Unexploded bombs from the second world war are getting more dangerous

A study of unexploded shells from the second world war has shown that one of the explosives they contain is becoming more sensitive to impacts, meaning it could be set off if they are dropped during disposal. This explosive, called Amatol, was widely used during the first and second world wars, and is still in some of the ammunition being used during the Russian war in Ukraine.

“Based on our findings, we can say that it’s relatively safe to handle, but you can’t handle it as like TNT,” says Geir Petter Novik at the Norwegian Defence Research Establishment. “It can certainly go off if it’s dropped, as opposed to TNT.”

There are millions of tonnes of unexploded ammunition around the world, some in old ammunition dumps and some in shells and bombs that failed to detonate after being fired or dropped. There is a widespread misconception that this unexploded ammunition is becoming less dangerous over time, says Novik. In a 2022 study, he tested samples of TNT and PETN – two of the most widely used high explosives – taken from bombs and shells from the second world war (WWII), and found there was no deterioration in their explosive properties.

Now, he has tested the impact sensitivity of five samples of Amatol taken from unexploded WWII bombs and shells found in Norway. The test involved dropping weights from different heights on small samples to see what it takes to make them explode. All five samples were more sensitive to impacts than expected for Amatol, with one sample being four times more sensitive.

The findings will change how he and his team handle unexploded ordnance, says Novik. For instance, when clearing dumps, they will transport smaller quantities at a time. He now plans to try to find out why the impact sensitivity is increasing. “We suspect that it’s the formation of sensitive crystals or salts,” he says.

This could be a result of contaminants from the manufacturing process, or reactions with the metal casing as the lining deteriorates, or it may be simply due to ageing.

Amatol is the name used to describe explosives made of a mixture of TNT and ammonium nitrate. It was developed in 1915 by the Royal Arsenal in the UK when supplies of TNT ran low. Many countries stopped using Amatol when TNT production ramped up towards the end of the second world war, but in places such as the Soviet Union it continued to be used. “Several types of Soviet-era ammunition that are currently being used in Ukraine are known to contain Amatol,” says Novik.

It is impossible to say what percentage of unexploded ordnance around the world contains Amatol, he says. But it was in five out of 20 bombs and shells he cut open for this study, and he has also found it in the vast majority of unexploded WWII bombs he has examined.

There are many cases of unexploded ordnance exploding. For instance, in 2023 a WWII bomb exploded in Great Yarmouth in the UK during disposal. In 2004, New Scientist revealed that an unpublished risk assessment had concluded that a sunken US ship in the Thames estuary containing 1400 tonnes of high explosive posed a substantial threat to a nearby town. Surveys of the ship in 2023 found that plans to remove its masts could be risky, and they have been delayed.