FIGURE 2.4 CNEA’s high-density LEU-aluminum dispersion targets. These targets have been used since 2002 to produce Mo-99 in Argentina. The target is approximately 15 cm in length. SOURCE: Courtesy of Pablo Cristini, CNEA, Argentina.

FIGURE 2.4 CNEA’s high-density LEU-aluminum dispersion targets. These targets have been used since 2002 to produce Mo-99 in Argentina. The target is approximately 15 cm in length. SOURCE: Courtesy of Pablo Cristini, CNEA, Argentina.

To meet these criteria, targets are fabricated in a wide variety of shapes and compositions to meet the needs of individual Mo-99 producers. Targets may be shaped as plates (Figure 2.4), pins, or cylinders. Target compositions include uranium metal, uranium oxides, and alloys of uranium, nearly always with aluminum. Metallic targets are typically encapsulated in aluminum or stainless steel to protect the chemically reactive uranium metal or alloy and to contain the fission products produced during irradiation. This encapsulation is referred to as the target cladding.9 Sometimes an intermediate barrier material such as aluminum or nickel is used to separate the cladding from the U-235 target material. Table 2.2 summarizes the types of targets used or planned to be used in the future by different producers.

Irradiation of Targets in a Nuclear Reactor

Mo-99 is produced in the uranium-bearing targets by irradiating them with thermal neutrons.10 Some of the U-235 nuclei absorb these neutrons, which can cause them to fission. The fission of the U-235 nucleus produces two but sometimes three lower-mass nuclei referred to as fission fragments. Approximately 6 percent of these fission fragments are Mo-99 atoms (Figure 2.5).

9

The target has a “sandwich” structure: The metal cladding is the “bread” and the uranium-bearing material is the “meat.”

10

A thermal neutron is a low-energy neutron of about 0.025 electron volts at room temperature. This energy is typical for neutrons in light-water (i.e., ordinary water) reactors.



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