geiger readings in excess of 100 cpm or 0,70 uSv/hr in rain

No problem, glad that information was useful.

so was Gunderson inaccurate when he commented that spikes were being seen in BC last year from the burning of radioactive material in Japan?

Yes, that was not true. Based on our measurements and others, the radioactivity in North American rain never had enough radioactivity from Fukushima to be detectable with a Geiger counter. The highest amounts of radioactivity due to Fukushima in rain was in the latter half of March 2011 after the major radioactive releases occurred. But even those amounts were far too small to detect with a Geiger counter. Also, we saw firsthand with our own rainwater samples that there haven't been detectable levels of fission products from Fukushima since April 20, 2011 in our area.

Another dimension to this is that if there had been even trace levels of non-natural radioactivity in rain, all sorts of official sources would have noticed and would have been asking where it came from. This kind of situation happened in Europe last November when very low levels of Iodine-131 were detected in air in the Czech Republic and other parts of Europe, then the IAEA was able to ascertain that it was caused by a release from a medical isotope facility in Hungary. These sorts of situations cannot be covered up since there is so much monitoring going on all over the world.

another claim I've seen lately is that the radon could be a product of the decay of Ur 238/235 from the MOX fuel

That claim is wrong. Even if there were trace particles of reactor fuel around (and I am not aware of definitive proof of this), this could not cause any increase in radon. Nuclear fuel is actually really bad at producing radon since there's hardly any Radium-226 in it. Radium-226, a decay product of U-238, is responsible for producing radon gas (specifically, Rn-222). Ra-226 production is hindered by multiple bottlenecks in the decay chain between U-238/U-234 (both in nuclear fuel) and Ra-226: U-234 has a 245,500 year half-life, meaning that it takes a long time to produce the next isotope down, Th-230; then Th-230 has a 75,000 year half-life to decay to Ra-226; then Ra-226 itself has a 1600 year half-life to decay into Rn-222.

However, there is Ra-226 in the soil and rocks all around us since natural uranium has had billions of years to overcome those bottlenecks to produce it. This natural radium can produce radon in abundance. When we have measured soil, we have found about 15 decays of Ra-226 per second per kilogram, and that's about typical for soil. Each decay of Ra-226 produces an atom of Rn-222, so a cubic meter of soil produces roughly 22,000 atoms of Rn-222 every second, many of which diffuse to the surface. In the air, this ends up amounting to a typical radioactivity concentration of 0.4 picocuries per liter of air outdoors, or 15 radon decays per second in a cubic meter of air (source: the EPA). Each radon decay leads to a chain of further alpha and beta decays until the nucleus reaches Lead-206, which is stable. These decay products are collected by rainfall and yield radiation that can be seen by Geiger counters.

So the bottom line is that radon has always been in our environment in larger quantities than many people realize. Fukushima did not — and cannot — add radon to the environment.

Mark [BRAWM Team Member]