Medical radioisotopes produced with the world’s most power-dense reactor

Demcon successfully conducts record-breaking experiment

The supply of medical radioisotopes, typically produced in old nuclear reactors, has had different issues over the last decade. As a result, cancer patients risk waiting longer for diagnostics or therapy, while their health is deteriorating. The SMART project (Source of MedicAl RadioisoTopes) – under the leadership of the Belgian Institute of Radio Elements (IRE) and technical guidance of ASML develops a sustainable and reliable alternative based on accelerator technology. A crucial contribution comes from technology developer and producer Demcon: the design of a target that can withstand the extreme heat and radiation involved in the production of these radioisotopes. At the beginning of February, a dedicated team, for which Demcon supplied essential knowledge and leadership, successfully demonstrated the feasibility of liquid metal cooling for this target under those extreme conditions. The experiment set a world record for the highest continuous power density ever sustained by any human-made object. With this, the project surpassed a critical milestone on the road to the commissioning of the SMART factory.

Millions of cancer patients worldwide depend on nuclear medicine, i.e. the application of medical radioisotopes for their treatment. Medical radioisotopes are short-lived radioactive substances used for diagnostics (imaging) and therapy (radiation). Their production, with uranium as raw material, takes place in nuclear reactors that are nearing their end-of-life. These are no longer reliable, while production must take place 24/7 because of the short half-lives of the radioisotopes.

SMART project
Technetium-99m (^99mTc), coming from the mother isotope ⁹⁹Mo, is the most widely used radioisotope for diagnostics in nuclear medicine. An alternative production method for this radioisotope came from an unexpected direction. While exploring free-electron lasers (FELs) as extreme ultraviolet (EUV) light sources for next-generation lithography, ASML realized that electron accelerator technology could be used to produce ^99mTc (FEL-based EUV generation was abandoned later). IRE, the world’s largest supplier of medical radioisotopes, had started the SMART project and teamed up with ASML to turn this idea into a commercial production facility. IRE leads an international collaboration of over 25 research institutes and high-tech companies, including technology developer and producer Demcon, working on the sustainable and reliable production of radioisotopes using accelerator technology, while maintaining compatibility with the current downstream supply chain – the so-called generators.

Electron accelerator
In the new concept, ⁹⁹Mo is produced by irradiating the non-radioactive molybdenum-100 (¹⁰⁰Mo) with an intense beam of accelerated electrons. Compared to conventional production with nuclear reactors, this alternative requires no enriched uranium and produces hardly any long-lived radioactive waste. Current electron accelerators used in this field do not meet the specifications for large-scale production of ^99mTc for nuclear medicine with the current generators. That requires a superconducting, high-power linear electron accelerator. The accelerator fires high-energy electrons on the target composed of ¹⁰⁰Mo-enriched molybdenum, which is then transformed into ⁹⁹Mo. After irradiation, the target is processed and distributed globally in the form of so-called generators. At the hospitals, the ^99mTc – that is used for imaging, is harvested from those generators.

Extensive cooling required
IRE commissioned Demcon to develop the exposure unit, including the target, and the harvesting system. One of the biggest challenges was the design of a target that can survive the extreme heat load and radiation injected by the electrons, Johannes Jobst, senior mechatronic system engineer, explains. “To be compatible with the current generator technology, the initial activation must be high enough. This level of activation requires focusing the 3MW beam on a target no larger than a matchbox. Without extensive cooling, the target would evaporate instantly. Only liquid metal provides sufficient cooling power, due to a high specific heat capacity and conductivity. After investigating several solution scenarios liquid sodium was chosen as the coolant for the SMART factory. However, it is flammable and corrosive, which makes it difficult to handle.”

Successful tests
In theory and in computer simulations, sodium cooling is an effective cooling method, Bas Vet, senior mechatronic system engineer, continues. “However, the extreme intensity of the beam pushes the thermo-mechanical stress and radiation damage to the boundaries of what the strongest materials can handle. To prove the target can survive we built a demonstration on a 1:1,000 scale of the SMART exposure unit. As the SMART electron accelerator is still under development, we scouted for facilities that could host this demonstration. Since power density and other exposure parameters during this demonstration had to match the conditions in the SMART factory, only a few facilities in the world could provide a sufficiently intense beam. Luckily, Helmholtz Zentrum Dresden-Rossendorf was enthusiastic to provide access to their ELBE electron accelerator and run our experiment. From the 1st to the 5th of February, we used this facility to expose our target for 115 hours to a 30-kW beam. With success, as our on-scale target survived the extreme conditions.”

World record
With the successful test, a world record has been set, Vet claims. “To the best of our knowledge never before has anyone even attempted to inject so much power per unit volume into a target continuously over days, while keeping it intact. For comparison, we deposit nine orders of magnitude higher power density in our target than is produced in the solar core.”

Market entry
“The SMART project reached a fundamental milestone and one of the main technical risks have been mitigated,” Demcon project manager Ricsi Horvath concludes. “This has been a group effort by a huge team working on the mini-target under the leadership of IRE and technical guidance of ASML. Over the past two years more than 150 people within Demcon, in collaboration with over ten contractors in the region and six international partners contributed to this success. There are still challenges ahead concerning radiation sensitivity and damage, cooling and shielding, but with this major risk mitigated, we engage them with confidence.