Aluminum is a favorite material for applications in tritium production and reactor plants.This chapter discusses the applications of aluminum in a reactor plant.

EO 1.27 STATE the applications and the property that makes aluminum ideally suited for use in reactors operating at:

a. Low kilowatt power

b. Low temperature ranges.

c. Moderate temperature range

EO 1.28 STATE why aluminum is undesirable in high temperature power reactors.

Applications

Aluminum, with its low cost, low thermal neutron absorption, and freedom from corrosion at low temperature, is ideally suited for use in research or training reactors in the low kilowatt power and low temperature operating ranges.

Aluminum, usually in the relatively pure (greater than 99.0%) 2S (or 1100) form, has been extensively used as a reactor structural material and for fuel cladding and other purposes not involving exposure to very high temperatures.

Aluminum with its low neutron capture cross section (0.24 barns) is the preferred cladding material for pressurized and boiling water reactors operating in the moderate temperature range. Aluminum, in the form of an APM alloy, is generally used as a fuel-element cladding in organicmoderated reactors. Aluminum has also been employed in gas-cooled reactors operating at low or moderately high temperatures. Generally, at high temperatures, the relative low strength and poor corrosion properties of aluminum make it unsuitable as a structural material in power reactors due to hydrogen generation. The high temperature strength and corrosion properties of aluminum can be increased by alloying, but only at the expense of a higher neutron capture cross section.

In water, corrosion limits the use of aluminum to temperatures near 100C, unless special precautions are taken. In air, corrosion limits its use to temperatures slightly over 300C. Failure is caused by pitting of the otherwise protective Al(OH)₃ film. The presence of chloride salts and of some other metals that form strong galvanic couples (for example, copper) can promote pitting.

Aluminum is attacked by both water and steam at temperatures above about 150C, but this temperature can be raised by alloying with small percentages of up to 1.0% Fe (iron) and 2.5% Ni (nickel). These alloys are known as aerial alloys. The mechanism of attack is attributed to the reaction when the hydrogen ions diffuse through the hydroxide layer and, on recombination, disrupt the adhesion of the protective coating.

Aluminum-uranium alloys have been used as fuel elements in several research reactors. Enriched uranium is alloyed with 99.7% pure aluminum to form the alloy.

Research has shown that radiation produces changes in both annealed and hardened aluminum and its alloys. Yield strength and tensile strength increase with irradiation. Data indicates that yield strengths of annealed alloys are more effected by irradiation than tensile strengths. The yield strengths and the tensile strengths of hardened alloys undergo about the same percent increase as a result of irradiation. Irradiation tends to decrease the ductility of alloys. Stressstrain curves for an irradiated and an unirradiated control specimen are shown in Figure 8. Figure 8 illustrates the effect of neutron irradiation in increasing the yield strength and the tensile strength and in decreasing ductility.

Figure 8 Effect of Irradiation on Tensile Properties of 2SO Aluminum

Summary

The important information in this chapter is summarized below.

Reactor Use of Aluminum Summary

Aluminum is ideally suited for use in low kilowatt power and low temperature reactors due to its low cost, low thermal neutron absorption, and freedom from corrosion at low temperatures.

Aluminum, with its low neutron capture cross section is the preferred cladding material for moderate temperature ranges.

Aluminum has been ruled out for power reactor application due to hydrogen generation and it does not have adequate mechanical and corrosion-resistant properties at the high operating temperatures.