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In regards to project Orion, using conventional explosives for a radiation free launch and in the range of 0.03 up to 0.35 kilotons.

What is the smallest amount of fissionable material needed to start nuclear fusion of Deuterium + Helium3?

If not able to initiate fusion with conventional explosives i am looking to find out the minimum amount of fissionable material to initiate fusion for a nuclear pulse pellet with the least radioactive fallout possible.

John
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  • This isn't on topic for this site, and isn't really a good format anyways. Including more details would be helpful, and you'd want to ask it at http://physics.stackexchange.com – PearsonArtPhoto Mar 24 '16 at 11:49
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    Simple test: Fusion requires 8-figure temperatures to get a reasonable reaction rate. How do you propose to get 8-figure temperatures from chemical explosives? You simply don't have the energy density you need. – Loren Pechtel Mar 26 '16 at 01:51

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The amount of fusionable material isn't a problem: you can initiate fusion inside a tiny pellet containing milligrams of fusionable material if you compress it enough. But you need a facility the size of the National Ignition Facility to ignite it.
The only place where we have working fusion at a small scale is inside thermonuclear weapons. These use a fission bomb to initiate the fusion reaction. If there were a way to ignite fusion using conventional explosives, I suspect we'd have seen that in bombs by now.

Hobbes
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    We have working fusion on the desktop--just at a rate far too slow to be of use for either power production or spaceship propulsion. – Loren Pechtel Mar 26 '16 at 01:50