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<p>The US had way better nuclear safety than other countries, in my
view partially because of the interest in stuff that goes boom.
Here is a little story from the other side, that started with this
email from Stuart Armstrong:<br>
<br>
<blockquote type="cite">I have a new expression: “a lump of
cadmium”.<br>
<br>
Background: in WW2, Heisenberg was working on the German atomic
reactor project (was he bad? see the fascinating play
“Copenhagen” to find out!). His team almost finished a nuclear
reactor. He thought that a reaction with natural uranium would
be self-limiting (spoiler: it wouldn’t), so had no cadmium
control rods or other means of stopping a chain reaction.<br>
<br>
But, no worries: his team has “a lump of cadmium” that they
could toss into the reactor if things got out of hand. So, now,
if someone has a level of precaution woefully inadequate to the
risk at hand, I will call it a lump of cadmium.</blockquote>
(Based on German Nuclear Program Before and During World War II by
Andrew Wendorff)<br>
<br>
It reminds me of the story that SCRAM (emergency nuclear reactor
shutdowns) stands for “Safety Control Rod Axe Man“, a guy standing
next to the rope suspending the control rods with an axe, ready to
cut it. It has been argued it was liquid cadmium solution instead.
Still, in the US project they did not assume the reaction was self
stabilizing.<br>
<br>
Going back to the primary citation, we read:<br>
<blockquote type="cite"><br>
To understand it we must say something about Heisenberg’s
concept of reactor design. He persuaded himself that a reactor
designed with natural uranium and, say, a heavy water moderator
would be self-stabilizing and could not run away. He noted that
U(238) has absorption resonances in the 1-eV region, which means
that a neutron with this kind of energy has a good chance of
being absorbed and thus removed from the chain reaction. This is
one of the challenges in reactor design—slowing the neutrons
with the moderator without losing them all to absorption.
Conversely, if the reactor begins to run away (become
supercritical) , these resonances would broaden and neutrons
would be more readily absorbed. Moreover, the expanding material
would lengthen the mean free paths by decreasing the density and
this expansion would also stop the chain reaction. In short, we
might experience a nasty chemical explosion but not a nuclear
holocaust. Whether Heisenberg realized the consequences of such
a chemical explosion is not clear. In any event, no safety
elements like cadmium rods were built into Heisenberg’s
reactors. At best, a lump of cadmium was kepton hand in case
things threatened to get out of control. He also never
considered delayed neutrons, which, as we know, play an
essential role in reactor safety. Because none of Heisenberg’s
reactors went critical, this dubious strategy was never put to
the test.<br>
</blockquote>
(Jeremy Bernstein, Heisenberg and the critical mass. Am. J. Phys.
70, 911 (2002); <a class="moz-txt-link-freetext" href="http://dx.doi.org/10.1119/1.1495409">http://dx.doi.org/10.1119/1.1495409</a>)<br>
</p>
<p>This reminds me a lot of the modelling errors we discuss in the
“Probing the improbable” paper <a class="moz-txt-link-freetext" href="https://arxiv.org/abs/0810.5515">https://arxiv.org/abs/0810.5515</a> ,
especially of course the (ahem) energetic error giving Castle
Bravo 15 megatons of yield instead of the predicted 4-8 megatons.
Leaving out Li(7) from the calculations turned out to leave out
the major contributor of energy.<br>
<br>
Note that Heisenberg did have an argument for his safety, in fact
two independent ones! The problem might have been that he was
thinking in terms of mostly U(238) and then getting any kind of
chain reaction going would be hard, so he was biased against the
model of explosive chain reactions (but as the Bernstein paper
notes, <i>somebody </i>in the project had correct calculations
for explosive critical masses). Both arguments were flawed when
dealing with reactors enriched in U(235). Coming at nuclear power
from the perspective of nuclear explosions on the other hand makes
it natural to consider how to keep things from blowing up.<br>
<br>
We may hence end up with lumps of cadmium because we approach a
risk from the wrong perspective. The antidote should always be to
consider the risks from multiple angles, ideally a few adversarial
ones. The more energy, speed or transformative power we expect
something to produce, the more we should scrutinize existing
safeguards for them being lumps of cadmium. If we think our
project does not have that kind of power, we should both question
why we are even doing it, and whether it might actually have some
hidden critical mass.<br>
</p>
<br>
(I want to get a lump of cadmium to have in my office.)<br>
<br>
<pre class="moz-signature" cols="72">--
Dr Anders Sandberg
Future of Humanity Institute
Oxford Martin School
Oxford University</pre>
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