Nuclear Energy & Waste Management Flashcards
Nuclear power
1) Requires radioactive fuel: unstable isotopes of Uranium (235U) and Plutonium (239Pu) for the decay to lighter elements
2) Fission of atoms releases neutrons and lots of heat energy
3) Emitted neutrons absorbed by other unstable isotopes in reactor = chain reaction
4) Reactors use heat generated by fission to produce steam; steam turns a turbine; turbine generates electricity
CanDu reactors
CanDU reactors use “heavy water” (D2O) instead of “normal” water because heavy water slows neutrons down and controls reaction speed
Heavy water purified via a high energy, multi-step process; 20% of reactors’ running cost is heavy water production; 340,000 tonnes of water is required per tonne heavy water
a, B, and y particles
a: He nucleus, but skin protects us from these
B: electrons can go through 1/2” of wood
y: through lead, through you, etc
235U releases an a particle and has a half life of ~700million years
Exposure to low doses of radiation has unexpected health effects (hormesis)
Storage of nuclear waste at reactor sites?
1) Wet storage cools/protects from high energy radiation
2) Dry storage, used after 2 years wet, still warm to the touch
Adaptable Staged Approach
1) Onsite: waste is kept onsite until an appropriate central site is found
2) Central site storage: waste is moved to a central site suitable for deep burial (this would take 20 years of constantly transporting nuclear waste)
3) Deep geological disposal
Current Canadian waste storage plan
Site selection based on community acceptance, technically sound, adequate transport, no seismic activity, no minerals/mining, large pluton (solid, intact rock), ground water not oxidizing (trickling down from surface)
Suggested site is on the Canadian shield where bundles could be placed in containers and radiation would be kept in until water corrodes the metal or it is deformed by the heat
Instant release nuclides
More volatile elements will evaporate and distill at cooler edges on surface of container and will be closest to water when the container eventually fails, while non-volatile elements cannot dissolve until CO2 crystals dissolve, which are almost insoluble in water
Current standard of exposure limit
< 1 in 1 million cancer risk = < 0.05 mSv/year, but average background radiation = 2.4 mSv/year
4 steps for treating sewage
1) Primary treatment: removal of solids vis settling and screening process
2) Secondary treatment: biological process to remove organics by aeration-aggravation (2 reasons for removing organics are that organics easily oxidize and absorb Cl, and they help remove Fe from water)
3) Tertiary treatment: removal of nutrients and cosmetic cleanup by chemical treatment processes like coagulation (alum sticks to small particles) and removal of PO4 using FeCl3
4) Disinfection/removal of bacteria: method chosen depends on cost, water type being treated, distance treated water must travel, etc.
i) Chlorination can cause a bad smell and taste and a small amount of chloroform is formed, and requires residual Cl left in the water
ii) Ozonation has a high cost, ozone must be produced onsite, required a high energy input and residual Cl
iii) UV radiation allows organic materials to absorb UV so water must be very clear, and no residual is needed to prevent infection
Measures of water quality
1) BOD (biological oxygen demand)
2) TSS (total suspended solids)
3) Total P
4) Toxins
Ideally, all 4 criteria are measured, but usually only 1 or 2 are
Waste water treatment in Ottawa
Robert O Ricard Centre in East Ottawa filters waste water via:
1) Screening
2) Settle and skim film
3) Add O2 to remove organics
Water is tested for P content and treated as required (May-Nov NaClO is added before release, and the centre produces its own electricity and heat by combusting CH4 and CO2 released
