Paul van Gerven
25 February

Thanks to a Demcon-designed cooling system, a major potential obstacle to producing medical isotopes outside nuclear reactors has been removed.

At the beginning of February, a 30-kilowatt beam of high-energy electrons was directed at a millimeter-sized piece of molybdenum in a facility in Dresden. That kind of abuse would normally evaporate the metal almost instantly, but it survived the bombardment for the better part of a week. The cooling system designed by Demcon has passed its first big test.

“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,” says Bas Vet, senior mechatronic system engineer at Demcon.

The result means that an alternative method conceived by ASML to produce a workhorse isotope for nuclear imaging, technetium-99m (Tc-99m), is one step closer to reality. That’s great news, considering that the aging of nuclear reactors is increasingly threatening the supply security of the radioisotope used to diagnose heart disease and cancer.

Demcon medical isotopes
Particles generated from this ELBE linear accelerator generated so-called bremsstrahlung, which knocks out a neutron from molybdenum atomic nuclei, leaving the radioisotope Mo-99. Credit: HZDR/Jürgen Jeibmann

Radioactive waste

Technetium-99m is a short-lived form of technetium-99, which upon decay sends out gamma rays. Its half-life of 6 hours is long enough to perform nuclear imaging but short enough to be eliminated from the body relatively quickly. However, with that kind of half-life, hospitals would need a near-constant supply, since it’s impossible to keep stock. That’s why Tc-99m is supplied as molybdenum-99 (Mo-99), which decays to Tc-99m with a much more convenient half-life of 2.75 days.

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Mo-99 is normally produced by the fission of enriched uranium in research reactors, such as the Dutch High Flux Reactor in Petten. A couple of years ago, ASML stumbled upon an alternative method while evaluating the potential of free-electron lasers as EUV light sources: hurling high-energy particles at ‘regular’ molybdenum (Mo-100). With the support of the Veldhoven-based equipment manufacturer, the Belgian producer of radiopharmaceuticals IRE now leads the international SMART consortium to commercialize accelerator-based radio-isotope production (link in Dutch). This method requires no enriched uranium and produces hardly any long-lived radioactive waste.

Mitigated

IRE commissioned Demcon to develop the exposure unit, including the target. One of the biggest challenges was the design of a target that can survive the extreme heat load and radiation injected by the electrons. “To be compatible with the current generator technology, the initial activation must be high enough. This level of activation requires focusing a 3 MW beam on a target no larger than a matchbox. Without extensive cooling, the target would evaporate instantly,” says Johannes Jobst, senior mechatronic system engineer at Demcon.

Only liquid metal proved capable of providing sufficient cooling power, due to a high specific heat capacity and conductivity. After investigating several options, Demcon chose liquid sodium as the coolant.

To test whether sodium could stand the heat, Demcon built a 1:1,000 scale exposure unit and asked the Helmholtz Zentrum Dresden-Rossendorf, home to superconducting linear accelerator ELBE, to try and shoot it to bits. The Germans gave it all they got for 115 hours, but the downsized target survived.

“The SMART project reached a fundamental milestone and one of the main technical risks has been mitigated,” Demcon project manager Ricsi Horvath concludes. “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.”