with the same Mo-99 yield, heat management requirements would also be the same.

Another consideration for direct replacement is reactor irradiation capacity. Most of the world’s supply of Mo-99 is produced by irradiating HEU targets in multipurpose, multiuser facilities (Table 3.2). Reactor operators’ ability to accommodate larger numbers of LEU targets could be limited because of other user demands on reactor resources.

Increase the Mass of U-235 in the LEU Target by Increasing Target Size

Additional U-235 could be incorporated into an LEU target by increasing the volume of the target material (i.e., the target meat). This approach would reduce target throughput requirements in the reactor but would not substantially change the other material throughput requirements described previously. Also, space limitations in the reactor target irradiation positions might preclude the use of substantially larger targets.

Increase the Mass of U-235 in the Target by Changing the Composition of the Target Meat

The HEU targets3 used for most current Mo-99 production are uranium-aluminum alloys (Table 2.2) having uranium densities approaching 1.6 g/cm3. To obtain an equivalent mass of U-235 in an LEU target of the same size, a uranium density of about 8 g/cm3 would be required. Higher-density LEU targets could be made of several materials:

  • Uranium metal targets. Argonne National Laboratory has led the development of a uranium metal target (Figure 7.1) in cooperation with several organizations. Recent progress is described by Vandegrift et al. (2007), Bakel et al. (2008), and Wiencek et al. (2008).4,5 The target consists of a thin (typically 100- to 150- micron) LEU metal foil wrapped in an

3

Although the focus of this discussion is on targets, the same considerations apply for the conversion of reactor fuel from HEU to LEU as will be discussed in Chapter 11. Targets and fuels have the same basic sandwich design and differ primarily in size and configuration.

4

The primary participants are Comisión Nactional de Energía Atómica (CNEA, Argentina), MURR (United States), and Indonesian National Atomic Energy Agency (BATAN). CNEA is providing advice on target design and has carried out tests on irradiated foils. BATAN and the Australian Nuclear Science and Technology Organisation (ANSTO) have also test-irradiated these foils. MURR is evaluating target fabrication approaches and modeling target thermal properties. In early November 2008, MURR also began irradiating and processing small (5 g) targets.

5

Compagnie pour l’ Etude et la Réalisation de Combustibles Atomiques (CERCA, France) is also investigating LEU foil targets in cooperation with the Missouri University Research Reactor (Allen et al., 2007).



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