The Large Hadron Collider (LHC) is to have an early-warning system installed to guard against a repeat of the catastrophic fault that caused the world’s largest atom-smasher to break down nine days after it was switched on in September.

CERN, the European particle physics laboratory near Geneva, is to fit the accelerator with 100 miles (160km) of cables and 2,000 crates of electronic monitors, so that engineers will be alerted to potentially hazardous abnormalities before they can cause serious damage.

The £4 billion “big bang machine”, which was switched on to global acclaim on September 10, was shut down after a huge helium leak caused extensive damage to many of its magnets.

The accelerator was then mothballed for repairs until next May or June, and its first experiments to investigate the fundamental nature of the Universe are not scheduled to begin until July 2009 at the earliest. The total cost of the repairs and refit is likely to be more than £30 million.

The official report into the accident, published yesterday, has confirmed that it was caused by a faulty connection between two of the LHC’s superconducting magnets, which bend beams of particles around a 17-mile ring at close to the speed of light.

This led to the release of about a tonne of the liquid helium that cools the magnets to 1.8C above absolute zero. A build-up of pressure then caused further damage to many other magnets in the affected sector: inspections of 57 magnets have shown that 53 need to be replaced.

CERN officials have decided on two measures to prevent a similar mishap in future. The early-warning system will alert the control room to abnormal patterns of electrical resistance that could presage another accident. The magnet network will also be fitted with pressure-release valves, so that a fault in one need not damage its neighbours.

James Gillies, CERN’s director of communications, said: “The report found that most of the damage did not occur in the actual incident, but afterwards because the LHC was unable to release the extra pressure from the escaped helium. We are therefore also going to install pressure release systems in the magnets that bend the beam.”

The total bill for the repair will be approximately 35 million Swiss francs (£20 million), as anticipated by a preliminary investigation that finished last month. In addition, CERN will have to spend a further 10 to 20 million Swiss francs (£5.5 million to £11 million) on replenishing the stock of spare magnets and parts used in the refit.

Dr Gillies said these costs could be met within the LHC’s existing budget. “We will not be going to our member states asking for more money,” he said.

Of the 53 magnets that need to be replaced, 28 have been brought to the surface, and all should have been removed by the end of the year. Two new magnets have already been installed, and all will have been replaced by the end of March.

The connections between the new magnets should be finished by May, allowing the LHC ring to begin powering up. A new beam should be inserted in July or August, allowing experiments to begin, though these will at first take place below the LHC’s maximum power.

For the moment, new pressure release valves will be added only in the three sectors that were warmed up to investigate and repair the fault. Similar devices will be installed in the other five sectors when these are warmed up for maintenance. Dr Gillies said: “The mood here is very stoical. We would obviously have preferred that this hadn’t happened, but it has and we’re confident that we are going about fixing the problems the right way.”

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