Dear Arjun,
What is HEU and is it used in all reactors?
-- At Sea in Seattle

Dear At Sea:

Back in the fifteenth century, the Castillian court was cursed with a king who could not talk. So desperate were his courtesans to hear the king speak, that every time he sneezed they would intone "His Eminence Utters!" and throw a party. This got boring. As the king aged, they began to just say "HEU" when he sneezed, and finally, just "aych ee ooo!"

In the nuclear arena, HEU refers to highly enriched uranium. Natural uranium contains three isotopes (or forms) of uranium, namely uranium-238, uranium-235 and uranium-234. Of these only uranium-235 is fissile, thereby making it an essential ingredient for creating nuclear chain reactions, necessary for nuclear weapons and capable of generating power. (The only practical substitute for uranium-235 in this role is plutonium-239.)

HEU refers to uranium that has been enriched to greater than 20 percent uranium-235. Above this level, it can theoretically be used to make nuclear weapons. However weapons grade HEU generally contains more than 90 percent uranium-235. HEU is also used in naval reactors because the allows the same mass of fuel to generate a given level of power for a longer period of time. Another way of looking at it is that a smaller mass of fuel is needed to generate the same amount of energy.

Most reactors do not use HEU because it is expensive fuel. Civilian power reactors use "Low Enriched Uranium" (LEU), which contains up to about 5 percent uranium-235, or natural uranium, which has just over 0.7 percent uranium-235. Besides nuclear weapons, HEU is most commonly used in naval reactors and in some research reactors. Research reactors are those used for basic and applied research, as well for training. World-wide, about 200 test and research reactors use HEU as fuel.¹ Of these reactors, about 66 may send their spent nuclear fuel to the US, according to a list supplied to IEER by the DOE.² Research reactors can be converted from HEU to LEU use.

Since HEU can be used to make a nuclear weapon, many countries are concerned about the possible diversion of HEU from research reactors to weapons use. In 1978 the U.S., which has been the largest supplier of HEU to the world, initiated the Reduced Enrichment for Research and Test Reactors (RERTR) program. Under this program, the U.S. is converting most of its own research reactors to LEU use and has stopped exporting HEU for research reactors abroad. The program also encouraged foreign countries to use low-enriched uranium (LEU) instead. The U.S. has proposed accepting foreign spent research reactor fuel. The DOE is preparing a programmatic environment impact statement on spent fuel management, and one on foreign research reactor spent fuel, as well.

Yrs. etc.

Arjun, a.k.a. Dr. Egghead