While there are numerous lessons for nuclear power plant designers and operators to be learned from the experience at Fukushima, there is one of particular importance and relevance to the operators of multi-unit Candu installations in Canada. That is, to quote from the summary of the second report by the Japanese Government to the IAEA,  the need to “ensure the engineering independence of each reactor at sites having more than one reactor.”

Aside from single reactors in New Brunswick (Point Lepreau) and Quebec (Gentilly-2), Canada’s nuclear plants are concentrated in Ontario at three multi-unit sites; Bruce, Darlington and Pickering. A significant feature of these installations is that the reactors share the same, single containment system; four (or in the case of Pickering, six) operational reactors are linked to a single containment building (the vacuum building). The purpose of containment arrangements is, as the name implies, to contain– to prevent the uncontrolled release of–  radioactive material in the event of an accident. The typical approach to this for pressurised water reactors (PWR), and single-unit Candu pressurised heavy water reactors, is to enclose the reactor in a massive pressure-tight concrete structure (the containment building) designed to withstand the peak pressure following a break in the reactor’s pressure boundary. The containment building includes provisions for controlling pressure through water sprays (to condense steam discharged from the reactor) and through controlled release of gases through filters to remove harmful radioactive species. Typically PWR containment buildings have design pressures of the order of 90 psi. Essentially then, the containment building is designed to resist the peak pressure resulting from a nuclear accident, then control that pressure.

The approach for multi-unit Candu reactors in Ontario is somewhat different. Instead of enclosing each reactor in a building designed to resist peak pressure, all the reactors are linked by a pressure relief duct  to a single structure maintained at a very low pressure, the vacuum building. Like a conventional containment structure, the vacuum building is provided with a water spray system to condense steam, and a filtered air discharge system to control internal pressure over the long term. However, rather than resist the pressure peak following a major pressure boundary failure, the vacuum containment system is designed to reduce it. In the event of a pressure boundary failure in one reactor building, fast-acting valves open, linking the vacuum building with the reactor building, so that the vapour and gases released from the reactor are drawn into the vacuum building and the pressure subsequently stabilises at below atmospheric pressure. This does appear to be a very elegant approach– especially with the post-accident pressure inside the containment envelope being below atmospheric. However it is true that the system cannot be described as providing “engineering independence” of each reactor. At the Bruce and Darlington installations four reactors are served by a single containment building. At Pickering, just outside Toronto, six operational reactors depend on a single vacuum building. In the event of an accident at one reactor that called upon the containment system, all the remaining reactors would be required to shut down promptly since they would have no access to any containment provisions. This of course is inherent to the design, and doubtless the risk of abruptly removing about 3000 MW(e) of generating capacity from the Ontario power system has been assessed and found to be acceptable.  And it could be argued that in a reasonable length of time an accident unit could be isolated from the containment system, and the vacuum building be returned to service allowing restart of the undamaged reactors– though it is questionable how easily such a task would be accomplished under post-accident conditions.

But all that aside, there is the question of what happens in a common-mode event resulting in multiple reactor failures. The design pressure of the containment envelope at Pickering is reported to be 6 psi gauge. That is, in the absence of the vacuum building, the containment envelope surrounding the reactors can withstand a peak pressure of 6psi above atmospheric.  By way of comparison, the hydrogen explosion at Three Mile Island Unit 2 in 1979 gave rise to a pressure peak of 28 psi– which the containment structure comfortably handled. In light of the Fukushima experience it is clear that the operators of multi-unit Candu installations in Canada should be applying their minds to this matter. Urgently.