Which country will become the next to explode a nuclear bomb?

Top officials of the Carter administration feel the biggest proliferation threat today comes from Pakistan.

After France last summer backed out of an agreement to sell Pakistan a plutonium reprocessing plant, deposed President Zulfiqar Ali Bhutto admitted that his motivation in buying the facility had been to give Pakistan a nuclear weapon capability.

Bhutto, in an extraordinary document smuggled out of the jail cell where he is being held, said the reprocessing plant would have been used to separate the plutonium needed for weapons from the spent fuel produced by Pakistan's only atomic power station.

"We have the brainpower. We have the nuclear power plant in Karachi. All we needed was the nuclear reprocessing plant," Bhutto said. "We were on the threshold of full nuclear capability when I left the government to come to this death cell."

Although France has now withdrawn from the deal, it provided Pakistan with complete blueprints for the reprocessing plant in 1976.

"The temptation for the Pakistanis is still there," a top U.S. official said. "The French have nipped in the bud the short route to proliferation, but the Pakistanis will probably explore a variety of other avenues."