Without this Power Source, Voyager would not have lasted this long!

MolaKule

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Voyager 1:


 
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OVERKILL

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Awesome...thankyou...nuclear power of all types need a refresh and new public image.

(As an aside, I pulled apart a dump found wine cooler - to make a biultong machine - and they have a relatively massive themopile in them...might mess with that on the weekend)

Yes it does. Saw some good news today about nuke plants in the US getting recognized, and rewarded, for their displacement of fossil emissions. That's a step in the right direction, though it appears too late to save Indian Point, but that was politics/corruption.
 
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Awesome...thankyou...nuclear power of all types need a refresh and new public image.

(As an aside, I pulled apart a dump found wine cooler - to make a biultong machine - and they have a relatively massive themopile in them...might mess with that on the weekend)
I was watching something on Fukushima and the solution to what went wrong (tsunami knocking out backup generators at ground level) was so simple as easy. H2O is needed to amplify the neutrons used in the reaction. The rods sit in a container of water and at the bottom of the container is a drain that is frozen by an apparatus that uses electricity - if the power goes out for any reason, the ice melts, and the water drains causing the reaction to stop. It seems so simple and it's how modern-day reactors are designed. As horrible as Fukushima was, it was an old reactor design. It is CAN be done safely and at a fraction of the cost to the environment.
 

OVERKILL

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I was watching something on Fukushima and the solution to what went wrong (tsunami knocking out backup generators at ground level) was so simple as easy. H2O is needed to amplify the neutrons used in the reaction. The rods sit in a container of water and at the bottom of the container is a drain that is frozen by an apparatus that uses electricity - if the power goes out for any reason, the ice melts, and the water drains causing the reaction to stop. It seems so simple and it's how modern-day reactors are designed. As horrible as Fukushima was, it was an old reactor design. It is CAN be done safely and at a fraction of the cost to the environment.

The solution (more water) has been around for a very long time. CANDU's (Fukushima was a BWR) have massive makeup water tanks in the tops of each unit and the units have so much water in them that you can flood the SG's and passively cool the units if the pumps all couldn't be powered.

BUT, had the seawall been upgraded, the emergency generators, which had been recommended to be relocated to behind the plant, would not have had their fuel tanks washed away and would have been operational. Tepco did none of the safety upgrades to the plant that had been recommended, that's the real cause of the tragedy.
 

Astro14

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I was watching something on Fukushima and the solution to what went wrong (tsunami knocking out backup generators at ground level) was so simple as easy. H2O is needed to amplify the neutrons used in the reaction. The rods sit in a container of water and at the bottom of the container is a drain that is frozen by an apparatus that uses electricity - if the power goes out for any reason, the ice melts, and the water drains causing the reaction to stop. It seems so simple and it's how modern-day reactors are designed. As horrible as Fukushima was, it was an old reactor design. It is CAN be done safely and at a fraction of the cost to the environment.
Water moderates, it doesn’t amplify. It slows the neutrons so that they’re better absorbed by the uranium, facilitating the chain reaction.

While the removal of the water would change the neutron absorption, the removal of the water would also allow the heat to build.

When the primary fission is stopped, by insertion of moderating rods, or whatever control mechanism is used, the production of heat does NOT stop. Many of the immediate daughter products of the reaction have a very short (hours) half life and continue to decay, making tremendous heat. You can shut down the reactor, but the heat continues to be produced, at very high levels for days afterwards (it’s a curve, but close enough).

That’s what happened at Fukushima - with a power loss, the moderating rods dropped fully into the reactor, absorbing neutrons and stopping uranium fission. As designed. A fail safe mechanism.

BUT - The power loss also caused the coolant circulation pumps to stop. And heat was still being produced. The water boiled off. The fuel rods melted and pooled at the bottom of the containment vessel and kept making heat, from decay, not necessarily from uranium fission, though that would increase as the fuel pooled, and they melted through the bottom of the reactor.

There are ways to make reactors safe, but in this design, you have to maintain coolant circulation for days after shutdown to manage the heat of decay.
 

OVERKILL

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Water moderates, it doesn’t amplify. It slows the neutrons so that they’re better absorbed by the uranium, facilitating the chain reaction.

While the removal of the water would change the neutron absorption, the removal of the water would also allow the heat to build.

When the primary fission is stopped, by insertion of moderating rods, or whatever control mechanism is used, the production of heat does NOT stop. Many of the immediate daughter products of the reaction have a very short (hours) half life and continue to decay, making tremendous heat. You can shut down the reactor, but the heat continues to be produced, at very high levels for days afterwards (it’s a curve, but close enough).

That’s what happened at Fukushima - with a power loss, the moderating rods dropped fully into the reactor, absorbing neutrons and stopping uranium fission. As designed. A fail safe mechanism.

