NsFFAGs and ADSRs what they are and why you need them 1. The current nsFFAG programme 2. Thorium...
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Transcript of NsFFAGs and ADSRs what they are and why you need them 1. The current nsFFAG programme 2. Thorium...
nsFFAGs and ADSRswhat they are and why you need them1. The current nsFFAG programme2. Thorium ADSRs and their advantages3. Matching the two
Roger BarlowManchester University
Seminar 10th June 2009
Accelerators 101
Cyclic accelerators use dipole magnetsp=0.3 B R
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RF acceleration
Positive kicks to the particles every time round
fRF= N fcirculating
Problem: putting these togetherHow to keep p = 0.3 B R with increasing p?How to keep RF frequency in sync with changing particle revolution frequency
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Solution 1: the Cyclotron
• p=0.3 B R – let R increase, B constant• frequency is constant
Continuous current
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Solution 2: The Synchrotron
p=0.3 B R – let B increase, R constant
B
t
Pulsed current
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Solution 3: the FFAG
Field varies in spaceBut not in time – Fixed FieldIncrease in R is
medium/smallp=0.3 B R holds
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Focussing
Particles in a bunch divergeNeed to focus using
quadrupole magnet
Problem: a quad which focusses in X defocusses in Y
Solution: a pair of quads has a net focussing effect
Alternating Gradient(aka Strong Focussing)
10th June 2009 7Roger Barlow: nsFFAGs and ADSRsFFAG field contains quadrupole components
Accelerators 101 (contd.)
Off-axis particles oscillate about the reference orbit: Betatron Oscillations
Tune: Number of betatron oscillations per turn
Integer Tune (Resonance) =death
Imperfections:Errors in position, current etc in a magnet means a particle gets the wrong ‘kick’Over many turns this smears out – if the particle is a different points on its betatron oscillation each time
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Scaling
Synchrotron during acceleration cycle: p=0.3 B R
• Bending dipoles and focussing quadrupoles carry same (increasing) current
• The optics – prisms and lenses – looks the same• Tune stays constant• Setting the tune to something sensibly non-
resonant means it stays there
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Scaling FFAGs
FFAG dipole and quadrupole fields not automatically in step
To achieve needs complicated and slowly-varying magnetic field (B ~ Rk) – and hence large beam pipes
Built in 1950s for electrons – superseded by synchrotrons
Now revived for protons in Japan
Why were FFAGs abandoned? Increase in momentum in FFAG ring limited to factor of 2-5(?) by geometry. Synchrotron AC magnets have much larger dynamic range – better for highest energies
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Nonscaling FFAGs
FFAGs accelerate fast (hence great for muons):Limited by RF power, not by magnet rampingIf we can go through a resonance quickly
enough it may not matter.Drop scaling requirement – simpler and more
compact.Will it work? Only one way to find out
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EMMA
Electron Machine with Many Applications10-20 MeV electron accelerator42 cells. 19 RF cavities.Accelerates in ~16 turns
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nsFFAG Benefits
• High currents – like a cyclotron• High energies – like a synchrotron• A ring not a disc – cheaper than cyclotron• Simple DC magnets• Very large acceptance• Fast acceleration time
Smaller, more compact systems for proton acceleration – applications in medicine and
power
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Part 2. Shrinking our Carbon Footprint
We all know fossil fuels are BAD because1.They cause climate change2.They are increasingly concentrated in
countries with dodgy politics3.They are going to run outAlternatives (windmills, solar power, improved
insulation, retreat to the middle ages) can’t supply the deficit without ….
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Nuclear Power
Fossil fuels will need to be replaced by a basket of alternativesIt is hard (impossible?) to put such a basket together without nuclear power
Big issues (real or in the eyes of the public?) with:•Safety: Chernobyl and 3 Mile Island•Waste disposal. Storage for millenia - NIMBY•Proliferation. Rogue states and terrorist organisations
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Safe Subcritical Reactors
Conventional:Run with k=1 exactly
k<1 stopsk>1 explosion
Sub CriticalRun with k<1
Use accelerator to supply extra neutrons
Hence: Accelerator Driven Subcritical Reactor (ADSR)
Each fission absorbs 1 neutron and produces ~2.5
Some neutrons lost, leaving k neutrons to produce k fissions
10th June 2009 16Roger Barlow: nsFFAGs and ADSRs
ADSRs
“Manifestly Safe”Switch off accelerator and
reaction stopsEnergy balance is OK: need 5-
10% of power to run accelerator
Can use Thorium as fuel
Accelerator
Spallation Target
Core
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Thorium
Fertile, not fissile 232Th +n 233Th233Pa233U • Abundant. (Like lead) and spread around• Much smaller waste problems (no long-lived
actinides)• Proliferation resistant
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Energy Amplifier (Rubbia)
Idea has been around for years
Nobody’s built one yet!
Feeling is that the accelerator is the weak point.
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Waste from ADSR
Needs storing – but not foreverMinor Actinides (Np, Cm, Cf) are not produced
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Transmutation
• Neutron flux can burn actinides produced by conventional reactors. MYRRHA project.
