It rained heavily that weekend, writes Fintan O'Toole. The first indication that the radioactive cloud had reached Ireland came from an air filter sample taken in Glasnevin, Dublin, on the Saturday.
There was something in the air: iodine-131, caesium-137, caesium-134, ruthenium-103, ruthenium-106, tellurium-132, barium-140: all radioactive isotopes. There was also a small amount of plutonium. The level of caesium-137 was about 40 times higher than it had been before the cloud arrived. The rain carried these radioactive particles down into the soil, increasing tenfold the amount of radioactive caesium that was already there from nuclear bomb tests in the 1950s and 1960s.
Soon, testing of milk samples was showing that there was a significant level of radioactive contamination, especially from farms in the south midlands and in the northwest. The levels of iodine-131 in milk in Ireland dropped gradually throughout May, and had virtually disappeared by the end of the month.
But contamination with caesium was still detectable as late as October. It was especially noticeable in cheese and in milk powders of the kind that is used to make baby formula.
Vegetables collected in places as far apart as Donegal and Wicklow also showed significant levels of radioactive contamination. As late as December, sheep tested in upland areas were still found to be contaminated with caesium.
Unlike RTÉ's fine drama-documentary Fallout, this is not a what-if narrative. It happened. The explosions at the nuclear power station at Chernobyl 20 years ago tomorrow sent a cloud of radioactive material across Europe. It reached Ireland on May 2nd, 1986.
Its effects were still measurable a decade later, when sheep in the northeast, northwest and south of Ireland continued to show radiation levels higher than the recommended maximum. Those effects have probably not yet gone away: just a fortnight ago the British Food Standards Authority reported that sheep in upland areas there are still showing contamination from Chernobyl. One in 10 sheep going to market here in Ireland is still tested for radiation.
In some respects, Chernobyl may lie behind the success of Fallout. The drama is convincing because the people playing the roles of refugees from a putative nuclear accident at Sellafield are completely believable. This surely has something to do with the way most of us can now imagine the drama's scenario. This kind of catastrophe is the background radiation of our lives. Invisible but potent, it has contaminated us all since the early morning of July 16th 1945, when the first nuclear explosion was detonated at the Trinity Site in New Mexico, fusing the desert sand into green glass and bringing a new thing into the world.
Sometimes, during the Chernobyl or Three Mile Island incidents, or in periods of heightened tension during the Cold War, that thing has been at the centre of our collective consciousness. Mostly, though, it has been at the back of our minds, a reality that we recognise but prefer not to think about.
The opening sequence of The Simpsons, in which Homer's nuclear accident is just part of family life, is emblematic of contemporary life.
It is true, of course, that when we do think about it, we tend to create our own chain reaction, in which one thought rapidly and inexorably generates an explosion of hysteria.
We forget that we live with background radiation all the time, and that some houses in Wicklow get doses from the natural environment that compare unfavourably with Homer Simpson's. (Natural radiation may cause 1 per cent of all cancers, whereas nuclear radiation may cause 0.002 per cent.) We forget that the byproducts of other ways of generating electricity - like coal or peat-burning power stations - have probably caused far more deaths than the radioactivity created by nuclear power stations. We forget that the risks involved in daily life are vastly greater than the risks of dying because of a nuclear accident.
Yet none of this makes the current shift back towards nuclear power as a viable and ethical way to deal with global warming and the oil crisis any more sane. It is not just hysteria that makes people fear the consequences of a nuclear accident. Such accidents have happened before and they will happen again.
Human incompetence, terrorist attacks, the transport of nuclear materials and natural disasters (the Asian tsunami in December 2004 actually hit India's Kalpakkam nuclear power plant) are all real. So is the astronomical cost of cleaning up the radioactive mess that nuclear power stations leave behind. The British Nuclear Decommissioning Agency estimates the cost of fulfilling its current remit at £62.7 billion. And that waste remains dangerous for, in some cases, hundreds of thousands of years - a legacy we have no right to leave to future generations. Sherlock Holmes famously maintained that the way to solve a crime was to start by excluding the impossible.
In solving the gathering energy crisis, we have to start by excluding the unacceptable - and that includes nuclear power.