Hell-Broth

Today, For a charm of powerful trouble ...  The University of Houston presents this series about the machines that make our civilization run, and the people whose ingenuity created them. 

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     That line by Shakespeare’s witches is followed by “Like a hell-broth boil and bubble.”  Most of us know boiling only from watching pans on our stove.  But here’s the thing: What we see on the stove is less than a twentieth of how intense boiling can actually get.  “Hell-broth” hardly begins to cut it.  And that intensity is something we need to make use of.

Which brings us to nuclear reactors.  Our nuclear powerplants come in two basic flavors.  BWRs or Boiling Water Reactors, and PWRs or Pressurized Water Reactors.  They provide about nineteen percent of America’s power.  Over half of our emission-free power. 

Inside a BWR reactor.  Image courtesy of Wikimedia commons

Now: Consider what goes on inside a reactor.  We trigger a nuclear reaction in a bundle of rods.  They emit vast heat that we must safely put to use. How?  Well, think about a teakettle.  Water carries heat away from the bottom.  The bubbles first sing as they grow and collapse, warming the water.

But once they reach a full rolling boil, the water above quickly finishes heating.  At that point, heat removal has become far more efficient.  Now bubbles are sweeping heat away from the bottom of the kettle.

What boiling looks like, just after a pot stops “singing”.  Photo by JHL

With that in mind, let’s now push the heating up to nuclear reactor levels. We must carry so much heat away from those reactor rods!  How to make them turn water into the steam that drives steam turbines.  Well, we need to boil water at rates that dwarf any kitchen kettle.

Two examples of intense boiling at very high heat transfer rates.  Photos by JHL

How much heat do you suppose we’re talking about here: First, our tea kettle: There we might reach an hourly rate of around a hundred kilowatts per square meter. But we need an hourly rate more than a thousand kilowatts per square meter in our reactor.  A hell-broth indeed!  No longer just bubbles leaving the surface.  Now it’s intense jets of steam.  It’s like comparing a candle to a blow torch.

Yet another wrinkle here: It might seem odd that we should undertake such Herculean cooling with plain old water.  In fact, we’ve considered other liquids.  Engineers have actually built working reactors that are cooled by the liquid metal, Sodium.  But they remain uncommon.   

I’m sure you’ve heard the old engineering mantra, KISS – or K I S S, for “Keep It Simple, Stupid”.  Sure, that saying does have it limits.  So much machinery has to be complicated.  But still: Water is our friend.  It is the dominant component of our biosphere.  Moving constantly between solid, liquid, and gas. 

So remember, when you turn on your lights this evening: A portion of that light is there just because our friend, water, (thank you Shakespeare) can “Like a hell-broth boil and bubble.

I’m John Lienhard, at the University of Houston, where we’re interested in the way inventive minds work.  

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For an extensive treatment of boiling, see: J. H. Lienhard V and J. H. Lienhard IV. A Heat Transfer Textbook. Ch.9: Heat transfer in boiling and other phase-change configurations. Available online here

Nuclear power in the United States - Wikipedia

Boiling water reactor - Wikipedia

Pressurized water reactor - Wikipedia

Sodium-cooled fast reactor - Wikipedia

This Episode first aired on August 10, 2026.