The Geiger counter ‘almost exploded’ (Picture: Getty/Metro)

A radioactive rock sat as part of a classroom display without anyone noticing for dozens of science lessons.

Nobody had realised that it was a lump of uranium, the metallic element which is used in nuclear reactors and even to produce atomic bombs.

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It was giving off thousands of millisiverts of radiation into the school, a far higher amount than occurs naturally, but teachers only found out when an anti-nuclear campaigner Thomas Neff came into the science lab at Missionaries of the Sacred Heart School in Salzburg, Austria, to give a talk.

To help in his lecture, he brought a watch with him from the Sixties, which contained small amounts of radium so that its dial would light up in the dark. They were popular several decades ago, when people did not know as much about the dangers of radiation.


It was radioactive uranium (Picture: Salzburg.gv.at)

Neff also brought a Geiger counter, used to measure the levels of radiation, so he could demonstrate how it responded to the watch.



Even though the watch was carefully sealed, the Geiger counter still showed 1,200 counts per minute, which is 20 times the normal value.

Was it dangerous? The rock wasn’t going to give anyone acute radiation poisoning, but wasn’t exactly something you’d want to find at school. According to Thomas Neff, if you carried it around in your bag for a year you would get around 210 millisievert exposure. That compares to an average exposure of just 2.8 millisievert a year in Austria. The maximum exposure allowed in the US (for people who have to work with radiation) is 50 millisievert. A dose of 100 millisievert per year is enough for a likely increase in cancer risk. But you would need a dose of 2,000 per year to get radiation poisoning.

Then something unexpected happened.

When he took the counter around the room to measure the low levels of radiation which occur naturally, the counter started to go nuts when he walked past a group of display rocks, minerals and fossils.

It almost ‘exploded’, going up to 102,000 counts per minute – around 100 times as much as the watch gave off.

Realising something was wrong, he stopped the lecture and alerted the school.

Pupils were evacuated while experts could work out if they were in danger.

The discovery led to an alert around more schools in Salzberg, which unearthed 38 more lumps of uranium on display in school geology collections at 11 different schools

All of them a now being kept in safe storage.

A radiological laboratory in Austria said: ‘The radioactive screening and the risk assessment show a basic risk potential in Salzburg schools in case of improper storage.’