The way the human body is powered, is it more like electronic engineering or a nuclear reactor?

It’s like a boiler that burns the food for fuel.

We are like a hybrid gas/electric vehicle or we could be described as a fuel cell. We consume raw materials and through chemistry we convert them into energy and molecules needed to maintain homeostasis.

@pleiades It’s a pretty simple formula: Calories in; Calories out and waste by products.

Go a day without fuel (food) and see how much reserve you have left.

I would say that it is like an engine.

C6H12O6 + 6O2 → 6CO2 + 6H2O + energy

That is a combustion reaction. The main difference is that car engines use explosions for physical movement, while cells control the reaction and use it to create other forms of chemical energy.

Chemical engineering.

In some ways, it could be argued that it does resemble a rechargeable battery, though one that needs to be fed periodically. Energy production in the human body is not as simple as “calories in, calories out”, and it’s not as straightforward as a combustion reaction. There are other processes which produce energy in our bodies, such as fermentation and anaerobic respiration.

@PhiotPi Is right on. It is like a conventional internal combustion engine. The energy content is food is very close to the bomb calorimeter data results you’d get if you simply burned it. Fats and oils are converted to energy/work as if you burned them. Each human is slightly more or less efficient at the conversion but in general we are pretty good.
Interestingly, the photosynthesis process in plants is sadly inefficient – in the 1.5 to 2.5% ranges. Man – made solar cells much better. The cheap ones are 6% while the good, expensive ones are pushing 18%.

The first life form that harnesses nuclear energy as its internal power source will take over the planet in no time.

Both.

The human nervous system finds lots of parallels in electrical engineering but not the power plant. As well noted above, we run on combustion.