BUT - The power loss also caused the coolant circulation pumps to stop. And heat was still being produced. The water boiled off. The fuel rods melted and pooled at the bottom of the containment vessel and kept making heat, from decay, not necessarily from uranium fission, though that would increase as the fuel pooled, and they melted through the bottom of the reactor.

There are ways to make reactors safe, but in this design, you have to maintain coolant circulation for days after shutdown to manage the heat of decay.

Yes, I probably should have addressed that in my post too. A light water reactor uses regular water as a moderator. Other designs use different material, the CANDU uses deuterium (heavy water) which has a greater neutron slowing ability, allowing the use of natural (unenriched) uranium. Light water requires LEU in order to maintain fission, but military reactors used for propulsion use higher levels of enrichment to facilitate longer fuel cycles, reducing the frequency of refuelling events.

Another very effective moderator material is graphite, which also allows the use of natural uranium. The UK Magnox design is one such unit, which also does online refuelling like a CANDU, but probably the most famous is the Soviet RBMK design used at Chernobyl.

Control rods are used to, as the name implies, control the fission process, maintaining a certain level as well as reducing or increasing power levels by insertion or removal.

There are also shutdown rods which are only used to terminate the chain reaction, these are separate from the control rods. The CANDU 950 design for example had:

For reactor control:
- 27 stainless steel Adjuster rods
- 4 cadmium/stainless steel control absorbers

For reactor shutdown:
- 36 cadmium/stainless steel shut-off rods
- 8 horizontal nozzle tubes for liquid poison injection, which is gadolinium nitrate

At Fukushima, not only was there no way to power the pumps, due to the fuel tanks for the emergency generators being swept away, but from what I recall, the seawater cooling pumps were severely damaged as well, so even if there was power, there was no way to get cool water into the plant. On top of that, this caused issues with the used fuel storage pool, which they were having problems maintaining the water level in with no circulation or makeup water.

As you noted, the decay heat curve has a taper to it and its intensity is the highest right after fission is terminated. It takes around 8-10 years for decay heat to sufficiently decrease to a level where liquid cooled storage is no longer required and the bundles can be shuffled into dry casks. They of course still continue to make heat in the casks, but the level is quite low.

An interesting diagram of the heat transport system at Bruce:
Bruce Heat Transport System.jpeg
 
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Water moderates, it doesn’t amplify. It slows the neutrons so that they’re better absorbed by the uranium, facilitating the chain reaction.

While the removal of the water would change the neutron absorption, the removal of the water would also allow the heat to build.

When the primary fission is stopped, by insertion of moderating rods, or whatever control mechanism is used, the production of heat does NOT stop. Many of the immediate daughter products of the reaction have a very short (hours) half life and continue to decay, making tremendous heat. You can shut down the reactor, but the heat continues to be produced, at very high levels for days afterwards (it’s a curve, but close enough).

That’s what happened at Fukushima - with a power loss, the moderating rods dropped fully into the reactor, absorbing neutrons and stopping uranium fission. As designed. A fail safe mechanism.

BUT - The power loss also caused the coolant circulation pumps to stop. And heat was still being produced. The water boiled off. The fuel rods melted and pooled at the bottom of the containment vessel and kept making heat, from decay, not necessarily from uranium fission, though that would increase as the fuel pooled, and they melted through the bottom of the reactor.

There are ways to make reactors safe, but in this design, you have to maintain coolant circulation for days after shutdown to manage the heat of decay.
Yeah sorry. I was talking about liquid fluoride thorium reactors where the molten salt is water-like. Same idea…power outage or overheats for any reason the frozen plug melts and the molten salt drains from the reactor quenching the reaction. It’s considered “walk-away safe”.
 

OVERKILL

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The solution (more water) has been around for a very long time. CANDU's (Fukushima was a BWR) have massive makeup water tanks in the tops of each unit and the units have so much water in them that you can flood the SG's and passively cool the units if the pumps all couldn't be powered.

BUT, had the seawall been upgraded, the emergency generators, which had been recommended to be relocated to behind the plant, would not have had their fuel tanks washed away and would have been operational. Tepco did none of the safety upgrades to the plant that had been recommended, that's the real cause of the tragedy.
To add to this, this is what the CANDU 6 looks like inside, #3, which is a huge space between the secondary containment and the top of the unit is the dousing tank for supplying makeup water.

Screen Shot 2021-05-06 at 11.52.37 AM.jpg
 
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Voyager 1:


The production facility at Savannah River Site used to produce the nuclear components of RTG's for various space probes and such over a number of years has recently undergone a large project concerning decontamination/decommissioning . There were lots of nasty leftovers from production of RTG's.
 
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