Dr Zoomie! I was watching some time-travel show the other day and there was a plot line that had something to do with taking plutonium from a nuclear weapon back to Germany in WWII and then using it to power the time machine? I’ve got to say it sounds a bit far-fetched – can you even use weapons-grade plutonium in a small nuclear reactor? Oh – and the bad scientist said it would keep it running for 30 years? Is there anything in there that’s maybe a little correct?
Doing some searching I found out that the show you’re watching is called Timeless, streaming on Amazon Prime (as of the date of this post). As you note, it’s a series about time travel and, in the first season, it’s episode 3 (Atomic City) where the bad guys steal a plutonium core from a nuclear weapon. It’s the next episode (Party at Castle Varlar) where the bad scientist (I know – although most scientists are honorable, good, fun, personable, attractive, and able to give a good back rub, there are a few bad ones, the majority of whom seem to find their way into television or movies) turns it into a power supply for the bad guys’ time machine, saying it’ll keep it running for 30 years. So let’s take a look at this, one area at a time, to see how plausible a plot device it is. And not to worry – we’ll be looking at it qualitatively, meaning I won’t get into any much math.
To start, a critical mass of Pu-239 weighs about 10 kg (22 pounds) and will be a sphere about the size of a golf ball (plutonium is almost twice as dense as lead). This seems almost too small to have to worry about, which might be why the plutonium sphere stolen in the show looked to be able the size of a softball. That being said, in an actual weapon the plutonium is in the form of a hollow sphere (sorry – I don’t know the dimensions), so this might not be too far from the actual case.
In a bomb the plutonium gives up its energy all at once in a massive explosion; a 20 kT nuclear explosion (to pick a number out of the air) will release just over 23,000 megawatt-hrs of energy in a tiny fraction of a second; the same 10 kg of Pu-239, if allowed to decay to stability (and if my calculations are correct) it will release about 5500 MW-hrs of energy annually via emission of alpha and other radiations, initially at an hourly rate of a few hundred kW. While this is a bit shy of the 1.21 GW (1210 MW) used by Doc Brown’s time machine in Back to the Future, it’s a fair amount of energy…not to mention that time machines are fictitious and can use whatever amount of energy the plot will allow.
But here’s the thing – we can get that much power without needing to fission a single atom – that’s simply the energy given off by the alpha decay of the plutonium at an initial rate of about 30 kW per hour or about 263 MW-hr annually – enough to provide the energy needs of over 50 people. If the plutonium is used, instead, in a nuclear reactor it’ll produce more energy but it will run out more quickly – possibly in only a few years.
So using a critical mass of Pu-239 as a radioisotopic thermal generator produces less power over centuries or millennia than does using it as a reactor for a handful of years or as an explosive for a fraction of a fraction of a second. And, if I may, it’s a lot easier to control radiation exposures to the crew as an RTG since alpha decay won’t produce any appreciable radiation dose at all. And then there’s also the fact that an RTG has few (or no) moving parts, unlike the cooling, steam production, electrical production, and other systems needed to keep a reactor humming along safely.
And did I mention that there’s much less need for bulky and heavy radiation shielding for a plutonium RTG and elaborate radiation safety precautions than for a reactor, which produces gamma and neutron radiation in copious amounts? There would be some neutron radiation from spontaneous fission that would need to be shielded, but water or plastic is a bit lighter than lead, so the RTG approach still comes in smaller, lighter, and simpler than a reactor. Reactors are more fun, of course! But for the long run, an RTG probably makes more sense, unless the (apparently) bad guys have a long-term supply of plutonium.
The bottom line is that this is a reasonable use of a bomb’s-worth of plutonium, provided you can accumulate enough energy to make a time jump (while keeping the crew alive) in a reasonable amount of time.