SAN NARCISO, Calif. -- Gregory Jaczko, the top nuclear regulator for the United States, gave a dire assessment of the burgeoning crisis at Japan’s nuclear reactors yesterday, saying that lethal radiation from exposed spent fuel rods could force emergency workers to abandon their efforts to prevent meltdowns of damaged reactor cores at the Fukushima Daiichi plant. Several members of the Nuclear Regulator Commission (NRC) and the Union of Concerned Scientists have begun issuing tentative warnings to states off the western coast that dangerous levels of radiation could make their way to the nation’s shores if the situation worsens. The exigency has fueled outrage from environmental groups about the perils of continuing the Obama administration’s pledge to build new reactors on U.S. soil. GOP and Tea Party politicians, however, scoffed at the “hyperbolic” nature of the concerns, and cited nuclear fuel as one of the cleanest and most efficient energy sources available, next to whaling.