Topic 6: Nuclear fuels Flashcards
What is chemical vs nuclear energy? differences?
chemical: breaking and joining of atomic bonds, with an electron transfer. Atoms maintain their identities
Nuclear: changes in forces that hold together atomic nuclei. The character of atom or element has changed
What is radioactive decay? what is a half life?
- spontaneous disintegration of an atom
- releases atomic particles, heat energy, gamma radiation
- decays from parent to daughter nuclide
- Half life- time take for half original volume of parent to decay to daughter
What is radioactivity?
- the propensity for a material to undergo radioactive decay, based on ratio of protons to neutrons in nucleus
What are isotopes?
- atoms of the same element with different numbers of neutrons - thus different atomic mass
what is ionizing radiation?
- emitted subatomic particles or electromagnetic waves
- energetic enough to detach electrons from atoms or molecules
What is alpha radiation?
- emission of alpha particles, 2 protons and neutrons
- very destructive but travels short distances
- can be stopped by paper
What is beta radiation?
- emission of beta particle, high energy electron
- travels further than alpha, but less destructive
- can be stopped w few cm metal
What is gamma radiation?
- emission of gamma rays, photons of very high frequency and very short wavelength EM spectrum
- far-travelling, very destructive to cell tissues
- can be stopped w thick shield of lead/concrete
What is nuclear fission
- breakdown of large nucleus into smaller daughter nuclei
- small amount of matter converted into large amounts of energy during fission
- bc mass of atomic nucleus is less than mass of constituent protons/neutrons = missing mass is converted into energy
Fission is a type of decay. it cannot be slowed, but it can be accelerated. How?
- bombarding radioactive nuclei with neutrons
- having sufficient numbers of radioactive nuclei so natural rate of neutron emission increases decay rate
- ex. bombardment of 235U results in a self sustaining chain reaction, so fission creates more fission. run-away rain reaction may cause nuclear bomb
Where are some uranium sources found on earth?
- igneous sources - found in low concentration crustal rocks, concentrated in fine stage residual melts and pegmatites. may be bound in minerals (apatite). May be found in rich veins next to granites (skarns)
- metamorphic sources- deposited in skarns, adjacent to granitic intrusions
What is hydrogenic uranium? Where is it found and how is it formed?
- it is our main economic source of uranium
- formed when reduced U4+ is oxidized into U6+, this combines with oxygen to form UO2
- relies on very specific formation environment
- found in surface and oxic ground waters
What is detrital uranium?
- Precambrian fluvial sandstones and conglomerates
- low oxygen precambrian atmosphere results in little or no conversion of U4+ to U6+, preserves un-leached uranium = presents as uraninite ore
How do we find uranium on earth?
- standard prospecting tech
- also gross count survey (Geiger-Muller detector)
- gamma ray spectrometry survey (detects radiation specific to different elements)
what us beneficiation and enrichment of uranium?
Beneficiation:
- uranium ore is very low grade in U3O8, less than 1%. thru mechanical and chemical concentration, a concentrate of enriched U3O8 is produced (called yellowcake)
enrichment:
- production of nuclear fuel (235U) from yellowcake
What is gaseous diffusion?
- U3O8 converted to uranium hexafluoride gas (UF6), by being passed thru porous barriers
- INCREDIBLY expensive. ~30% total nuclear fuel cost.
What are nuclear fuel rods?
- enriched UF6 oxidized into UO2 and converted to ceramic powder
- compacted into small fuel pellets, and loaded into metallic fuel rods, then placed into elements.
What is reactor burning?
- the actual process of producing nuclear energy
- fuel rods loaded into reactors
- irradiated with neutrons to initiate fission, chain reaction occurs, which produces energy for 3-5 yrs.
- rods are replaced when they are no longer able to support chain reaction
What are waste rods and how are they dealt with?
- nuclear fuel rods that are not longer able to support chain reaction (all fissionable material is used up)
- when removed, they are still ‘hot’ and contain highly radioactive but short-lived isotopes
- temporarily housed in water tanks to cool and absorb radiation for several months
- afterwards permanently stored in geologic strata (buried)
What is the natural geologic reactor??
- geologic evidence for fossil natural fission reactors in gabon west africa
- natural hydrogenic uranium deposit, that has depleted 235U content and increased nuclide content - which suggests former nuclear fission
- occurred around 2 billions yrs ago
what are power generating reactors
- machines that help slow neutrons (via light elements) to capture natural uranium and support/cause fission
What are some examples of power generation reactors?
