A question of fact and fission

The Cold War - and the principle of mutually assured destruction - may be over, but research into nuclear weaponry continues apace. The US, for example, is to study four new types, according to an agenda leaked from last month's US Strategic Command meeting. These include an enhanced radiation ("neutron") bomb; a bunker-buster, the Robust Nuclear Earth Penetrator (RNEP); a "mini-nuke" battlefield weapon with an explosive yield equivalent to 100 tons of TNT; and an "agent-defeat weapon" designed to incinerate anthrax spores or nerve gas. The weapons that have grabbed most headlines are the RNEP and the mini-nuke. In the public debate, these have been conflated into a single weapon, able to destroy buried dictators without collateral damage. In fact, there is no such device.

The Cold War - and the principle of mutually assured destruction - may be over, but research into nuclear weaponry continues apace. The US, for example, is to study four new types, according to an agenda leaked from last month's US Strategic Command meeting. These include an enhanced radiation ("neutron") bomb; a bunker-buster, the Robust Nuclear Earth Penetrator (RNEP); a "mini-nuke" battlefield weapon with an explosive yield equivalent to 100 tons of TNT; and an "agent-defeat weapon" designed to incinerate anthrax spores or nerve gas. The weapons that have grabbed most headlines are the RNEP and the mini-nuke. In the public debate, these have been conflated into a single weapon, able to destroy buried dictators without collateral damage. In fact, there is no such device.

When Congressional supporters of the nuclear bunker-buster enthused about a bomb that could be safely dropped on a bunker in the middle of a city without harm to innocent people above ground, physicists reacted with scorn. At the depths attainable by a ground-penetrating bomb, said Professor Sidney Drell, the director of the Stanford Linear Accelerator, not even the smallest nuclear explosion could be contained underground. America has an earth-penetrating nuclear bomb, the B61-11. In tests, it has never penetrated more than 20 feet of earth.

Robert Nelson, a nuclear-weapons expert at Princeton University, says there is no novel technology around the corner that will bring an impressive leap in penetration. "The rules of long-rod penetration say a steel rod hitting concrete can penetrate about 10 times its own length. But even that would require an impact velocity liable to melt the casing or destroy the warhead."

Supposing scientists can burrow twice as deep as their best efforts to date, their bomb will detonate 50 feet underground. Proponents of the RNEP talk of attacking bunkers as deep as 1,000 feet. A bomb that could shatter a bunker 950 feet below it without broaching the ground 50 feet above would be a wonder weapon indeed.

In the 1960s, the US government carried out a series of tests known as Operation Plowshare, to study the feasibility of using nuclear weapons to build substitute harbours and canals if war or catastrophe destroyed existing facilities. These tests showed that a one-kiloton bomb had to be buried 250 feet deep to contain the blast. For five-kiloton devices, the minimum safe depth was 650 feet. Surface leaks were common - and those bombs were buried in sealed shafts. A falling bomb inevitably leaves a hole, making the scenario more akin to tests that left the shaft unsealed. These were known as "Roman candles".

Most nuclear weapons are designed to burst high above their target, so that terrain features cannot shield their victims from blast and heat. Then, a rising column of hot air sucks up irradiated dirt from the ground, which returns to earth as fallout. A Roman candle explosion ejects a plume of dust far more radioactive than typical fallout. It spreads horizontally in a powerful base surge. The effect is similar to that of the "dirty bomb" used by hypothetical terrorists.

"All this talk about safely containing explosions is a red herring," says Michael Levi, a science fellow at the Brookings Institution. "The politicians who support these weapons cling to this claim because it makes them seem more acceptable. Their opponents cling to it because it's so easily debunked as bad science, and makes a convenient stick with which to beat the whole project. But the scientists who are actually developing these bombs don't seriously claim they can be used without collateral damage."

In fact, when it comes to destroying bunkers, the weaponeers are relying on the proven recipe of sledgehammer force. "Current plans for the RNEP envisage a warhead as large as two megatons," says Levi. Opponents argue that since it could kill millions if used in an urban setting, the enemy need only place his bunkers in his cities and "self-deterrence" still applies. The agent-defeat weapon is also a bunker-buster, but is designed to destroy chemical and biological weapons stocks with heat. It could work but, like the RNEP, it would kill anyone living nearby.

The Pentagon has befriended the RNEP and the other projects, and the Bush administration also backs the weapons, which fit in with its recent announcement that it will no longer necessarily refrain from using nuclear weapons against non-nuclear states. But the germ of the idea dates back to the Clinton administration, and comes from the scientists themselves.

The early Nineties were hard for the nuclear labs. Demoralised by Salt (Strategic Arms Limitation Talks), Start (Strategic Arms Reduction Treaty) and the Comprehensive Test Ban Treaty, they faced the end of the Cold War. Then came the Gulf War and the smart bomb. Pundits began forecasting that precision-guided munitions could make whole classes of nuclear weapons redundant. Redundant weapons could mean redundant weaponeers.

In late 1991, two nuclear scientists published an article in Strategic Review entitled "Countering the threat of the well-armed tyrant: a modest proposal for small nuclear weapons". They argued that existing weapons were so destructive that no president would consider using them unless the US came under direct nuclear attack. The scientists proposed making less destructive nukes that could be more readily used. By blurring the distinction between conventional and nuclear weapons, they argued, the process of escalation could be made smoother, and the deterrent would gain credibility.

This argument horrified many on Capitol Hill, and in 1994 they passed the Spratt-Furst amendment, which banned the development of weapons of less than five kilotons' yield. But the projects never went away. Each year, the government has allowed research into low-yield weapons, arguing that studying the concept is different from developing the weapon. This May, Congress repealed the Spratt-Furst amendment. Andscientists will need to test devices such as the RNEP before they can certify them. Last week the US government reaffirmed at a UN conference in Vienna its opposition to ratification of the Comprehensive Test Ban Treaty. Instead, the Bush administration is seeking funds to ensure that testing could be resumed within 18 months of a go-ahead from the government.

The mini-nuke, the bunker-buster and the agent-defeat weapon are positively cuddly compared to most weapons in the armoury. It suits the labs that their products become more acceptable. They want a bomb they can feel good about. And for the customer, the whole point of the self-deterrence argument is to build bombs you can use without feeling too guilty.

At first sight, the odd one out is the neutron bomb, designed for maximum radiation and a small blast. The idea was floated in the Eighties, but abandoned in the face of popular protest, after it was branded as the bomb that leaves cities standing but kills everyone in them. Its real purpose was to destroy invading Soviet armoured formations without killing millions of West Germans in the process, because it produced minimal fallout. It's a nuclear option that the South Koreans might find acceptable today.

Arguments for the new nuclear weapons change with the political weather. There is less talk of self-deterrence now, and more focus on buried weapons of mass destruction or Afghan-style terrorist tunnel hideouts. But the push from the labs is steady, because it is based on institutional need. "I don't think they fear losing their jobs, but they fear being shunted into non-nuclear work," says Levi. "They will always identify new weapons needs and gaps in the armoury. It's their job. They're like any scientists angling for a grant."

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