Overview of Chapter
chemistry 100- October 29, 2001 (Monday)
- Began with "cartoon of a reactor"
- fuel elements (pellets of enriched Uranium)
- moderator (graphite, water) -- to slow the neutrons
- control rods (cadmium, typically)
- to absorb neutrons and slow the rate of fusion
- can vary power production by withdrawing or inserting control rods
- can cause emergency shut down by inserting rods ("scramming")
- on problem at Chernobyl-- reactor overheated, cracked the graphite, and the holes for the control rods were no longer available = total loss of control.
- this is the reactor assembly-- the part that gets hot (it is enclosed in a steel shell)
- the rest of the plant is almost same as a coal fired plant
- (exception: nuclear plant indirectly transfers heat to the water to keep the water from passing through the radioactive area in the reactor core.)
- heat is used to generate steam
- steam runs turbine which runs a generator
- turbine and spent steam has heat that must be lost
- largest part of a nuclear reactor is the heat exchanger or cooling tower.
- Nuclear Power is an important economic force
- __% of US electricity (25%?)
- __% in France, for example (75%)
- Economic Reasons
- fuel and transportation costs less than coal
- typically fewer, larger power plants
- no fossil fuels-- no CO2, SO2, NOx
- Objective Evaluations
- sometimes hard to come by
-
- Compare a Coal vs. a Nuclear Electrical power Plant
- a typical coal plant releases more radioactivity
- traces of heavy elements in fly ash
- both are similar beyond the heat source
- heat boils water
- steam drives turbines
- turbines drive electrical generators
- heat is exchanged with surroundings
- river or lake water
- large air cooled heat exchangers
- can build small/medium/large coal fired generators
- generally impractical to make small nuclear plants
- can make them small
- (submarines and aircraft carriers)
- but not cheap
- tied into national power grids
- critical for large reactors-- major power supplier
- go off line, perhaps unexpectedly
- go off line to refuel or upgrade
- often weeks, months, years
- Downside of Nuclear Power Plants
- fear of radiation (NIMBY)
- hard to get sites approved
- last new US facility: 19xx
- fear of catastrophic failure
- Chernobyl and 3Mi Island
- Kursk and other seagoing disasters
- US design: containment vessels
- coal plant accidents more common
- but damage is local, short term
- fear of small leakage
- system closed, nothing should escape
- intense scrutiny
- needs more highly trained technical staff
Specific issues
- 1. failure modes and risk assessment
- 2. spent fuel
- 3. actual economics
- 4. lifetime of a nuclear plant
Spent Fuel
- The fuel is enriched uranium
- 0.7% 235U upgraded to, say, 25%
- U-ore is relatively cheap and abundant
- enrichment is expensive
- Fuel would be to supply power for decades
- but reaction products accumulate
- they collect and divert needed neutrons
- fuel is useless after, perhaps, 25% of 235U is used
- Must refuel
- shut down and remove old fuel
- install new fuel and restart
- Used Fuel
- can be chemically processed
- impurities are not U, chemically separated
- fuel is still enriched 235U
- Processing
- difficult since the fuel is highly radioactive
- processing is by machinery, remove control
- serious problems if any leakage
- costly process
- plants like Fernault (near Cincinnati) disasters
- economically worse than fresh Uranium
- environmental disasters
Spent Fuel
- now mostly just stored for disposal
- too radioactive for routine disposal
- (treatment residues also need storage)
- Storage Facilities
- None- for high activity wastes
- now stored on site at each reactor facility
- (temporary storage, 20-30 years now)
- Need secure location for thousands of years
- no ground water
- no earthquakes
- no human habitation for time > human society to date
- Underground, sealed
- encapsulate wastes (e.g., in glass or concrete)
- Yucca Mountain ?
Relicensing of Nuclear Reactors
- Most were designed for 30-40 year life
- design life, in part, period before failure is likely
- Most were built in the 60's
- Most nearing the end of their design period
- Now, push to grant 10-20 year extensions
- Some say, automatically
-
(Also true of Aircraft-- many commercial airliners are 30-40 years old.)
Continuous testing, replacement of critical components
Shutting down and Dismantling
- No real history of decommissioning
- some Naval reactors simply deep sixed
- Core components radioactive for 1000 years
- probably costs $0.5-1 billion to decommission
- costs less to refurbish and run
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