TE/MSC Group Meeting – Safety Day CERN, 18 September 2014 [email protected]@cern.ch Page 1/18 Contents 1 – Collision hazard 2 – Tool handling

TE/MSC Group Meeting – Safety Day

CERN, 18 September 2014 [email protected]

Page 1/18

MAGNETIC MEASUREMENT LABORATORY cern.ch/mm

Contents

1 – Collision hazard

2 – Tool handling

3 – Impact on human health

Working Safely with the Magnetic Fields

of the Accelerator Magnets

Marco Buzio

mailto:[email protected]



Page 2/18


Collision hazard – what can go wrong




Page 3/18


Collision hazard examples

Relatively common accidents in the MRI

(Magnetic Resonance Imaging) field.

One fatality on record

(O2 bottle)

Worst case at CERN:

LEP’s L3 0.5 T magnet

(currently in ALICE)

none injured.




Page 4/18


Tool handling inside a magnetic field

uniform field in the gap

tools align to the magnetic field

r

B

gradient field in the end regions

tools are pulled into the magnetic field

in the fringe: B 3 mT for sources 100 mT (Directive 2013/35/EU)

non-magnetic

CuBe tools




Page 5/18


Magnetization of ferromagnetic objects

force per unit volume is much stronger on elongated objects

Sforce

widely spaced poles little demagnetization (N0)

N

Hd

force NS

Hd

closely spaced poles

strong demagnetizing field

(e.g. sphere N=1/3)

H=Hext+Hd, Hd=-NM

𝑀=𝝁𝒓−𝟏

1+𝑁 (𝝁𝒓−𝟏)𝐻 𝑒𝑥𝑡[ 𝐴 /𝑚]

H field (irrotational i.e.H=0)

NS inside and outside the material

B field (solenoidal i.e.B=0)

closed field lines

Hint

B

Hd demagnetizing field

Hext external field

M magnetization




Page 6/18


Safety classification of magnetic materials

• Non-magnetic and weakly magnetic (r 1-10)

- Elements: aluminum, copper, titanium

- Bronze, brass, beryllium copper, aluminum bronze

- Austenitic + high Ni/Cr/Mo stainless steels e.g. 316 (annealed)

- Virtually all polymers and glasses, most ceramics

• Strongly magnetic (r 10-5000)

- Elements: iron, nickel, cobalt

- Low-C (soft) e.g. ARMCO steel

- Ferritic (e.g. 409) and martensitic (e.g. 420) stainless steels

- Most other steel types

- NiFe alloys e.g. permalloy, mumetal (r up to 106

)

- Ferrites (Mn/Ni/Zn ceramics)

• Permanent magnets (Br 1.5 T)

- Ferrites, AlNiCo, rare-earth ceramics (NdFeB, SaCo)

large

magnetic forces




Page 7/18


Radioprotection instrumentation and magnetic fields

individual dosimeters = passive sensors → no problem

fixed induced

activity monitors

may be affected

(e.g. ATLAS)

→ calibrated in situportable survey meters, electronic dosimeters:

impact of B varies by type (e.g. B<50 mT)

→ may fail or give inaccurate readings

KTT: DGS/RP/SP + Politecnico di Milano are developing field-

compatible dosimeters ( 1 T for now)

→ 4 prototypes available on demand




Page 8/18


Interference with welding process

• plasma arc- or electron beam-based welding processes are sensitive to local and

ambient magnetic field

• lower-current methods (e.g. TIG) tend to be most sensitive

• problems appear already between 1 and 4 mT: instability, “arc blow” (deflection), molten metal spray arc welding impossible above 20-40 mT

example: moderate arc blow

(source www.twi.co.uk)




Page 9/18


Improvised current leads

• Also related to live magnets: risk of electrical arcs in case of sudden lead disconnection

• Energy stored in the inductor E = ½ LI2

is released very quickly → risk of electrical shock

and irreversible damage to the insulation




Page 10/18


Incorrect current lead connection - 1

Accident occurred on a SPS main dipole test bench in bldg. 867, during a 6kA rampup (2012)

