OSAKA — While Japan debates the possible restart of two nuclear reactors amid warnings of summer power shortages, the plant that prompted the shutdown of the country's entire nuclear industry is still a cause for concern. Fukushima Daiichi's reactor No. 4, which was badly damaged by a hydrogen explosion last March 15, houses 1,535 uranium fuel assemblies and, some activists and politicians have warned, is vulnerable to collapse if the plant is rocked by a powerful earthquake.