Thermochemistry and
Nuclear Power
Jonathan Lee
Thermodynamics
First 2 laws provide the main constraints on any
power system.
We can’t produce energy or reduce entropy.
Exergy is the useful measure of efficiency.
Entropy is related to temperature. High
temperature implies less entropy.
In a Thermal system
We want high output exergy for high efficiency.
If we want low grade energy, we should get the
required entropy from the environment.
Hence heat pumps for heating if possible.
Chemistry
Reactions occur spontaneously when the
entropy change involved is positive.
Chemical bonds tie up both entropy and energy.
The ratio of entropy to energy in chemical bonds
is higher, so for the same exergy input more
“energy” can be given out.
Electrical Grids
Need to balance supply to demand
Many renewables are poor on this, Solar and
Wind particularly so.
A grid needs to absorb fluctuations and have
backup generation
Why this is a problem
Normal renewables are very variable.
In the 2006 California heatwave, the
aggregate state generation was under
4% rated capacity for a week.
According to E.On Netz, German wind
produced between 0.1% and 32% of
daily peak load in 2003, and 0.2% to
38% in 2004.
Backup generation is needed.
E.On Netz data also shows changes of
10-16MW/min occur for long periods
(several hours)
So Backup must be very responsive
Possible solutions
Energy storage of some variety
- Pumped Storage, Molten Salt (≈0.1MWh/ton)
Altering other production to match quickly
- Pumped Storage, Gas Turbines
- For most thermal plants, altering production
quickly isn’t possible.
Why we still need Hydrocarbons
Lots of uses, especially if people don’t want to
change lifestyles:
• Plastics
• Lubricants
• Feedstocks in other industry (Haber-Bosch)
• Aviation
• Arguably cars
Transport
PHEV/BEV are limited by global lithium supply.
Vehicle production is roughly x10 too high.
NiMH/Lead Acids can be used, but heavier and
less efficient.
Greening fuels cleans things up now. Swapping
to new vehicles will take at least 15 years.
How to make them?
Fisher-Tropsch synthesis:
H2 + COx → Oxy-hydrocarbons + H2O
Clearly this needs Hydrogen. Related reactions
can form CO or C as output from CO2.
Biology
Limited by low temperature and lack of
choices.
Making Hydrogen
Electrolysis converts low entropy electricity to
higher entropy chemistry. 30-45% efficient.
High Temperature Electrolysis is a little better, as
some energy comes from heat.
Thermochemistry: 65% total exergy efficiency is
possible without combined processes or
particular cunning.
Sulphur Iodine Process
High temperatures –
>800°C at one point
Multiple reactions at
different temperatures
Efficiency limited by need
to drop temperature
between stages
Combined Hydrogen and Power (CHyP)
Consider using the thermal gradient between
stages to drive electrical generation.
All exergy in the system can now be used. Exergy
either heats chemicals, and is scavenged as
they are cooled, or it drives electrical
generation.
Responsiveness
Under CHyP, whatever doesn’t go into the S-I
process produces electricity.
The exergy consumed in S-I can be varied simply by
throttling flow rates in the heat exchangers, at a
slight cost in overall efficiency.
Hence electrical production is decoupled from
energy production in the plant.
Generalisation
There is nothing special about S-I in this. Any
endothermic thermo-chemistry works, Eg
• Haber-Bosch – Both H2 use and heating.
• Blast furnaces – Injecting hot CO
• Cement Kilns – Simple heat.
Conclusion on energy
If we have a high temperature heat source, we
can produce hydrogen for Fischer-Tropsch
Generalised CHyP lets us use ‘baseline’
generators to handle short term fluctuations.
This allows more renewable usage, if they’re
economically viable.
High Temperature Power
Steam plants are limited to 600°C by the creep
strength of steel. Gas turbines run hotter.
If we want no CO2 output, then we need to
capture and store or go nuclear.
Problems with CCS imply nuclear
Nuclear Power
We can’t use steam to cool, and we want an
unreactive coolant that doesn’t need pressure
- Ionic salts are good, Fluorides in particular
- LiF and BeF2 mixtures can run from 400°C to
1300°C without pressure
Avoiding bomb materials and enrichment would
be bonuses
Thorium Cycle
Naturally pure Th232 is bred to fissile U233
Thorium is 3 times more abundant, and all Th232
No enrichment, and U233 ends up with U232
contamination, so it’s unsuitable for weapons.
The breeding can be done with thermal neutrons,
so much easier to handle. <0.1 neutrons/fission
excess in any design.
Molten Salt Reactors
Integrate the fuel into the primary coolant loop,
as both will be molten and thus should be
unreactive ionic salts.
This lets you reprocess the fuel continuously.
Hexafluoride volatility and distillation make
the process straightforward.
Fuel breeding
External blanket of Thorium salts absorbs neutrons
to produce fuel.
Th232+n → Th233 → Pa233 (22m)→ U233 (27d)
Further neutron absorption is bad – U234 isn’t fissile.
In Chlorides, the Pa233 can be extracted trivially as
PaCl4 is volatile. Hence essentially pure U233 can
be extracted.
Why do this?
Long term waste output is low if
reprocessing is continuous
0.1% of that of a light water reactor
per unit power
Most waste is short half life fission
products.
Waste is less radioactive than natural
uranium ores after around 300 years.
Avoiding separate coolant loops allows
higher power density. Also helped by
high fissile content of fuel.
Smaller cores mean they’re easier to
build and shield.
Safety
Small core and liquid fuel give lots of options.
Fuel can be physically removed easily and with
passive mechanisms, which prevents core
overheats.
First order stable with respect to power output
and voids.
The LFTR concept
Liquid fluoride cooled, thorium breeder reactor.
Optionally Chloride blanket to allow high purity
fuel production
High temperature allows high efficiency.
Passively safe, and few neutrons emitted.
Very little waste produced.
LFTR-CHyP
High temperature baseline power station being
utilised to provide responsive power.
Hydrogen for oxyhydrocarbon production or
immediate industrial use.
With CHP, desalination or other low
temperature processes, exergetic efficiency is
high.
Large scale renewables become viable.
Thank you for listening
Any questions?