What hinters us in manufacturing “miniature” atomic generators?

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So I always wondered what is the issue that countries/companies do not look on manufacturing “mini” atomic generators?

I know that there exist small atomic generators maybe but they are like the size of a house or at least of a room.

Why not using e.g 10 grams or even 1 gram of uranium or any other suitable element to make really small generators that could fit e.g inside a bike or a small car?

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Making a nuclear reactor for controlled fission is already extremely difficult to do at scale. It would be virtually impossible to miniaturize.

Using the heat off of uncontrolled fission, like the Mars rovers do, is problematic because it relies on having some *really* radioactive material.

It would be a combination of all but impossible and very dangerous.

There are nuclear powered submarines with fairly small reactors.

There are also RTGs that are used in space probes. Slightly different technology to steam boiler type reactors. These don’t produce a lot of power, but last for a very long time.

But to answer your question on stuff for inside a bike/car. Economies of scale. You can shrink the uranium used to 1 gram, but you can’t shrink the pipes, pumps, valves, safety equipment, control systems, electronics etc. to the same amount. It is better to use a big power plant to create electricity first, and then transfer that power into an electric bike or car that has batteries.

Because it doesn’t matter how much of your 10 grams is U235, 10 grams of uranium will never go fissile. It takes about 100 lbs of it to even get critical mass. Plutonium is better, but it’s still about 23 lbs. It’s really not something that can be miniaturized any more than it already is.

And that’s before you put high radioactivity elements in the hands of people without the expertise necessary to handle it, in the case of the bike/car examples.

In all, 0/10, would not recommend

The power you generate depends on the geometry of the reactor elements. Each splitting Uranium atom fires off neutrons and you need those neutrons to fit other Uranium atoms for the reactor to keep running. The neutron direction can’t be controlled, because they have neutral charge, you have to have a Uranium atom that coincidentally happens to be in the right place. As a result, even for solid Uranium, you need a goodly amount – called a critical mass. That’s 52kg for U235 (the kind that’s hard to get). In a giant reactor, this isn’t a problem, but the atoms themselves don’t miniaturize.

You also need to consider that these reactors only generate heat, in a radioactive liquid. You’d need to extract that heat, hopefully without spilling any radioactive waste. That’s going to take pumps and pipes and “not going to fit on a bike” stuff.

Small, molten salt based nuclear reactors have existed, but their development has suffered due to more government interest in large fast breeder reactors. In technology as complex, risky and expensive as nuclear power there is not a lot of diversity possible and the larger reactors got the advantage in funding, as far back as the 1960s. But China and other countries are now developing the small reactors again and we might see them become popular, but AFAIK in the energy game there is more interest on renewables, and rightly so as they are far safer.