THE ELECTRICITY SUPPLY SYSTEM ASPECT

Problems concerning all sector performance

Nuclear wastes storage options

Nuclear wastes management is very serious problem. Although fission technology is well known and developed, generation IV fission reactors are to start operation in near future and nuclear safety level in new reactors is undoubtedly high, nuclear wastes problem has no satisfying solution that allow to stop considering influence of nuclear power plant on environment. The matter is worrying because, the fission reactors has been operating since years 50's of last century and nuclear wastes are accumulated and stored in temporary depositories. Using the word "temporary" is particularly unsuitable term, because i.e. in the USA, spent fuel and all radioactive wastes are stored through all reactor's life time.

After removing spent fuel from the reactor it requires a storage with intensive cooling system due to existing of after-heat. Fuel rods after reload consists large amount of highly radioactive fission products. Majority of energy gained by fission is transferred to the coolant in the reactor core. But part of them is being released long time after the reaction occurs because of radiation and the fact that it is impossible to stop all reaction undergoing in the fuel at one moment. From the core, spent fuel is transported to surface storage with efficient cooling system. For this purpose water pools are appropriate. They are 12 m deep and keep spent fuel at temperature of around 40ºC. Pools are first but very long stage of wastes management. Spent fuel is required to be kept under water from several years to decades to reduce radioactivity and after-heat.

Spent fuel can be in general treated in two ways. It can be transported to deep geological depository or to recycling facility. Second solution is expensive and not very popular worldwide. The issue was presented in point 2.4.2.2 of the thesis. Nevertheless fuel recycling is less harmful for the environment, rational in relation to resources management and compatible with sustainable development policy. According to [92] projections, amount of spent fuel in storage will be steadily increasing and will reach 450 000 tons of heavy metal (t HM) in 2020. Around of 120 000 tons will be reprocessed, the rest of 320 000 tons will be directed for storage. Today it is 320 000 t HM discharged globally. 95 000 t HM has been reprocessed and 225 000 t HM are stored. Global capacities for reprocessing are only at level of 5 000 t HM annually.

There are several types of nuclear wastes. Its division is shown below [19], [21], [93] [94]

  • Very low level wastes - VVLW. Very low radioactive wastes with radiation alpha, beta, gamma below 400 Bq/kg. Granit rock emit radiation of 7000 Bq/kg.
  • Low level wastes - LLW. Low radioactive wastes the reactors and from small radioactive sources, occurring in science, medical and industry facilities. They bring only 1% additional radioactivity worldwide, although they are major part of wastes removing from the reactor ( 90%). They are stored in shallow geological repositories with radiation control and physically protection. Requirements for LLW repositories are lower than for HLW.
  • Intermediate level wastes - ILW. Medium radioactivity in the UK. Radiation alpha and beta above 1,2x107 Bq/kg. Radiation gamma above 4x107 Bq/kg. Radiation at these levels is dangerous. ILW are covered with asphalt or they are pitched. Next, they are moved to containers and deposited in deep underground repositories. They can be stored with HLW.
  • High level wastes - HLW. Highly radioactive wastes that arise in the fuel elements at reactor's operation and during procedures of nuclear weapon construction. They are very dangerous. They consist long term fission products. Management of HLW bring many problems and end solution has not be discovered yet. The ideas are to ensure special wastes containment and disposed them deeply underground.
  • Transuranic wastes - TRU. Wastes from nuclear fuel reprocessing and recycling facilities and from nuclear weapon industry. They are as harmful as HLW and they require similar storage solutions.

    • For the HLW and TRU storage, multibarrier system is proposed - EBS (the engineered barrier system). First step is wastes vitrification (that is embedded in the glass structure). It is hardly soluble and radioactive nuclides are well combined. According to researches vitrification give very strong and endure product resistant to mechanical stresses and water activity. Scientist said that they are able to keep radioactive nuclides combined even for 1000 years with minimal radioactivity leakage to the water or ground. Wastes are formed in a compact cylinder that can be placed in a sealed cooper or stainless steel container. As long as container is not broken, any nuclides can not get outside to the environment. Nevertheless corrosion is a problematic issue and eventual tectonic movements, that can not be predicted, especially that containers have to be stored for thousands years. Additionally container's endurance can turn out to be much worse that vitrification product. The container is surrounded by bentonite clay. It protects vessel from small rock's movements and set it in one place. Moreover the clay separate the container from water and it acts like a filter of nuclides. It binds radioactive particles with itself. Last important barrier is final geological repository meeting with very rigorous requirements. Rocks should effectively protect containers from water and mechanical damages. In caves, hydrochemical conditions are favourable and permanent what is their comparative advantage. [93]

    There are several options for a final geological depository of spent fuel. The most significant are [83]:

  • Östhammar project, undertaken in Sweden - with the licensing process coming to the end, and plans for operation in 2023
  • Olkiluoto project, undertaken in Finland - operation in 2020
  • Bure project, undertaken in France - operation in 2025
  • Yucca Mountain project - construction terminated by the US Government, but with continuing licensing process

    • However, to date, none of final geological repository is in operation.

    Radioactive spent fuel is not the only nuclear waste. All solid, liquid and gas wastes, containing radioactive particles or contaminated by radioactive materials, are called nuclear waste. Hence all nuclear industry is a large source of threatening wastes. Moreover all nuclear power plant's elements after decommissioning are wastes as well as usable equipment in the power plant. [93] The amount of low level wastes after decommissioning is average 10 000 m3. [82] Major radioactivity in nuclear power plant is caused by activation process. Steel elements normally not radioactive, gain radioactivity due to long neutrons bombardment. [94]
    [7] [19],[21],[82],[83],[91],[92]

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