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decay-heat.html
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---
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
---
<div class="row">
<div class="col-md-8">
<p>Why can’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’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’s below 1% within a day and continues to drop.
The problem is, in a 3 large 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’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 "Interpreted" right next to (n,fis.ene.release).</li>
</ol>
<h1>References</h1>
<ol>
<li>Tobias, A. "Decay heat." <a href="http://www.sciencedirect.com/science/article/pii/0149197080900025">Progress in Nuclear Energy 5.1 (1980): 1-93.</a></li>
</ol>
</div>
</div>