I personally would love for more devices in an open design rather than the GUI one Alb has for the Nuclear Reactor now (meaning separate the units so that the players can decide how they want to build their engines rather than just the structures around it, making it look like its function). The problem is that Alb said he is "not adding any new blocks" to IC2, and this may require one (or more?) new ID for reactor components (such as the turbine, pressure tank, etc. below). The way I picture it is as follows:
Air Pipe
Allows for pressurized gas (for now, refrigerant and steam are the only gas I'll talk about, but more could be added). Along with the gas type flowing through the pipe, you'd need 3 other values to move in code: liquid, gas, and heat. 'Liquid' is how much of the 'gas type' is in the pipe at that location that is in liquid form, and 'gas' is how much is in its gas form. As the pipe 'heat' goes up, its liquid turns to gas if there is room, causing pressure...or if it goes down, it turns gas into liquid if able. The formulas for the values "move" across the pipes similar to liquid in waterproof pipes. The values themselves are limited:
'liquid'
from 0 to 100
'gas'
from 0 to 800
'heat'
from 0 to 1350
will increase slowly if near lava/fire/engine/etc (reducing those things in heat, or turning lava into obsidian over time)
will decrease slowly if near ice/water/cooler/etc (increasing those things in heat, or destroying source blocks)
if above boiling point of liquid type, 'liquid' -1, 'gas' +2, and 'heat' -1 (so that it doesn't boil all at once, requires steady heat over time)
if below condensation point of liquid type, 'liquid' +1, 'gas' -2, and 'heat' +1
if below freezing point of liquid type AND 'liquid' is more than 49, pipe turns into "Broken Air Pipe"
if above 1350, pipe melts (connected pipes become "Broken Air Pipe" and this pipe is destroyed, expelling its ingredients upon deletion)
if below 270, nearby water will start to freeze
'pressure'
not a value, but a calculation, formula is 'gas' / ( 100 - 'liquid' )
a value of 1 represents atmospheric pressure at sea level/room temp
a value above 8 turns the pipe into a "Broken Air Pipe" and harms nearby players
Boiling points
water 275 + 100 * Pressure
refrigerant 200 + 50 * Pressure
Condensation points
water 270 + 100 * Pressure
refrigerant 195 + 50 * Pressure
Freezing points
water 270
refrigerant 170
Broken Air Pipe
All liquids entering this part of the pipe is attempted to be expelled into the nearby area, 'gas' and 'liquid' values are not transferred to other pipes, but other pipe values are transferred to it ('heat' value is still transferred across pipes).
Safety Valve
If the pressure reaches above 7, 'gas' is reduced by 1
Expansion Valve
Allows 'liquid' and 'heat' values to move across this pipe (one-way), but 'gas' is blocked. In an AC unit setup, when the liquid moves into the new area, it will quickly expand, causing the pressure to be decreased and the boiling point to lower, which results in boiling and the lowering of the temperature of the pipe.
Compressor
Requires power, allows 'gas' and 'heat' to move across this pipe (one-way), but 'liquid' is blocked. This engine will also create 'heat' while running on the output side (+1 per tick?)
Inline Pump
Requires power, moves 'gas', 'heat' and 'liquid' across this pipe (one-way). This engine will also create 'heat' while running on the output side (+1 per tick?)
Radiator
'heat' in the pipe is reduced by 10 if near air, 50 if near water, or 150 near ice (but ice melts). 'heat' will not reduce below 300
Bleeder Pipe
Attempts to move 'liquid' to only 1 of the output faces (only going to the other faces if unable), but 'gas' is allowed to traverse along the other directions (and will not go to the liquid side). 'heat' will move as normal to all faces.
Turbine
Moves 'gas', 'heat' and 'liquid' across this pipe (one-way). Each 2 'gas' that is moved, 1 is turned into 2 'liquid' and 1 'heat' is removed and power is made (any 'liquid' that is moved reduces the power output, so best to have a bleeder pipe).
Air Tank
Allows 'gas', 'heat' and 'liquid to move in and out, but is limitted to 10 'gas' in/out and 5 'liquid' in/out at a time. Connections at the top will attempt to move 'gas' out first, while 'liquid' is attempted at the bottom. 'heat' is reduced by 1 per tick.
With these things, you can now make nuclear and boiler generators possible, by just adding 'heat' to the pipe system. Above values are just an example situation, easily changed/configured for balancing. An actual powerhouse generator could be designed and crafted to not only look awesome, but make the players have a sense of accomplishment.
Example setup would be a loop containing: Air Tank (bottom to) Inline Pump, Heater, Bleeder Pipe (water back to air tank), Turbine, and Radiator (back to air tank). A better setup would involve a line of Bleeder Pipes after Turbines repeated until there was no more 'gas' and then feed back to the tank. Why refrigeration? It can be used to quickly cool an overheating system, create lakes of ice, or a 'Stirling Engine' can be created that creates power based on the difference of heat between 2 pipes (the 'heat' values are averaged out for the pipes). Just a suggestion anyway.
EDIT: This probably is better as a BC addition, but I sometimes get carried away with my ideas and mods that I use tend to blur together...oh well.