INHERENT SAFETY

SLOWPOKE (to which we promised to come back in last month's "Canadian" issue) stands for Safe Low Power Critical Experiment, though it is no longer an experiment, but a proven success. SLOWPOKE is a tiny nuclear reactor producing 20 kW of heat and no electricity; its job is to generate neutron radiation with high flux and low cost for a variety of purposes, such as production of shortlived radioisotopes in medical therapy and diagnostics.

Experiments conducted at the Los Alamos Scientific Labs in 1967 made a Canadian scientist, Dr John Hilborn, realize that a large neutron flux could be obtained from a tiny (2 lbs) mass of enriched uranium immersed in water as coolant and moderator, when it is surrounded by beryllium reflectors.

Let's go over that again. When a U 235 nucleus absorbs a neutron, it splits and spits out several neutrons of its own. If at least one of these, on the average, splits another U 235 nucleus, you have a split'n spit chain reaction; the reactor goes critical.

There are two reasons why this does not easily happen: One is that the spat out neutrons are too fast; they must be slowed before they are capable of splitting further atoms, and this is done by the moderator - - water, or more precisely, the hydrogen nuclei in the water. When a neutron hits one, it bounces off more slowly than it came in, like a tennis ball hitting a foam mattress. The other reason is that the neutrons, fast or slow, simply get lost by leaving the volume of uranium; but in a SLOWPOKE reactor they are bounced back by the beryllium mirrors like tennis balls by a concrete wall.

The core is, in fact, a little cylinder of fuel rods no bigger than 9 ins in diameter and 9 ins in height. It sits in a container (shown on the next page), which in turn is suspended in a pool of water in the user's basement, about 20 ft deep, and 8 ft in diameter. Samples to be irradiated are lowered into the vicinity of the core through one of two tubes; and the water in the outer pool gets warm by contact with the reactor container.

Such fission products are present in SLOWPOKE's fuel rods, too; but even without coolant, they cannot raise the temperature anywhere near the melting point of the metal, because the power density (heat produced per cubic inch per second) is insufficient.

SLOWPOKE thus works without safety back-up devices; "it is safer without operators than with them," Dr Hilborn told this writer in an interview.

The bare fact of the SLOWPOKEs running for a decade is no mean advantage. The HTGR at Fort St. Vrain [AtE Jan 81], for example, is also inherently protected from meltdowns and therefore does not need a massive containment building. But only last month was it allowed to go to 70% of its rated power. Yet every time a pump fails or it "leaks" a microcurie, the local environmental reporters still hope, with dilated pupils and drooling mouths, for a meltdown. How tragic to be dull-witted; with a little more ability, they might have become editors of the classified ad pages.

[More: R.E. Kay and J.W. Hilborn, "SLOWPOKE: A demonstration of simplified reactor control and unattended operation" (p.453) and "The selflimiting power excursion behavior of a SLOWPOKE reactor," (p.481-2), ANS Transactions vol. 17 (1973); "Can SLOWPOKE do what CANDU can't?" Canadian Business, Aug. 1980; "Downsizing nuclear plants," Business Week, 10 Nov 80, p. 144; interested readers should also follow the excellent Canadian journal Ascent.]