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Reactor channel layout

  1. Mar 15, 2017 #1
    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.

  2. jcsd
  3. Mar 18, 2017 #2


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    I will attempt to answer questions.

    All nuclear reactors heat a working fluid in the core. The fission energy is released as heat in the fuel material, and the heat is conducted through the cladding into the coolant. The coolant transports the heat, either to a turbine, or to a heat exchanger, which exchanges heat from the primary coolant to a secondary loop.

    A BWR boils water in the core, and boiling can start at about 1 m into the core (active fuel has 4.27 m height). The amount of boiling depends on the power level in the fuel/channel. BWR fuel operates at a pressure of about 72-73 bar, with a saturation temperature of about 284-286ºC. Each BWR fuel assembly is enshrouded in a channel to ensure the coolant passes along the fuel rods. Adjacent assemblies may operate at greater or lesser power with different levels of boiling. The fuel rods are maintained in a square array by spacer grids and/or upper and lower tie plates. The tie plates are connected by different methods depending on the supplier. One supplier uses selected fuel rods as tie rods, another suppliers uses the central water channel as a structural member, and a third supplier has small fittings at the top and bottom of mini-bundles, which are separated by a watercross, with upper handle and lower end fitting fastened to the channel. Earliest BWR fuel lattices where 6x6 or 7x7 with some unique 9x9 or 11x11 designs, but gradually, the 7x7 designs evolved to 8x8, 9x9, 10x10 (current standard) and recently 11x11 in the same design envelope. One supplier started with 8x8 and skipped to 10x10, while the other two designers (and their affiliates) progressed from 8x8 to 9x9 to 10x10.
    http://mragheb.com/NPRE 402 ME 405 Nuclear Power Engineering/Boiling Water Reactors.pdf

    PWRs operated at pressures of about 155-158 bar, slightly more than twice that of BWRs. The assemblies do not have shrouds, since there the coolant is usually single phase (liquid), although some nucleate boiling may occur toward the upper portion of the hottest fuel assemblies. The active fuel region is either 3.66 to 4.27 m, although some of the earliest reactor had slightly shorter (~3 m) cores. The coolant inlet temperature is ~280-292ºC, and coolant exit temperature is ~315-330ºC, depending on pressure, flow rate and power density. Most PWR fuel rods are maintained in a square array by spacer grids; the exception is VVER, which uses a hexagonal or triangular lattice. The guide thimble tubes are mechanically attached to upper and lower nozzles composed of stainless steel. Control rods fall from above into the guide tubes in certain assemblies in the core.

    BWRs and PWRs shutdown periodically to refuel the core and perform maintenance. The area above the core is flooded with about 10 m of water.

    CANDU reactors have horizontal coolant channels or pressure tubes. The core consists of 12 or 13 with fuel about 0.5 m. The CANDU fuel assemblies had 19 fuel rods (1, 6, 12), then later 28 (4, 8, 16) and now 37 (1, 6, 12, 18) fuel rods. The assemblies or bundles are loaded in sets of 4 or 8, or odd numbers if the core has 13 assemblies across the core. The refueling machines lock onto the pressure tubes, which allow the fuel assemblies to be loaded while the reactor continues to operate. The outlet header pressure is 10 MPa or ~ 100 bar (intermediate between BWRs and PWRs.) Inlet temperature is ~266ºC and the outlet temperature is ~310ºC. The pressure tubes are Zr-2.5Nb, although some plants have used Zircaloy-2 in the past. The ends of the pressure tubes are rolled/expanded into the end fittings. CANDU units have either 380 or 480 channels.

    https://canteach.candu.org/Content Library/19720114.pdf
    http://www.nuceng.ca/candu/pdf/17 - Fuel.pdf

    The CANDU pressure tube is centered in a calandria tube with a gas space in between. The heavy water in the calandria is at a lower pressure and temperature than the coolant in the pressure tube.
    https://canteach.candu.org/Content Library/19980102.pdf
    https://canteach.candu.org/Content Library/19980103.pdf
    https://canteach.candu.org/Content Library/20044203.pdf

