Guide: RBMK Fuel Recycling

With all nuclear fission reactors, the RBMK's fuel rods eventually deplete with operation and need to be replaced. Unlike the generic reactors however, the rod's effectiveness in heat and flux production decreases with depletion, as fissile fuel decreases and neutron absorbing waste increases. For this reason, depending on the fuel and the setup of the RBMK, fuel will most likely deplete and lose effectiveness to a level where it can no longer produce sufficient heat to boil steam and produce electricity before it reaches complete depletion. This is why you are able to recycle fuel with only a minimum of 1% depletion.

When to Recycle

For most instances, fuel should be removed and replaced when it is no longer capable of contributing to the neutron flux and/or heat of the reactor. For more "research reactor" style designs where depletion is desired, such as for Bismuth production, then depletion must be at least 80% ideally. RBMK fuel has 5 increments of depletion for the spent pellets, which increments in percentages of 20% depletion in the rod.

Removing the Spent Fuel From Its Rod

After finding which rod you want to recycle, you can either replace it immediately while running (which may be dangerous depending on the setup) or you can shut down the reactor and then replace it. In order to have the fuel pellets removed and be recycled however, the rod's skin and core temperature must be below 50°C. Fuel rods cool quickly in the RBMK, so you can let them sit until sufficiently cooled. If the rod is self-igniting, then the heat will persist and Xenon-135 will accumulate, which may be undesired for recycling. In this case, you should remove the rod and place it into a Spent Fuel Pool Drum where it will cool down without reacting. It's slower, but you can have 12 rods cooling per drum.

Once the fuel is at a depletion level greater than 1% and is cool enough, you can simply place the rod in your player 2x2 inventory crafting grid to extract the spent fuel. An empty RBMK fuel rod will be left and your 8 spent fuel pellets (equivalent to the 8 billets used to craft the rod initially). It should be noted that while these recipes do not appear in NEI, they still exist. The spent fuel pellets cannot be reused for fuel regardless of depletion level, they can only be recycled.

Note about Xenon-135

When a fuel rod loses strength in flux or at least when a reactor shuts down, it will likely accumulate a large amount of Xenon-135 poison. This Xenon-135 can be extracted when recycling, which will slightly reduce the proportions of the other products. If you do not want the Xenon-135, you can simply expose the fuel to a neutron source rod and burn it off to a level where it will not be extracted.

How to Recycle

Spent fuel pellets are recycled in the Laser Isotope Separation Chamber (SILEX). The outputs are chance/proportion based. You can view the proportions of the recycling materials in each RBMK fuel's page and in the Nuclear Waste pages. For the most part, spent fuel results in some unspent fuel material and various forms of nuclear waste with their proportions depending on depletion level. Each operation results in a nugget's equivalent of the specified material, so each pellet would have total 6 operations for the 6 nuggets the original billet was. For instance: Uranium Fuel's Barely Depleted (20 - 40% depletion) is 70% unspent Uranium Fuel, 12% Uranium-235 Long-Lived Waste, and 18% Uranium-235 Short-Lived Waste. Simply change the concept of the numbers being proportions to chances of output per operation on the aforementioned or any other spent fuel pellet.

Trivia

Some fuels do not have recycling recipes yet, but most if not all should have spent fuel pellets. You can check if a fuel has a recycling recipe if it both has spent fuel pellet forms and those forms have SILEX recipes.
While the depleted rod may be extremely radioactive, the spent fuel pellets are usually not quite as radioactive, even as a whole.