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Couldn't we have a lead box lined with these radiation to electricity converters with a small amount of radioactive material in the center, and have an energy generating device that would last for thousands or even millions of years? Imagine putting the sun in a box lined with solar cells, but on a much smaller scale.
Is there a reason this wouldn't work?
I think there a similar concept for providing energy to space missions built around a plutonium isotope.
And why turn it off? Even if the maximum output was only seldomly required, convert the excess energy into a reserve (like green hydrogen) would make sense.