- magnox reactor
- CANDU reactor
- AGR’s (advanced gas-cooled reactors)
- boiling and pressurized water reactors
- fast breeder reactors
What are some potential future technologies we can use in nuclear reactors?
- small modular reactors (miniaturized reactor tech, much smaller scale for places with modest power needs, require no human intervention to maintain.)
- fusion reactors (controlled fusion of nuclei to release huge amounts of energy)
What is fusion?
joining of two lighter atoms to make a heavier one
What are some causes of nuclear accidents?
- inherent complexity (the more complex a system, the more possible fail points exist)
- design/modelling errors (any unexpected behavior of reactor or fissionable material may exceed a systems ability to cope, any emergencies not forseen will not have plans in place to help solve them)
- mechanical failure (equipment not built for encountered conditions, or poor maintenance/cost-saving results in failure)
- human error (design stage, improper operation, improper decommissioning)
- intentional sabotage or attack
- natural disaster (major nat disasters can cause damage to reactors and disable safety mechanisms)
What are some types of nuclear accidents?
- loss of coolant (which can cause overheating of the reactor or steam explosion is water is cooled)
- melt-down (reactor core becomes too hot, causes fuel to melt, can melt thru reactor housing and into the ground)
- Waste fuel accidents (failures in cooling ponds for waste fuel rods, or low-grade leaks from reprocessing plants/fuel containers)
- processing accidents (low-grade leaks and danger of criticality)
What was chernobyl? What caused it?
- Ukraine, 1980’s
- the worst nuclear accident in history, level 7 event
- experiment for emergency systems causes reactor power output to become unstable, core overheats and reactor heats resulting in huge surge of reactant power and output
- steam explosion causes more increase in reactor power output, then a second explosion causes a runaway fission cascade, graphite moderator catches fires - which disperses radioactive material
What were the main causes of chernobyl?
- human error, bad design, no containment structure
What was the worst nuclear accident in North America?
- Three mile island, Pennsylvania in 1979
What occurred at three mile island?
- loss of coolant, partial core meltdown, major contamination of coolant fluid and power station site.
- minor hydrogen explosion due to exposed fuel rods reacting with steam
- radioactive steam vents into the atmosphere
- contamination occurring at plant and around plant
What was the worlds 2nd level 7 nuclear disaster?
Fukushima Daichi boiling water reactors in Japan in 2011
What occurred at Fukushima?
- plant built on subduction zone
- earthquake causes tsunami, damaged onsite equipment
- reactors 1,2,3 SCRAM’d, 4,5,6 were already shut down
- tsunami wave flooded backup generators
- emergency battery ran out, reactors 1,2,3 overheat and meltdown
- hydrogen explosions occur
- release of radiation into ocean
What is SCRAM?
A rapid emergency shutdown of a nuclear reactor
What are the front-end sources of nuclear waste?
- waste generated in fuel processing
- mining waste (not very radioactive)
- depleted uranium
What are the back-end sources (after fission) of nuclear waste?
- spent fuel rods, containing fission products
- neutron poisons
What are low level nuclear wastes?
- paper, rags, tools, clothing
- these contain small amounts of shortlived radioactivity
- more of a precautionary thing
- typically dealt with by shallow burial and incineration
What are intermediate level wastes?
- higher radioactivity than low level waste,
- resins, chemical sludge, reactor fuel cladding, contaminated materials from reactor decommission.
- can be solidified in concrete or bitumen for disposal
- typically deposited/buried deeper underground if long-lived contaminants
What are high-level wastes?
- waste produced by nuclear reactors
- fission products, elements from reactor core that are highly radioactive and thermally hot
What are disposal methods for high-level radioactive waste? intermediate treatment, for materials with concerns over long-lived radioactive elements
- vitrification (radioactive waste bonded with glass in steel containers, welded shut. Stable for 1000+ yrs)
- ion exchange (ferric hydroxide used to concentrate radioactive contaminants into sludge, mixed with concrete + buried)
- synroc (mineral-based substrate absorbs liquid high-level waste, turns into synthetic rock)
What are disposal methods for high-level radioactive waste? For long term management and disposal
- re-use and reprocessing (some elements extracted for industry applications, such as food irradiation, radio-medicine, or nuclear assisted hydrogen production methods for oil recovery)
- transmutation (placing highly radioactive elements in breeder reactors to bombard with neutrons and convert to less dangerous elements)
- space disposal (but very dangerous and costly)
- Geologic disposal (burial of nuclear waste into deep geologic formations
