OOPS

tlb wrote:

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.

You are correct. They are part of the process. Light water is used as both coolant/moderator.

kzt wrote: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.

How do all these plasma pipes work, that RFC implies are needed everywhere? The only way that I can imagine is to use artificial gravity to confine it to the center of a tube. Magnetic confinement is very difficult even when some of the plasma does not become electrically neutral and impact the wall. What is the longest that a tokamak has kept a plasma stable (they were working on them back when I was in grad school)?

tlb wrote:How do all these plasma pipes work, that RFC implies are needed everywhere? The only way that I can imagine is to use artificial gravity to confine it to the center of a tube. Magnetic confinement is very difficult even when some of the plasma does not become electrically neutral and impact the wall. What is the longest that a tokamak has kept a plasma stable (they were working on them back when I was in grad school)?

Beats me. Last thing I want breaking during battle is a pipe holding 50 million K plasma at 15,000 bar.

cthia wrote: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!

I'm not aware of any reactor design were fuel rods are inserted/removed operationally in order to control reaction rates[1]. If you want more reaction you remove more of the control rod (or rods), if you want less you insert more of the control rod (or rods).

Soviet RMBK reactors, like Chernobyl, still used fixed fuel rods (only removed during refueling; not as a way to moderate power output). Moderating the reaction still used movable control rods sliding up and down through the fixed fuel elements. However those control rods did have a bizarre and counterproductive design; where the control rods were tipped with a long graphic 'displacer'.

That actually locally increased the chain reaction because RBMK reactors were also void positive reactors; a void in the water (a weak neutron absorber) would increase the chain reaction and thus the power output. The US NEC required all power reactors to be void negative so that displacing coolant would reduce the reaction (usually because the reaction relied on the water slower the emitted neutrons to the point where they were likely to chain reaction. Neutrons with too much energy won't actually be very likely to split uranium). So thanks to the combination of void positive design and graphic tipped control rods inserting a control rod would locally increase the reaction before the boron carbide (a much stronger neutron absorber) portion of the control rod arrived to actually moderate the chain reaction. That wasn't a major problem during careful normal operation, where you'd keet the control rods partially inserted. But became a huge problem if you had withdrawn them. When this style of control rod was fully withdrawn inserting it to moderate the reaction would first cause a reaction increase!
And if you try to SCRAM the reactor when all the rods are withdrawn then you get a reaction increase simultaniously across the entire insertion face of the reactor as every control rod, including the extra SCRAM rods, first shoves the current neutron absorber out of the way. And of course as a void positive design as the heat reaches the point it starts boiling the cooling water that creates additional gas voids which increase the reaction even more.

Chernobyl was operated so far outside its safe operating parameters it's nearly unbelievable. Seriously, if a novel claiming that had been published several years before it'd have been laughed off as ludicrously contrived. They'd been doing an unsafe test and then cut it off early and tried to ramp back up to production power well before the neutron poisoning I'd mentioned in a previous post had passed. So they'd withdrawn all the control rods in an attempt to overcome that - then once overcome the neutron poison started burning off too quickly causing excessive chain reactions and overheating so they SCRAMed which compounded the problem due to the aforementioned graphic tipped control rods making the runaway reaction even worse - it didn't survive long enough for the main body of the control rods to absorb enough neutrons to break the cycle.

But back to reactors in general. The fuel assembly may, or may not, contain some fixed neutron absorbing cladding or wrapping - that's very reactor design specific. For example the first reactor was air cooled and it was the graphite blocks the fuel assembly was installed in, and that the control rods slid through, that provided the fixed 'baseline' neutron absorption - not the fuel rods themselves. But I've never heard of fuel rods called moderator rods.



[1] Molten salt reactors don't even have fuel rods, and there are some refector designs where moving an external reflector up and down the static fuel assembly controls location and rate of fissioning.

kzt wrote:

tlb wrote:How do all these plasma pipes work, that RFC implies are needed everywhere? The only way that I can imagine is to use artificial gravity to confine it to the center of a tube. Magnetic confinement is very difficult even when some of the plasma does not become electrically neutral and impact the wall. What is the longest that a tokamak has kept a plasma stable (they were working on them back when I was in grad school)?

Beats me. Last thing I want breaking during battle is a pipe holding 50 million K plasma at 15,000 bar.

Which is why I question the practicality of a ship's power plant being shutdown. In current nuclear power plants, individual reactors are shut down at different times. A dependable power source is needed to continue to supply Fail-Safe systems and normal power needs, that a generator may not be able to safely/reliably handle. That mistake is also part of the problem that led to Chernobyl.

On a HV warship, power would still be needed for fail-safe systems as well. And for the enormous power to achieve the author's designs.

cthia wrote:Which is why I question the practicality of a ship's power plant being shutdown. In current nuclear power plants, individual reactors are shut down at different times. A dependable power source is needed to continue to supply Fail-Safe systems and normal power needs, that a generator may not be able to safely/reliably handle. That mistake is also part of the problem that led to Chernobyl.

On a HV warship, power would still be needed for fail-safe systems as well. And for the enormous power to achieve the author's designs.

However the plasma pipes work, they are everywhere; so a ship being repaired could easily have their fusion reactors shut off, while power is supplied by the station. Then the station could also supply the plasma to restart fusion. No problem!

PS. I refuse to accept that the microchips in fail-safe systems and other electronics are run by plasma; I expect some form of electricity is also needed and could be supplied by the station before shutdown.

Jonathan_S wrote:

cthia wrote: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!

I'm not aware of any reactor design were fuel rods are inserted/removed operationally in order to control reaction rates[1]. If you want more reaction you remove more of the control rod (or rods), if you want less you insert more of the control rod (or rods).

