Science Scene - Nuclear Fuel Reprocessing

Since President Carter shut down the U.S. reprocessing program in the 1970s, U.S. policy has been to take used power plant fuel and bury it. In 1987, Congress designated Nevada's Yucca Mountain as the final resting place for the country's nuclear waste, but the facility there has never opened. Now President Obama has proposed to stop federal funding for Yucca -- which is strenuously opposed by Nevada politicians -- while a new federal commission reviews Congress' old policy. This would sound like a bad thing for the nuclear power industry, but it may actually be a hidden favor. There is a lot of usable material remaining in the spent nuclear fuel, not enough to support power operations without being reprocessed and mixed with more fuel with greater energy capacity (kind of like taking the logs off the fire after they are 3/4 burned and burying them in your yard). If the fuel is reprocessed to extract maximum energy potential, then the remaing waste is much smaller in volume and will decay faster (imagine finishing burning the logs from the fire and burying just the ashes).

Below is more information on nuclear fuel reprocessing for your reading pleasure, so grab some no-doze, toothpicks, Red Bull, intravenous caffine, and settle in :o)

Fuel Reprocessing at a glance

A key, nearly unique, characteristic of nuclear energy is that used fuel may be reprocessed to recover fissile and fertile materials in order to provide fresh fuel for existing and future nuclear power plants. Several European countries, Russia and Japan have had a policy to reprocess used nuclear fuel, although government policies in many other countries have not yet addressed the various aspects of reprocessing.

Over the last 50 years the principal reason for reprocessing used fuel has been to recover unused uranium and plutonium in the used fuel elements and thereby close the fuel cycle, gaining some 25% more energy from the original uranium in the process and thus contributing to energy security. A secondary reason is to reduce the volume of material to be disposed of as high-level waste to about one fifth. In addition, the level of radioactivity in the waste from reprocessing is much smaller and after about 100 years falls much more rapidly than in used fuel itself.

In the last decade interest has grown in recovering all long-lived actinides together (i.e. with plutonium) so as to recycle them in fast reactors so that they end up as short-lived fission products. This policy is driven by two factors: reducing the long-term radioactivity in high-level wastes, and reducing the possibility of plutonium being diverted from civil use - thereby increasing proliferation resistance of the fuel cycle. If used fuel is not reprocessed, then in a century or two the built-in radiological protection will have diminished, allowing the plutonium to be recovered for illicit use (though it is unsuitable for weapons due to the non-fissile isotopes present).

So, what is the difference between reprocessing for power reactors versus weapons grade materials?

The first difference is the length of time the material is in the reactor. As the reaction goes on, some of the U238 turns into Pu239, going through some intermediate steps. If the material stays in much longer, some of the Pu239 turns into Pu240.

Pu240 makes the material unsuitable for weapons because it fissions spontaneously. When the bomb mechanism combines the fissile material into a critical mass, the fission of Pu240 causes it to pre-detonate, causing the material to separate, and the bomb only burbs instead of exploding.

So, in production reactors, the fuel has to be extracted in some short time, months instead of years, and sent to the separation facility. In power reactors, the fuel stays in for years and accumulates a high proportion of Pu240. To make the spent fuel into bomb material requires isotope separation on top of chemical separation. If a country has the resources to separate the plutonium isotopes, it would be better off separating uranium isotopes, because that would save it the trouble of operating a production reactor.

The second difference is the chemical process. For the weapons program, the whole point of processing was to separate out the plutonium. For commercial power, that's not necessary. It's cheaper and easier to keep the plutonium mixed with the leftover uranium.

How does it work?

Old nuclear fuel assemblies -- highly radioactive, elongated packages of metal rods that once energized some of France's 58 nuclear power plants -- are gripped by large mechanical arms. They are hoisted by cranes and placed on belts that move them along in the dim orange light. The machinery works to prepare the assemblies to be lowered into four giant pools.

There they will sit, with about 13 feet of demineralized water above them, a bath to shield and cool them, for about three years. Then more machines will lift them out, chop them up and put the pieces to be dissolved in vats of nitric acid. The fissioning of the fuel in the power plant, or the splitting of uranium atoms to release energy, has created a large family of elements, called fission products. The goal of this process is to find and recycle the ones that still contain more energy -- the plutonium and the uranium.

Spent fuel rods also contain elements that have relatively little energy, but plenty of long-lasting radiation. These include americium, curium, cesium and iodine. They are sent off to be immobilized -- hopefully for thousands of years -- by imbedding them in glass logs. Employees here monitor and operate their robotic helpers from a bank of computers housed in lime-green metal coverings.

What is the timeframe and cost for this technology?

Areva, General Electric Co. and another unnamed vendor have asked the NRC to develop licensing procedures for reprocessing plants by 2012. Areva officials say the earliest a reprocessing plant could be built in the United States would be in the 2020-25 time frame, and that such a plant would cost about $20 billion to $25 billion.