Materials in Nuclear Energy Applications

Volume I

C.K. Gupta

• Publisher: CRC Press
• Year: 2018
• Print ISBN: 9781315895741
• eBook ISBN: 9781351091749
• Topic: Technology & Engineering
• Subtopic: Electrical Engineering & Telecommunications
• Index: Technology & Engineering

The text combines an account of scientific and engineering principles with a description of materials and processes of importance in nuclear research and industry. The coverage includes fuel materials, control and shileding materials, and so on - in fact, for most of the important pasts of a reactor.

Chapter 1

STRUCTURAL MATERIALS

I. INTRODUCTION^{1, 2, 3, 4, 5}

Structural materials can be shown as a group listed in seven main categories into which the primary components and materials of nuclear fission reactors are classified. The structural materials of nuclear reactors provide the physical containment (for fuel protection), mechanical strength, and structural support for reactor components. The main basic materials consist of the fuel cladding, pressure vessel, fuel coolant channels, core support plates, coolant piping system, and control element mechanism. Selection of structural materials depends upon the functional requirements and varies with different types of nucelar fission reactors.

For example, reference can be drawn to cladding materials. The term cladding refers simply to the outer jacket of the nuclear fuel and serves as a barrier to the release of fission products. The cladding materials should fulfill the following: (1) mechanical and chemical stability with the fuel on one side and with the coolant on the other side, (2) high-temperature mechanical strength, (3) good heat transfer properties, (4) ability to accommodate radiation damage, (5) ability to withstand high heat loads without melting and losing mechanical integrity, and (6) favorable nuclear properties. The most important among these for a material to be used as fuel cladding in a thermal reactor is that it should have reasonably low thermal neutron absorption cross section. If the absorption cross section is too large, the material cannot be chosen no matter what mechanical and other advantages it may possess. This consideration alone limits the list of materials to a great extent. From consideration of the neutron absorption cross section of materials decreasing with increasing neutron energies, one of the advantages of the fast reactor system is the fact that the neutron economy is not seriously affected by absorption in structural materials. As a result, the materials list need not be as limited as in the case of thermal reactor systems. In fact, volumes up to 20% of a wide variety of structural materials can be incorporated without appreciable sacrifice of neutron economy.

The fissile nucelar fuels with the nonfissile cladding materials on them, as one unit, are called the fuel elements. They are recognized as the fundamental reactor core material. It is in this site where the most important event takes place. Within the core of an operating reactor, the fission heat is generated in the fissile fuel material and conducted into the coolant that flows past the fuel elements. The fuel elements have long lives as compared to the conventional types of fuel. They are the replaceable components of the reactor. The residence of fuel elements in reactors varies widely. It varies, for example, for about 1 year in fast reactors to 7 years in Magnox reactors to between 3 to 5 years in light water reactors. The criteria of an ideal fuel element require (1) thermal, irradiation and mechanical stability; (2) good corrosion resistance; (3) ease of fuel fabrication and fuel reprocessing; (4) good neutron economy and high fuel burnup; and (5) long lifetime service and low cost. To date, there is a large variety of nuclear fuel elements. These may be classified according to such diverse criteria as fuel material composition, fuel element shape, type of fuel-cladding contact, reactor type, etc. Table 1 gives the classification of fuel elements on the basis of the various reference points as stated. From the standpoint of heat transfer, heat removal, and ease of fabrication, the basic considerations of geometry and economics of the fuel elements are (1) large surface to volume ratio, (2) coolant removing the heat with maximum efficiency from the fuel element surface, (3) high coolant pressure-drop configuration permissible only if it is accompanied by a proportional gain in the heat transfer rate, and (5) simple geometry with ease of fabrication and low cost.

Table 1
CLASSIFICATION OF FUEL ELEMENTS

Basis of reference points	Classification	Remarks/example
Fuel composition and cladding materials	Metallic fuel/metallic clad elements	Metallic uranium is used in a magnesium alloy cladding
	Ceramic fuel/metallic clad elements	UO2, (U,Pu)O2, or (U,Th)O2 is clad in zirconium alloy or in stainless steel; UC or (U,Pu)C is clad in stainless steel
	All-ceramic fuel elements	UO2, (U,Pu)O2, or (U,Th)O2 grains are coated with SiC and pyrocarbon layers embedded in a graphite matrix
Fuel nature	Metallic fuel elements	The fuel is a metal or an alloy
	Ceramic fuel elements	The fuel is a nondiluted ceramic
	Dispersed fuel elements	The fuel is a highly diluted alloy or a ceramic material with a low density of fissile material
Shape	Cylindrical fuel elements
	Solid fuel elements	Metallic fuel elements or ceramic fuel pellets are used
	Tubular or annular fuel elements	A metallic fuel rod or tube is used
	Plate fuel elements Flat Curved	The element is generally made of highly enriched ^235U fuel clad with aluminum
Type of contact	Mechanic-contact fuel elements	The contact is achieved by either pressing or rolling
	Metallurgical-bonding fuel elements	The materials in contact are bonded by diffusion
	Intermediate-layer fuel elements	An intermediate liquid, solid, or gaseous medium is used for improvement of fuel cladding heat transfer

