OOPS

ThinksMarkedly wrote:A fission reactor, on the other hand, never completely stops: even if you remove the rods, they will still generate fission and you need to store them somewhere. You might as well keep them inside the reactor producing energy, which you can then dissipate as heat.

Unless the Honorverse has stimulated fission of almost non-radioactive material.

Aren't the rods in a fission reactor for control (like a throttle)? So you would push the rods all the way in, rather than taking them out, to minimize the power.

tlb wrote:

ThinksMarkedly wrote:A fission reactor, on the other hand, never completely stops: even if you remove the rods, they will still generate fission and you need to store them somewhere. You might as well keep them inside the reactor producing energy, which you can then dissipate as heat.

Unless the Honorverse has stimulated fission of almost non-radioactive material.

Aren't the rods in a fission reactor for control (like a throttle)? So you would push the rods all the way in, rather than taking them out, to minimize the power.

There are the fuel rods made up of uranium pellets. These are what he is referring to.

Control rods indeed do as you say, tlb.

tlb wrote:

ThinksMarkedly wrote:A fission reactor, on the other hand, never completely stops: even if you remove the rods, they will still generate fission and you need to store them somewhere. You might as well keep them inside the reactor producing energy, which you can then dissipate as heat.

Aren't the rods in a fission reactor for control (like a throttle)? So you would push the rods all the way in, rather than taking them out, to minimize the power.

I probably used the wrong term. I did not mean the moderator rods. You're right that if you want a powered reactor at minimum power, you'd push the moderator rods all the way in, or as far as you dare without causing other consequences like stalling the reactor (if that is possible).

Anyway, I meant the fissionable fuel. Because it spontaneously fissions, even in storage it is being spent and you need to capture the radiation it produces, lest it harm people or equipment. So you wouldn't remove the fuel from the reactor, unless you're decommissioning the reactor, replacing the fuel or performing major maintenance in it in the first place. You may as well keep it where it is: in the reactor, so the fission products and the energy can be channelled without harm.

cthia wrote:

tlb wrote:Would ships st the stations have their reactors at some minimum power level or perhaps even shut off?

Why would the reactors be shut down? Idling at nominal power levels perhaps.

@KZT. Holy calculations Batman!

Probably depends on how long they're docked at the station.
But being docked and able to run off station power gives your engineering department a change to pull types of maintenance on all your reactors that can only be done while they're powered down. Even on ships large enough to carry at least one excess reactor (for redundancy) you don't really want one pulled apart for routine maintenance when you're out on deployment; you might unexpectedly need to engage the enemy sooner than you could get it buttoned back up and running.

So anytime you're docked for long as a friendly station in a secure area I'd expect engineering to be trying to shut down anything they could in order to get ahead on preventive maintenance.

Plus simply turning it off saves fuel and operating life of the reactor - delaying the next time you'd need to fuel or perform maintenance.


Now if you're just there for a few hours to load supplies, or exchange personnel, then it probably doesn't make sense to power everything down. But there for even a day or two and shutting down reactors makes sense. (Though I'd hope they could come up from dead cold faster than a steam boiler could. A warship holding at 4 hour notice to sail was actually at a quite high state of readiness; with it's boilers lit off at holding at standby power. From totally cold boilers it could take most of a day to be ready to sail!!!)

The control rods are NOT the moderator rods. The fuel rods are both the coolant and moderator rods.

Control rods reduce the reaction rate. Reactors are sometimes shut down to replace spent or depleted fuel rods, but other reactors remain running to supply the necessary power to support the fail-safe systems.

cthia wrote:There are the fuel rods made up of uranium pellets. These are what he is referring to.

Control rods indeed do as you say, tlb.

It is an unfortunate matter of terminology that they are both called rods. I should have been able to decipher which was meant by the rest of what was said.

cthia wrote:The control rods are NOT the moderator rods. The fuel rods are both the coolant and moderator rods.

Control rods reduce the reaction rate. Reactors are sometimes shut down to replace spent or depleted fuel rods, but other reactors remain running to supply the necessary power to support the fail-safe systems.

The moderator (and cooling element) need not be part of the fuel rod. For example, heavy water can moderate and cool a fission reactor. I read that light water absorbs too many neutrons, so needs enriched uranium before it can work as a moderator.

ThinksMarkedly wrote:I probably used the wrong term. I did not mean the moderator rods. You're right that if you want a powered reactor at minimum power, you'd push the moderator rods all the way in, or as far as you dare without causing other consequences like stalling the reactor (if that is possible).

