Vern. Lecture 1b Flashcards
(22 cards)
Name three sources of nuclear waste
- Nuclear fuel cycle
- Fission products: U, Np, Pu
- Contaminated machinery
What is interim waste storage?
Safe storage until long term management/disposal option is available. Surface or subsurface waste facilities.
Briefly describe the three level classifications of radioactive waste
Low level waste: Small amounts of short lived radioactivity; hospitals, industry. ~90%, <1% radioactivity
Intermediate level waste: short and long lived; resins, chemical sludge, ~7%, ~4% radioactivity
High level waste: highly radioactive and often thermally hot; fission products, transuranic elements, ~3% volume, ~95% radioactivity
Give an advantage and three disadvantages of near surface disposal at ground level for low-intermediate radioactive waste.
When is it used?
Advantage: easy access if things go wrong
Disadvantage: No significant barriers, affected by long term climate change, natural hazards
Used for radioactive waste of short half life (up to 30 years)
List five methods of intermediate and high level waste disposal that have not been implemented
Long term above ground storage Disposal in outer space Rock melting Disposal at subduction zones Sub seabed disposal (not permitted)
What is the publicly accepted management option for long-lived and high-radioactive waste?
Deep geological disposal
What is a mined repository storage shaft composed of?
From the centre out: waste, casing, impermeable rock, bentonite backfill
Give four reasons why bentonite backfill is used in a mined repository storage shaft
- Ultra low permeability
- Self healing
- Pore spaces too small for bacteria to get through
- Strong adsorption properties, escaping radionuclides would be adsorbed onto clay
What depths are geological repositories?
Mined repositories: 300m-1 km,
Very deep boreholes: 4-5km
Describe the final waste insertion process of deep borehole disposal
- Insert the final run of casing
- Emplace the first batch of HLW canisters
- Pump in the special grout and allow it to set
- Insert bentonite clay
- Insert anotehr batch of canisters, pour the grout and allow to set
- Repeat until bottom km of borehole is filled
How is the borehole sealed in deep borehole disposal?
- Insert some backfill (crushed granite)
- Insert heater and melt backfill and wall-rock to seal the borehole
- Pour in more backfill and seal the borehole again
- Repeat as often as required then fill the rest of the borehole with backfill
What is the key criticism of borehole drilling?
Drilling always creates a damage zone around borehole
What characteristics are required in rocks used in deep disposal?
Low permeability - granite, mudstone, low permeability chalk
Low permeability and not susceptible to fracture - salt
These rocks will be surrounded by higher permeability rocks
What are the advantages and disadvantages of immobilisation in glasses?
Advantages:
Glass structure has a short range order, but no long range order
Radioactive species can go into the network or other sites e.g. alkalis into Na site
Open and random structure can accomodate most of the periodic table
Disadvanatages:
Borosilicate glasses are metastable and will corrode when contact with waterr or humid air
What is a major drawback in immobilisation in ceramics?
Crystalline structures can accommodate only a few radionuclides in very specific locations
Describe encapsulation in cements
Physically surround metal component with set cement
Escaping radionuclides may be immobilised by being incorporated in cement phases
What is the benefit in encapsulation in cement?
Cements act as diffusion barrier providing sorption and reaction sites and maintaining very high pH which reduces radionuclide solubility
How can bacteria compromise storage?
Can live in a diverse range of environments
How can acid production by microbial processes compromise storage and how is this countered?
Iron and sulphide oxidising bacteria generate acid however this needs an oxidising environment
This has the potential to corrode matrices/canisters/cement
Cements can be used to limit microbial activity as they produce a high pH environment
Why can organic degradation compromise storage?
Causes breakdown of organic waste matrices
Generation of gases, soluble organics, fatty acids
Gas generation causes pressure increase
In the early phase (Near field) of development what prevents microbial infiltration on the storage unit?
- Bentonite buffer prevents microbes migrating to waste surface
- adjacent to HLW, too hot and dry with too much radiation for microbial activity
What microbial contamination dangers are present in the near field to mid late field?
- Cooling/less radiation/more water increases microbial activity
- Potential migration of microbial metabolites from distal bacteria to waste
