Because this was a reactor that operated on water that was already at its boiling point, this also meant that the pressure inside the reactor was rising as well.

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

The Nuclear Industry Association (NIA) said the UK does not currently have the capacity to build parts such as reactor pressure vessels.

BBC: Sizewell C proposal

In February 2009, pressure levels spiked inside Reactor 1, forcing the release of steam through an emergency valve.

WSJ: Japan Plant Had Troubled History

In addition, the modular design could make it more difficult to inspect and maintain key components, many of which would be located within the pressure vessel at the heart of the reactor, he said.

CNN: U.S. backs project for small nuclear reactors

Eight of the more than 9, 700 tubes in one of the unit 3 generators failed a pressure test, while six tubes in unit 2's reactor needed to be plugged, the NRC has found.

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The drywell is a large steel pressure vessel that looks like a giant upside-down pear and holds the reactor and primary pumps, and the wetwell is a large toroidal vessel that looks like a donut.

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