Unleashing copper-encased plastic explosives on a suspended cage of steel that weighs more than 400,000 pounds certainly meets Joe Jacobson’s definition of cool.

Not quite “Call of Duty” cool, mind you, but the Kansas Turnpike’s demolition-by-blasting sure beats memorizing multiplication tables at school.

“It’s like lightning,” the beaming third-grader said, moments after watching explosive charges rip through the steel so quickly — at more than five miles per second — that their flash easily could missed with a blink.

The remnants of Monday morning’s explosions, of course, will be around awhile.

The turnpike’s hired contracts are busy demolishing what remains of two original bridges that cross the Kansas River. Crews already have built one new bridge to carry turnpike traffic, and once the old bridges are gone work will begin on a second span.

It’s all part of a $130 million project that already has overhauled the West Lawrence interchange, replaces several other bridges, and soon will close and rebuild the East Lawrence interchange at the edge of North Lawrence.

Monday’s blast was the fourth such event conducted in recent weeks, and could be considered among the most important. That’s because once the fallen steel is removed — a process that began soon after the smoke had cleared, shoving the twisted beams into piles for recycling — another crew can start boring as much as 60 feet below the ground alongside the river.

Such holes will be the first of several that reach down to bedrock, to accommodate new concrete-and-steel supports for a bridge expected to be built and open by spring 2011.

Crews still need to pull steel pieces from the river before lining up the final blast to remove such metal: several sections that connect the river banks. That blast will be the fifth such event, tentatively planned for sometime during the week of Jan. 11.

Contractors need to be done working in the river by May 15, to comply with environmental permits.

“That’s why this is so important,” said Ron Nadvornik, project manager for Perry-based Hamm Inc., the project’s general contractor.