CIVE40010 Energy & Environmental Engineering Flashcards
which countries are currently produce the most amount of nuclear energy ?
- USA
- France
- China
- Russia
- South Korea
what percentage of UK energy need is currently supplied by nuclear ?
17 %
what is a PWR ?
nuclear
pressurised-water reactor
- pure water heated to very high temp. through fission
- kept under v. high pressure
- converted to steam by a steam generator
- turbines -> generators -> electrical power
describe nuclear fission
- neutron induced process
- Nucleus fragments into two or more smaller
elements (fission products), generating heat (KE -> heat through collisions with fuel lattice) and other neutrons, - Only happens for large nuclei (eg Uranium, Plutonium, etc) known as fissile (particular isotopes only)
- chain reaction (fission event produces servral neutrons), however likelihood of chain reaction depends on neutron velocity and the fuel content (fission cross-section).
what is the fuel for nuclear fission reactors ?
usually, uranium (either U235 or U238 isotope)
how do you increase likelihood of nuclear fission chain reactions ?
- increase amount of fuel, enrichment (~4%)
- slow down neutrons, moderation
two types of nuclear fission reactors ?
Thermal Reactors (most used)
- high velocity neutrons slowed into thermal region using a moderator material (eg, graphite, light water H20, heavy water D20)
Fast Reactors
- no moderation required, use of enrichment
How is nuclear fission rate described ?
Effective neutron multiplication factor
(keff): how many neutrons from one
fission, cause another?
* keff > 1 fission rate increases exponentially
* keff < 1 fission rate decays
* keff = 1 criticality – the operating state of a
power reactor.
Describe the structure of a fission reactor?
- uranium contained as uranium dioxide (UO2) stacked in fuel rods and installed in fuel assemblies which form the reactor core
- control rods (material that absorbs nuetrons so keff<1) to stop reaction when required)
- enclosed in reactor pressure vessel
- pump coolant in (cold) and comes out hotter (in PWR, water)
what are the three sources of nuclear waste ?
mining & fuel fabrication
radioactive waste
storage, GDF
what are the five main reasons for waste arising ?
nuclear
- mining and milling of uranium ores
- spent fuel discharged from reactors
- reprocessing of fuel
- Decommissioning
- Military
- Medical and engineering applications
what are the three categories of waste ?
- LLW (low level waste) 91% radioactive waste - discarded equipment, tools, protective clothing
- ILW (intermediate level waste) 9% radioactive waste- stripped/leached remains of cladding or PCM
- HLW (high level waste) 0.1% radioactive waste - fission products
depending on radiation levels
(note : uk classification has vllw (very low level waste))
describe radioactive waste management
HLW requires immobilisation followed by** storage underground**, methods include :
Vitrification in glass
- Commonly borosilicate glass
- Some components are immiscible in glass
Encapsulation in ceramics
- SYNROC is the most common (composite)
- Fluorite, Pyrochlore , Phosphate
Mixtures of glass and ceramic - composite
what are key functions of deep geological nuclear disposal ?
1) Isolate waste from near-surface
processes and human activities,
2) Protect the biosphere,
3) Limit release from progressively
degrading waste package,
4) Disperse and dilute flux of longlived radionuclides.
what are different fuel types ?
nuclear
- Natural uranium metal
- Uranium dioxide
- Thorium, other
note : PWR uses uranium oxide, thermal neutron cycle and light water as coolant
what types of reactors are there ?
nuclear
- PWR pressurised light water reactor (high safety, loss of coolant = loss of moderator = reaction shut down)
- AGR advanced gas cooled reactor ( uranium dioxide fuel with graphite core as moderator, using gas as coolant )
- BWR boiling water reactor ( lower pressures, cheaper, no steam generator)
- PHWR pressurised heavy water reactor
currently all use thermal neutron cycle with moderator
future tech :
- advanced reator (Gen IV), using closed fuel cycles (‘fast reactors’)
- small modular reactors
describe difference between closed & open fuel cycles
nuclear
open : fuel used once & discarded
modified open : part separation of actinides and fuel reprocessing
closed (full recycle) : full separation of actinides from fuel, therefore waste only contains fission products
half-life significantly reduced for actinide separation
what is ccs ?
CO2 can be captured
from a process or the
atmosphere and
permanently stored
geologically
how much ccs do we ideally need ?
To meet COP21 targets, we
need to capture a total of ~10 Gt
CO2 per year.
~2000x 5 MMtpa projects?
(currently at about 20 plants)
what are the ccs categories
Post-combustion capture
– Separate CO2 from gas mixture after combustion
Pre-combustion capture
– Separate CO2 from the production of H2, combust the H2
Oxy-fuel combustion
– Combust fuel in O2, no further separation needed
what is a TRL
technology readiness levels
- must advance through series of scale-up steps
- TRL 9 is ‘ready to go’
- from initial research to commercial readiness
Describe post-combustion capture methodology ? (inc. advantages & disadvantages)
amine-based chemical absorption
- uses 2x columns solvent regeneration & co2 scrubbing columns
- outputs compressed CO2 for storage
advantages :
* Retrofittable
* Mature technology
* Experience with ‘large-scale’ in the O&G industry
* Flexibility – range of operating conditions
disadvantages :
* High CapEx – large gas volumes → large equipment
* Parasitic energy (uses ~ 20% energy output)
* Solvent disposal
describe amine degradation ?
Solvent susceptible to chemical degradation in O2, SO2, CO2, high temperatures
Solvent losses to environment – vapourisation, entrainment
Degradation products could present health risks
– Amides, aldehydes, nitrosamines, nitroamines
Release mitigation measures
* Reclaimers to reverse thermal degradation (more energy required)
* Water wash and demisters to minimise entrainment
* Waste management practices – treatment/disposal
what is adsorption ?
another method of post-combustion capture
* Adhesion of species/molecules to a solid surface
* Governed by electrostatic interactions
* Porous solids are used in a cyclic process to separate
gas mixtures
– Pores give more surface area per unit volume
– Enables re-use of the adsorbent
Adsorption is proportional to pressure/concentration
– Reducing the pressure reverses the process
– Pressure/vacuum-swing adsorption (PSA, VSA, PVSA)
* Adsorption is exothermic
– Heating the adsorbent reverses the process
– Temperature-swing adsorption (TSA) (much faster, less co2 recovery)
* Solids (beads) are traditionally packed in a vessel to contact with gas
– Pressure drop limits gas throughput per vessel
explain pre combustion co2 capture
- React fuel with steam & small amount of O2
– Produces ‘syngas’ – CO2, CO, CH4, H2
– Syngas further reacted to produce CO2 and H2 - CO2 is separated from H2
- H2 is used as a fuel
- First part of process is identical to conventional H2 production process
‘hydrogen production’ , complex
options:
– Pressure swing adsorption
– Cryogenic separation
– Physical absorption
– Chemical absorption
