Why don't we have nuclear batteries?

Every two years someone post a new nuclear battery that is a tiny radioactive source between two "solar panels". It has like 100mW, so it can only power a watch or something even smaller. For example see https://news.ycombinator.com/item?id=38989690

I guess to scale them to a car size device, the two main problems are security and heat management. (Big objects (like elephants) are difficult to keep cold because they don't have enough surface are for the heat to escape.)

If you’re talking about a small nuclear reactor the problem is that you get a lot of radiation in the form of neutrons which have a certain scale length to stop which is a few meters —- you could make a reactor with a core that fits in the space of a car engine but the radiation shielding will make it shipping container sized.

Apart from the regulatory/safety issues, it's not surprising that if it's hard to miniaturize the system (turbine, condenser if closed loop, control system, etc.) in an economical way.

There do exist smaller nuclear batteries which use another principle for power generation though: https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_ge... They output around 40-100W/kg