decay-heat.html

---
layout: default
title: What is afterglow/decay heat?
subtitle: Why you can't just turn off nuclear reactors
category: details
description: To understand nuclear safety one must first understand decay heat.
author: nick
image: /img/decay_heat_power.png
byline: true
date: 2011-10-20
last_modified_at: 2023-11-28
---

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    <p>
      Why can&rsquo;t we just turn off nuclear reactors if something goes wrong?
      Well, we can stop the chain reaction very quickly by putting in the
      control rods. But, there&rsquo;s this extra energy called
      <em>afterglow heat</em> or <em>decay heat</em> that is released even with
      the control rods in, and we have to carry this heat away or else the
      reactor will get too hot and compromise the containment systems, possibly
      releasing radioactive particles into the environment. Decay heat removal
      is the key factor in many nuclear accident scenarios, and so reactor
      designers and operators work hard to ensure cooling is robust.
    </p>

    <figure>
      <img
        class="img img-fluid w-100"
        src="/img/decay_heat_power_opt.svg"
        alt="The
relative power during a reactor shutdown showing decay heat"
        title="The relative
power during a reactor shutdown showing decay heat"
      />
      <figcaption>
        <p>
          <strong>Figure 1. </strong>Relative power of a nuclear reactor before
          and after shutdown. The decay heat decreases over a long time and must
          be cooled.
        </p>
      </figcaption>
    </figure>

    <p>
      When large atoms like Uranium or Plutonium fission, the vast majority of
      the energy released comes from the two smaller atoms (called fission
      products) flying out at high speeds. Once these fission products slow
      down, their nuclei often remain in high-energy states, which slowly
      undergo
      <a
        href="{% link
radioactivity.md %}"
        >radioactive</a
      >
      decay, producing a little bit more heat. When the control rods enter a
      nuclear core to shut down the chain reaction, all fissions stop and the
      fission products stop flying around, but the fission products remain
      radioactive and will produce heat no matter what.
    </p>

    <p>
      As physics would have it, the decay heat power level is usually about 6-7%
      of the full power of the reactor immediately after a shutdown. Then it
      decays exponentially such that it&rsquo;s below 1% within a day and
      continues to drop. The problem is, in a large 3 GWt plant, 1% of full
      power is still 30 million Watts, and that requires a lot of cooling.
    </p>

    <h2>The Safety Issue</h2>
    <p>
      Since there&rsquo;s effectively no way to immediately shut a nuclear
      reactor all the way down, the cooling systems must operate in some fashion
      after a shutdown or else the fuel will heat up above its melting point
      and...melt, possibly releasing radioactive nuclides into the environment.
      This is what happened at
      <a href="{% link fukushima.html %}">Fukushima</a>. Emergency diesel
      generators as well as some passive systems are typically relied upon to
      provide this cooling. In some advanced designs such as sodium-cooled
      <a href="{% link fast-reactor.md %}">fast reactors</a>, the large vat of
      low-pressure liquid metal allows natural circulation to provide all the
      decay heat removal without any generators or pumps.
      <a href="{% link msr.md %}">Molten salt reactors</a> have fluid fuel that
      can be drained into a passively-cooled tank that helps ensure cooling.
      Advanced light water reactors have large pools of water high up where
      gravity can provide cooling for a long time if the generators fail.
    </p>

    <p>
      In risk assessments, loss of decay heat removal accidents are usually the
      highest-risk scenario to release radiation to the public. If someone could
      discover a way that nuclear fission would result in stable fission
      products instead of radioactive ones, safety and cost issues of nuclear
      energy would disappear. Unfortunately, this is likely impossible.
    </p>

    <a id="references"></a>
    <h1>See Also</h1>
    <ol>
      <li><a href="{% link radioactivity.md %}">What is radioactivity?</a></li>
      <li><a href="{% link fukushima.html %}">The Fukushima accident</a></li>
      <li>
        <a
          href="http://www.nndc.bnl.gov/sigma/index.jsp?as=235&lib=endfb7.0&nsub=10"
          >National Nuclear Data Center</a
        >
        -- Find out how much energy comes out in which form. Click a fissionable
        element like U or Pu. Then click a good isotope like U-235 or Pu-239.
        Then click the little link that says &quot;Interpreted&quot; right next
        to (n,fis.ene.release).
      </li>
    </ol>

    <h1>References</h1>
    <ol>
      <li>
        Tobias, A. &quot;Decay heat.&quot;
        <a
          href="http://www.sciencedirect.com/science/article/pii/0149197080900025"
          >Progress in Nuclear Energy 5.1 (1980): 1-93.</a
        >
      </li>
    </ol>
  </div>
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