Nuclear reactors provide an efficient source of thermal neutrons for Mo-99 production. This is why all major Mo-99 producers irradiate their targets in nuclear reactors. The amount of Mo-99 produced in a target is a function of irradiation time, the thermal neutron fission cross section for U-235,11 the thermal neutron flux12 on the target, the mass of U-235 in the target, and the half-life of Mo-99. For typical reactor thermal neutron fluxes on the order of 1014 neutrons per square centimeter per second, irradiation times of about 5 to 7 days are required to achieve near-maximum Mo-99 production in the targets.

Beyond these irradiation times, the amount of Mo-99 produced in the targets approximately balances the amount of Mo-99 being lost to radioactive decay, so further irradiation is not productive (see Sidebar 3.1). Even at maximum production, only about 3 percent of the U-235 in the target is typically consumed. The remaining U-235 along with the other fission products and target materials are treated as waste.

Dissolution and Mo-99 Recovery

Once the targets are removed from the reactor, they are cooled13 in water typically for half a day or less before being transported to the processing facility in shielded casks. Once at the processing facility, the targets are placed into hot cells (Figure 2.6) for chemical processing. Processing is carried out quickly to recover the Mo-99 to minimize further losses from radioactive decay. About 1 percent of the Mo-99 produced in the target is lost to radioactive decay every hour after irradiation.

The apparatus in the hot cell used to process the targets and recover the Mo-99 (Figure 2.7) consists of a container for dissolving the targets, which is connected to tubing and columns for subsequent chemical separations to isolate Mo-99. The components can be easily replaced or reconfigured by a human operator using remote manipulators. The most expensive part of the separation facilities are the hot cells themselves. Hot cell facilities can cost tens of millions of dollars to construct.14 The separation apparatus

11

Fission cross section is usually expressed in barns, where 1 barn = 1 × 10−24 cm2. This cross section is related to the probability that the nuclei will capture a thermal neutron and cause fission.

12

Neutron flux is a measure of the intensity of neutron radiation. It is defined as the number of neutrons crossing a unit area of one square centimeter in one second (neutrons/cm2-s).

13

Cooling is a safety measure to prevent the target from being damaged because of high temperatures, to provide time for short-lived fission gases to decay, and to reduce overall radiation doses in the target processing system.

14

For example, Ralph Butler, director of the Missouri University Research Reactor (MURR), estimated that it could cost between $30 million and $40 million to construct a new hot cell facility for Mo-99 production at MURR. The facility would have two processing lines with three or four hot cells plus one additional common hot cell. This cost estimate was character



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