Usama bin Laden's Al Qaeda network held detailed plans for nuclear devices and other terrorist bombs in one of its Kabul headquarters.

The Times discovered the partly burnt documents in a hastily abandoned safe house in the Karta Parwan quarter of the city. Written in Arabic, German, Urdu, and English, the notes give detailed designs for missiles, bombs and nuclear weapons. There are descriptions of how the detonation of TNT compresses plutonium into a critical mass, sparking a chain reaction, and ultimately a thermonuclear reaction. 

Both President Bush and British ministers are convinced that bin Laden has access to nuclear material and Bush said earlier this month that Al Qaeda was "seeking chemical, biological and nuclear weapons." 

The discovery of the detailed bomb-making instructions, along with studies into chemical and nuclear devices, confirms the West's worst fears and raises the specter of plans for an attack that would far exceed the Sept. 11 atrocities in scale and gravity. 

Nuclear experts say the design suggests that bin Laden may be working on a fission device, similar to Fat Man, the bomb dropped on Nagasaki. However, they emphasized that it was extremely difficult to build a viable warhead. 

While the terrorists may not yet have the capability to build such weapons, their hopes of doing so are clear. One set of notes, written on headed notepaper from the Hotel Grand in Peshawar and dated April 26, 1998, says: "Naturally the explosive liquid has a very high mechanical energy which is translated into destructive force. But it can be tamed, controlled and can be used as a useful propulsive fuel if certain methods are applied to it. A supersonic moving missile has a shock wave. That shock wave can be used to contain an external combustion behind the missile ..." 

The document was one of many found in two of four Al Qaeda houses which had been used by Arabs and Pakistanis and even reportedly by bin Laden himself. The houses — two in the Karta Parwan district and the others further to the east — were abandoned on Monday as Taliban units and their allies fled the city. 

Attempts had been made to burn the evidence, but many documents still remained. They included studies into the development of a kinetic energy supergun capable of firing chemical or nuclear warheads, external propulsion missiles, preliminary research on the creation of a thermonuclear device, as well as a multitude of instructions for making smaller bombs. 

There were also studies into Western special forces' hostage rescue techniques, phone numbers for industrial chemical and synthetic producers, flight manuals, aerodynamic research, and advanced physics and chemistry manuals. 

The houses were identified by local people. Looters had concentrated on more appetizing objects, ignoring foreign language documents that were of no use to them. 

Bin Laden sees it as his "religious duty" to obtain a nuclear bomb. In an interview with a Pakistani journalist last week, he threatened: "If America used chemical or nuclear weapons against us then we may retort with chemical and nuclear weapons as deterrent." 

Intelligence agencies already have indirect evidence from defectors, middlemen and scientists of bin Laden's obsession with obtaining or producing a nuclear device. 

Al Qaeda agents are known to have spent more than $1 million trying to obtain enough fissile material to make a "dirty bomb" that, if detonated with TNT in a populous area, could kill thousands and contaminate it for decades. 

Intelligence sources told The Times last month that bin Laden and Al Qaeda had acquired nuclear materials illegally from Pakistan. And at least ten Pakistani nuclear scientists have been contacted by agents for the Taliban and Al Qaeda in the past two years, according to reports. 

Fears that bin Laden has components for a nuclear weapon is believed to lie behind the warnings from Bush and British Prime Minister Tony Blair that he would commit worse atrocities than the suicide assaults in America if he could. Blair's spokesman said: "Bin Laden would have killed 600,000 people on Sept. 11 if he could have done. This underlines again why he has to be stopped."