If the bomb exploded at ground level instead of high above the city, the main difference would be an enormous crater four hundred metres across and seventy metres deep. All the dirt, rock, or masonry excavated would be made into radioactive dust and small debris. The larger particles would quickly descend in the immediate vicinity, and the finer particles and dust would descend in minutes or hours, mainly downwind from the site of the explosion.

The radiation dose to people exposed to this fallout would depend upon many factors, and would be enough to be lethal to anyone in the open or in a frame house for several hundred kilometres downwind. A simple basement "fallout shelter" would afford good protection. It would be necessary to spend a week or more in a fall-out shelter, and it would be impossible to judge when it would be safe to leave without a radiation survey meter or advice from public health authorities.

The area of blast damage would be smaller by perhaps a half, compared with an air-burst, though an earthquake effect would add to structural damage to buildings. The number of immediate deaths might be about half of those from an air-burst, but unless survivors could find protection from fall-out there would be many deaths from radiation sickness days or weeks after the bomb.



A TEN-KILOTON BOMB DETONATED AT GROUND LEVEL

If a bomb in the 10- to 20-kiloton range (the likeliest terrorist bomb) were to be exploded near ground level or in a ship in the harbour, the areas of blast, heat, and burn damage would be much smaller, perhaps reaching out to only one-tenth of the distances estimated for the one-megaton air-burst. The numbers of immediately killed and severely injured people would be counted in thousands, not hundreds of thousands.

Exploded on land, the bomb would vaporize all people and buildings in the immediate vicinity, and make a crater that might be as much as one hundred metres in diameter. If in the harbour, there would be a crater in the harbour floor and a tidal wave. The outstanding feature would be a radioactive downpour because much of the water in the harbour would be made radioactive and thrown high into the air as fine and coarse spray.

The explosion at ground level of this type of bomb would probably not cause a firestorm, so rescue operations for the injured might have some degree of success.

In either case, radioactive fallout would be serious, and might make the city, and an area of countryside stretching tens of kilometres downwind, uninhabitable for weeks or years. There would be a number of deaths from radiation sickness, for which there is really no effective medical treatment. The total amount of radioactivity might be comparable with the Chernobyl disaster, more or less depending on many circumstances.



THE ENHANCED RADIATION WEAPON OR "NEUTRON BOMB"

This is a small 'hydrogen bomb' in the 1- to 10-kiloton range without the outer casing of depleted uranium, which in an ordinary hydrogen bomb stops the neutrons that are formed and converts them into additional explosive power. The result is a spray of neutrons that is lethal for a distance of a few hundred metres. These neutrons, unlike the X-rays from the explosion, penetrate a considerable thickness of concrete or steel protection, like defence posts or the sides of a tank. They are designed for 'battle-field' use, not for use against cities. It is commonly said that neutron bombs spare buildings, but we believe this is a misconception. The blast effect would be reduced by half, and would still be enormous.



HOW COULD THIS SORT OF "ONE-BOMB" SCENARIO DEVELOP?

It is worth considering what circumstances might result in one or just a few nuclear bombs exploding, as opposed to a major nuclear war.

We hope, but we cannot be sure, that a nuclear attack by one of the "great powers" against a smaller country (which has been threatened several times since 1945) would never be carried out for any reason whatever.

There have been serious risks of war involving smaller military powers with nuclear weapons, such as India, Pakistan, and Israel. Clear or veiled threats of nuclear attack have been made by these countries, and might be again. Such use would most probably be directed at cities, and the bombs delivered by aircraft or relatively short-range rocket. It might be air-burst or ground-burst, with bombs in the ten- to one-hundred kiloton range.

Accidental or unauthorized launch of an intercontinental missile or a submarine-launched missile from one of the big nuclear arsenals might destroy a city with a bomb in the range of 100 kilotons to 1 megaton.

A terrorist type of attack is perhaps the most likely risk, and might be done by criminals for blackmail or ransom, or might be directed by an unidentified hostile government against a country too powerful for a declaration of war to be considered. It is possible that a 'hydrogen bomb' might be acquired from one of the superpower arsenals, and delivered by ship to the harbour of a port. More likely is a bomb in the ten-kiloton range exploded at ground level in a city, or in a ship.

An accident to a nuclear weapon, such as dropping it down a silo or from an aircraft, would not cause a full-scale nuclear explosion, but could scatter kilograms of plutonium by detonation of the high-explosive charge. To cause a nuclear explosion, the charge has to be detonated absolutely simultaneously all round the nuclear core, which is done by special electric circuits. Accidental detonation by a shock would not do this, but one wonders whether an electrical fault or a lightning stroke could ever do it.



FINAL COMMENTS

The above description was set in the context of a North American city. As proliferation of nuclear weapons continues, there is a greater risk that a tropical city may be attacked.

In such circumstances, the deaths and injuries from firestorms and flash burns would be higher than in the North American context, because many of the dwellings would be of light construction, and a higher proportion of the population would be likely to be in the open at the time of the explosion.

The distances quoted from ground zero are derived from a number of secondary sources, which do not all agree. Basically the numbers are derived from United States government measurements made during the years before 1963, when test nuclear explosions were permitted in the atmosphere.

It does not really matter if some of these distances are not accurate. Similarly, even if the estimates of deaths and injuries are considerably over-stated, the consequences of exploding a nuclear bomb and giving rise to a disaster even approaching this magnitude - anywhere on earth - remain completely unacceptable.

The only way to abolish this risk is to get rid of all the nuclear bombs in the world.