The shield building is intended to protect the reactor from damage and provide a radiation barrier, in the event of an accident.

WSJ: NRC Approves Toshiba Reactor Design

In one of the examples cited in the report, the Nuclear Regulatory Commission (NRC) has twice raised the acceptable level of radiation damage reactor vessels can sustain, after several plants violated or came close to violating the original standard.

FORBES: The Biggest Threat to Nuclear Power: 'Pencil Engineering'

The water in the reactor is susceptible to damage from radiation, causing it to split into its components, hydrogen and oxygen.

FORBES: Explainer: What Caused The Incident At Fukushima-Daiichi