Recently I got a whole new central heating system. Apparently it's all very efficient (over 90%). The boiler is used to produce hot water that's piped round the house to radiators, and also to provide instantaneous hot water for the taps. However, since it is gas fired, it throws out rather a lot of CO2, which depending on your point of view is either bad or irrelevant. For the sake of this thread, lets assume that it's bad. So I was thinking, a boiler doesn't have to be particularly powerful, I believe mine has a peak power output of 28 KW, so what would be the feesability of using a small amount of radioactive material to heat the water, rather than requiring a constant supply of gas? I understand that such a device would require shielding, however since only a small amount of power is required, only a small amount of fuel would be required, therefore hopefully the shielding wouldn't need to be excessive. There are questions that I don't have answers to though, such as what's the smallest amount of fuel that could be used? If there's too little it simply won't chain react, if there's too much it would be wasteful and possibly dangerous. What the fuel might be is another question, I gather that Plutonium trumps Uranium when it comes to chain reactions, however the government might get a bit antsy about handing out Plutonium. Thallium springs to mind as it is abundant in the crust and fairly useless as a nuclear bomb, though of course any radioactive fuel could find an application as a dirty bomb. Just to be clear I was thinking that such a device would provide energy only in the form of heat, which would simplify it over a whole electrical power plant design. Similar in design to perhaps a nuclear powered aircraft which would in theory just heat the air for thrust as a sort of ramjet without combustion. Also, about the fuel, would it be possible to include enough fuel for the device to run until way past when it is obsolete and needs to be replaced? This would side-step the issue of having the home owner getting their hands dirty/irradiated. If we assume an average power output of 1KW, then that's 3.16 TJ of energy over 100 years (far more than should be needed to ensure it never needs to be refueled). But I'm not done yet, when I get thinking I tend to think in several directions at once, so I also thought, what if it did include an electrical generator? How small could a nuclear powered generator be made? If it's not possible to make a home generator small enough, perhaps it would be possible to build neighbourhood generators which would generate electricity and steam to be delivered to homes. The steam would be mixed with cold water to provide the hot water for central heating and the taps. It seems that huge power plants throw away most of their energy as heat, whereas a home or neighbourhood plant could put this "wasted" energy to good use. I'm curious as to what anyone has to say in terms of problems associated with such systems (other than public opinion) and ways in which they might be solved. For example, nuclear power plants are monitored by engineers with lots of letters after their names, but could a mini reactor be monitored by software to a sufficient extent that it could run autonomously with only routine maintenance?