    CANDU-600 reactors are considered with moderator heat load varying from 120 to 160 MWth, and moderator outlet temperature (from calandria) varying from 80 to 100°C.
    Ref: Moderator heat recovery of CANDU reactors

    The RBMK, which is a graphite moderated, water cooled reactor is a different beast. It uses vertical pressure tubes inserted down through the graphite blocks, and the pressure tubes keep the cooling water separated from the graphite. The fuel assemblies are smaller in diameter than CANDU fuel. The RBMK has 1661 pressure tubes for fuel.

    Good overview. It shows a fuel element with a central tie rod for holding the end fittings together, which allows the fuel assembly to be lowered into the core or retrieved. The tie rod is surrounded by 18 fuel rods in two rows of 6 and 12 rods.

    There are 1693 fuel channels and 170 control rod channels in the first generation RBMK reactor cores. Second generation reactor cores (such as Chernobyl-4) have 1661 fuel channels and 211 control rod channels.

    The nominal temperature of the cooling water at the inlet of the reactor is about 265–270 °C (509–518 °F) and the outlet temperature 284 °C (543 °F), at pressure in the drum separator of 6.9 megapascals (69 bar; 1,000 psi).

    startup neutron sources (12)
    control rods (167)
    short control rods from below reactor (32)
    automatic control rods (12)
    pressure tubes (1661)

    Description of system - RBMK-1500

    Reactivity Control System




    Description of Ignalina (RBMK-1500)

    Russian RBMK Reactor Design Information

    Reactor core (diameter is about 12 m and height is 7 m) is situated in the central part of the graphite masonry and is formed by zirconium process channels (outer diameter is 88 mni and wall width is 4 mm), passing along the axis of the graphite column, forming square grid with spacing 250 by 250 mm. Zirconium CPS channels (diameter is.88 by 3 mm) are installed in this grid. Altogether there are 2488 graphite columns in the core. Ofthem, 1693 (lst phase) and 1661 (2nd phase) have process channels; 179 (1st phase) and 211 (2nd phase) have CPS channels; 12 have energy generation control detector channels; and 4 have fission chamber channels.

    In the process channels, there are two types of fuel cells (Figure 4). The first type of fuel cell has a channel (dia = 7 mm) in the central carrier zirconium rod (tube) for installation of radial energy distribution detectors, gamma chambers, absorbing rods, or local automatic control system detectors; outer diameters of all device located in the channel are 6 mm. The second type of fuel cell does not have a channel in the carrier rod. The external diameter of carrier rod is 13 mm; carrier tube is 15 mm.

    A fuel cell consists of 2 fuel assemblies mounted on the central rod (tube). Each fuel assembly includes 18 fuel element rods (length is 3.46 m at 20°C), a frame, and mounting hardware.

    Graphite heat is 5.5% of the core thermal power. Most of the heat is removed by the primary coolant in the pressure tube (channel) through the "hard contact" graphite rings (Figure 5). It is nearly 80% to 85% of all graphite heat. The rest of it is removed by the control rod coolant in their channels.

    The gas circulation (70% to 90% He + 10% to 30% nitrogen) almost does not remove graphite heat. It only creates the condition for its effective removal (the so called "heat bridge").

    I'm not sure about the welding; perhaps brazing. The pressure tubes are formed from a zirconium alloy for neutron economy. I suspect the end-fittings are stainless steel.

    Iron, nickel and chromium are soluble in beta zirconium, and the transition from alpha phase to beta phase begins around ~820-830°C (alpha transus) with full beta phase at ~965-980°C (beta transus) for Zircaloys with nominally ~0.0012 oxygen. So, it could be possible to form a weld between a zirconium alloy and steel near the beta transus. On the other hand, the alpha transus is well above normal annealing temperature. Beyond some mechanical process, I'm not sure how the joints between the Zr-alloy pressure tube and steel are made.
    Last edited: Mar 18, 2017
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