• Also destroy most-problematic fission products (e.g. 99Tc: soluble, T½=211,000 Y) by ‘Adiabatic Resonance Crossing’. Lead moderator to ensure neutrons hit the resonance for absorption
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Accelerator requirements
Proton Energy ~ 1 GeV gives ~20 spallation neutrons per proton. For 1GW thermal power:• Need 3 1019 fissions/sec (200 MeV/fission)• 6 1017 spallation neutrons/sec (k=0.98 gives 50 fissions/neutron)• 3 1016 protons/sec Current 5 mA. Power = 5 MWReliable! Spallation target runs hot. If beam stops, target cools and stresses
and cracks: no more than 3 trips per year
Compare: PSI cyclotron: 590 MeV, 2mA, 1MWISIS synchrotron: 800 MeV, 0.2mA, 0.1 MWSeveral trips per day
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Reliability: the 3rd Frontier
In the real world:Accelerators often trip for seconds/hours/days.
They are complicated systems operating in real world environments
But there are complex real world pieces of apparatus trip that trip rarely. Planes, computers, radio sets…
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How to achieve Reliability
Reliability must be paid for:• Parallelism• Robustness under failure• Under-rating• Preventive MaintenanceMust throw money accurately at the problemNeed thorough understanding of complete system
and to learn from experience (with prototypes?)
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Accelerators for ADSRs
Cyclotron
Energy too high for classical cyclotron. On the edge for other types
FFAG
Looks like the answer
“Cyclotron currents at Synchrotron energies”
Simplicity = reliability
Linac
Can do the job. But VERY expensive
Synchrotron
Current far too high.
Complicated (ramping magnets)
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Proliferation: Issues and Questions
“Thorium fuel system does not produce weapons”
• Explains why nuclear power went the U/Pu route back in the 1950’s
• Solves today’s dilemma of states like Iran
Is it true?1. ‘Dirty bomb’2. ‘Little boy’ type device3. ‘Fat man’ type device
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“Dirty Bomb”
(Spent) fuel rods will contain fission productsDispersal over civilian areas would cause panic, expense,
and few fatalities It is thought that during the 1960s the
UK Ministry of Defence evaluated RDD*s, deciding that a far better effect was achievable by simply using more high explosive in place of the radioactive material.
Wikipedia
* RDD: Radiological Dispersion Device
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Enriched Thorium
Can you build a bomb from Thorium, the counterpart of the 235U device?
No
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233U device
In principle possibleCritical mass ~15kgNo spontaneous fission problems: simple gun-
type device233U ratio in fuel stabilises after about 5 years.
Extract chemically from Thorium
10th June 2009 29Roger Barlow: nsFFAGs and ADSRs
Together with 232U232Th(n,2n) 231Th 231Pa then 231Pa(n,) 232U 14 mb for neutron energies above threshold ~6 MeV
233U(n,2n) 232U4 mb for neutron energies above threshold ~6 MeV
Fast neutrons from tail of fission spectrum – or spallation
10th June 2009 30Roger Barlow: nsFFAGs and ADSRs
232U : makes 233U unworkable
232U decays with a half life of 69 y, producing 228Th which decays producing a 2.8 MeV ray. Really nasty stuff
50 ppm 232 in 233 gives (long term) ~2 rem/hr for a worker 0.5m from a 5kg sphere. Health and safety limit 5 rem/y. Lethal doses 200-1000 rem
It is also bad for electronics
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Possible loophole
• Can’t separate U isotopes• Can chemically isolate the
intermediate 233Pa.• Wait (27 d half life) for it to
decay to pure 233U• Some MSR schemes use just
this
10th June 2009 32Roger Barlow: nsFFAGs and ADSRs
Ionium to the rescue
• Ionium is 230Th• It does not occur in Thorium, which is pure
232• It does occur in Uranium, part of the 238U
decay chain• ‘spike’ Thorium with Ionium: get Pa and 232U
10th June 2009 33Roger Barlow: nsFFAGs and ADSRs
Proliferation: Conclusions
• Safety depends on design• Advantage to have all fuel exposed to fast
neutrons to ensure 232U concentration• Ionium may be needed• Building a device will be very difficult.
Technology beyond the reach of back street terrorists, detectable by WMD inspectors
10th June 2009 34Roger Barlow: nsFFAGs and ADSRs
SummaryADSRs provide a possible form of Nuclear Power that
avoids the problems of• Critical accidents• Long-lived waste• ProliferationFFAGs may provide the best accelerator technologyWe (UK, Particle Physicists, Manchester, Cockcroft
Institute) are working hard to make it happen
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Formation of the Thorium Energy Amplifier Association: Universities and labs and industry
A research consortium aimed at • Networking (website, workshops)• Sharing knowledge, within and outside UK• Arousing interest in Research Councils, Whitehall, etc.• Collaborative response to funding opportunities• Design of a Thorium ADSR, aimed at power generation with
transmutation as bonus. FFAG is baseline accelerator If you’re interested, see www.thorea.orgNext Meeting: Daresbury – July 10th
A way forward
10th June 2009 36Roger Barlow: nsFFAGs and ADSRs