THE US is likely to build the world's fastest supercomputer capable of carrying out 1.8 trillion calculations a second - to ensure that it never again needs to carry out underground nuclear tests. The us $46 million computer will be built for the us Department of Energy by Intel, the world's largest chip maker, together with State-owned national laboratories.

It would take a mere mortal working around the clock for 57,000 years to carry out calculations which the computer will do in one second. It will be as powerful as the largest 56-,000 mainframe Computers in the world put together.

At its heart will be Intel's P6 microprocessor, due later this year to replace the company's high performance Pentium chip, used in many of today's personal computers.

The new supercomputer will use 9,000 P6 chips linked together to create whit is known in the industry as a 'massively parallel processor" capable of 1.8 trillion calculations a second. But in comparison, a single top-end Pentium chip manages just 150 million.

The us Government's Sandia National Laboratory, which is working with Intel on the project, already owns one of the two fastest computers - an Intel Paragon capable of 140 billion calculations a second. Oak Ridge, another us lab, has one which does 150 billion a second.

The supercomputer is needed to fulfill President Clinton's recent pledge that us nuclear weapon safety will be maintained without resorting to underground testing. France says its current nuclear test programme, which is facing worldwide opposition, is needed to garner the information to enable it to witch to computer-based simulation of nuclear tests.

cllomputer simulation will be a principal means for ensuring the safety, reliability and effectiveness of the us nuclear deterrent," said Victor Reis, assistant secretary for energy pro- grammes at the us Department of Eriergy.