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  • W.B.C.S. Examination Notes On – Unloading And Cooling – General Science Notes.

    Spent reactor fuel is extremely radioactive, and its radioactivity also makes it a source of heat (see above Fueling and refueling LWRs). When the spent fuel is removed from the reactor, it must continue to be both shielded and cooled.Continue Reading W.B.C.S. Examination Notes On – Unloading And Cooling – General Science Notes.

    This is accomplished by placing the spent fuel in a water-storage pool, or spent-fuel cooling pool, located next to the reactor. The water in the pool contains a large amount of dissolved boric acid, which is a strong absorber of neutrons; this ensures that the fuel assemblies in the pool do not go critical.

    Pools vary in size; older spent-fuel cooling pools are able to accommodate only about 10 years’ worth of spent fuel. As the pools fill up, more spent fuel storage is needed. Additional storage space can be gained by loading spent fuel into the pool more densely than originally planned, by building a new pool, or by removing the oldest fuel assemblies from the existing pool and storing them in air-cooled concrete and steel silos—called spent-fuel storage casks—located aboveground.

    This last method becomes feasible after fuel has been stored in cooling pools for two or three years, because radioactivity and the rate of heat generation decrease rapidly over this period. Dense storage in existing pools and casks tends to be less expensive and more economical for utilities than building new pools.

    Reprocessing

    Both the converted plutonium and residual uranium-235 in spent fuel can be recycled by chemically reprocessing the fuel and extracting the specific elements of interest. Reprocessing not only provides a means to recycle nuclear fuel, but it also can reduce the volume and radioactivity of the waste material that must ultimately be eliminated by some method of permanent disposal.

    One motivation for reprocessing is ultimately to provide a “closed-loop” fuel cycle within the nuclear industry. Closed-loop refers to recycling with 100-percent efficiency of all materials that are fabricated for use as nuclear fuel (including the most commonly used fuel, uranium dioxide pellets). Though the goal of 100-percent efficiency has yet to be attained by any country’s nuclear industry, a closed-loop fuel cycle is not an unrealistic ambition, based on current progress in reprocessing technology. Many benefits would result from fuel recycling, including lower cost for fuel (once the recycling infrastructure was in place) and reduced quantities of spent fuel to be stored on reactor sites around the world.

    Reprocessing methods

    The most common method for reprocessing, known as the PUREX (for plutonium-uranium extraction) process, begins with dissolving the spent fuel in nitric acid and contacting the acid solution with oil in which tributyl phosphate (TBP) has been dissolved. TBP is a complexing agent for uranium and plutonium, forming compounds with them that bring them into the oil solution. A physical separation of the (immiscible) oil and acid serves to remove the desired products from the nitric acid solution (which still contains all the fission products). The uranium and plutonium are then washed out of the TBP back into a water solution and separated from each other by various means to the degree desired. Thus, reprocessing produces three product streams: (1) a purified uranium product, (2) a plutonium product that may be either pure or mixed with uranium, and (3) a waste stream of fission products dissolved in nitric acid.

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