Made by SuperChicken17 From: https://www.reddit.com/r/factorio/comments/1ga8dvh/losses_nuclear_testing_in_20_with_blueprint/
I did some testing in creative mode with the new ability to read temperature and fuel out of reactors. It is pretty easy to only use heat as storage now, forgoing steam tanks.
I used a fairly standard 4 reactor setup as a test bench. Loading at lower heat means that you need less heat storage in order to losslessly store a set fuel cells. I found that putting the refuel temperature much lower than 600 degrees would result in short drops in power output during high utilization, which is no good. 600 degrees was perfectly stable at full utilization though, and gives a pretty good headroom for extra heat.
To try and make it fully lossless even in the most unrealistic circumstances, I tested completely removing the power load as soon as the reactors refuel at 600 degrees. I then would watch the heat climb. Any heat over 1000 degrees means that energy is being lost. I found I had to add a few non-necessary heatpipes as storage. With the current setup, the reactor maxes out at about 970 degrees in the 'load fuel, then remove all power draw' scenario. No steam tanks required.
I made the plant usable without logistics robots, as I find sometimes I am desperate for extra power right now, before I have robots online. Fuel cells go in on one belt spent cells come out on the other. Water comes in from the side opposite the input belts.
Manually priming the reactor isn't necessary. So long as there is fuel on the belt and the inserters have power, it will start itself up with a synched insertion.