What are “dirty bombs” and why is Russia talking about them?

Western officials quickly dismissed Russia’s claim over the weekend that Ukraine was planning to use a so-called dirty bomb on its own territory. The United States and its allies have released a series of statements accusing Moscow’s leadership of making “patently false allegations” to create a pretext for escalating the war.

But the intense exchanges of the past few days have renewed attention on the very concept of the dirty bomb.

It is a type of weapon that was thought out and tested more than three quarters of a century ago, in the early years of the atomic age, but never used by a military force.

Following the September 11 attacks, government officials sometimes warned that terrorists might build one out of radioactive materials used in many commercial industries, and dirty bombs became a bogeyman in the public consciousness, an object of fear.

The more formal name for the bombs – radiological dispersal devices, or RDDs – offers a fairly straightforward description of what these weapons are and how they work.

These are basically improvised bombs that use conventional high explosives to spread radioactive material in the surrounding area. But the fact that no army is known to have one deployed in its arsenal is a good indicator that they are not useful on the battlefield.

The most commonly imagined version is usually small enough to fit in a backpack and contains perhaps 20 pounds or less of explosives, with a smaller mass of radioactive material placed on top.

That’s about it, although such a weapon could potentially be much larger.

There are, however, inherent problems with this concept that limit a bombmaker’s chances of success. First, the size of the main charge: use too much explosive material and the radioactive material could be largely consumed by the intense heat when the bomb explodes. Use too little and the device won’t spread the radioactive material very far.

Another consideration is that only a few radioisotopes, which are commonly used for medical purposes or for power generation, are suitable for use in this type of device.

A radiological dispersal device is not a “nuclear weapon” in the classic sense, because there is no fission, no fusion, no massive release of energy, and no destruction of urban craters.

If everything works correctly – and as an improvised bomb there are many potential points of failure – a dirty bomb detonates radioactive material into small pieces and releases it into the surrounding air. It produces a localized contamination problem, not a global one.

People who inhale or ingest radioactive dust could be injured or killed, and contaminated buildings should be bulldozed and sent to a landfill. Excavators would likely dig up the radiated soil a meter deep and chop down nearby trees – which would also be sent to landfills.

But many effects would depend on atmospheric conditions.

Temperature gradients would affect how high the plume of radioactive material could rise through the air, and wind speed and direction would determine how far it could spread from the explosion.

An academic article published in a US military magazine on the subject in 2004 noted that “economic and psychosocial effects are likely to be the most severe damage mechanisms of any use of an RDD”

“Fear of ionizing radiation is a deep and often irrational legacy of the Cold War,” the report says. And while an attack with this type of device is unlikely to “cause mass death,” it has the potential to “cause great panic and enormous economic loss.”

There is a significant risk for the bomb maker when building this type of weapon.

For the radiation from the weapon to be lethal, the radioisotope used should have an intensity strong enough to harm people.

If bomb makers acquire radiological material that is in a shielded container – that is, a ship built in such a way as to prevent the emission of harmful rays – they would have to make a crucial decision: whether or not to attempt to remove radioactive materials.

Bomb makers may think that removing the shielding will allow for better dispersal of radioactive material. But it can expose them to harmful ionizing radiation at very close range for long enough to cause real harm to their bodies – a fundamental danger of working around radioactive materials.

The closer the attacker gets to any dangerously radioactive material – and building such a device would usually require being within reach to do so – the more intense its harmful effects would be. Bomb-makers could be exposed to a lethal dose of radiation before they even finish their work.

Then there’s the possibility of the attacker being discovered en route to planting the bomb, with a strong radioactive source potentially triggering detectors along highways and bridges that alert law enforcement.

According to government documents, the U.S. military experimented with radiological dispersal devices at Dugway Proving Ground in Utah from 1948 to 1952, but that work was eventually abandoned.

Other than these tests, during which radiological dispersal devices of various types were constructed, there is no evidence that the U.S. military or any other force ever deployed such a weapon for combat use.