Hi, help me out with this one , I have wanted to understand this for a long time, I know different reactors have different approaches to active zone design but here I want to understand how the fuel channel of a RBMK reactor works, I know there are types of reactors like BWR in which I assume there is one giant pressure vessel and into that vessel at the bottom half there is the active zone consisting of multiple individual closely packed fuel assemblies or cassettes. in each cassette there are multiple zirconium metal rods into which uranium dioxide pellets are stacked if I remember correctly from a manual I saw. This approach seems more understandable its sort of similar like throwing a hot electrical heater into a tea pot and after a while the water begins to boil at the surface, I assume that since a BWR is under pressure inside the vessel the water first boils off only at the very top but if one increases or allows the temperature to increase the boiling starts to happen lower in the vessel and it can also under some circumstances boil in the active zone region right ? Although I assume this is usually avoided as it would cause reactivity instabilities? Now the reactor that puzzles me more is the PWR and the RBMK, also the Canadian CANDU , actually tell me if i'm wrong but to me it seems the RBMK and CANDU are similar in their physical and geometrical configurations the difference is in the moderator and coolant mostly correct? They both seem channel type , one is vertical one is horizontal but that doesn't affect the nuclear properties of the reactor I guess, what affects them is probably the use of graphite between each channel in an RBMK while coolant is ordinary light water and the use of heavy water as a moderator in the CANDU as well as coolant. Now here's where my question begins, they are both channel type reactors now lets forget about the heavy water and graphite moderator inbetween the channels as that is easy to imagine its layout , how are the channels formed? I have seen many pictures and videos about the RBMK (probably so famous due to Chernobyl) from what it seems the reactor reminds me a giant pipe organ. basically the large water pipe directly exiting the main circulation pumps is the split into about 1600+ smaller stainless steel pipes and each closely side by side with each next one goes horizontally approaching the reactor bottom then they take a U turn like a kitchen sink siphon and go up (I assume this approach is made to minimize the welded parts with 90 degree L angles) then each pipe seems to widen a bit and go all the way up some 14 meters, of which the lower 7meters are the active zone and next 7 meters is the part where the water is allowed to boil I assume or something like that. I have seen in pictures that at the upper part where the pipe is joined in the upper biological shield it has a welded L angle and a pipe exits sideways from each channel and probably goes to the steam separator located next to the reactor hall. I've talked with nuclear engineers that have worked on the RBMK , I asked how come those welds haven't come open and he said something that they perfect a weld technique for joining stainless steel with zirconium , is this correct? I assume the part of the pipe in the active zone must be zirconium because neutron transparency is important at that part? So basically they have to make 1600+ welds from each channel and all the pipes go sideways in two split directions.half go one way the other half other way. Ok heres the part I don't get even more, I haven't yet managed to get to Chernobyl so I can only look at pics or videos, the upper cover plate of the reactor has all the pipes both fuel channels and technological ones (neutron detectors etc) going through the cover plate , now I have seen the long round shaped uranium fuel rods that are inserted into each fuel channel , here is what I don't get, I don't remember how much pressure there was in an RBMK fuel channel but there is definitely some steam pressure there as the water boils, how do they seal the upper part where each fuel rod is inserted against hot water and steam leaking? surely the idea is that all that hot steamy water exits the welded side pipe and into the steam separator but what does the fuel rod seal looks like? Also while on the same RBMK topic , how is the fuel arranged? I assume the same round uranium dioxide pellets enriched to about 2.4% stacked one upon another forming long rods , in each fuel channel there are multiple rods with even distance from one another in a hexagonal geometry. Now is the fuel cladding applied to each individual stack of pellets or are the pellets with cladding inserted into a larger pipe which is then inserted into the coolant channel , ? by watching this video I cannot clearly understand , look at the part where he measures radiation from the empty stored fuel rods with screw type upper endings. (starts about 10:30 into the video) In CANDU for example I understand only the fuel/coolant channels are under pressure while the intermediate space between each channel is simply filled with heavy water as moderator but is not under pressure, correct? How about the connections and seals in a CANDU , since they too move physical fuel rods into each pressurized coolant channel I assume they would have similar seals and working like the RBMK at the ends where the reactor cover/end plate is where the fuel loading takes place? I will probably have more questions regarding this , the same I want to know and understand about the PWR, does the PWR has an individual coolant channel under pressure for each fuel rod/channel and then besides that the whole intermediate space between each coolant channel is filled with water under pressure? Would that mean there are two loops of water in the active zone of a PWR , one that goes through each fuel channel and the other that goes between all the channels? I got confused I haven't read long about the PWR's and wikipedia seems rather weak on the PWR explanation. From the other hand it doesn't seem logical for the PWR to have pressurized fuel channels because then why would they need the big pressure vessel which can withstand all that pressure, but then again if that is so the PWR seems very similar to BWR, ok I hope you will explain the difference. I would much appreciate good high resolution or simply good enough photos of actual channels and seals etc, that is the best way to understand the exact geometry which is what I want. thanks.