Basic understanding of Graphite-Water reactors

[mccannta]

TL;DR

Where is the coolant in a graphite-water reactors and how can water act as a moderator in water-water reactors, but not in graphite-water reactors?

From my reading about the Chernobyl reactor disaster, I am trying to get a basic understanding of a graphite-water reactor. From what I understand, the graphite acts as a neutron moderator - essentially slowing fast neutrons down to enable greater probability for an additional fusion reaction. The water acts as a coolant to keep the optimal thermal characteristics and so the fuel rods, graphite blocks and the rest of the reactor doesn't melt. So the graphite acts to moderate neutrons/increase reactivity and the water acts as a coolant.

But... water can also act as a neutron moderator as in water-water reactors common in the rest of the world. I get the attraction: water acts as a moderator but in the case of a coolant leak or if the water boils out, the fuel loses moderation and falls below critical mass. It acts like a deadman's switch. I've seen pictures of theses reactors; a big pool where you can see the tops of the fuel rods holding the uranium.

1) What I don't understand is where is the water is in a graphite-water reactor? From the HBO miniseries, I can see that the graphite blocks have cylindrical holes in their center which, ostensibly I assume, surround each long cylindrical fuel assemblies which are filled with Uranium. These reactors are huge so I imagine a the core being a massive tank filled with towers of graphite blocks that surround each fuel assembly. Is this correct? If so, where is the water?

Also, the fission produces immense heat which needs to be cooled by the water. But to keep the water from boiling out too quickly, it must be pumped in/out very fast and under great pressure. With all the graphite blocks surrounding the fuel rods, I just don't get where the water is and how it moves through the reactor fast enough to not all boil out.

The other aspect of graphite-water reactors I don't understand is the positive void coefficient. I understand that in a water-water reactor the water acts as a moderator, slowing neutrons to enable and sustain criticality. And the water also acts as a coolant. And if the water heats too much to generate steam bubbles, neutron moderation is reduced and reactivity falls. If enough of the water turns to steam or coolant is lost, the reaction dies out completely - thus the negative void coefficient. However, in a graphite-water reactor, they suffer from the positive void coefficient; where the water turning to steam doesn't slow reactivity but increases it.

2) How can steam bubbles in one reactor reduce reactivity (water-water) and in another steam acts as a catalyst for a potentially runaway reaction (graphite-water)?

Sorry if this is too simple a question, I am a layman with no background in nuclear physics.

 

[Astronuc]

Summary:: Where is the coolant in a graphite-water reactors and how can water act as a moderator in water-water reactors, but not in graphite-water reactors?

1) What I don't understand is where is the water is in a graphite-water reactor? From the HBO miniseries, I can see that the graphite blocks have cylindrical holes in their center which, ostensibly I assume, surround each long cylindrical fuel assemblies which are filled with Uranium. These reactors are huge so I imagine a the core being a massive tank filled with towers of graphite blocks that surround each fuel assembly. Is this correct? If so, where is the water?

Also, the fission produces immense heat which needs to be cooled by the water. But to keep the water from boiling out too quickly, it must be pumped in/out very fast and under great pressure. With all the graphite blocks surrounding the fuel rods, I just don't get where the water is and how it moves through the reactor fast enough to not all boil out.

The water is in pressure tubes that encase the fuel assemblies in an RBMK. The pressure tubes, usually made of a Zr-based alloy are connected to headers and pipes that circulate water through a closed loop. A simple diagram is found here - https://www.world-nuclear.org/infor...-power-reactors/appendices/rbmk-reactors.aspx.

Heat is removed from the core and extracted through a heat exchanger, or steam generator, for the purpose of driving a steam turbine.

The amount of moderation depends on the volume of the moderator. In a water-water reactor, the assemblies of fuel rods are surrounded by water. In a graphite-moderated reactor, the volume of graphite is greater than the volume of water, so the moderation is mostly in the graphite. The water, primarily the oxygen in the water molecule, absorbs neutrons, such that when there is a void (i.e., liquid becomes steam which displaces liquid), fewer neutrons are absorbed by water, and the moderation continues in the graphite, so there is a net positive reactivity.

A void in a water-water reactor reduces the moderation. Another aspect is fuel temperature, such that when the fuel temperature increases, there is increased absorption of fast neutrons by what are called resonances in the uranium (primarily U-238) and plutonium (primarily Pu-240). Resonances relate to energies at which the absorption (n,γ) cross-section is greater than energies just a few keV less or more, and often greater than at thermal energies. For some historical background on resonance absorption - https://www.osti.gov/servlets/purl/4271369