Plutonium in Perspective

Of benefit and risk

Nuclear power plants this year will generate more than 200 billion kilowatt-hours of electricity - enough to meet the needs of about 24 million households. Most of this energy will be produced by the fissioning or splitting of uranium atoms in their fuel. But up to one-third will come from another fissionable element - plutonium - created in the fuel during reactor operation. Not all this material, however, is consumed while producing power. In the fuel rods routinely removed from these reactors over the year, nearly five tons of fissionable plutonium will remain. Some nine tons already have accumulated, locked within spent fuel rods being held in storage. Reclaimed for use in current and advanced nuclear power reactors, the increasing inventory of this man-made fuel offers an energy supply greater than all domestic coal, oil and gas.

Putting this resource to work, however, is being delayed by controversy over plutonium, depicted by some as a new element: the most hazardous substance known, the ready ingredient for a terrorist bomb.

In fact many tons of plutonium have been produced, recovered, purified, fabricated, stored and transported for more than 35 years in government, military and research programs. It has been more intensively studied, its qualities are better understood, than many materials in common industrial use.

And during this time there has never been known a fatality due to plutonium's toxicity nor a theft for unlawful weaponry.

Genesis of plutonium

Although minute amounts occur in nature, essentially all plutonium is artificially produced. In today's reactor used to generate electricity, it is a by-product of the fission chain reaction taking place in uranium fuel. Here readily fissionable Uranium-235 has been enriched from its natural concentration of 0.7 percent to approximately 3 per cent. Relatively non-fissionable Uranium-238 makes up the balance.

When U-235 splits into two lighter atoms, it releases heat for the production of electric energy and an average of 2.4 neutrons. One of these particles must fission another U-235 atom to sustain a chain reaction. The remaining neutrons may be captured by surrounding fuel and structural materials or may escape from the system. If one collides with an atom of plentiful U-238, fission occasionally occurs. Most often the particle will be absorbed to form U-239. Through radioactive decay, this transforms to Plutonium-239. Similar processes yield other plutonium isotopes, atoms of the same element but with different atomic weights. Two of the more plentiful isotopes - Pu-239 and Pu-241 - undergo fission as soon as they form. Near the end of the fuel's useful life, in fact, plutonium contributes about as much energy as does uranium. Nuclear power plants operating today, then are partially fueled with plutonium generated in place. About half the plutonium escapes fission and, together with residual U-235, remains in the fuel removed from the reactor. Operating at full capacity for one year, a 1,000-megawatt facility will discharge about 435 pounds of fissionable plutonium.

Properties paradoxical

Plutonium is a metal, one in the series of transuranic elements - those heavier than uranium.

And many of its characteristics are common to other metals.

In pure form it is hard and brittle, like cast iron, and can be similarly melted, molded and machined. But as an alloy with most other metals, it is so soft it can be drawn into wire or rolled into foil.

Plutonium combines also with many nonmetallic elements to form stable compounds. Produced in a reactor as an oxide, it has a melting point above 4,000(F and can be dissolved only by concentrated acids at elevated temperatures.

Other properties are unique.

All plutonium isotopes are radioactive, throwing off subatomic particles to reach a more stable, balanced state. Pu-239, for example, decays by emitting alpha particles. After 24,390 years, half the original quantity transforms to U-235. Passing th rough 15 decay stages, some involving elements with half-lives of a few thousandths of a second, Pu-239 finally becomes stable, non-radioactive lead.

The alpha particles given off in this process travel less than 1.5 inches in air. Captured in the crystals of plutonium metal, this radiation makes the material self-heating.

Alpha activity is the source also of plutonium's extreme radiotoxicity. If the material enters the body- by inhalation, ingestion or absorption through cuts- these particles deliver all their radiation energy in a very short space, damaging sensitive internal tissues nearby. On the other hand, these particles cannot penetrate the outer, protective layers of skin, and so plutonium is not a hazard as long as it remains outside the body.

Finally, Pu-239 and Pu-241 join naturally occurring U-235 and man-made U-233 as the only readily fissionable isotopes available in sufficient quantity for commercial utilization.

Uses many, varied

Due to its unique properties, plutonium has found application in many and diverse fields.

In thermoelectric generators, heat from spontaneous disintegration of Pu-238 is converted directly into electricity. These compact, long-lived nuclear batteries are especially suitable for cardiac pacemakers, satellites, navigational beacons and remo te weather stations.

Compounded with certain other metals, plutonium emits a constant number of neutrons over extended periods. IN this form it has been used for oil well logging, reactor start-up and instrument calibration.

By far the greatest potential utilization of plutonium, however, depends on its fissionable nature: producing heat for the generation of electricity.

About 98 per cent of the plutonium in spent fuel at the time of discharge from the reactor can be reclaimed; the balance decays away prior to processing or remains in waste products.

Then plutonium oxide can be blended with natural uranium, containing less than one per cent U-235, to make a fuel equivalent to enriched uranium. This mixed oxide can replace the three per cent U-235 fuel used in today's light water reactors.

Recycling both uranium and plutonium recovered from spent fuel would reduce the amount of uranium ore concentrate required by the nuclear industry up to the end of the century by approximately 400,000 tons or about 23 per cent. This quantity of fuel material, if used in light water reactors, would generate as much electricity as some seven billion tons of coal.

Plutonium is also the principal fuel for the fast breeder reactor now being developed. Here Pu-239 is mixed with uranium left over from the enrichment process, about 99.7 per cent U-238. This is transformed to new plutonium, much as in conventional reactors. The breeder, however, is designed to minimize the loss of free neutrons, increasing the efficiency of the conversion process. In fact more plutonium is produced than consumed in generating electricity. Eventually the reactor creates enough ne w fissionable material to replenish its own fuel and operate another breeder of the same size.

Extracting 60-70 per cent of the energy potential in uranium, compared to 1-2 per cent for current reactors, and permitting use of low grade, higher cost ores, breeder technology can extend domestic nuclear fuel resources from decades to centuries. A nd the key to this advanced utilization is plutonium.

Hazards readily controlled

Like most materials, plutonium presents potential risks together with its immense benefits. However, these hazards are neither unique nor more difficult to control than those of many other substances routinely handled by industry.

Indeed plutonium is toxic: in significant quantity it can cause injury, including cancer many years later, if it reaches tissues within the body.

But plutonium is not particularly hazardous: it can not harm people it does not reach.

And for more than three decades engineered and procedural safeguards have proved effective in stringently isolating plutonium from the public.

Similarly, carefully developed and strictly enforced security systems have substantially reduced the likelihood of obtaining nuclear materials for illicit purposes.

Continuous upgrading of equipment and methods based on these experiences will make certain that the risks of a commercial nuclear industry utilizing man-made plutonium are minimal.

The benefit is very great, offering society for the first time the ability to create a fuel resource, one that can provide an essentially unlimited supply of vitally needed energy.