Soviet RMBK reactors, like Chernobyl, still used fixed fuel rods (only removed during refueling; not as a way to moderate power output). Moderating the reaction still used movable control rods sliding up and down through the fixed fuel elements. However those control rods did have a bizarre and counterproductive design; where the control rods were tipped with a long graphic 'displacer'.

That actually locally increased the chain reaction because RBMK reactors were also void positive reactors; a void in the water (a weak neutron absorber) would increase the chain reaction and thus the power output. The US NEC required all power reactors to be void negative so that displacing coolant would reduce the reaction (usually because the reaction relied on the water slower the emitted neutrons to the point where they were likely to chain reaction. Neutrons with too much energy won't actually be very likely to split uranium). So thanks to the combination of void positive design and graphic tipped control rods inserting a control rod would locally increase the reaction before the boron carbide (a much stronger neutron absorber) portion of the control rod arrived to actually moderate the chain reaction. That wasn't a major problem during careful normal operation, where you'd keet the control rods partially inserted. But became a huge problem if you had withdrawn them. When this style of control rod was fully withdrawn inserting it to moderate the reaction would first cause a reaction increase!
And if you try to SCRAM the reactor when all the rods are withdrawn then you get a reaction increase simultaniously across the entire insertion face of the reactor as every control rod, including the extra SCRAM rods, first shoves the current neutron absorber out of the way. And of course as a void positive design as the heat reaches the point it starts boiling the cooling water that creates additional gas voids which increase the reaction even more.

Chernobyl was operated so far outside its safe operating parameters it's nearly unbelievable. Seriously, if a novel claiming that had been published several years before it'd have been laughed off as ludicrously contrived. They'd been doing an unsafe test and then cut it off early and tried to ramp back up to production power well before the neutron poisoning I'd mentioned in a previous post had passed. So they'd withdrawn all the control rods in an attempt to overcome that - then once overcome the neutron poison started burning off too quickly causing excessive chain reactions and overheating so they SCRAMed which compounded the problem due to the aforementioned graphic tipped control rods making the runaway reaction even worse - it didn't survive long enough for the main body of the control rods to absorb enough neutrons to break the cycle.

But back to reactors in general. The fuel assembly may, or may not, contain some fixed neutron absorbing cladding or wrapping - that's very reactor design specific. For example the first reactor was air cooled and it was the graphite blocks the fuel assembly was installed in, and that the control rods slid through, that provided the fixed 'baseline' neutron absorption - not the fuel rods themselves. But I've never heard of fuel rods called moderator rods.



[1] Molten salt reactors don't even have fuel rods, and there are some refector designs where moving an external reflector up and down the static fuel assembly controls location and rate of fissioning.

Neither am I! That is part of the design process. As I stated upstream, the number of fuel rods in a fuel assembly is what characterizes the output of the reactor. I was simply responding to what I thought at the time was an honest mistake on TM's part. I was giving him the benefit of the doubt. He couldn't have meant removing the control rods - which would have the opposite effect of what he stated.*

Fuel rods ARE removed when it is their time to be replaced. And they do regulate (in the manner in which I thought TM meant moderate in that capacity) energy output. But, in the design process, and not - as you say - operationally.

*Although I don't see any reason a plant cannot operate on fewer fuel rods, less than the 200 or so which a design might call for. Albeit, just because I see no reason why, doesn't mean there isn't.

cthia wrote:*Although I don't see any reason a plant cannot operate on fewer fuel rods, less than the 200 or so which a design might call for. Albeit, just because I see no reason why, doesn't mean there isn't.

Practicality, one would assume. You'd have to have the heavy duty radiation shielding around the area the fuel rods moved to as well as the operational position they're normally in. Plus due to the thermal load of the short life fission products you'd still have to cool the fuel rods the same whether they're in the "in" position or the "out" position. I suppose it could theoretically be done, but why?

Galactic Sapper wrote:

cthia wrote:*Although I don't see any reason a plant cannot operate on fewer fuel rods, less than the 200 or so which a design might call for. Albeit, just because I see no reason why, doesn't mean there isn't.

Practicality, one would assume. You'd have to have the heavy duty radiation shielding around the area the fuel rods moved to as well as the operational position they're normally in. Plus due to the thermal load of the short life fission products you'd still have to cool the fuel rods the same whether they're in the "in" position or the "out" position. I suppose it could theoretically be done, but why?

I always try to consider the human element and the foibles of man. I suppose some underfunded outfit may remove spent fuel rods before realizing they have none to replace them, or the shipment was hijacked or whatever. What do they do? Shut the plant down because they won't be able to achieve the maximum rated power output?

Also, do realize that spent fuel rods are placed in their own holding tank. See: Used Fuel Management.

cthia wrote:I always try to consider the human element and the foibles of man. I suppose some underfunded outfit may remove spent fuel rods before realizing they have none to replace them, or the shipment was hijacked or whatever. What do they do? Shut the plant down because they won't be able to achieve the maximum rated power output?

Also, do realize that spent fuel rods are placed in their own holding tank. See: Used Fuel Management.

I misread what you were posting and thought you were suggesting someone might build a reactor that kept the control rods in place and moved the fuel rods. Like I said, probably possible but why would anyone want to try it?

As for running a reactor short of fuel rods (but in an otherwise sensible manner, not like the above), it might be possible if they rearranged the rods to form a critical mass in the new configuration. But just pulling a quarter of the rods and expecting it to work... probably isn't going to work. Fuel rebalancing when refueling a reactor is a very ticklish issue as it's possible to make the reactor either underperform or burn too hot to control - more likely the former than the latter, thankfully.