a story lives forever
Sign in
Form submission failed!

Stay signed in

Recover your password?
Form submission failed!

Web of Stories Ltd would like to keep you informed about our products and services.

Please tick here if you would like us to keep you informed about our products and services.

I have read and accepted the Terms & Conditions.

Please note: Your email and any private information provided at registration will not be passed on to other individuals or organisations without your specific approval.

Video URL

You must be registered to use this feature. Sign in or register.


Edward Teller: Like a spoilt brat


How the warm neutron principle works
Freeman Dyson Scientist
Comments (0) Please sign in or register to add comments

In the normal course of events, you depend on neutrons being absorbed in the fuel of a reactor in order to produce fission, but a lot of the neutrons then get absorbed outside, just... what they call 'parasitic absorption', or are absorbed in the stuff that's moderating the reaction, or in the coolant, and in the case of a light water reactor a lot of the neutrons are absorbed in the water that's cooling it. And... so it's always a balance between the neutrons that are absorbed in the water and the neutrons that are absorbed in the uranium. In a normal reactor which is cooled with light water, if the thing goes supercritical so that the fuel gets extremely hot, the thing can actually blow up because the neutrons that come into the fuel are still cold, they've been exposed to the water, so if the thing is blowing up in a fraction of a second, the water is still cold, the fuel is hot, but that doesn't affect the temperature of the neutrons. So the neutrons are still cold, that means the fission cross sections are still high and the thing continues to be supercritical and it blows itself up. Well the warm neutron principle says if you could put the moderator in mixed with the fuel, then you'd be in much better shape. What happens then is that the... moderator, which in this case is zirconium hydride – we chose as the stuff to moderate the neutrons – is heated up as fast as the uranium. The uranium and the zirconium hydride are mixed in the solid fuel elements so that as fuel heats up, the hydrogen is heated up within a microsecond, and so the neutrons that come into the hydrogen get slowed down and the fuel elements are heated up too. And once the neutrons are warmed up, then the fission cross section goes down, and then the water which is also necessary remains cold and so the neutron cross section in the water remains high. So it means the neutrons which have a lower cross section in the fuel and still the high cross section in the water, will stream out of the fuel elements into the water, and the fuel elements will immediately stop being super-critical. And... so it's a simple thing, but it's a very lovely thing and this was actually demonstrated; after we had invented the reactor we actually built it and Simnad... Massoud Simnad, who is an Iranian chemist who built the actual... did the chemistry of the fuel elements and actually made the fuel elements, was an employee at General Atomic, so he actually did the hard work after we had left, but within two years we actually had a working reactor and we demonstrated it at a public... ceremony in San Diego.

[Q] And the energy output is roughly...?

Well, Niels Bohr actually pushed the button to start the thing at this public demonstration, and what it did was to pull out the fuel elements explosively with compressed air, so that the thing became highly supercritical; this was all done in a public situation with a big crowd of people sitting there, and the reactor was exposed to the public view. And... it went up to 1000MWs within a millisecond, which is equal to the power output of a huge power station, and it shut itself down within 2 milliseconds, without any moving parts. I mean, that's the beauty of it, that the shut down is caused just by the laws of physics, not by any kind of engineering, so that it's totally automatic; after you've thrown out all the controls the things goes highly supercritical and it's exploding with a multiplication time of a millisecond. But this warm neutron effect actually just quenches the reaction within two milliseconds and so then the thing sits down and bubbles away at about 1MW, which is its design power. So it's supposed to operate at 1MW; it was actually operating a million times that power level for a millisecond, but it didn't do itself any harm and it didn't hurt anybody in the crowd. So it was a triumphant success, and... so it was great fun to do. It was a case again of an elegant idea actually working and... so since then the reactor has actually been used a great deal, in hospitals for producing short-lived isotopes, and the company is still in business and they're still making the reactor and they're still selling it. It's one of the few reactors which actually has made... made a profit for the company that builds it.

[Q] And this was your first hands-on experience in an engineering project?

Yes. In fact, really, the only one. I mean, I've never done anything like that since.

Freeman Dyson (1923-2020), who was born in England, moved to Cornell University after graduating from Cambridge University with a BA in Mathematics. He subsequently became a professor and worked on nuclear reactors, solid state physics, ferromagnetism, astrophysics and biology. He published several books and, among other honours, was awarded the Heineman Prize and the Royal Society's Hughes Medal.

Listeners: Sam Schweber

Silvan Sam Schweber is the Koret Professor of the History of Ideas and Professor of Physics at Brandeis University, and a Faculty Associate in the Department of the History of Science at Harvard University. He is the author of a history of the development of quantum electro mechanics, "QED and the men who made it", and has recently completed a biography of Hans Bethe and the history of nuclear weapons development, "In the Shadow of the Bomb: Oppenheimer, Bethe, and the Moral Responsibility of the Scientist" (Princeton University Press, 2000).

Tags: General Atomics, TRIGA, Massoud Simnad, Niels Bohr

Duration: 5 minutes, 32 seconds

Date story recorded: June 1998

Date story went live: 24 January 2008