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

10th June 2009 2Roger Barlow: nsFFAGs and ADSRs

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


Solution 1: the Cyclotron

• p=0.3 B R – let R increase, B constant• frequency is constant

Continuous current


Solution 2: The Synchrotron

p=0.3 B R – let B increase, R constant

B

t

Pulsed current


Solution 3: the FFAG

Field varies in spaceBut not in time – Fixed FieldIncrease in R is

medium/smallp=0.3 B R holds


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


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


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Built in 1950s for electrons – superseded by synchrotrons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


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


EMMA

Electron Machine with Many Applications10-20 MeV electron accelerator42 cells. 19 RF cavities.Accelerates in ~16 turns


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


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 ….


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


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


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


Thorium

Fertile, not fissile 232Th +n 233Th233Pa233U • Abundant. (Like lead) and spread around• Much smaller waste problems (no long-lived

actinides)• Proliferation resistant


Energy Amplifier (Rubbia)

Idea has been around for years

Nobody’s built one yet!

Feeling is that the accelerator is the weak point.


Waste from ADSR

Needs storing – but not foreverMinor Actinides (Np, Cm, Cf) are not produced


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


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


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…


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?)


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)


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


“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


http://en.wikipedia.org/wiki/UK_Ministry_of_Defence

Enriched Thorium

Can you build a bomb from Thorium, the counterpart of the 235U device?

No


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


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


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


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


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


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


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


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


http://www.thorea.org/

NsFFAGs and ADSRs what they are and why you need them 1. The current nsFFAG programme 2. Thorium...

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