LETTERS TO THE EDITORS
Not So Fast with Thorium
To the Editors:
Robert Hargraves and Ralph Moir’s article “Liquid Fluoride Thorium Reactors” (July–August) was a pleasure to read but stirred concern in me. I have studied many reactor concepts dating to the 1960s. The authors correctly note advantages to the thorium fuel cycle. Given the commercial failures of the thorium-based high-temperature gas-cooled reactor (HTGR) and the demise of the thorium-based Shippingport light-water breeder reactor (LWBR), however, I don’t envision the liquid fluoride thorium reactor concept playing a central role. The developmental, technical, safety, regulatory and financial challenges are probably insurmountable.
U.S. nuclear reactors are constructed with solid fuel, metal cladding, water coolant, high integrity pressure vessels and piping, and concrete and steel pressure containments. For sound reasons, the Nuclear Regulatory Commission required their designers to assume that the system’s largest pipe could instantly rupture and release reactor coolant to the containment. It was assumed that large fractions of the reactor core fission products and any hydrogen generated would be released. In the case of the HTGR, this included potential graphite-water reactions (yielding hydrogen) and graphite-air reactions (yielding fire) in the core.
With the liquid fluoride thorium reactors (LFTRs), a total loss of coolant is equivalent to a total loss of the liquid core, fuel and blanket materials to the containment. Since the liquid fluoride operates at temperatures of 800 degrees Celsius, it is quite likely that UF4, ThF4 and fission by-products would react with other materials to cause a criticality event, major fires and/or explosions. I find it hard to believe that anyone would endorse building new reactors using such a chemically complex, potentially unsafe, environmentally hazardous, and unproven technology.
Las Vegas, NV
Drs. Hargraves and Moir respond:
A criticality event would not occur during a total rupture of the reactor vessels because the materials would leave the compact geometry that permits criticality. Also the neutron fission cross sections will reduce as the materials leave the reactor and moderator, thereby hardening the neutron spectra. By design, the reactor room is steel-lined with strongly sloping floors leading to drains. Spilled molten salt would flow to holding tanks designed to treat such an event as “normal” rather than a big “accident.” The continuous chemical processing removes most fission products, especially the gaseous ones that would build up a pressure as they are created, reducing that hazard. The amount of fissile material within an LFTR is a fraction of that within today’s water-cooled power reactors or proposed liquid-metal-cooled fast breeder reactors. The LFTR needs only a low-pressure containment structure, perhaps below ground.
Many of the LFTR chemical processes were pioneered at Oak Ridge and Argonne national laboratories and are used in the aluminum and uranium fuel manufacturing industries, but not within today’s U.S. power reactors. Acceptance of LFTR, “the chemist’s reactor,” will require new skills within the NRC, the nuclear industry and the utilities. These LFTR safety features require validation that can only be achieved by a concerted research and development effort—estimated to cost less than a billion dollars excluding new reactor construction—to bring the technology to a level exceeding that already demonstrated at Oak Ridge.