Spent nuclear fuel, also called the used nuclear fuel, is a nuclear fuel that has been irradiated in a nuclear reactor (usually at a nuclear power plant or an experimental reactor) and that must be replaced by a fresh fuel due to its insufficient reactivity.
An irradiated fuel is due to presence of high amount of radioactive fission fragments and transuranic elements very hot and very radioactive. Reactor operators have to manage the heat and radioactivity that remains in the “spent fuel” after it’s taken out of the reactor. In nuclear power plants, spent nuclear fuel is usually stored underwater in the spent fuel pool on the plant. Plant personnel move the spent fuel underwater from the reactor to the pool. Over time, as the spent fuel is stored in the pool, it becomes cooler as the radioactivity decays away. After several years (> 5 years), decay heat decreases under specified limits so that spent fuel may be reprocessed or interim storaged. At first glance, it is difficult to recognize a fresh fuel from an used fuel. From mechanical point of view, the used fuel (irradiated) is identical as the fresh fuel. In most PWRs, used fuel assemblies stand between four and five metres high, are about 20 cm across and weighs about half a tonne. In contrast to the fresh fuel, which are simply shiny, the oxide layer forming on the surface of used fuel assemblies during the four-year fuel cycle makes them dark. Moreover, Cherenkov radiation is typical only for spent nuclear fuel. The glow is visible also after the chain reaction stops (in the reactor). The cherenkov radiation can characterize the remaining radioactivity of spent nuclear fuel, therefore it can be used for measuring of fuel burnup.
The key feature of LWRs fuel cycles is that there are many fuel assemblies in the core and these assemblies have different multiplying properties, because they may have different enrichment and different burnup. Generally, a common fuel assembly contain energy for approximately 4 years of operation at full power. Once loaded, fuel stays in the core for 4 years depending on the design of the operating cycle. During these 4 years the reactor core have to be refueled. During refueling, every 12 to 18 months, some of the fuel – usually one third or one quarter of the core – is removed to spent fuel pool, while the remainder is rearranged to a location in the core better suited to its remaining level of enrichment. This is usually done by the refueling machine. The removed fuel (one third or one quarter of the core, i.e. 40 assemblies) has to be replaced by a fresh fuel assemblies. Over time, as the spent fuel is stored in the pool, it becomes cooler as the radioactivity decays away. After several years (> 5 years), decay heat decreases under specified limits so that spent fuel may be reprocessed or interim storaged.
The refueling operation is divided into six major phases:
- reactor disassembly,
- fuel handling,
- reactor reassembly,
- preoperational checks, tests,
- reactor startup.
Storage in Spent Fuel Pools
Spent fuel pool (SFP) is storage pool for spent nuclear fuel from nuclear reactors. Spent fuel pool may be located inside the containment building or inside the fuel building (outside the containment building). When located outside the containment building, the two areas are connected by a fuel transfer system which carries the fuel through a normally closed opening in the reactor containment. In this case spent fuel is removed from the reactor vessel by a manipulator crane and placed in the fuel transfer system. In the spent fuel pool, the fuel is removed from the transfer system and placed into storage racks. After a suitable decay period, the fuel can be removed from storage and loaded into a shipping cask for removal from the site. Spent fuel pools are typically 12m or more deep, with the bottom equipped with storage racks designed to hold fuel assemblies removed from the reactor. A reactor’s pool is specially designed for the reactor in which the fuel was used and situated at the reactor site.
Spent fuel pools are fitted with stainless steel and aluminum racks that hold the fuel assemblies and are lined with stainless steel to prevent leaking. There are no drains that would allow the water level to drop or the pool to become empty. The plants have a variety of extra water sources and equipment to replenish water that evaporates over time, or in case there is a leak. Plant personnel are also trained and prepared to quickly respond to a problem. The water serves two purposes: it cools the fuel and shields workers at the plant from radioactivity. Although water is neither high density nor high Z material, it is commonly used as gamma shields. Water provides a radiation shielding of fuel assemblies in a spent fuel pool during storage or during transports from and into the reactor core. Although water is a low-density material and low Z material, it is commonly used in nuclear power plants, because these disadvantages can be compensated with increased thickness.
To conserve space, in all plants open storage racks were replaced with so called high-density racks that incorporate (boron-10) or other neutron-absorbing material to ensure subcriticality. Using such racks, fuel assemblies can be stored in about one half the volume required for storage in standard racks.
Safety of Spent Fuel Pool
Safety of AFR spent fuel pools stands on various criteria. These criteria are usually very similar to the criteria for AR pools and may be grouped according to following aspects:
- Subcriticality. Fulfillment of this criterion is based on:
- the design of the spent fuel pool,
- requirements on boric acid diluted in water,
- limiting of stored fuel (e.g. fuel enrichment, assembly burnup)
- Cooling. Fulfillment of this criterion is based on:
- the design of the spent fuel pool (e.g. no drains below the top of the stored fuel elements),
- requirements on water level in the pool,
- requirements on active cooling elements (heat exchangers and heat sink).
- Radiation Shielding. Fulfillment of this criterion is based on:
- the design of the spent fuel pool (water and concrete shielding),
- the design of surrounding working area
- requirements on water level in the pool,
- Integrity. Fulfillment of this criterion is based on:
- the design of the spent fuel pool,
- the design of surrounding working area,
- ensuring periodic inspections
Interim Storage of Spent Nuclear Fuel
Interim storage is a temporary solution that plays a central role in the management of the most highly radioactive materials: spent nuclear fuel and vitrified waste resulting from reprocessing such fuel. Since spent nuclear fuel is compact, plant operators are able to store fuel assemblies for a long time. It must be noted, the spent nuclear fuel is due to presence of high amount of radioactive fission fragments and transuranic elements very hot and very radioactive. Reactor operators have to manage the heat and radioactivity that remains in the spent fuel after it’s taken out of the reactor. In nuclear power plants, spent nuclear fuel is usually stored underwater in the spent fuel pool on the plant. Plant personnel move the spent fuel underwater from the reactor to the pool. Over time, as the spent fuel is stored in the pool, it becomes cooler as the radioactivity decays away. After several years (> 5 years), decay heat decreases under specified limits so that spent fuel may be reprocessed or interim storaged.
Interim Storage and Wait and See
Most countries with nuclear programs use some type of interim storage as their back end strategy. They have explicitly decided to take a “wait and see” approach to spent fuel management, leaving their spent fuel in interim storage, which leaves both the reprocessing and direct disposal options open for the future. The wait and see option, as the name implies, proposes interim storage until some solution for permanent storage and disposal will be developed in the future. For many operators, the spent fuel represents also a strategic material, since the spent nuclear fuel still contains about 96% of reusable material. Storing spent fuel and waste for several years allows heat release and radioactivity to subside. Despite the continued debate over the future of the fuel cycle, a quiet consensus has developed that for the near term, simply storing spent fuel while continuing to develop more permanent solutions is an attractive approach.
Currently, spent fuel is being stored in at-reactor (AR) or away-from-reactor (APR) storage facilities and we can identify two basic solutions of interim storage: