A copy of the October study was obtained by The Canadian Press under the Access to Information Act.

CSIS relies mainly on previously published research and analysis in assessing the threats, though brief passages were deemed too sensitive to disclose.

The Sept. 11, 2001, terrorist attacks on the United States raised fears that extremists could crash a jetliner into a nuclear reactor or get their hands on material to craft a rudimentary dirty bomb, or radiological dispersal device.

"The technical capability required to construct and use a simple RDD is practically trivial, compared to that of a nuclear explosive device or even most chemical or biological weapons," the CSIS study says.

A homemade radiological weapon could consist of a conventional explosive laced with radioactive material commonly found at universities, medical and research laboratories or industrial sites.

Several isotopes used in applications including cancer treatment and industrial radiography have been identified as possible sources. However, CSIS notes, much would depend on the material's half-life, the amount of radioactivity present, the portability of the source and the ease with which it could be dispersed.

Experts say such an explosion, while claiming few initial casualties, could spread radiation over a wide area, contaminating several city blocks, sowing panic and wreaking economic havoc.

Canadian organizations have quietly spent hundreds of millions of dollars since 9-11 to secure nuclear reactors, mines, research facilities and laboratories that handle radiological material.

CSIS contends detonation of a crude bomb is "undoubtedly the most likely" terrorist scenario involving radioactive sources.

"Indeed, it is quite surprising that the world has not yet witnessed such an attack," the study says, adding "it appears that we are positively overdue for one."

The intelligence service points to the notion terrorist thinking has shifted from the desire to inflict mass casualties to "one of inflicting severe economic damage."

Despite the assessment, the study provides little sense of the actual likelihood of a radiological or nuclear strike, said Prof. Wade Deisman, a criminologist and director of the University of Ottawa's national security project.

A more detailed CSIS analysis would be needed to develop such a measuring stick, Deisman said.

"They need to have an idea of how to prioritize their responses to threats based on their probabilities. And I still am far from convinced that they have any sense of that."

Security agencies need to assure the public they have a grasp of the risks, systems in place to protect key facilities and the resources to respond to emergencies, Deisman added.

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