Cool diagram of a nuclear reactor cooling system


If you're still trying to get your head wrapped around how nuclear reactors like the ones in Fukushima operate, and what went wrong after the Tohoku earthquake, Mother Jones editorial fellow Joe Kloc might be able to help. He's put together a nifty diagram of the cooling system from a GE Mark 1 nuclear reactor—the kind found in 5 of the 6 reactors at Fukushima Daiichi. By coloring in the pipelines he demonstrates how coolant is supposed to flow through the reactor normally, and how it's supposed to flow during various emergency shutdown situations.

This version of the diagram shows normal operation.

Mark I Reactor Running Normally: Recirculation loops (RED) keep pressurized water circulating through the uranium core of the reactor. When water is heated by the uranium core it turns to steam. It passes through the steam separator and dryer assemblies positioned above the core (ORANGE) and then moves through the steam pipe. The steam is used to turn a turbine connected (PURPLE) to an electrical generator. It is then turned back into liquid by a condenser and cooled by a pipe (GREY) of circulating cold water. The water is then pumped back into the reactor, where the process begins again.

I have attempted to create a diagram of the reactors based on the US Nuclear Regulatory Commission's Boiling Water Reactor Systems Manual, which contains maps of the various Mark I emergency systems. In places where the manual was unclear, I used Japanese news broadcasts. The drawing is not to scale and the layout of the pipes entirely my own (though their location in relation to the various containment walls is based on the USNRC manual). To my knowledge, the diagram is accurate to the extent that a New York City subway map is accurate. It shows the various components, connections, and relationships between the emergency water systems.


  1. It’s all so astonishingly crude.. heat makes steam from water, steam turns turbine, turbine generates electricity… waste heat is vented off as steam is condensed.. So basic, and so inefficient… Amazing that we still haven’t discovered a more efficient way of turning heat into electrickery! And indeed WHY do we have wasted heat, why isn’t it used for something else. (Microgenerators?)

    1. Crude, but it’s actually the way most power is generated: coal, natural gas, oil or nuclear power. Hydro turns turbines without heat but you need nature to make a big waterfall, wind requires hot air to move around a bit and solar requires the clouds to step out of the way for most of the time.

      If you or anyone else can find a way to use that waste heat, there’s a bundle of money waiting for you, I’m sure. A larger pile awaits if there’s a way to use the waste products of nuclear, coal, natural gas or oil…

      1. “A larger pile awaits if there’s a way to use the waste products of nuclear, coal, natural gas or oil…”

        The entire class of substances we call “plastics” may be considered as being “waste” products from oil processing, can they not?

        Well, maybe they were “waste” once. Now they are just inexpensive materials.

  2. This shows beautifully how the reactor vessel is (very typically, BTW) misdesigned.

    Any problem of any sort that causes the fuel to melt results in puddling, which in turn creates critical mass.

    Chambers and containers are invariably shown as being constructed with flat bottoms (to save money, by simplifying construction) or with concave bottoms (to save money, by allowing containers to meet pressure requirements through shape rather than through thickness). This encourages puddling.

    If I make a round chamber, it spreads pressure perfectly, and thus gets a higher pressure containment rating. This is false safety, as the chamber concentrates material by gravity and thus increases the pressure and heat and radiation beyond the rating, particularly at the lowest point.

    If I make a chamber with an expensive, heavily sculpted floor – something that will cause material to separate and flow into separate containment pockets – I can accommodate easily predictable failure modes. If it’s built properly, a meltdown will be a controlled shutdown without external cooling fluids. If you really engineer the hell out of it, you could even accommodate shifts in gravitational alignment (in case the bedrock underneath the plant departs from level).

    But nuclear technology is obsolete from top to bottom. It’s only slightly more refined than Victorian coal boilers, really. The 21st century has less use for terrestrial fission plants than we have for buggy whips… we need to leave fission and petroleum burning behind us and embrace modern sustainable technology. Stop polishing turds.

  3. Smaller generators that utilize waste heat could also provide more reliable back-up power for cooling systems, making both reactors and storage pools safer.

    1. Japan prohibits using the power plant for its own emergency electricity, and I believe the US is similar.

      1. Do you have a source for that? As the events were unfolding I was curious to know why they didn’t do exactly that after they lost external, then diesel, then battery power. I’m having a hard time understanding why it wouldn’t be possible, let alone disallowed in emergencies.

        With respect to the torus with water in it at the bottom of this style reactor, can someone enlighten me as to the utility of that portion of the design?

        1. Whup! Latter portion of my question withdrawn. The linked article hadn’t finished loading so I didn’t see the explanation that its a part of the emergency high and low suppression system to feed water directly onto the core.

      2. I’m not talking about heat from fission. I understand that the main turbines go down when the control rods shut down the reactor, and that power has to be seen as unreliable. My point is that even in those circumstances there’s still a lot of decay heat, and the plant needs active cooling to dissipate that heat in a non-catastrophic manner. Why not utilize some of that waste heat to power the cooling measures? If there’s not enough waste heat to power the heat exchangers, then the heat exchangers presumably aren’t necessary, and we can do without them.

  4. Nice pictures. Now if someone can explain how you can cool the thing by spraying water on it from the outside I would be most happy. (Assuming the wessel hasn’t completely cracked and the spraying is not only for the spent fuel pool.)

  5. Sadly, the Homer Simpson character may be all too real, especially in the Japanese Nuclear Industry. There’s been a whole catalogue of stupid errors over the years…..

  6. I too see a Ghostbusters proton pack. Egon describes it as an “unlicensed nuclear accelerator”. Maybe the prop designer was partially inspired by a diagram like this, trying to imagine what a nuclear reactor you could wear on your back would look like.

  7. The way it’s colored, it appears a robot with four orange legs is bringing me a pitcher of frozen margaritas. Would that all diagrams communicated such blessings.

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