The new Japanese prime minister, Shinzo Abe, recently told a television interviewer that he wants to see new nuclear reactors built in Japan. He said that these should be “totally different” from the current ones, which are light water designs of the pressurized-water or boiling-water variety. He should consider the Canadian CANDU. It is totally proven: CANDUs are providing, right this minute, all of the output in the “nuclear” category in Figures 1 and 2 to the left. As for the CANDU’s ability to handle possible accident scenarios in an earthquake zone such as Japan, here is a video by the Canadian Nuclear Safety Commission that describes how a CANDU station would handle an event such as the one that brought down the Fukushima Daiichi plant in northeastern Japan in March 2011.
The CNSC video does not mention a fundamental, critical safety design difference between the CANDU and its light water cousins. Light water designs all feature a single heavy reactor vessel in which all the fuel, arranged in a single large assembly made up of many individual rods configured vertically, sits immersed in ordinary water (H2O). The CANDU by contrast features a horizontal configuration of many individual pressure tubes in which many smaller assemblies, consisting also of many individual fuel rods, are inserted, surrounded also by water but in this case heavy water (D2O).
In the event of a loss of coolant accident (LOCA) and a meltdown of the fuel, there is tremendous pressure inside whatever pressurized container holds the fuel. When the water level drops below the top of the fuel, whether the fuel is arranged vertically as in most LWRs or horizontally as in the CANDU, steam forms and fills the volume that was once filled with liquid. This creates huge pressure.
Such a scenario is at the bottom of the fears of nuclear meltdown. The fears centre on the possibility that if the pressure becomes extreme enough, the pressurized vessel holding the fuel will explode as it ruptures, violently ejecting radioactive fission products into the surrounding area.
Light water designs defend against this possibility by employing brute force. Pressure vessels are made of steel so thick that they can, it is hoped, withstand extreme pressure. The Fukushima-Daiichi event of March 2011 demonstrated that the hope was justified. Three 1970 vintage reactors melted down, and not one of the pressure vessels failed.
The CANDU designers defended against this highly unlikely eventuality not with brute force, but with guile. The consequences of the failure of a pressure vessel become more extreme as the vessel gets bigger. So instead of a single large pressure vessel as in an LWR, the CANDU features many, much smaller, pressure tubes—the CANDU 6 reactor has 380 of them. In the unlikely event of a large LOCA, where there is simply no coolant to prevent the fuel from overheating and melting, the CANDU’s fuel channels would over-pressurize and explode into the surrounding heavy water moderator. The calandria—the large horizontal cylinder that holds the pressure tubes and moderator—would then sag to the concrete floor of the reactor building, at which point the controlled depressurization of the reactor or vacuum building would take place as depicted in the CNSC video. There would, and could, be no failure of the containment. No fission products would be ejected uncontrolled into the surrounding area.
There is simply no possibility of the violent ejection of the reactor core, because that opportunity has been designed out of the equation.
The Japanese prime minister’s stated preference for new reactor designs reflects his desire to effect a political compromise between his need to assuage public fear of a meltdown and Japan’s urgent need for nuclear-generated electricity. The public must be mollified, even though the Fukushima Daiichi meltdowns have, after 1092 days, not produced a single human casualty.
It is not known what the PM meant when he said the new reactors would be “totally different” from the current ones. He might be referring to fast-neutron reactors cooled with liquid sodium, of the type Japan has been developing in collaboration with France and the U.S. That would be a huge step forward in the world energy scene.
But Abe has more pressing nuclear needs, beyond getting Japan’s 45 currently shut-down reactors back up and running. His electricity-starved and environmentally responsible country needs more than 47 reactors (there are two currently running). He needs a reactor design that is solidly proven, and that could dovetail with the current light water fuel cycle. This rules out every single current design on offer in the world, except the CANDU.
The CANDU would also offer the more or less immediate ability to begin addressing any used fuel issues that might also be plaguing public re-acceptance of nuclear energy. The CANDU, through the proven DUPIC fuel cycle (DUPIC stands for Direct Use of Pressurized Water Reactor Spent Fuel in CANDU), could burn used fuel from Japan’s PWRs.
Suddenly, Japan seems on the verge of becoming the world’s leading nuclear nation. It is adept in fuel recycling already, and has been at the forefront of fast reactor development, which would represent an elegant solution to the long-lived heavy isotopes in used fuel. And by adding the totally proven and inherently safe CANDU to their technological repertoire, Japan would finally achieve true flexibility in all their nuclear options.