Boiling Water Reactor Mark I 140 EU/t Fuel Eff=7/7 (designed by a nuclear energy engineer in real life)

First off, when using the reactor planner, please send us a link to your design (See that "Copy URL" button there?) rather than a screenshot. It's actually easier on both ends- it only takes a few clicks to post a URL on the forum, and only a few more to put it back into the reactor planner.

For the record, here are the links to your designs:
http://www.talonfiremage.pwp.b…ph5wrzw2eq7unk0k1hgan5n9c
http://www.talonfiremage.pwp.b…g3vweyqatwsmlxbpb1q3rwtfk

Second, neutronreflectors are big traps. They're rather expensive, and the only way to replace them when they wear out is by using quite a lot of materials each cycle, and those materials require UU-matter to make, if you want to run your reactor at high efficiency indefinitely without using using expensive and/or unreliable mining methods, bees, or anything else outside the scope of IC2. UU-matter requires a large amount of EU to produce, which seriously hurts your efficiency. I don't know the exact numbers, but the cost of replacing four thick reflectors every cycle is surely a large portion of the energy the reactor actually produces, probably reducing your effective efficiency to 3 or 4.

Third, your reactors are incredibly expensive for reactors that can't match the output of two of these efficiency-3.3 reactors.

But, I'm not saying that your design won't work- it won't explode or anything- I'm just saying that your resources would be better spent on reactors that produce more energy at lower efficiency.
The Official Reactor Plans sticky thread contains a lot of good designs- I'd suggest looking there.

If you want high efficiency AND high EU/t, your only option is the Constantly Reapplied Coolant System (CRCS for short, check the DDoS thread for more information), which requires a fair amount of knowledge about how reactors work and a lot of automation. It's not for those new to IC2.

That design with the quad reflector quad cell is about as old as quad cells themselves are And yes, as Someone Else 37 already said, according to the "Resources Needed" tab of the reactor planner, the actual efficiency of this reactor is less than 3.

...or it would be in classic IC2. Unfortunately the planner is not updated to IC2 Experimental. However, if it was, your efficiency would be even lower because uu-matter costs increased tenfold. Even saving the 40 copper per quad cell doesn't make up for that.

You made the normal beginner mistake of focusing too much on the fuel rod configuration. That's actually the least important part in reactor design for IC2. What you are actually required to design is a cooling system. In this case, you need to build a cooling system capable of venting 448 heat per second while using as little space and resources as possible in the process. And as far as cooling system designs go, yours is quite inefficient. You can easily get that much out of 4 chambers with room to spare and almost 400 copper ingots less than your variant (again only valid for classic IC2). Can probably be made cheaper still with some effort. Unfortunately knowledge of real-life reactors is of no use in this minigame...

If you are looking for a high efficiency setup, it basically boils down to: are you willing to pay for it in copper, tin and coal? If yes, then you better have it lying around, because making it out of uu-matter will destroy your efficiency. But even then, you could be doing something else with that coal (like making diamonds, or generating EU from it), and you could be doing something else with the copper and tin (like building a second reactor).

If you're not willing to accept massive running costs (which is the saner decision in most actual use cases), then this is the efficiency king in classic IC2, running at 4.23 after all costs are paid (copper for the multicells in this case). In IC2 experimental, it is still great for uranium, but ultimately nothing beats MOX reactors which can go up to efficiency 20 on internal cooling alone.

CRCS reactors are a chapter for themselves, but they can reach numbers that internal cooling reactors couldn't dream of.