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ASK AN EXPERT on Â鶹´«Ã½ Channel

Thanks for submitting your questions! We had Dr. Gordon Edwards, an expert and critic of the nuclear industry, answer several of them on Â鶹´«Ã½ Channel. You can watch the video above.

Because so many questions were submitted, Dr. Edwards took the time to give written responses to a few more. You can read them below:

Salt and Vinegar asked: How do Japan's nuclear reactors compare safety wise to Canada's CANDU reactor? What would happen if this was a CANDU reactor instead -- would there be any different outcome?

Answer: In a 1989 document addressed to the Treasury Board of Canada, the Atomic Energy Control Board (Canada's nuclear regulator) wrote "CANDU plants cannot be said to be either more or less safe than other types." In some ways CANDU reactors are safer (e.g. better containment) in other ways they are less safe (e.g. tendency to have a power surge when the coolant is lost), and in some ways there are comparable (e.g. spent fuel pools are not protected by the containment structure). But all commercial reactors will melt down if core cooling is unavailable for a protracted period of time.

Howard in Victoria, B.C. asked: If all four reactors at Fukoshima Dai-ichi Power Plant were to meltdown, what amount of radiation would be released compared to the release of radiation after the Atomic bombs dropped on Hiroshima and Nagasaki?

Answer: Much of the radiation associated with an atomic blast is in the form of heat (setting combustible materials on fire in all directions), neutrons (very penetrating particles which are particularly dangerous to living things), and gamma radiation (like X-rays only much more powerful). This blinding flash of destructive radiations has no counterpart in a nuclear reactor accident.

However, the fallout from an atomic bomb -- the radioactive material that is present in the mushroom cloud and disseminated into the environment -- is much less that the radioactive inventory in a large commercial power reactor. In both cases, hundreds of different radioactive poisons are created -- they are in fact the broken pieces of the atoms that were split to obtain energy. Since a reactor uses about 100 tonnes of uranium as fuel, and a bomb uses only a fraction of that amount, one can understand why one reactor, after one year of operation, produces radioactive materials equivalent to the fallout from 1,000 Hiroshima-type bombs.

Each spent fuel pod in the Fukushima plant contains even more radioactivity than the core of the reactor, because the irradiated fuel from several previous years of operation are stored there. For example, the total amount of radioactive cesium in four of these reactors, including the spent fuel pods, is greater than the amount of cesium released by all the nuclear weapons tests that have so far been conducted.

Colin Wookey in Kamloops, B.C. asked: Shouldn't there be signs by now the situation at the Fukoshima Dai-ichi nuclear power plant is improving if these reactors weren't going to have a complete meltdown? What are the chances of preventing a complete meltdown in any or all of the four reactors? How many times worse will the fallout be than the disaster at Chernobyl was if/when all four of the reactors have a complete meltdown?

Answer: Actually there are some signs that the situation may be improving as of Thursday evening, March 17. Today is the first day for which the situation does not seem even worse than the day before. Electrical power has been partially restored on-site which may allow regular pumping to resume. And the radioactive decay heat, which has been driving temperatures upwards in both the reactor cores and the spent fuel pods, is getting weaker as the very short-lived radioactive materials have undergone radioactive decay. Whether the heroic measures to restore cooling will be successful now depends a great deal on the geometry of the fuel rods. If they have crumbled together or melted together in such a way that the cooling water cannot penetrate into the interior of the overheated mass, then melting will continue even though the outside of the molten mass is being cooled. There is no clear indication that such an "uncoolable geometry" has been reached however, and so the restoration of pumped cooling should stabilize the situation.

Bouf asked: My son is about 800 km. south of Tokyo, what are the chances that he could be contaminated if there was a meltdown or explosion of the nuclear plan?

Answer: This depends on many things, such as which way the wind is blowing, whether it is raining or not (rain brings the contamination down to the earth), and of course how much of the overhead containment structure is preserved, thereby partially or totally blocking the escape of radioactive materials to the atmosphere. But some degree of contamination would be expected for anyone who is in the direct path of a cloud of radioactive steam and vapours up to a distance of several hundred kilometers and even beyond. This would not be enough to cause radiation sickness but could cause health problems among some of those exposed many years in the future. Heavily contaminated areas close to the reactor could remain uninhabitable for many decades.

Nathan asked: If millions of people are crowding in the airports down in Japan to get away from there and come over here to Canada and they are infected with toxic radiation even without knowing it, could they spread the toxic radiation on to us?

Answer: Good question. Since the radioactive materials in the cloud consist of such tiny particles, like the haze of cigarette smoke but invisible, they can easily cling to buildings, soil, plants, skin -- and clothing. If the contamination is heavy the gamma radiation will set off radiation monitors -- but not everybody has a radiation monitor. On the other hand highly toxic materials like plutonium give off little or no penetrating radiation and are therefore extremely difficult to detect; such materials can be carried by the unsuspecting person wearing contaminated clothes. In some cases, workers at nuclear facilities have inadvertently carried radioactive contamination into their homes because of contaminated skin or clothing that was not detected by the radiation monitors at the nuclear plant. In some such cases, furniture and bed-clothes in the workers' homes had to be taken away and stored as radioactive waste material.