6kA current leads

fluxmeter

MB dipole

6kA copper

connection box




Page 11/18


Incorrect current lead connection - 2

molten Cu

bad connection copper melts circuit opens electrical arc explosion, flame

fastening bolts

freshly broken

long-time broken

(cause of the fault)




Page 12/18


Magnetic field effects on human health

• Iron in human blood: 3 g (red cells) 1 g (ferritin) (typ. adult male values)

isolated atoms, no ferromagnetic domains negligible forces

• conducting fluid elements induced currents

magnetic drag flow slows down (7% @ 5T)

small blood pressure increase (3% @ 8T)

small effects no health hazard of whole-body field immersion B8 T

(confirmed by epidemiologic studies in the MRI field)

v = flow speed

(max 2-3 m/s in the ascending aorta)

B = magnetic field

(worst case = horizontal)

E=v × B J = E

electric field, induced current

dF/dA = J B = magnetic drag force


http://brucelashleydpm.files.wordpress.com/2012/03/bigstock_human_circulatory_system_full_30332087.jpg



Page 13/18


Interaction of DC fields with implants

orthopedic implants

plates, screws, rods

neural/bone

stimulators

infusion pumps

heart valves,

pacemakers

hearing aids

cochlear implants

IUD

eye implants

splinters

implants may malfunction or dislodge (especially if recent)

magnetic anus

needles

bullets, shrapnel

Co-Cr implants

braces

vascular clips

(e.g. aneurysm)

jewelry, piercings




Page 14/18


Interaction of magnetic fields with pacemakers

• Normal pacemaker function: sense electrical cardiac pulses, if needed provide pulses at an

appropriate intensity and rate

• A reed switch can be magnetically closed from outside to:

- disable pulse sensing and go into fixed-frequency mode (asynchronous pacing)

- go into programming mode

• Uncontrolled switch behavior if B > 0.7 mT

competitive rhythms discomfort, arrhythmia, death

• AC fields may interfere with pulse detection/generation electronics

pacemakers, implantable defibrillators (ICD) etc

exposure to B > 0.5 mT is absolutely forbidden

reed switch

external field magnetization and closure of contacts

field sources > 0.5 mT are ubiquitous (office magnets, electrical components, machinery ….)




Page 15/18


Exposure limits at CERN according to IS36.2

B > 200 mTB 0.5 mT

heart implant

(pacemaker, defibrillator)

B 10 mT

general public

(generic implants)

B 200 mT

employees

(all categories)

40 h /week OK

conservative limit

takes into account

potential long-term effects

occasionally OK

need authorization of

Medical Service/RSO




Page 16/18


Safety perimeter around magnets

• fringe field radiates from the gap as far as 45 gap lengths (more if the coils are exposed !)

• for non-saturated magnets, minor leakage only from the yoke

• safety perimeter measured and documented in some cases (e.g. main PS units)

B(r)

r

1/r3

decay rate

(far field)




Page 17/18


Safety signs

WARNING: flashing light + delimitation

magnetic field 0.5 mT

field map showing 0.5 mT and 10 mT boundaries must be exposed

and communicated to HSE

(this is done in CMS and ATLAS: not possible in TE/MSC labs!)

a whole enclosed area can be marked

as restricted (authorization needed)

bldg. 181

bldg. 375 – ISR tunnel




Page 18/18


Safety Form OHS 0-0-3 Occupational Hazards (for staff members)

• To be compiled at least once a year (MARS

interview) or upon function changes

• For our typical sources,

tick boxes 604 and 605

(somewhat unclear formulation, will be updated to separate RF from quasi-

DC sources)


Documents

TE/MSC Group Meeting – Safety Day CERN, 18 September 2014 [email protected]@cern.ch Page 1/18 Contents 1 – Collision hazard 2 – Tool handling