Since it's proably required for proper testing and balance discussion, here a sum up of the reactor system:
The reactor does not have any sort of own/external cooling anymore.
It still contains (3+chambers)*9 inventory slots, as usual.
During a reactor tick, it will straightforward iterate through the slots (from top left to bottom right), calling a "updateTick" method on every item (if the Items are implementing IReactorComponent). EVERY IC² core compenent is coded by using that interface, thus it's quite mighty and sufficient for Addon Developers.
For reasons you will grasp upon reading this, Nuclear Output was HALVED (5EU/t / pulse) to compensate for the much more powerful and efficient reactors you can now build.
Uranium
There are 3 tiers of Uranium Cells: Single, Dual, Quad.
Single tells are your casual Uranium Cells, pulsing once + once for every pulseable object surrounding them. The heat generated by a cell is calculate by SSUM(pulses)*4.
SSUM :=
SSUM(1) = 1
SSUM(2) = 3
SSUM(3) = 6
SSUM(4) = 10
etc
Heat will be distributed to all surrounding components which accept heat (by default all which have a damage bar). If there is none, heat will be applied to reactor hull.
Dual cells are effectively 2 cells in the same slot (whole calculation will be doubled) and as well get +1 additional pulse for themselves.
Quad cells are 4x cells, which gain +2 additional pulses each, permitting efficiency 7.
Coolant Cells
are heat storages.
10k, 30, 60k.
They do not dissipate heat, don't cool and don't perform heat management on their own.
Plating
There are three kinds of plating.
The 'Integrated Plating' grants +1000 maxHeat to the Reactor, as well as reducing explosion range AND the chance to trigger heat-based effects (melting rock, setting stuff on fire) to 0.95 (multiplicative).
There are two modified Plating versions. The Heat Plating grants +2000 maxHeat, but only 0.99 modifier, whilst the Explosive Plating grants +500 maxHeat and 0.9 modifier.
Platings DO NOT take or redistribute any sort of heat and accordingly have no damage bar and are, as a result, stackable. Stacking them inside of a reactor does not have any effect.
HeatSwitch
HeatSwitches aka Heat Distributors are the new HD's of the old system, coming in various versions.
The standard Heat Switch can store 2500 heat, has a sideTransfer rate of 12 and a coreTransfer rate of 4.
All HeatSwitches work the same way: They calculate the % of heat stored in all surrounding tiles, themselves and the reactor hull, calculate a median and then attempt to reach that median on all components. A heatSwitch will first shift around (component <-> switch) the heat of adjacent components, to a max of sideTransfer. Then he will try to balance the heat between itself and the reactor to a max of coreTransfer.
The 'Core Heat Switch' does have a sideTransfer rate of 0 (thus no heat balance between adjacent components), but a coreTransfer rate of 72, and a maxHeat of 5000.
The 'Spread Heat Switch' does not have a coreTrasnfer, but instead 36 sideTransfer, and a maxHeat of 5000.
Lastly, the Diamon Heat Switch has a sideTransfer of 24 and a coreTransfer of 8, and a maxHeat of 10000.
Opposed to the old HD's, the switches do NOT dissipate heat, have a LOW heat storage and do go by %, not my static values. F.e. you have a core heat switch (5000 max) and a reactor with some plating (20000 max). The system has a total of 5000 heat. The switch will balance 1000 heat to itself and 4000 to the reactor, resulting in 20% heat for itself and the reactor.
Vents
As you probably notice, there were so far no components providing any sort of cooling, at all. That's what Vent's are for.
Vents have a maxHeat of 1000 and a selfCooling and a reactorTransfer rate.
A vent will always first draw heat from the reactor in height of it's reactorTransfer rate, regardless of it's own heat level. They do not 'balance' as heatSwitches do. Second, they will reduce their own heat by the selfCooling rate, venting the heat into the air = Mystically gone.
Basic Vents do merely have a selfVent of 6.
CoreVents have 5 selfVent and 5 reactorTransfer (effectively applying continous -5Heat/tick to the reactor hull.
Golden CoreVents are tricky to use. They provide an amazing 20 selfCooling, but have 36 reactorTransfer. Effectively, this means they will always melt themselves if the reactor has enough heat avaible. It's up to you to figure out how to use them properly.
Diamond Vents have 12 selfCooling, but 0 reactorTrasnfer again.
There is one special, the SpreadVent. It can NOT take up any heat. However, it automatically cools down all adjacent components by 4 per tick.
Breeding
Breeding didn't change much from how it was before. Uranium cells have a 25% chance of generating depleted uranium (or you can create 8 depleted uranium from one ingot and 8 cells). Mixed with coaldust, this turns into an isotope cells. Isotope cells require 10000 pulses from uranium cels to become enriched uranium. Enriched uranium can be used for crafting of nukes or for new uranium cells.
However, there was one important change: Reactor heat does not add a x2 bonus for each, but merely a +1. Thus an isotope will need 10000 ticks on heat 0-2999, 5000 for 3000-5999, 3333.3 for 6000-8999, etc. The reasoning behin dthis: With heat plating, you can easyly create Reactors well past the 30k.
Heat Packs
However, to reach a constant 30k setup, you would need ridicolous amounts of lava in the old system. That's why you now can craft HeatPacks/HeatCells. These things are sort of cheap and stackable. Placing them inside of a reactor will cause them to heat up all surrounding components by 'stackSize' (=the amount of heatcells).
They will keep doing that, until the components heat level reached stackSize*1000. This way you can easyly configure your reactor to remain on a specific heat level.
Be advised you should use coolant cells next to the heatpacks, as heating f.e. vents to 30k doesnt really work.
Reflectors
Good efficiency is something hard to obtain, usually requiring many uranium cells and thus creating unstable setups.
Neutron Reflectors solve this problem. An uranium cells nearby will consider Reflectors similar to Uranium cells and send out additional pulses. Whilst this increases the Uranium Cells heat output, the Neutron Reflector itself will of course not produce additionally heat (opposed to a second Uranium Cell).
Neutron Reflectors have a limited life length of 10000 ticks. You can, however, craft a 'Thick Neutron Reflector' with a lifetime of 40k ticks. Be advised: Neutron Reflectoers surrounded by multiple cells will diminish faster (2 cells adjacent to the same reflector will deplete it in half of a cell cycle).
Surrounding a single uranium cell with 4 Reflectors will grant it efficiency class 5.
Condensators
Condensators are a special tool to reduce Reactor heat. They come as Redstone and as LapisLazuli versions (latter one being an upgrade of former).
Condensators will accept any amounts of heat from surrounding components (though they don't balance heat around themseles), and INSTANTLY disperse the heat by using their fuel.
A Redstone Condensator can absorb 20k heat, refilling it (crafting) with redstone will restore 10k of it's capcity.
LapisLazuli Condensators can absorb 100k heat, redstone refills 5k and Lapis Lazuli 40k.
And now go forth and create even more industrial-hayoish reactors!
PS: For testing purpouses, the Nuclear Reactor'S tickrate is increased to 20 ticks per second (opposed to 1).
This way you can test setups and balance. Be advised this effectively 1/20th's the energy value of used Uranium!
(as usually, the reactor emits EU every tick, whilst only updating the setup every 20th tick)