Fail-safe reactors are too costly

Graphics: Marijn van der Meer

Nuclear reactors that don’t melt down, even if all cooling fails, can be made. But the development of the high temperature reactor has been slowed because other types are cheaper.

The Fukushima-1 reactor boils away several tens of thousands of litres of water per day, estimates Dr Jan Leen Kloosterman, of the Reactor Institute Delft (Applied Sciences faculty). The amount of heat that a switched-off nuclear reactor produces is initially about 6 percent of its full thermal power. That may not sound like much, but as Fukushima shows, it can lead to a desperate struggle to cool away the reactor’s heat. The people staffing the reactor know only too well the consequences of insufficient cooling: pressure building up, forced radioactive steam releases, hydrogen formation and explosions, melting of the core, possibly even damaging the reactor vessel, resulting in a much-feared melt-down or the ‘China syndrome’.
Over time the heat production diminishes. In a day’s time the power diminished by about 90 percent, but it will take six months before the heat production diminishes another 90 percent. All the time forced cooling of the fuel rods remains necessary to prevent releases of radioactive material into the surroundings.

After Fukushima, it seems logical to ask if nuclear reactors can be constructed so that they do not collapse disastrously when the cooling breaks down. Well, they can be, but the power of this ‘inherently safe reactor’ is limited and its price appears to be prohibitive.

“Siemens worked on it in the 1980s,” says Dr Kloosterman, adding however that eventually the company dropped the project in favour of a water-cooled reactor, because this seemed less expensive. South Africa has been working on its own design for an inherently safe high-temperature reactor (HTR), but the project was recently cancelled because of the financial crisis. China has bought the entire German HTR-research inventory, including the fuel factory, and has continued the development from a 10-megawatt demonstration plant into a 250-megawatt prototype, which is expected to be completed by 2015. About this demo plant, Dr Kloosterman says: “They just shut off the cooling and stand back. You see the temperature in the core rise, but nothing happens. Three days later the temperature eases down again.”

The magic of this inherently safe reactor is in its fuel packaging: three layers of heat-resistant material cover tiny uranium oxide bullets measuring only half a millimetre across. The porous inner layer captures any gaseous products and prevents their release. About 15,000 of these millimetre-sized fuel balls, known as TRISO particles, are contained in a carbon sphere about as big as a tennis ball. These balls are heat resistant up to 1.600°C and do not release any of their contents into their surroundings.
A reactor fuelled with these fuel spheres, also known as a pebble bed reactor, is inherently safe as long as the temperature in the core remains under 1.600 °C in all circumstances. This condition restricts the freedom in designing the reactor.

Calculations from Dr Kloosterman’s section show that the width of the active core should be limited to about 1 metre and enclosed between two thick cylinders of graphite on either side. He explains: “The reactor has to be slender in order to have enough surface to give off the heat to the air.” The typical design for a 250-megawatt HTR reactor measures 6 metres wide and 11 metres high.

Under normal operation, circulating helium, which powers a generator, cools the reactor core. About 40 percent of the heat is converted into electrical power. The operational temperature of the outgoing helium is about 800 °C, while the outer temperature of the reactor vessel is 300 °C (400 °C without cooling).

A HTR power plant will probably have a modular structure. A number of 250 megawatt pebble bed reactors in parallel will be needed to produce the same amount of power as a competitive light water reactor. To produce 1.500 megawatt of power, one would need to put 15 HTR’s in parallel, which is probably much more expensive than one EPR (European Pressurised Water Reactor – under construction in Finland).
“That’s why Siemens stopped the development,” Kloosterman says. It also explains why none of the reactors ordered under the recent nuclear renaissance is safe without cooling. If by 2020 an inherently safe reactor hits the market, it will have been ‘Made in China’.