The Mark I Containment System in BWR Reactors

March 16, 2011

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.

* Read “Setting the Record Straight on Mark I Containment”

A BWR reactor: The schematic above shows the torus at left, which is doughnut-shaped.

* Read our most recent update on the nuclear energy situation in Japan

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Willem Jan Oosterkamp

Its the economy stupid, not the engineers.

At the Doodewaard nuclear power station build in 1968, now deconmissioned. we had an emergency condenser. It had for 180 MWth a capacity of about 120 m3. Equivalent to 1 600 s full power or 15 h decay power. The unit could be refilled byfire truckwith a stand pipe outside the reactor building.

Apparently Fukushima I had a simmilar condensor. A mixture of hydrogen and oxygen is formed by the splitting of water due to radiation during normal operation. Even after shutdown this mixture is produced.
This mixture will collect in the emergency condenser and make this condenser inoperable.

The composition of the mixture may well depend on the water chemistry of the reactor (Is hydrogen added?).

As far as I know we did not perform a longterm functioning test of the emergency condensor at the Dodewaard BWR.

I think with todays knowledge a vent line with filter and delay line should have been added.

Once rapid oxidation of zircaloy cladding, grids and channels occurs producing pure hydrogen, then the emergency condenser will no longer work.

Venting hydrogen into the reactor building is not a sound technical solution. I assume that the containment has failed, barring better information on the Fukushima plants.
Henry Morozumi

I have been asking what happened to GE transportation dept offer to send GE turbine
generators to Fukushima. You people never answered question.

Have they ever shipped to Japan? If not what prevented you to send them to Japan?

Henry Morozumi
Retired GE employee
Sean-GE

Henry, I work at GE and here’s the some additional information on the generators that you mention.

An update that we published on April 6 noted that “GE has now made more than 20 gas turbine units available for Japan, including 10 flexible-use TM2500 aero-derivative engines. These will be ready for operation by the beginning of this summer, when demand – and the potential for blackouts — will be the greatest.” The rest of the update can be viewed here: http://www.gereports.com/an-update-on-ge-disaster-relief-efforts-in-japan/

Thanks
Malcolm W. Adams

In light of the Fukushima disaster, what can you tell us about the safety of the Hamaoka Nuclear Power plant in Shizuoka in the event of a similar unprecedented natural disaster, i.e., M9.0 earthquake and tsunami of the same scale as the one in Fukushima? There are increasing calls for it to be shut down by people all over Japan as a preventive measure considering that the plant sits on a major quake fault line that is predicted to experience a major quake in the near future. Is GE concerned about this possibility and if so, what should be done as a precaution to avoid what happen in Fukushima.
Thank you.
BobinPgh

I wonder if any engineer can answer this question I came up with. I understand that in GE reactors there is a steam engine that can drive a pump that can keep the reactor core cool without using any electricity. Did these engines work at fukushima? One reason why not might be that the exhaust from the steam engine, as I saw in another diagram leads into the torus. But if the water in the torus is too hot, then there is no temperture difference and no steam cycle and so the engine stops. Would it be possible to place this exhaust pipe, if not outside, somewhere else? Could it go up the “hardened vent”? There must be someway to make this work, that could be modified for existing GE plants. I think it would also help to move at least some of the spent fuel to dry storage and GE plants should be first in line for this.
donald person

this is now academic. it seems certain that in the event of even a partial meltdown as
indisputably happened at Fukushima Unit i on Mar 12 2011 – that the CRD stub tube penetration
locations rapidly acted a fast conduit for molten fuel to spread w/o any hindrance to the containmnet floor.
It is almost certain that rapid ablation of these penetrations accelerated the escape of core materials.

Lesson: The real flaw in the BWR design was unfortunate assumption that no core melt could ever happen.
Because when it does – bottom entry control starts to look really really BAD.

Dear GE: You can erase this comment, suppress the details of this critical design weakness in the GE/Hitachi BWR reactor but you can’t erase the facts as they will eventually come out elsewhere.
stu minahan

GE: do you have any update on the physical state of the Torus at FD U 2. Those of us operating mark I are concerned that a gross pc failure could dwarf the hardened vent debate. how could a detonation occur in a nitrogen environment? Did they do the modifications that we all did?
Please advise
Stu Minahan VP Operations Entergy
John Orosz

My entire career has been in the oil and power-generating sector. I have a plan for to sealing the breached Daiichi reactor(s) without risking life or limb. My plan is not a quick fix solution; it would be a significant undertaking. I estimate that once the planning, mobilization and site set-up phases are completed it would take approximately 120 to 160 days to permanently seal a unit.

Would GE have any interest in my plan?