The NASA Puff proposal would be more aptly described as a boosted fission engine. From their intro [1] This synergy has been observed in the development of

The NASA Puff proposal would be more aptly described as a boosted fission engine. From their intro [1] This synergy has been observed in the development of other fission-fusion devices. You may be wondering what "other fission-fusion devices" were ever developed. The answer is of course, nuclear bombs. The first fission-fusion device was not a thermonuclear bomb, but a boosted fission one [2]. In such a device, there's a little bit of Deuterium and Tritium in the center of the Plutonium pit. When the Plutonium starts undergoing fission, the temperature increases to tens of millions of degrees, and the D-T fusion reaction happens; the energy produced by that reaction is negligible. What the reaction does is release a huge amount of neutrons that go and split nuclei of Plutonium, therefore boosting the yield of the fission in the bomb. The idea with Nasa's Puff proposal is the fusion is triggered first, using a plasma confinement device, similar to a Tokamak, but linear, not toroidal. The device is well below breakeven. But you don't care about the energy produced by the fusion, only that enough neutrons are released to go on an split a bunch of fissile nuclei. Few people are aware, but not only Uranium and Plutonium (and other heavy nuclei) can undergo fission, but also Lithium. Lithium can't sustain a chain reaction, but if external neutrons (or protons) are provided, it is happy to split and release a respectable amount of energy in the process. Pound per pound about as much energy as Uranium and Plutonium. The end result is a very hot plasma of millions of degrees that is expelled and provides huge thrust and specific impulse. Of course the ejected plasma will be radioactive, and that's why this design can't be used on Earth to produce energy, or other useful things. But in space, blowing some radioactive stuff is not a problem: it will simply spread out and go on and on forever and ever. 99.9999999% of it will not encounter anything for billions of years. The few particles that will, by luck of the draw, encounter something will be no different than other cosmic particles coming from supernovas, or other sources. [1] https://www.nasa.gov/puff/intro https://www.nasa.gov/puff/intro [2] https://en.wikipedia.org/wiki/Boosted_fission_weapon https://en.wikipedia.org/wiki/Boosted_fission_weapon