Effective reactor design

I will give it a go.

First I feel I should explain a little about general reactor design and terminology etc. I will use Uranium 235 as the example.

U235 is fissile (meaning that an atom of it will very likely undergo fission when the nucleaus is struck by a highly energetic neutron, or otherwise if we wait long enough it might or might not spontaneously fission) when the U235 atom fissions, a number of things are created. A number of neutrons that were in the nucleus are spat out at nearly the speed of light(C). The remnants of the split atom are the fission products - these are different elements depending on their new atomic number, these likewise fly off at a relative paltry fraction of C (because they are much heavier than lone neutrons) now if you add all the masses of the fission products and free neutrons together you don't quite get the mass you started with, because some of the mass (neutrons & protons)has changed into energy in the form of photons (they have incredibly high energy we call these gamma rays - a bit like sunburn times a million)and of course kinetic which in the subatomic scale is the effect we percieve as heat (the molecules in hot things are vibrating, the faster the vibration the hotter it is) and of course this vibrating kinetic energy we call heat is what we are going to use, because we have a great many exceedingly clever ways to change heat into other forms of energy to do our work for us

Criticality, I will define as the threshold where a density of U235 atoms in a finite volume hits a point where if we cause an atom of it to fission by say firing a neutron at it, then the likelihood that the neutrons created would generate another fission which would generate another, then another etc becomes 100%. This would be a self sustaining chain nuclear reaction - at this point to increase power we just fire a few extra neutrons in until there are a large enough of self sustaining fissions happening to heat the fuel up enough to drive our energy transfer system.

Moderator - in a reactor this is a material that has a high likelihood of interacting with the fast neutrons (when they are created they are fast by default - nearly lightspeed). Fast neutrons are not at all very likely to hit a nucleus and cause another fission - but interactions (which could be thought of as sort of deflections) with other atoms take away some of their speed. When they are slowed, their likelihood of striking a U235 nucleus increases. So this is the function of the moderator. It slows down the neutrons to increase the chances of more fissions. So perhaps now we need less density of fuel to sustain a 1>1 reaction.

The coolant, well this stops our machine from melting, exploding and catching fire. It also provides the primary way to move the heat to where we are going to transform it into electricity. Here's the trick though, the coolant can also be a moderator (semi/heavy water)or it could be a neutron absorber (sodium) to a great range of values depending on the particular element/compound chosen. This is why your choice of coolant will affect the reactivity bandwidth.


1. Depends on usage, but as you've already proven, higher temperatures mean more efficient radiators, and radiators are going to weigh more than the reactor (providing you design a reasonably compact reactor) Personally for high output 500MW+ designs I would use an outlet temp of >2000K otherwise your ship is going to be more radiator than ship. For lower output designs this may trade off more favourably - but ultimately it is a trade off. High temp radiators also have the mandatory glow like a giant fusion reactor feature, like it or not.

Genrally, outlet temp should be high. A 50MWe reactor with a 1200K outlet and an efficiency of maybe 30-40% means ~70MWt to radiate off, with an outlet fof 2200K and efficiency of 10% there would be ~450MWt which seems a lot more - the trick is that the hotter radiators discard much more heat (MWt) much quicker - so that 70MWt @1200k actually needs bigger radiators than the 450MWt @2200k.

2. The primary coolant has an effect on the reactivity of the core because it is essentially in between the core elements right in the middle of the reaction. Every chemical element or compound has particular qualities when it comes to interactions with radiation. Absorbtion cross section, deflection likelihood etc.. all particular to each element in the chemical you choose. Sodium has a reasonably large absorbtion cross sections, meaning it will slightly decrease the reactivity of the core when used as primary coolant. It has other qualities however that do make it a very solid choice.

3. Generally you are correct, though different shapes have slightly different characteristics. Too far from a sphere (ie diameter much greater than length or vica versa) and the core elements will get too spread out (overall density of fuel atoms will be lower) and reactivity will drop. a slight difference between diameter and height seems to help with the external radiation output.

4. Moderators explained above, depending on the coolant or by design you can omit this completely (you can make a 'fast reactor' which is one that uses no moderator and relies almost entirely on the relatively lower likelihood of fast neutrons causing a fission to maintain it's reaction). control rods should contain strong neutron poiosons with a very large absorption cross section so they can take neutrons out of the sustaining reaction and slow it down. Boron, boron and boron. Choose the compound to tolerate the temperature you want to run at. This will very likey be Boron Nitride, or Hafnium Diboride for very high temps. Inrease the amount until the reactivity bandwidth becomes a line instead of a point, and increase until the lower side is at least as far from the critical point as the higher side. (This is a personal recomendation - a reactor that has a reactivity bandwidth of just below critical to a significant distance beyond it will be able to inrease power very quickly, but even when all control rods are scrammed into the core it can only decrease power from whatever level it is at very very slowly, you can see this would not be an ideal feature)

5. Fuel mass and enrichment. This game treats the non fissile part of the fuel as filler. 100Kg of U235 @ 9.7% enrichment generates exactly the same amount of heat as 10Kg at 97%. For weight saving there is no reason I know of not to use max enriched fuel always.

And the radiation flux. I guess you could think of this as your primer, remember we need to fire some neutrons in to start the reaction off, unless we wait for a spontaneous fission - and spontaneous fissions are never going to get enough 1>1 fissions happening together to generate the heat we need. So the flux sets the number, with with our primer in place and the reactor brought to full power, the number of free neurons zipping around in the reactor core is a number. A rather large number involving E because we don't want to write a whole line of zeros. The larger this number, the more neutrons, the more reactions, and the more heat. Set this as high as possible, balance against your operating temperatures, and the mass of the enriched fraction of your fuel. A higher flux will burn the fuel quicker. A moderator increases reactions which will burn fuel quicker, but in a design where the density of U235 would be get too low too quickly in lower enriched fuel for instance it can actually increase the length of the fuel burnup process.

And finally thermocouples.

I don't know how they work, and Im sure some of the monstrous objects I've designed would be practically impossible in reality. So I'll just say, you're so close but I don't know where you got the idea that diamond would be good. The best material pair in my expereince is Tungsten/Osmium - it doesn't matter which way around unless there is some extreme temperature on one side of the system that one element can't handle. The shape of the thermocouple has some effects on it's efficiency but generally I design them to fit into my ships, so moderately long and tall for the most part.

Just to finish up, I will add that in my opinion, U233 is easier to design with due to its natural innate characteristics, and is much better than U235 for high power reactors.