02/06/2013 - 05:40

How to dispense with highly enriched uranium

About 85 percent of all diagnostic nuclear medical procedures, amounting to 30 million procedures a year, utilize technetium 99, a metastable medical isotope. This is the decay product of molybdenum 99, which is considered the most important radioisotope employed in nuclear medicine.

Molybdenum 99 can be produced in several ways, but the most efficient means for medical applications is the fission route -- using thermal neutrons produced in a nuclear reactor to irradiate targets that contain uranium. This results in the splitting (or fission) of uranium atoms into various stable and radioactive isotopes, one of them being molybdenum 99. This isotope is ultimately transported to manufacturers of technetium 99 generators, which deliver the generators to nuclear medicine centers. There, technetium 99 is employed to label molecules for use in diagnostic procedures.

Either highly enriched uranium (HEU) or low-enriched uranium (LEU) can be used as the target (the material at which neutrons are fired to induce fission in the reactor core). Use of HEU carries inherent risks for nuclear proliferation and terrorism. Using LEU reduces these risks significantly.

High to low. Argentina's National Atomic Energy Commission (CNEA), which is responsible for carrying out nuclear research and development and also for applications and services like radioisotope production, began producing molybdenum 99 in 1985, using targets enriched to more than 90 percent in the isotope 235. During 1988 and 1989, the commission's RA-3 reactor -- then as now the most important reactor in Latin America for producing radioisotopes -- was converted to rely on LEU fuel in order to address international proliferation concerns. The next step in HEU minimization was to develop a procedure for molybdenum production using low-enriched uranium targets.

The first challenge was to develop a suitable LEU target that would yield at least as much molybdenum as was obtained using the HEU target. Toward that end, a joint project was carried out by the groups at CNEA responsible for fuel elements, target manufacturing, and chemical processing of targets for separation and purification of molybdenum. Several uranium compounds were tested until, finally, an LEU target was developed with geometry similar to that of the former HEU target but with higher uranium content.

To begin using these new targets, several changes were required in the facility's separation and purification processes, but only slight modifications in infrastructure were needed. In 2002, commercial production of molybdenum 99 from LEU targets began. The entire conversion project had been carried out by the commission's staff, with no external technical or financial support. Today, CNEA produces high-quality molybdenum 99 that meets all of Argentina's demand and one-third of Brazil's. Additionally, around 15 percent of the molybdenum 99 produced is exported to the Latin American region beyond Brazil in the form of technetium 99 generators.

The Argentine experience with conversion from HEU to LEU has often been presented internationally as evidence that conversion is technically and economically feasible. Examples include a 2009 study mandated by the US Congress, "Medical Isotope Production without Highly Enriched Uranium"; the publications of the High-Level Group on Security of Supply of Medical Radioisotopes, an initiative that addresses global supply issues and also conversion to LEU targets in fission radioisotope production; and the US Energy Department program known as Reduced Enrichment for Research and Test Reactors.

CNEA's current efforts to build a new research and production reactor, as well as a new fission radioisotope production plant with a higher capacity, will help increase the availability of medical radioisotopes produced with LEU targets. This will help avert supply crises for technetium 99 -- in 2009, problems at a Canadian technetium facility helped spark such a crisis -- and will also contribute to the global initiative for minimization of highly enriched uranium.

Encouraging conversion. International efforts to minimize the use of HEU in the civilian sector are moving in the right direction. Several initiatives mentioned above are powerful instruments for publicizing the feasibility and importance of conversion to LEU; so are efforts by the International Atomic Energy Agency to coordinate meetings and establish research projects on this subject. On a national level, countries with a strong demand for fission molybdenum products should demonstrate a preference for suppliers that use LEU and impose restrictions on products based on HEU.

CNEA has contributed strongly to the conversion process -- not only by converting its RA-3 and RA-6 reactor cores and its molybdenum 99 production process to LEU, but also by successfully transferring to entities abroad its LEU production technology. These include the Australian Nuclear Science and Technology Organisation and the Atomic Energy Authority of Egypt.

Taken together, national and multilateral efforts are helping to create a better understanding of the importance of eliminating HEU in civilian applications. This contributes to making the world a safer place.