The USGS Research Reactor in Denver: What a TRIGA Reactor Does for Earth Science

At the Federal Center in Denver, Colorado, the U.S. Geological Survey runs a nuclear research reactor: a pool-type reactor fuelled with low-enriched uranium, known as the Geological Survey TRIGA® Reactor (GSTR). It exists to support USGS science, using methods only a reactor makes possible to study rock, plant and animal specimens sent from around the world.

Cerenkov radiation, the "blue glow", from the reactor in operation. Image from the USGS.

A small reactor with a big reach

The reactor has operated since the late 1960s, serving the USGS and universities across the country. It is the only research reactor of its type in the Department of the Interior and the only research reactor within 350 miles of Denver. Its design resembles the research and training reactors at many U.S. universities. It can run continuously at 1,000 kilowatts (thermal) and can be pulsed to a peak of about 1,600 megawatts, making it an intense source of neutrons. It is also the only facility in the United States that can perform automated delayed neutron analysis, which detects fissile and fissionable isotopes.

The reactor’s control console. Image from the USGS.

What the neutrons do

Most of the work is irradiating samples. Neutrons from the reactor strike a specimen and change the nuclear make-up of its elements, a "transmutation". Measuring what the sample then emits is neutron activation analysis, and it detects most elements at a few nanograms or less, with no chemical processing before or after. The results help trace where mineral deposits came from. Irradiating rocks and minerals also lets scientists work out their ages. The services include:

• neutron irradiation for argon isotopic dating;
• neutron activation analysis;
• uranium and thorium analysis by delayed neutron counting;
• making radioisotopes, and gamma spectrometry.

Containers that carry samples to the reactor. Image from the USGS.

Safety

Much of the reactor's safety is built into its design, which prevents overheating or a meltdown. Radiation monitors with automatic alarms are placed throughout the building, and the control system watches power, temperature and other readings. If any operating limit is exceeded, the reactor shuts itself down in a fraction of a second. An air-monitoring system in the reactor bay detects airborne contamination, seals the space off, and filters all of its air so that nothing escapes. The U.S. Nuclear Regulatory Commission checks the operation every year. The staff also advise researchers across the country on designing experiments, and the USGS has a memorandum of understanding with the Colorado School of Mines for joint research.

Three projects

Dust on coral reefs. Reddish-brown soil was harming coral reefs off Jamaica, and reefs matter: they feed people and shield tropical shores from storms. Neutron activation analysis "fingerprinted" the soil as a mix of volcanic ash from the Lesser Antilles island arc and dust blown from Africa. African dust is also a main parent material of soils in the Florida Keys, the Bahamas and Barbados. For the first time, the method could tie distant climates together this way.

Fallout from Fukushima. The reactor accident in Japan in March 2011 released radioactive material that travelled around the world. The GSTR analysed hundreds of water and filter samples to help find what fission products had settled across the United States, and how much.

The Three Sisters volcanoes in Oregon, from the air. Image from the USGS.

The age of volcanoes. Geologists at the USGS Cascades Volcano Observatory use the reactor to build eruption histories of active volcanoes in the western United States, Alaska and Hawaii. In the Three Sisters field west of Bend, Oregon, three stratovolcanoes are ringed by lava flows, several of which erupted in the last 4,000 years. Argon dating showed that two of the three volcanoes formed at the same time, between 48,000 and 15,000 years ago, and the third is much older. The finding suggests the big volcanoes are less likely to erupt soon, and that the more recently active lava flows pose the greater hazard.

What comes next

Better measurement of the reactor's neutrons keeps improving the precision of age dating for many laboratories. The Colorado School of Mines planned to install a neutron radiography instrument with the reactor in 2012, the only one within 450 miles of Denver.

Sources

• Timothy M. DeBey, Brycen R. Roy and Sally R. Brady, "The U.S. Geological Survey's TRIGA® Reactor," USGS fact sheet; rewritten in hubnx's own words. Images from the USGS; a satellite photo of Fukushima from a company is left out.