Anyway, I meant the fissionable fuel. Because it spontaneously fissions, even in storage it is being spent and you need to capture the radiation it produces, lest it harm people or equipment. So you wouldn't remove the fuel from the reactor, unless you're decommissioning the reactor, replacing the fuel or performing major maintenance in it in the first place. You may as well keep it where it is: in the reactor, so the fission products and the energy can be channelled without harm.

The fuel does spontaneously fission - though at a fairly low rate. Still it take quite a while for the decay chains from any single fission to work their way down to stable elements; and they produce a lot of heat while doing so. That's why spent fuel rods need to be kept in a cooling pond because even without active chain reaction splitting of the uranium the short lived elements it splits into continue self-fissioning through their short half-lives. (The fact that the mass of water also nicely absorbs the radiation they're putting off is a nice bonus)

Probably the closest thing to "stalling" a fission reactor is neutron poisoning, by creating too many short lived neutron absorbing elements; like xenon-135. Those make it very hard to control the reactor if restarted before letting them decay away - you have to pull the neutron absorbing control rods much further out than normal to overcome all that excess internal neutron absorption and get the reactor critical again and then as the neutron absorbing elements decay, or are burned away by all the neutrons the now critical reactor is producing, the reaction level can spike up dangerously forcing you to slam the control rods back down.
So in general if you shut a reactor down too quickly you simply have to wait out the neutron poisoning before it's safe to restart. (In some ways more like vapor locking an engine than just stalling it).
You can actually get a similar effect from a rapid and significant reduction in power output; you need to wait a specified length of time to let the reactor internal conditions stabilize before increasing power output again. (There are good reasons that nuclear power plants are run as base-load plants where they try to remain at constant power output at all times)

ThinksMarkedly wrote:I probably used the wrong term. I did not mean the moderator rods.

Yes, you DID mean the moderator rods, because that IS the correct rod. It is part of a whole, called the fuel assembly. The number of rods determine the output of the reactor.

ThinksMarkedly wrote:You're right that if you want a powered reactor at minimum power, you'd push the moderator rods all the way in, ...

NO!!! The moderator rods are fuel rods! You don't add more gasoline to a fire if you want a smaller fire. The fuel rods are also called "moderator" rods because they slow down the neutrons enough to support a chain reaction. Inserting more rods creates a bigger chain reaction!

Fission reactors produce heat from splitting an atom. It would be too costly and complicated to design a reactor that splits thousands of atoms each and every time from scratch. So a chain reaction is used instead. The domino effect.

Jonathan_S wrote:The fuel does spontaneously fission - though at a fairly low rate. Still it take quite a while for the decay chains from any single fission to work their way down to stable elements; and they produce a lot of heat while doing so. That's why spent fuel rods need to be kept in a cooling pond because even without active chain reaction splitting of the uranium the short lived elements it splits into continue self-fissioning through their short half-lives. (The fact that the mass of water also nicely absorbs the radiation they're putting off is a nice bonus)

Probably the closest thing to "stalling" a fission reactor is neutron poisoning, by creating too many short lived neutron absorbing elements; like xenon-135. Those make it very hard to control the reactor if restarted before letting them decay away - you have to pull the neutron absorbing control rods much further out than normal to overcome all that excess internal neutron absorption and get the reactor critical again and then as the neutron absorbing elements decay, or are burned away by all the neutrons the now critical reactor is producing, the reaction level can spike up dangerously forcing you to slam the control rods back down.
So in general if you shut a reactor down too quickly you simply have to wait out the neutron poisoning before it's safe to restart. (In some ways more like vapor locking an engine than just stalling it).

I see nothing wrong with your post except what I've highlighted. In some of the older reactor designs, which there are many still operational, you do not want to SCRAM (slamming the control rods back down) in that situation. It is what caused the Chernobyl disaster.

It is akin to slamming on your brakes when it first starts to rain. You get the opposite effect from what you intended. You speed up!

Jonathan_S wrote:Now if you're just there for a few hours to load supplies, or exchange personnel, then it probably doesn't make sense to power everything down. But there for even a day or two and shutting down reactors makes sense. (Though I'd hope they could come up from dead cold faster than a steam boiler could. A warship holding at 4 hour notice to sail was actually at a quite high state of readiness; with it's boilers lit off at holding at standby power. From totally cold boilers it could take most of a day to be ready to sail!!!)

My guess is that most of the ships that are not partially dissembled for heavy maintenance would be running a reactor. Most because David has really strongly implied that the only way to start up a fusion reactor is via a bunch of plasma from an already working reactor. So if you are supposed to be able to leave port within a few hours you need a reactor running.

Not to mention that the massive industrial area and the rest of the huge platform probably needs power too.