While events are still unfolding on the ground at the damaged Fukushima Daiichi Power Plant, GE continues to provide technical assistance to TEPCO through our joint venture partners in Japan and to the U.S. Nuclear Regulatory Commission (NRC), which is in turn providing assistance to the Japanese government. There are also some facts that GE Hitachi Nuclear Energy can attempt to clarify, such as those concerning the Mark I containment system in use at the reactors in the Fukushima Daiichi Power Plant.
The Mark I containment has a proven track record of safety and reliability for over 40 years and there are 32 BWR Mark I reactors operating as designed worldwide.
While the technology was commercialized 40 years ago, it has continued to evolve. Over the last four decades, the Mark I has been modified in the form of retrofits to address technology improvements and changing regulatory requirements.
All of the modifications were made in accordance with regulatory requirements. In the United States, for example, the NRC issued a generic industry requirement in 1980 for the Mark I containment that the industry used to make modifications.
We understand that all of the BWR Mark I containment units at Fukushima Daiichi also addressed these issues and implemented modifications in accordance with Japanese regulatory requirements.
The modifications made to Mark I containments include:
- “Quenchers” were installed to distribute the steam bubbles in order to produce rapid condensation and to reduce loads on the unit. In a reactor, exhaust steam is piped into a suppression chamber, which is known as the torus and is a large, rounded suppression pool that sits next to the reactor core. It is used to remove heat when large quantities of steam are released from the reactor. In the torus, the steam bubbles go under water. With the modification to the Mark I, the quenchers, which are also underwater, make steam bubbles smaller by breaking up the larger bubbles. This in turn reduces pressure.
- Another modification is the installation of deflectors inside the torus. When that steam goes in, the water level rises. The deflectors that were added break up the pressure wave that is produced and help relieve pressure on the torus.
- A further modification was made to the “saddles” on which the torus sits — basically the series of leg-like structures that support it. The construction was fortified, as was the steel, to accommodate the loads that are generated.