In this blog post, we’ll examine the concept of spent nuclear fuel, the nuclear fuel cycle, fuel characteristics by reactor type, wet and dry interim storage methods, and the key challenges Korea faces in spent nuclear fuel management.

Spent Nuclear Fuel and the Nuclear Fuel Cycle

Nuclear power continues to play a vital role in South Korea’s overall electricity production, accounting for approximately 30% of the total even recently. However, if the issue of spent nuclear fuel is not resolved, the sustainability of nuclear power generation could face serious constraints.
Spent nuclear fuel refers to fuel that has undergone nuclear fission inside a reactor and has lost its efficiency as fuel; it is highly radioactive and continuously generates decay heat. The nuclear fuel cycle can be divided into the front-end and back-end cycles. The front-end cycle refers to the process from uranium mining through refining, conversion, enrichment, and fabrication until the fuel is loaded into the reactor. The back-end cycle refers to the entire process of storing, processing, disposing of, or recycling spent nuclear fuel—the fuel that has been used in the reactor.

Fuel Characteristics by Reactor Type: PWR and PHWR

Currently, South Korea operates pressurized water reactors (PWRs) and pressurized heavy-water reactors (PHWRs), and the two reactor types use fuel with different characteristics. Natural uranium consists of approximately 99.3% uranium-238 (U-238) and approximately 0.7% uranium-235 (U-235), a fissionable isotope; U-235 is primarily responsible for causing nuclear fission.
Pressurized heavy-water reactors (PHWRs) use natural uranium (containing approximately 0.7% U-235) as fuel, whereas pressurized water reactors (PWRs) use enriched uranium, in which U-235 is enriched to about 4–5%. Consequently, PWRs require a relatively smaller total amount of fuel to produce the same amount of electricity, whereas PHWRs require more frequent refueling.
The fuel replacement methods also differ. PWRs operate by replacing about one-third of the loaded fuel approximately every 18 months, whereas PHWRs use an online refueling method, replacing small amounts of fuel almost daily even during operation. These differences have a direct impact on the amount of spent nuclear fuel generated and the methods used for its storage.
In summary, PWRs use enriched uranium (approximately 4–5% U-235) and replace a portion of the loaded fuel at regular intervals. In contrast, PHWRs use natural uranium (approximately 0.7% U-235) and continuously replace fuel even during operation. As a result, PHWRs generate relatively large amounts of spent nuclear fuel, and in many cases, the fuel is transferred to dry storage facilities after initial wet storage.

Interim Storage Methods: Wet Storage and Dry Storage

Since spent nuclear fuel generated by PWRs and PHWRs emits high levels of radiation and decay heat, it is first stored in water-filled storage pools to allow sufficient cooling. This is called wet storage, and the water serves to shield against radiation while simultaneously cooling the fuel.
In wet storage, the water effectively shields against radiation emitted by the spent nuclear fuel and absorbs decay heat to lower the fuel’s temperature. The heated water is then continuously circulated through a cooling system to maintain the proper temperature in the storage pool.
The most important aspect of wet storage is that water performs two functions simultaneously: radiation shielding and cooling. Because of this characteristic, spent nuclear fuel must undergo wet storage for a certain period before moving on to the next stage.
Dry storage is a method in which spent nuclear fuel, sufficiently cooled through wet storage, is dried and placed in storage containers made of concrete or metal, and then cooled using natural air circulation or similar methods. Dry storage is advantageous for the long-term storage of large quantities of spent nuclear fuel because storage facilities can be expanded relatively easily and operating costs are low.
However, dry storage can only be applied after the spent nuclear fuel has been sufficiently cooled, and it has the limitation that it is relatively difficult to directly inspect the condition of the fuel during storage. For this reason, in South Korea, there are many cases where large quantities of spent nuclear fuel—particularly from pressurized heavy-water reactors (PHWRs)—are transferred to dry storage facilities for management.