3D printing is increasingly finding use in the manufacture all sorts of end-use products, but it can also lend a hand to researchers on the way to reaching a final goal. A good example of this is a recent project that Oak Ridge National Laboratory (ORNL) in Tennessee, run by the Department of Energy (DOE) and already responsible for numerous 3D printing innovations, helped out with by providing research and development efforts.
The last time that the radioactive isotope molybdenum-99 (Mo-99), the parent of short-lived decay product technetium‑99m (Tc-99m), was made in the US was during the late 1980s…until now. Tc-99m, best known for imaging blood flow in cardiac nuclear stress tests, is the most used radioisotope in medical diagnostic imaging.
This winter, the FDA approved the first Mo-99 that’s produced domestically without using highly enriched uranium (HEU). For nearly ten years, the US Department of Energy’s National Nuclear Security Administration (NNSA) has been supporting efforts to make Mo-99 without using HEU.
“We wanted to help prepare for the commercial production of molybdenum-99 here in the United States at full-cost-recovery pricing. We were excited to assist domestic efforts that don’t use highly enriched uranium,” said Chris Bryan, who leads Mo-99 research at ORNL.
According to the NNSA, Mo-99 is used in over 40,000 medical procedures daily in the US, but is 100% supplied by foreign vendors, most of whom use HEU. To reduce how much the US relies on other countries for this, it provides funding for non-proprietary national technical support at several DOE laboratories, including Argonne National Laboratory, ORNL, Los Alamos National Laboratory, and the Savannah River National Laboratory.
Tc-99m loses potency in just 6 hours, while Mo-99 holds on a little longer at 66 hours. This kind of rapid decay is great for patients who don’t wish to be exposed to radiation for too long, but manufacturers who can’t stockpile Tc-99m are forced to deliver it before it grows too weak to produce high-contrast images. In this case, radiopharmacists use a device that runs a solution through a resin that’s been loaded with Mo-99; then, it releases Tc-99m, which is rushed straight to clinics and hospitals.
Wisconsin-based NorthStar Medical Radioisotopes and SHINE Medical Technologies both have cooperative agreements with the NNSA to increase domestic production of the isotope, and researchers from ORNL have also helped with several R&D projects aimed at making Mo-99 without HEU.
“NorthStar, along with other Cooperative Agreement holders, have benefited greatly from NNSA-supported technology development at the national laboratories. This work at ORNL exemplifies the value of the collaboration and will make our processes more efficient in using enriched molybdenum target material,” said James T. Harvey, Senior Vice President and Chief Science Officer at NorthStar.
NorthStar is producing Mo-99 through a neutron-capture process that uses stable molybdenum target material. It’s similar to SHINE’s project in that it uses an accelerator, but differs because no uranium is involved. Instead, an electron accelerator bombards a target enriched in Mo-100 for six days, which creates intense gamma rays that knock a neutron right out of the mixture, resulting in Mo-99. Helium gas flows through the system to remove heat, so the material used to make the targets needs to be tough enough to hold up under stresses, but still lightweight so it can dissolve quickly, in order for the isotope to be recovered.
The only problem is that enriched Mo-100 is not cheap. ORNL made the initial target, a disc only the size of a half dollar that still cost a few thousand dollars in raw material. In addition, impinging electrons in the accelerator convert less than 10% of the Mo-100, which means that NorthStar has to recover and recycle the rest.
ORNL’s Rick Lowden, a metallurgist whose team develops target materials and fabrication technologies, explained, “Every time you handle that powder, mill it, sieve it, spray it, you lose material. The goal is to lose zero.”
First, ORNL researchers mixed molybdenum powder with a water-soluble polymer, utilizing a spray-dry method to bind small particles into larger spherical agglomerates. Tough, quickly dissolving discs with tight dimensional tolerances were produced after pressing and heating the spray-dried powder. Unfortunately, when they heated the discs with a laser to simulate conditions inside NorthStar’s accelerator, they warped and twisted due to uneven heating across the material.
Working with ORNL materials scientist Jim Kiggans, Bryan and Lowden decided that they could solve a lot of problems by 3D printing the target assembly of discs in their holder, including the fact that disc diameters and thicknesses had to change as the design of accelerator parts did.
Bryan explained, “It was a moving target.”
ORNL teamed up with the designers from Los Alamos responsible for the accelerator system and target to 3D print representative shapes and assemblies out of stainless steel. These were sent to chemists at Argonne so they could dissolve in a process currently being developed to recover unconverted Mo-100. The process is sustainable, as the recycled precipitate was returned to ORNL so it could be processed into 3D printing feedstock for the next assembly.
Lowden said, “Now we have only four steps instead of dozens.”
As molybdenum has a high melting point of 2600°C, ORNL installed a special 400-watt laser in its Renishaw laser melting system; Renishaw also built a reduced-volume insert that could accommodate smaller amounts of Mo-100 to save on costs. ORNL also installed a 15,000-watt plasma system that could recycle material from laser melting by spray-drying molybdenum agglomerates, which produced dense spherical particles as 3D printing feedstock.
The ORNL researchers will now focus on characterizing the material for the target assembly.
“There’s not much data on 90 percent dense 3D-printed molybdenum. We’re basically blazing the trail, especially for such a unique application,” said Lowden.
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