Frothing seas will mark French blast

Nuclear-test row: Seismographers wait for the big explosion that will mean France has gone ahead with its controversial experiment

Papeete (Reuter) - For outside observers the only indication when France carries out its underground nuclear test in the Pacific will be a surge of froth rapidly spreading through the lagoon around the testing site and the swaying needle of seismographs recording the energy blast.

These tests, like the 134 which preceded them between 1975 and July 1991, will occur about half a mile underground in wells dug deep into the basalt foundation of Mururoa or Fangatufa atolls, 750 miles south of Tahiti, in French Polynesia.

The energy blast will be at the bottom of pits drilled using techniques borrowed from the petroleum industry. It could be several times greater than the atomic bomb exploded over Hiroshima but will be over in a millionth of a second.

Before that the nuclear device will be placed in a canister 60 ft long and 5ft in diameter and stuffed with recording instruments and optic fibres.

The nuclear heart of the experiment will arrive last on the test site and will be lowered into the pit from a specialised barge.

Cables and optical fibres are then connected to a remote command system and other data-collecting devices located on another barge. According to a French Defence Ministry document, this is the last moment the nuclear device can be retrieved if there are any problems with the equipment before the blast.

The next step consists of closing the lid on the well using rocks and a cement stop-gap. The cement stopper "is calculated so as to avoid all radioactive leakage", the defence document says.

The countdown starts and the bomb is triggered. In a nano-second the blast is measured. In the next moment, the container and its instruments are pulverised and the cavity created by the explosion is filled with liquefied rock.

Experts at France's Atomic Energy Commission say it is in this lava that "the essential radioactive residue is trapped". According to Alain Barthoux, director of the tests and now on Mururoa, a dozen different types of tests will be conducted using 100 to 200 instruments during each blast.

The tests seek to measure the emission of X-rays, gamma rays, and neutrons. After the bomb is triggered, a barge will retrieve a couple of kilograms of the lava from the explosion chamber. Analysis of tests, which will occur two to three weeks apart, will take up to a year to complete.

The New Zealand navy will be among the first outsiders to detect the huge shock waves set off by nuclear explosions.

It has hydrophones on its oceanographic ship, Tui, which has joined an anti-nuclear flotilla outside a 12 mile off-limits zone around the blast area.

Next in line is likely to be a small group of American seismologists thousands of miles away at Alice Springs, Australia,where monitors are buried in a shaft 200ft deep.

About 12 minutes after shock waves reach Alice Springs, slower sound waves will be picked up by a New Zealand seismological station in Rarotonga, in the Cook Islands, about 1,300 miles from Mururoa Atoll.