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.

Dead or missing:  over 20,000
Houses destroyed or damaged:  792,000
Public buildings destroyed or damaged: 18,000

Of course these had nothing to do with Fukushima, save that they resulted from the same natural phenomena that crippled the nuclear plant. No one would dispute the significance of the Fukushima reactor accident to the international nuclear industry, but surely against the grim actuality of death and destruction represented by those figures, the event and its consequences must pale into insignificance.

Just examining these examples, it is clear there is a slight difficulty with the Windscale accident receiving the same rating as Three Mile Island. It is true that in both cases the reactors were destroyed, but the Three Mile Island reactor remained totally contained and there were no uncontrolled releases of radioactive material. Radiological releases were essentially limited to noble gases and were, from a public health standpoint, inconsequential. The Windscale accident, on the other hand, involved an uncontained reactor (which caught fire) with an open-circuit cooling system (air) and an uncontrolled release of significant quantities of radioactive material. By inspection Windscale and Three Mile Island are quite different events, even if application of INES criteria gives them the same score.  This problem arises because the creators of INES  wished the scale to give a comprehensive picture of the severity and significance of the accident, reflecting not just the actual effects (eg levels of radiation exposure to people), but the potential effects. The International Nuclear and Radiological Event Scale User’s Manual notes:

Events are considered in terms of their impact on three different areas: impact on people and the environment; impact on radiological barriers and controls at facilities; and impact on defence in depth.

And goes on to point out that:

In major facilities with the potential (however unlikely) for a large release of activity, where a site boundary is clearly defined as part of their licensing, it is possible to have an event where there are significant failures in radiological barriers but no significant consequences for people and the environment (e.g. reactor core melt with radioactive material kept within the containment). It is also possible to have an event at such facilities where there is significant contamination spread or increased radiation, but where there is still considerable defence in depth remaining that would prevent significant consequences to people and the environment. In both cases, there are no significant consequences to individuals outside the site boundary, but in the first case, there is an increased likelihood of such consequences to individuals, and in the second case, such failures represent a major failure in the management of radiological controls. It is important that the rating of such events on INES takes appropriate account of these issues.

INES seeks to indicate the total severity and significance of an event, not just the experienced effect, and this is fair enough. An accident involving core melt, for example, undoubtedly indicates serious failures in either engineered or human systems (often both). But if the melted core is effectively contained, then it can also be argued that the event is less severe than if the core had not been contained– the experienced event is less severe than the potential event, which actually is a statement that could be made about almost any accident. Of course the INES scale has to cover all kinds of nuclear installations– ranging from commercial and industrial irradiators, through waste processing and fuel reprocessing installations to full scale power reactors.  And this can give rise to apparent anomalies too. The User’s Manual points out that

…the 1979 accident at the Three Mile Island nuclear power plant is rated at Level 5 on INES, and an event resulting in a single death from radiation is rated at Level 4.

At first blush this seems distinctly odd, bearing in mind the fact that no-one was killed by radiation at Three Mile Island. But the point here is that a relatively trivial fault which may lead to a lethal over exposure at an irradiation facility, for example, does not pose any significant threat to the external environment, whereas a core melt most certainly does… even if that threat is not realised.

From the point of view of assessing challenges to nuclear safety the INES rationale is rock solid. But from the point of view of communicating quickly, clearly and essentially correctly to the general public the immediate seriousness of a nuclear accident it is questionable. Notwithstanding the unarguable rationale of INES, it is difficult to equate Windscale, with its uncontrolled release of significant quantities of I-131 and other isotopes (which necessitated disposal of locally produced milk for a period), with Three Mile Island.

How much more difficult it is to accept the fact that the Fukushima accident is given the same INES rating as Chernoby! At Chernobyl the reactor exploded, ejecting incandescent fuel, graphite and structural materials. The remains of the reactor burned for ten days, totally open to the environment. There were 28 prompt deaths from radiation exposure among those who battled the initial fires. At Fukushima all the reactors shut down automatically at the time of the earthquake. Despite leaks from damaged containment structures, the reactor cores (in whatever state of destruction) remain contained. Releases of radioactive material have been significant and an evacuation zone exists around the plant. There have been no deaths from radiation exposure among the workers at the site.

Of course the rationale for the INES 7 rating is perfectly logical and consistent, but to equate Chernobyl and Fukushima is grossly misleading. The INES ratings require much better explanation if they are to be used to guide the general public and the news media. Virtually the only thing Chernobyl and Fukushima have in common is the gallantry and dedication of the people on site who worked so tirelessly to stabilise their stricken plants.

http://www.southshorenow.ca/archives/2011/032911/news/index002.php

http://www.cbc.ca/homestretch/episode/2011/03/29/nuclear-crisis/