Structural materials comprise (1) metals and alloys, (2) ceramics, and (3) cermets. Metals and their alloys, because of their ability to withstand the primary requirements, dominate as structural materials in different types of nuclear reactors. Most of the reactor structural materials are made of metals and their alloys, such as aluminum, magnesium, zirconium alloys, stainless steels, carbon steels, and nickel alloys. This chapter presents a description of these major categories of nuclear reactor structural materials.

II. ALUMINUM

Aluminum has been and still continues to be used as a cladding and structural material for teaching and research reactors. The chief requirement of a teaching or research reactor is high neutron flux for neutron economy, but not for power generation. The teaching and research reactors are therefore operated at low temperatures. The choice of aluminuum for application as cladding or structural material in thermal research and teaching reactors is based on advantages such as relatively low thermal neutron absorption, high thermal conductivity, high stability under irradiation, good corrosion resistance to air and water, fabricability and weldability, low cost, and abundant availability over disadvantages such as low melting point and low mechanical strength at elevated temperature.

A. Extraction

Aluminum oxide, of alumina (Al₂0₃), is the main source from which extraction of aluminum is accomplished by electrolysis. For the electrolysis, purified anhydrous Al₂O₃ is dissolved in cryolite (Na₃AlF₆) in an iron tank with carbon which functions as the electrolytic cell cathode. Inside the tank there are large blocks of carbon/graphite to serve as anodes. When electric current is passed through the cell, molten aluminum forms at the wall and bottom of the tank while the oxygen is liberated at the carbon anode to form carbon dioxide. The reactions at the cathodes and anodes are represented as

$\begin{array}{l}\text{A}\text{t}\text{t}\text{h}\text{e}\text{c}\text{a}\text{t}\text{h}\text{o}\text{d}\text{e}:\text{A}\text{l}{\text{F}}_3\rightleftharpoons \text{A}{\text{l}}^{3+}+3{\text{F}}^{-}\hfill \\ \text{A}{\text{l}}^{{}^{{}_3+}}+3{\text{e}}^{-}\to \text{A}\text{l}\hfill \\ \text{A}\text{t}\text{t}\text{h}\text{e}\text{a}\text{n}\text{o}\text{d}\text{e}:3{\text{F}}^{-}\to 3\text{F}+3{\text{e}}^{-}\hfill \\ 2\text{A}{\text{l}}_2{\text{O}}_3+12\text{F}=4\text{A}\text{l}{\text{F}}_3+3{\text{O}}_2\hfill \\ \text{C}+{\text{O}}_2\to \text{C}{\text{O}}_2\hfill \end{array}$

B. Properties

Aluminum used as cladding or structural material in thermal research and teaching reactors has excellent corrosion resistance to air and water coolant. Aluminum-clad metallic uranium fuel elements operated in such reactors for a number of years are only slightly corroded. The corrosion resistance of aluminum lies in the fact that aluminum and oxygen have a high affinity to form alumina which forms an adherent and impervious layer on the metal surface. The oxide film protects the metal from further attack unless removed or penetrated by chemical and mechanical means. Aluminum corrodes uniformly in the coolant water of thermal research reactors up to abut 220°C. At higher temperatures, hydrogen atoms produced by the radiation decomposition and corrosion reaction (H₂O → H + OH, 2Al + 30H → Al₂O₃ + 3H, H + 3H → 2H₂) diffuse into the metal and combine as molecular hydrogen. This can enhance the corrosion rate during the development of corrosion products and gas blisters spread on the metal surface. At relatively low temperatures, 200 to 250°C, the corrosion rate is small, and at relatively high temperatures, above 400°C, the corrosion rate increases with temperature and exposure and tends to break away.

Compatibility between aluminum and uranium i...

Table of contents

• Cover
• Title Page
• Copyright Page
• Table of Contents
• Chapter 1 Structural Materials
• Chapter 2 Moderator and Reflector Materials
• Chapter 3 Coolant Materials
• Chapter 4 Control and Shielding Materials
• Chapter 5 Radioactive Waste Management
• Chapter 6 Fusion Reactor Materials
• Index