracture in an un-irradiated environment may cleave and fracture excessively upon irradiation due to the increasing hardness and loss of ductility. 29