Liquids Can’t Melt Down

So I’ve been playing around a bit with nuclear reactor design, as one does. Thinking about the gap in the portfolio between the high-performance and high-unfriendliness molten-salt designs mentioned for use in power armor, and the low-power pebble-bed designs used for distributed medium-power applications, and wondering what exactly the sort of fission reactors the Empire used back in the old pre-fusion days for civil power.

Herein is the not-yet-canonical result, and I invite physicists, nuclear engineers, and so forth, to tell me all the places I’ve gone horribly wrong. Behold the LCGCR: the liquid-core gas-cooled reactor!

Basically, it’s a liquid fuel design (I’m considering here solutions of uranium and/or thorium salts, rather than molten salts; probably in water, unless there’s a more convenient solvent available.) to take advantage of their self-adjusting reactor dynamics. The formulation of the fuel solution is such that it only achieves criticality when inside the calandria containing the deuterium oxide (heavy water – ignore the D2 on the diagram, that’s a writo) moderator; elsewhere in the fuel loop it doesn’t have that. (The details of the calandria – such as the precise arrangement of moderator around fuel – and the control systems for tuning the reaction are omitted in this diagram.)


The fuel loop itself is how we keep the reactor running continuously and maximize fuel use. The liquid fuel continuously circulates through the reactor and the fuel regenerator (heat exchangers omitted for clarity). The fuel regenerator is where we filter neutron poisons and stable fission products that won’t burn any more out of the fuel, and top it up with fresh salts as required, ensuring that we can use all of the U/Th we put in and all their useful decay products too.

(As a safety feature, we have the core dump valve located right at the bottom of the fuel loop. In the event of something going horribly wrong with the plan, opening this valve empties the whole fuel loop into a safe-storage system split across multiple tanks, set up so that none of them can possibly achieve criticality and all can handle the decay heat of however much of the core they get.)

We get the heat out for use by bubbling an inert gas (helium seems to be a good choice, given its low neutron cross-section and susceptibility to neutron activation, meaning the primary coolant loop is probably clean enough to run the turbines off directly) through the salt solution in the calandria. After running the turbines, we feed it through a gas cooler and a gas cleaner, which latter removes neutron poisons such as xenon, and other gaseous products of the nuclear reaction, before returning to the reactor.

This is of course a very brief sketch of a design which I haven’t spent all that much time thinking about, but it seems to me to be roughly plausible and to have a few interesting advantages. Your thoughts, sirs?

Trope-a-Day: Going Critical

Going Critical: Averted.  In four ways:

Fission reactors in the universe are very well designed, ideally – although not always – to keep messy things like prompt criticality out of the possible performance envelope.  Some of them, the higher-power ones, can still quietly melt down (giving you basically a corium puddle in a highly refractory can to dispose of, but no major problems outside that), but most of them – like the ones they use in vehicles, for example – are pebble-bed designs that can’t even do that.

Fusion reactors depend on the continuous operation of their support systems to maintain the conditions that make the fusion reaction possible.  If they go wrong, even for extreme values of going wrong, what you get is a fizzle as the fusion plasma expands, loses its heat and pressure – all the more so if it escapes the envelope and touches the surrounding environment – and quenches.  A worst-case crash shutdown will screw up the inside of the reactor vessel, forcing you to replace the lining before you can restart, but it won’t penetrate it.

And no, they can’t go runaway.  There is a clever device built into the deuterium, etc., feed lines to stop that from happening.  It’s called a valve, which is attached to a big purely mechanical lever, which is labeled “IF SHIT HAPPENS, PULL”.

Matter/antimatter reactors by and large don’t do the equiavlent of going prompt critical, mostly because as long as you can pull the equivalent of said lever, the ambiplasma in the reactor vessel will quench much like the fusion reactor case.  (Remember, this isn’t Star Trek engineering – there’s always much more matter being fed in than antimatter, because it’s a lot easier to extract energy from hot plasma than from photons.  Thus, necessarily, no excess antimatter floating around inside the reactor core waiting to cause trouble.)  The remaining loose antiparticles that are there will chew the crap out of the inside of the containment, definitely, but it’s even heavier-duty than the fusion containment is, being designed for essentially this case.

Now, the storage cryocels where the antimatter’s stored, they can explode with great verve and drama, but that’s called “losing containment”, not “going critical”.

Singularity inductors don’t go critical because if the mini-black-hole falls out of the field knot and then through the containment, there’s usually stuff around for it to eat which will prevent it from going all Hawking-evaporatey on y’all.  Of course, you do then have a loose singularity chewing its way through your ship, station, habitat, or possibly even planet, so it’s not like your day isn’t going to suck anyway… but it won’t go critical.

At least not until it’s run out of stuff to eat.