Swedish reactor promises greater safety of nuclear power plants

Most of the scientists and engineers in the nuclear power industry believe they have a safe, efficient, and cheap product that should be accepted. They continue to point out that no one has been killed so far in a licensed nuclear facility - an enviable record achieved by few industries. This belief has prevented them from studying other designs. But it is clear that, whether true or not, this belief is not universally accepted. There are enough members of the public who are unconvinced to prevent further widespread use of nuclear power.

It is, therefore, refreshing when a new preliminary design that promises greater safety comes from a private manufacturing company.

In this case, it comes from ASEA-ATOM in Sweden. Built underground and with the whole reactor system immersed in water, the concept goes much farther than existing designs in preventing conceivable accidents. The reactor vessel must be much larger than the present stainless-steel tanks, and plans call for it to be made of prestressed concrete. Being underground, it should be immune from rocket attack and many other types of sabotage.

If the cost of the concept continues to appear favorable, and if no flaws are found in its inherent safety, this could be the reactor type for the next century - a design that could turn around the fortunes of the nuclear industry.

The first commercial nuclear reactors in the United States were designs modified from naval reactors. These naval reactors are compact, efficient devices that make compact, efficient nuclear electric power plants. To the nuclear enthusiasts, using uranium in a reactor is a simple, elegant way of producing electricity. But their enthusiasm has never provided a convincing answer to the persistent question ''Is it safe?''

It is true that well-designed nuclear reactors are inherently stable against a large number of perturbations. For example, an unusual increase of reactor power increases the temperature of the water, which moderates the nuclear-fission process. The rise in temperature reduces the water density. This , in turn, slows the rate of the fission reaction, reducing the reactor's power output and thus canceling the original power surge.

However, we can imagine some types of drastic changes that are not automatically compensated. If the water pipes break, the nuclear reaction stops. But the uranium fuel could be uncooled. Then, the heat from the radioactive decay could break the fuel rods or melt the uranium and release radioactivity. The present reactors have safety devices to put extra water into the reactors as needed. But these may not always work. At Three Mile Island, they were repeatedly switched off by the operators, who did not understand what was happening.

Thus there have been many pleas in the US to make a safer reactor. Perhaps it could be a reactor using graphite as a moderator and gas for cooling rather than water, as is done in the early English design. Perhaps it should be a new type entirely. But, until now, each proposed design has had its drawbacks. It began to appear that the inherent safety features of reactors could not be appreciably extended.

Now the Swedish design raises new hope. In this concept, the reactor, its pipe system, pumps, and steam generator (boiler) would be immersed in a half-million-gallon tank of water. If a pump or other part of the reactor system failed, the water circulation would change from pumped flow to natural convection and the water would circulate into the main tank. Water in this tank would contain boron intended to stop the nuclear reaction. Boron absorbs neutrons needed to keep the fission chain reaction going.

With this design, most of the hypothetical accident scenarios now discussed would lead to automatic mixing of these water systems and to shutdown of the reactor. Also, there would be enough water to cool the reactor for a week, after which, surely, the fire brigade, if no one else, would have arrived with more water. This counters the risk of overheating. At no time would the fuel rods have broken and released radioactivity.

The whole reactor vessel would be put underground so that leakages through the concrete would be slow. Many scientists have argued that reactors should be underground. This also makes the system almost immune to sabotage and even mortar attack.

It is, of course, possible to create an accident with this reactor. A saboteur could blow open the top of the big water tank and pump out the water. Anything built by man can be destroyed by man. But this is a hard task and would take appreciable time. It illustrates the fact that this reactor design allows the precious element of time in responding to any untoward event.

The Swedish design arose from a desire to have a reactor that could be placed in a city for district heating. The design team now is discussing the project with European and US colleagues in the hope that they will find any flaws. So far, none have been found.

This reactor is much larger than the conventional light water reactor, widely used in the US, and it costs correspondingly more. But in a power station, the nuclear reactor accounts for only a quarter of the cost of the whole power station. Thus doubling the cost of the reactor itself does not matter too much. Moreover, this increase in cost is matched by the fact that the inherent safety of the concept means that the rest of the power station need not be built to such high standards and could cost correspondingly less than it does now.

Since the basic nuclear part of the system is similar to that in the present light water reactors, the development cost should be moderate. Overall, the total cost should be about the same as for present designs. Also, this concept probably would be easier to license and could therefore be built more quickly.