Surplus U-233 now "treasure" for new isotope technologies



The outward appearance of Building 3019 has changed little over the years since this 1950s photo was taken. The mission of the Radiochemical Development Facility, however, has evolved significantly. The Graphite Reactor is in the background.



In a half-century-old building almost in the center of ORNL's main campus of research facilities, the U.S. government stores one of the most difficult-to-manage substances that's come out of the nuclear age.

To look at Building 3019, also called the Radiochemical Development Facility, one wouldn't think there could be much of anything special about it. However, secured behind its doors in shafts bored deep within concrete walls are canisters of highly radioactive uranium-233.

Like its uranium isotope cousins such as uranium-235 and uranium-238, it emits alpha particles, except it is a much stronger alpha emitter. A "contaminating isotope"—uranium-232—also emits high-energy gamma rays. Gamma radiation, as most staff members know from their general employee training, is much more difficult to shield. Gamma emission changes the whole ballgame when it comes to handling radioactive materials.

U-233 sounds like a big headache. But its Chemical Technology Division stewards in Building 3019 think it's worth more than gold.

The U-233-derived bismuth radioisotope is showing early signs of being a very effective and specific therapy for certain types of malignant tumors.

That's because researchers are reporting promising results with experimental cancer treatments using bismuth-213, which is derived from U-233. The bismuth radioisotope, when attached to monoclonal antibodies, is showing early signs of being a very effective and specific alpha radiation therapy for certain types of malignant tumors.

"When you start looking at the cancer treatment potential of uranium-233, it's a treasure," says Brad Patton, the Radiochemical Development Facility's manager and Radiochemical Technology section head.

ORNL's uranium-233 is classified as a "special nuclear material," meaning that it can be produced in quantities needed for nuclear weapons production. However, the radioisotope, which was produced at considerable expense by irradiating naturally occurring thorium-232 in nuclear reactors, has been declared surplus material. Plans are now being made to "dispose" of excess U-233—to make it unusable for weapons purposes.

Cancer treatment is a big actor in the excess U-233 disposal program, Patton says.

Like an adjacent building—the Graphite Reactor—Building 3019 was built in haste during the Manhattan Project. A pilot plant was needed to perform plutonium extraction processes. Weeks after the decision to build was made, workers broke ground on the series of vaults and thick walls that would receive irradiated slugs from the "X-10 pile" via a subterranean canal. Chemists extracted the first milligram amounts of plutonium in what was then called Building 205.

In the cells of the facility eventually renumbered 3019, processes like Purex, the plutonium recovery system used at the Hanford and Savannah River sites, were tweaked and perfected. Contaminated equipment from those days remains in some of the cells.

"You have to admire the foresight of the original designers who made it flexible enough for uses beyond the pilot plants," says Alan Krichinsky, of the Radiochemical Technology section. He points to old instrument panels, now mostly out of service, although some still perform monitoring activities for the facility.

On a quick tour through the building's "canyons," Krichinsky shows how engineers for Clinton Engineer Works (a wartime ORNL appellation) designed certain types of services, such as liquid pipes or air vents, to occur at certain levels in the cells. Many of these ports and fixtures are sealed, the pilot projects long since completed. The structure itself, however, is very functional.

"This building is built on bedrock," Krichinsky notes. "The uranium is stored in a ventilated, shielded and secure area in five feet of concrete."

It's the five-foot-thick concrete walls, thick enough to contain the radiation inside, that have come in so handy for U-233 storage.

It's the five-foot-thick concrete walls that separate the six room-sized cells, thick enough to contain the radiation inside, that have come in so handy for U-233 storage.

Krichinsky explains that 34-foot-long cylindrical shafts were bored down into one of the dividing walls. The U-233, in a sand-like, highly purified oxide form, is packaged in a variety of shapes from rectangular cigarette-case-shaped packets to heavy stainless steel or aluminum cylinders. They are then inserted into other cylinders and placed deep within the canyon's concrete for safe storage.

Any radioactively contaminated particle that finds its way into the air inside the building is taken up and routed through a complex and thorough system of filters. The purified air is then released through the tall stack just behind the building.

"Ventilation, shielding and, of course, security are the most important factors in a facility like this," Krichinsky says.

The bismuth-213 isotope that is holding so much promise for medical treatment is not derived directly from U-233. The isotope is actually extracted during a chain of processes that separates U-233, then progresses to thorium-229 to actinium-225 to bismuth-213, which is attached to a monoclonal antibody.

ORNL researchers including Saed Mirzadeh and Steve Kennel of the Life Sciences Division are directly involved in developing the isotope generators that physicians will "milk" (or process) to recover the bismuth-213 for clinical treatment.

In fact, the thorium processed to date actually has come from waste products left over from the original U-233 processing at 3019. According to Mirzadeh, the bismuth-213 project began with a Laboratory-directed R&D project to convert the waste into something useful, namely to extract thorium for eventual use in an actinium-bismuth generator.

Rose Boll of the Life Sciences Division and Dave Depaoli, Stanley Cooper and Linda Farr of Chem Tech did much of the original prototype work on the LDRD project with Kennel and Mirzadeh.

Chem Tech's Oren Webb has worked closely with the bismuth-213 project through one of the main missions of the Radiochemical Development Facility: the extraction of thorium-229 from the U-233. Remember the troublesome gamma radiation? That extraction process also removes the U-232 "daughter" isotope that is responsible for the gamma. For clinical uses, the extraction processes must be essentially perfect.

"U-233 is difficult to work with because of its gamma and fissile properties," Webb, an engineer in the Radiochemical Technology section, says. "There are safeguards you must observe. You have to limit the amount of material you are working with to minimize radiation exposure.

"This is a high-purity material because the radioisotope ultimately may be injected into a human. That purity comes through the bismuth-213 recovery process."

However, the challenge of separating thorium, in its own way, results in a pure process.

"Thorium is present in the uranium at very low concentrations," Webb says. "For every atom of thorium there are 50,000 atoms of uranium. So the separation step is critical."

The upshot is that the rather limited supply of U-233—there are only about two metric tons of U-233 compared with many, many more tons of U-235 and plutonium in storage around the globe—could go a long way toward making sick people well.

"Thorium-229 has a 7,000-year half life," Webb says. "So we have a limited but stable and reliable supply for generations."

Brad Patton lists the three main missions of Building 3019—safe storage of U-233, thorium extraction and the disposition of excess U-233, which mainly entailed processing it into forms for disposal. Another emerging mission could be the conversion of U-233 from the deactivated Molten Salt Reactor Experiment facility. Building 3019 is now storing the U-233 being extracted from the MSRE.

There is some potential use of U-233 for weapons, although there are no current plans. U-233 has been explored as a fuel for spacecraft on deep-space missions. Patton explained that a smaller mass of U-233 is required for a spacecraft reactor configuration than currently required for plutonium oxide. That savings in weight is very appealing to spacecraft designers who must deal with limited payloads.

The medical application appears to be the most promising, however. Early publicity on the cancer treatment experiments indicated that the isotope material came from Germany. Patton pointed out that, in fact, ORNL provided the raw material isotope to the German supplier from Building 3019 under a work-for-others project.

If successful, the bismuth treatment will require most of DOE's U-233 inventory. That inventory remains nestled in one of ORNL's most historic buildings—the American Nuclear Society declared Building 3019 a nuclear historic landmark several years ago. As ORNL researchers discover uses for the U-233 stored there, Building 3019 could be the scene of history yet to be made.—B.C.