Nuclear Bomb

Introduction

Mushroom

A nuclear bomb detonation in the UK is considered to be a very low probability. The 2020 National Risk Register mentions the use of an improvised nuclear device by terrorists (this is a nuclear explosion as opposed to a dirty bomb dispersing a radioactive source) “resulting in much greater numbers of casualties and widespread, long-term impacts of a magnitude above all other terrorist attacks”, but this is considered to be unlikely. A "larger scale CBRN attack", a category which includes an improvised nuclear device, is assigned a likelihood of 5 in 25 per 500 of "the reasonable worst case scenario of the risk occurring in the next year" which is 5 to 25 times higher than the estimated likelihood of a nuclear accident.

The Chemical, biological, radiological and nuclear (CBRN) response discussed on the Centre for Protection of National Infrastructure website is basic. The UK emergency services response to a CBRN event is outlined in a Joint Operations Principles document.

The consequences of such a detonation would depend on the nature of the attack. A single improvised bomb is likely to have a relatively low explosive yield, a nuclear device stolen from one of the nuclear powers would pose a far greater threat and an all-out attack from a superpower would be another matter entirely.

The explosive power of a nuclear bomb is measured in terms of the amount of TNT that would be required to give a similar size explosion. (A ton of TNT releases about 4.184×109 joules) There are a wide range of explosive yields available from nuclear devices from 0.01 kT for a tactical nuclear weapon to 50,000 for the Tsar Bomba, a Russian bomb that currently holds the record for the largest nuclear explosion (this written 28/2/22).

A Nuclear Explosion

A nuclear explosion has a number of identifiable steps:

  • Fireball - caused by the release of energy;
  • A wave of thermal radiation - very fast moving and capable of blinding, burning or killing outright. First degree burns (similar to bad sunburn) may be expected out to a distance of 3.2 km from a 10 kT bomb; second degree burns (blistering, possible shock and death if extensive and untreated) out to 2.4 km and third degree burns (destruction of tissue, shock and death without specialist treatment) out to 1 km (FEMAFEMA Skin burn range, https://emilms.fema.gov/IS3/FEMA_IS/is03/REM0502100.htm);
  • A shock wave that can cause considerable physical damage to anything in its way. At 0.4 km from a 10 kT explosion the wind could be 160 miles an hour, sufficient to suck people out of buildings;
  • An electromagnetic pulse - this can damage electricity supply and telephone systems as well as computers and other electrical systems. Wireless communications may be disrupted for a while even if the hardware is unaffected. A blast high in the atmosphere could damage any satellites within range. This is considered to be a tactical use of nuclear weapons, and one to be concerned about as it greatly damages the ability of a modern society to function without causing so much longer term fallout;
  • A mushroom cloud as everything within the fireball is vaporised and carried upward. This can include great quantities of dust if the blast is low enough to hit the ground and produce a crater;
  • Fallout - caused by the dust and vapour produced cooling, condensing and returning to ground level. This fallout can be intensely radioactive although the levels of radioactivity will drop rapidly, at least at first;

Survival

To survive a nuclear bomb it is necessary to survive a number of deadly effects:

  • The air blast (shock wave) caused by the energy released locally heating up the air and causing it to expand rapidly;
  • The thermal radiation produced in the explosion and the interactions with the atmosphere;
  • The neutron radiation produced in the explosion;
  • The gamma radiation produced in the explosion;
  • Any fire storm resulting from the blast;
  • The beta/gamma radiation from decay of fission products and neutron-induced radioactivity within the fallout.

The initial pulse of radiation may be sufficient to kill, or at least blind, people beyond the zone that is devastated by the blast. Survival is dependent on being far enough away from the bomb or well shielded. Those looking towards the bomb are obviously more likely to have their eyesight damaged, although this may be temporary for those “lucky” enough to be further away.

The effects of the blast travel more slowly than the flash. They can destroy buildings and kill people over an extensive range. Again survival is about distance and sheltering. Shelters would have to be robust to survive the blast. Underground would be better.

The blast can cause changes in air pressure, both sudden increases and decreases, which can be sufficient to kill. The blast and resulting fire can deplete the oxygen supply leading to death.

For the unprepared and unwarned survival of the blast is a matter of luck - not being too close to the bomb when it goes off or fortuitously being shielded. Thereafter survival of the next few minutes could be enhanced by quickly (very quickly!) finding better shelter from the thermal and radiation blasts and their effects. The advice given is “''if you can’t find shelter within a few seconds then lie on the ground and cover your head if possible. Do not look at the flash or fireball if you can help it''”.

The next issue is fallout - the radioactive dust and gases produced by the explosion. Survival is about putting as much distance or shielding between individuals and the fallout as possible. Evacuations should be away from the fallout. Any survivors in shelter will need advice, reassurance, and probably, medical attention. A good source of such advice is the USA Ready Government web-site.

For the few days or weeks after the explosion avoidance of the fallout is necessary to prevent the exposure to a large radiation dose. Evacuation away from the dust falling from the sky followed by decontamination is the quick route out. Staying in a fallout shelter until the radiation levels have dropped low enough not to be a great hazard is the slow way out. Those exposed to high doses of radiation or injured in the blast may be unlikely to survive without prompt medical care.

FEMA report the 7:10 Rule of thumb which states that for every 7-fold increase in time after detonation, there is a 10-fold decrease in the exposure rate. In other words, when the amount of time is multiplied by 7, the exposure rate is divided by 10. For example, let’s say that 2 hours after detonation the exposure rate is 4 Sv/hr. After 14 hours, the exposure rate will be 1/10 as much, or 0.4 Sv/hr. Recently published advice to potential responders to a nuclear detonation in the USA can be found on the internet.

Bomb Range

The figure shows predicted bomb damage ranges for a 10 kT, ground-burst nuclear explosion in an urban environment. The Moderate Damage zone from about 0.5 - 1.0 miles is described as “Significant building damage and rubble, downed utility poles, overturned vehicles, fires, many serious injuries, greatest lifesaving opportunities” (ref).

The film “War Game” (1960) is available on the internet. This shows a fictional docudrama depicting a hypothetical nuclear attack on Britain and shows what the local authorities would be up against. Dated, informative, occasionally accidently amusing, and terrifying, it is well worth watching.

A quick read of the “planning guidance for response to a nuclear detonation” prepared by the US Homeland Security Council should be enough to convince us that the situation facing responders would be terrible.

If interested see: A student guide to survival.