Introduction

The use of radionuclides in cancer research and treatment, as well as other medical procedures, has been given significant attention. This includes isotopes like iodine-131 (I-131) used in thyroid imaging and actinium-225 (Ac-225) used in targeted alpha therapy. Radioactive isotopes of numerous elements have been shown to offer great benefits in patient care. The applications of nuclear medicine utilize multiple types of radiation in patient treatment, many of which can only be sourced from radionuclides. With growing understanding and use of radionuclides, the demand for such materials is currently rising, and the access to some isotopes in the United States (e.g., Ac-225) remains at critically low levels. A possible remedy to the issue of supply are radioisotopes formed in nuclear reactors.

Liquid-fueled molten salt reactors (MSRs) are a type of nuclear reactor that uses a molten salt as both a fuel delivery system and as a coolant system. An important feature of MSRs is that researchers have access to fission and decay products generated from the fuel. Many of these isotopes are medically relevant, and different groups of isotopes are obtained depending on the nuclear fuel used. For example, a MSR fissioning uranium-235 (U-235) will generate meaningful amounts of I-131, molybdenum-99 (Mo-99), and other valuable medical isotopes. While U-235 is the most common fissile isotope of uranium used for nuclear fuel, U-233 is also fissile and produces significant quantities of Ac-225 when used as fuel in a MSR. MSRs are a potentially invaluable tool for increasing the availability of medical isotopes.

The Nuclear Energy eXperimental Testing Laboratory (NEXT Lab) is currently making strides to alleviate the low supply of medical isotopes through development of isotope extraction techniques from molten salts. NEXT Lab is undertaking efforts to construct and operate a U-235 Molten Salt Reactor (MSR) intended to serve as a source for radiopharmaceuticals. One isotope of interest that will be produced in significant quantities is Mo-99, which will be used as a training isotope for methods of isotope capture.

Mo-99 is a product of the fission of U-235. Mo-99 undergoes beta decay with a half-life of about 66 hours, at which point technetium-99m (Tc-99m) is produced. Tc-99m is particularly useful as a radiopharmaceutical, producing gamma radiation useful in diagnosing several tissue and organ system diseases.

Methods

As a multitude of isotopes will be present in the molten salt reactor as products of these fission reactions, a considerable number of which are medically useful, it is most apt to select one isotope with which to develop robust methods for the capture of other isotopes. Molybdenum, especially molybdenum-99, has been chosen to serve as the training ground for the development of isotope extraction methods. Three methods of extraction are being investigated and will be presented.

Results

Experiments are underway towards the extraction of valuable medical isotopes from molten salts on a smaller scale. The ability for molybdenum to be extracted from aqueous solutions via a phase transfer agent is currently under investigation. Early results show greater than 95% transfer of molybdenum, in the form of the molybdate anion, from an aqueous phase to an organic phase. Other methods are equally promising.

Conclusion

MSRs clearly hold significant promise for the production and distribution of radionuclides involved in cancer research and treatment. Efforts like those currently underway at the NEXT Lab are expected to make significant contributions to the domestic supply of medically-interesting isotopes. As the efficiency in methods for extracting medically-useful isotopes increases, the supply of radionuclides to medical centers will become more reliable. The efforts being undertaken at the NEXT Lab in isotope production, extraction, purification, and eventual distribution are a step toward making radiopharmaceuticals widely available.