TOKYO—Japanese regulators discussed in recent months the use of new cooling technologies at nuclear plants that could have lessened or prevented the disaster that struck this month when a tsunami wiped out the electricity at the stricken Fukushima Daiichi power facility.

However, they chose to ignore the vulnerability at existing reactors and instead focused on fixing the issue in future ones, government and corporate documents show. There was no serious discussion of retrofitting older plants with the alternative technology,...