Dirty Bomb - programme summary
The Horizon scenarios - page 3 of 4
To assess the impact of a strontium or caesium chloride dirty bomb, Horizon imagined two credible attack scenarios - one in London's Trafalgar Square, the other on the Washington DC underground train network - and commissioned expert opinion on the effect of each.
Trafalgar Square scenario
The fictional attack supposes a dirty bomb combining 4.5kg of Semtex plastic explosive with a large caesium chloride radiological source (activity rate: 74,000 gigabecquerels), detonated in the open air.
"Long term, people would have an increased risk of cancer, due to their exposure" |
Graham Smith, Enviros Consulting |
The blast itself kills the bombers and a handful of people nearby. The dust cloud from the bomb immediately carries the powdery caesium tens of metres up into the air. The plume travels downwind at 300m per minute.
As soon as the emergency services arrive on the scene, it becomes apparent that there is radioactivity present. Evacuation of the immediate area follows but there are few visible health problems. No one contracts radiation sickness, the problem instead is a potential cancer timebomb hanging over central London for a generation. How great?
Exposure to radiation is measured in sieverts (Sv), a unit that takes account of our bodies' varying susceptibility to alpha, beta and gamma radiation. 1Sv causes radiation sickness; 8Sv will kill you. Wherever you live there is natural background radiation, from the ground, the air, food and water. It typically amounts to 0.002Sv.
- Horizon's simulation suggests that anyone 5km away receives an additional radiation dose of just 0.001Sv (half as big as background); the cancer risk is marginal, rising by a factor of just 0.1%.
- People 1km distant sustain a 0.012Sv dose and run a cancer risk that is increased by 1%.
- It is only those who remain within 500m of the Square that face a major threat from the contamination. Anyone who continues to use the area within 200m of the bomb site for an extended period of time will receive a radiation dose 80 times background - 0.16Sv - sufficient to cause cancer in 1 in 7 people.
Washington DC Metro scenario
This fictional dirty bomb incident uses a smaller device in a more contained space. The amount of caesium is 74 gigabecquerels, one thousandth of the mass in the London scenario. It's also cruder, using just a firework as its explosive force.
"Even the smallest dirty bomb could be very disruptive" |
Dr Jack Caravelli, US Dept of Energy |
Detonated in a quiet corner of a subway station, the dispersal blast is over in a second and does not attract any attention. In fact, no one even realises there has been an attack, until the terrorists announce their action 24 hours later. In that time, thousands of commuters and hundreds of trains will have spread the radioactive caesium widely through the station and, more selectively, the entire underground network.
Modelling the health effects shows that the cancer risk to the general public is very slight.
- A typical commuter doubles their radioactive exposure, compared to background levels. Statistically they stand a 1 in 4,000 risk of contracting cancer in their lifetime.
- It is Metro staff who face graver concerns. After a day's work, their cancer risk is 1 case in 100 people.
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