LONDON (Reuters) - A successful attack on a British nuclear power plant would be "highly unlikely" to kill many people immediately but could cause large numbers of long-term cancers, according to a report.

The study into the threat of terror attacks on both civilian and military nuclear facilities said even an unsuccessful attempt could have an economic or social impact by affecting public confidence in nuclear power generation.

The report by the Parliamentary Office of Science and Technology, published on its Web site on Thursday, drew only on publicly available information and made no recommendations.

It said there was enough information in the public domain to identify ways attackers might effect a release of radioactive material, but not enough to draw conclusions on the likelihood of a successful attack, nor the size and nature of any release.

Anyone planning an attack on a nuclear power plant would have two choices: either blast through the thick concrete shielding its superheated radioactive core, or get past security and damage the site's critical safety systems.

The report played down the likelihood of either approach succeeding.

Citing a U.S. study, it said a September 11-style attack on a nuclear plant by a hijacked commercial airliner would probably fail because the aircraft would be unlikely to strike at the angle and speeds needed to cause sufficient damage.

And it said an attempt to damage safety systems would need a high degree of access, co-ordination and detailed plant knowledge.

If there were a release of nuclear material, its impact would depend on weather conditions and the efficiency of measures to protect people from radiation, including evacuation, sheltering and restricting food and water supplies.

Britain has 13 active nuclear power plants, with six in decommission, as well as large amounts of nuclear material at the Sellafield reprocessing plant in Cumbria, and at a former research site at Dounreay in Scotland.

Since the September 11, 2001 attacks, public access to nuclear plants has been greatly restricted and some information formerly in the public domain has been withdrawn, the study said.

A summary of the report can be found on the Internet at www.parliament.uk/parliamentary_offices/post.cfm .