[ExI] uncle spike's history lesson: clinton laboratories

Anders anders at aleph.se
Sun Sep 11 07:20:30 UTC 2016

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:

> I have a new expression: “a lump of cadmium”.
> 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.
> 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.
(Based on German Nuclear Program Before and During World War II by 
Andrew Wendorff)

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.

Going back to the primary citation, we read:
> 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.
(Jeremy Bernstein, Heisenberg and the critical mass. Am. J. Phys. 70, 
911 (2002); http://dx.doi.org/10.1119/1.1495409)

This reminds me a lot of the modelling errors we discuss in the “Probing 
the improbable” paper https://arxiv.org/abs/0810.5515 , 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.

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, /somebody /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.

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.

(I want to get a lump of cadmium to have in my office.)

Dr Anders Sandberg
Future of Humanity Institute
Oxford Martin School
Oxford University

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