CANDU and SLOWPOKE

While the United States - with its post-World War II enrichment technology and capacity - built power reactors using enriched uranium, the Canadians took a different approach. You may recall that deuterium (2H) reacts with oxygen to form "heavy water" - an unusually good moderator that bounces back and slows down neutrons that might ordinarily escape the reactor. The most interesting thing about the Canadian CANDU heavy water reactor - from the standpoint of community or home power plants - is that it uses natural (unenriched) uranium. This doesn't get them off the hook from an initial energy expenditure, however, since heavy water is expensive to separate - about $100 per pound and costing $100 million dollars for a full-scale 1,000 megawatt reactor. It does, however, eliminate the problem of enrichment. The CANDU design has many parallel fuel assemblies with the heavy water coolant/moderator flowing through each. To refuel the reactor, it doesn't need to be shut down; you just cut off the water to stop the nuclear reaction in a section isolated for refueling, and then change out the "spent" fuel assemblies.

["Spent" fuel assemblies aren't really spent at all - they have more than 95% of the initial fuel remaining with only a few percent of "daughters" that contaminate the rest and absorb the needed neutrons.]

Even more interesting from the standpoint of decentralization is the Canadian SLOWPOKE reactor, which is as safe and secure as a Sierra Club official working for the Environmental Protection Agency. [Safe LOW POwer Kritical Experiment - but it's not experimental anymore, having been in operation for more than twenty-five years. (Canadians may be great reactor designers, but they seem to have a little problem with their spelling.)]

Figure 34 shows a cutaway sketch of this "pool" type reactor - so named because it operates submerged in a pool of water. Unlike PWRs and BWRs, it does not have "defense in depth" - because it doesn't need it. The laws of physics provide it with more than enough protection.

The original design has a maximum operating temperature of 80 degrees Celsius with a cylindrical core about nine inches in diameter by nine inches in height. Surrounding the enriched-uranium fuel assembly are beryllium reflectors, which keep the reactor critical... as long as the water density remains high. If the reactor "heats up," the lower water density slows the reaction bringing the temperature back to the design point. [SLOWPOKE I and II have been operational for some time; the series is now up to V or VI, but I haven't been able to get much information on the later models.]

Suppose all the water evaporates or is sloshed out by an earthquake? Naturally, the reaction stops, as the moderator is gone. But also the power density is so low that nothing happens to the fuel. The reactor just goes dormant until someone takes an action to bring it back to life. [Typically the operators do not have access to the reactor.]

As Canadian scientist Dr. John Hilborn, who conducted experiments leading to the SLOWPOKE, said, "It is safer without operators than with them." [From an interview with Petr Beckmann, Access to Energy, Vol. 8, No. 9, May 1981, pp. 1-2.]

The original SLOWPOKEs were not designed as power reactors. Their heat output (which is considerably higher than any possible electrical output) is a mere twenty kilowatts, equivalent to about thirteen hair dryers. Their function, as mentioned, was not to produce electricity but to transmute certain materials into radionuclides, primarily for medical purposes. But the concept of a low-temperature, inherently safe, non-polluting, inexpensive-to-fuel, produce-power-where-you-need-it reactor is intriguing for those who would like to have energy independence. [Some electric utilities might oppose such a competitive concept, but they would, as mentioned, be in the best position to provide service for local power reactors.]

Isotopes of Hydrogen

Hydrogen offers a simple example of isotopes. Figure 4 shows three forms of the element: The first is just regular hydrogen, designated at H or 1H, which has one proton and no neutrons. [It has a name, protium, but it's only used by real physics nerds or trivia freaks.]

The second form, deuterium or 2H, is known as "heavy hydrogen." It has the obligatory single hydrogen proton, but also has a neutron in its nucleus. [Deuterium is the form of hydrogen that makes "heavy water," the substance on which many World War II spy novels are based. These stories are based on a factual February 1943 raid on the heavy water concentration plant operated by the Nazis in Vemork, Norway. While an earlier attempt resulted in disaster - all thirty-four commandos were either killed in glider crashes or executed by the Germans - the second raid was a textbook example of guerrilla warfare and bravery. Disabling the Vemork plant was a major setback for the Axis A-bomb program.]

Finally, the form of hydrogen known as tritium, or 3H, is seen to have two neutrons.

All three forms are known as isotopes with 1H making up 99.98% of the total hydrogen that we know about - and presumably in the universe. Both "regular" hydrogen and deuterium are stable isotopes, meaning that they don't change over time.

But tritium is anther story: it is an unstable isotope that will eventually decay or disintegrate into stable 1H. [All tritium undergoes "beta-decay" in which the neutron changes into a proton and an electron - a feat of amazing atomic legerdemain.] It is this process - where atoms go to pieces - that we normally call radioactivity.