Builders of the U.K.’s first nuclear plant in two decades are about to take a vital component and break it. The 110-ton spherical steel lid was destined to sit atop a reactor at the Hinkley Point site in Somerset. Instead it will be sacrificed to test the strength of a part already welded in place at similar atomic projects in France and China. The tests are essential after regulators found potential weaknesses in the steel used to contain radiation. The results may derail countries’ nuclear programs that are relying on the EPR reactors. They also threaten a generation of atomic plants that developer Areva SA has billed as the world’s safest.