Vern. Lecture 1b

Question 1

Name three sources of nuclear waste

Answer 1

1. Nuclear fuel cycle
2. Fission products: U, Np, Pu
3. Contaminated machinery

Question 2

What is interim waste storage?

Answer 2

Safe storage until long term management/disposal option is available. Surface or subsurface waste facilities.

Question 3

Briefly describe the three level classifications of radioactive waste

Answer 3

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

Question 4

Give an advantage and three disadvantages of near surface disposal at ground level for low-intermediate radioactive waste.
When is it used?

Answer 4

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)

Question 5

List five methods of intermediate and high level waste disposal that have not been implemented

Answer 5

Long term above ground storage
Disposal in outer space
Rock melting
Disposal at subduction zones
Sub seabed disposal (not permitted)

Question 6

What is the publicly accepted management option for long-lived and high-radioactive waste?

Answer 6

Deep geological disposal

Question 7

What is a mined repository storage shaft composed of?

Answer 7

From the centre out: waste, casing, impermeable rock, bentonite backfill

Question 8

Give four reasons why bentonite backfill is used in a mined repository storage shaft

Answer 8

• Ultra low permeability
• Self healing
• Pore spaces too small for bacteria to get through
• Strong adsorption properties, escaping radionuclides would be adsorbed onto clay

Question 9

What depths are geological repositories?

Answer 9

Mined repositories: 300m-1 km,

Very deep boreholes: 4-5km

Question 10

Describe the final waste insertion process of deep borehole disposal

Answer 10

• 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

Question 11

How is the borehole sealed in deep borehole disposal?

Answer 11

• 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

Question 12

What is the key criticism of borehole drilling?

Answer 12

Drilling always creates a damage zone around borehole

Question 13

What characteristics are required in rocks used in deep disposal?

Answer 13

Low permeability - granite, mudstone, low permeability chalk
Low permeability and not susceptible to fracture - salt
These rocks will be surrounded by higher permeability rocks

Question 14

What are the advantages and disadvantages of immobilisation in glasses?

Answer 14

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

Question 15

What is a major drawback in immobilisation in ceramics?

Answer 15

Crystalline structures can accommodate only a few radionuclides in very specific locations

Question 16

Describe encapsulation in cements

Answer 16

Physically surround metal component with set cement

Escaping radionuclides may be immobilised by being incorporated in cement phases

Question 17

What is the benefit in encapsulation in cement?

Answer 17

Cements act as diffusion barrier providing sorption and reaction sites and maintaining very high pH which reduces radionuclide solubility

Question 18

How can bacteria compromise storage?

Answer 18

Can live in a diverse range of environments

Question 19

How can acid production by microbial processes compromise storage and how is this countered?

Answer 19

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

Question 20

Why can organic degradation compromise storage?

Answer 20

Causes breakdown of organic waste matrices
Generation of gases, soluble organics, fatty acids
Gas generation causes pressure increase

Question 21

In the early phase (Near field) of development what prevents microbial infiltration on the storage unit?

Answer 21

• Bentonite buffer prevents microbes migrating to waste surface
• adjacent to HLW, too hot and dry with too much radiation for microbial activity

Question 22

What microbial contamination dangers are present in the near field to mid late field?

Answer 22

• Cooling/less radiation/more water increases microbial activity
• Potential migration of microbial metabolites from distal bacteria to waste