It will be used to model climate systems before being classified by the Army to work on nuclear explosion modelling.
"We replace our high-performance supercomputers every four or five years," said Andy White, leader of supercomputer development at Los Alamos.
"They become outdated in terms of speed, and the maintenance costs and failure rates get too high."
White explained that the use of two kinds of chips represents a revolution in supercomputing.
'Roadrunner' uses fewer than 20,000 processors while the next fastest computer, IBM's Blue Gene/L, uses over 200,000 and runs at half the speed.
The Los Alamos machine uses 7,000 multi-core Opteron processors and parses the data to 12,960 enhanced Cell processors using 57 miles of fibre-optic cabling. The device consumes three megawatts of power, enough to run 1,000 homes.