Dust studies at jet a widdowson Dust/flake collection at jet

Dust studies at JET

• A Widdowson

Dust/flake collection at JET

Analysis of dust removed from JET in 1986

Total mass of resuspended dust 2.29 mg, air concentration 11 µg.m-3, resuspension ratio 10-5

• Total mass of resuspended dust 2.29 mg, air concentration 11 µg.m-3, resuspension ratio 10-5

• Median mass aerodynamic diameter for Be aerosols: 4.01.9µm (not many)‏

• Aerosols (%): C 80.3, Be 0.03-0.7, Ni 1.2, Fe 16.7, Cr 1.3, Co 0.02

• Debris (%): C 27.5, Be 0.35, Ni 54.9, Fe 5.4, Cr 11.8, Co 0.08

• Initial T release on venting 200 MBq (1 MBq.m-3), thereafter wall releases 145 Bq.m-2.s-1

• T activity on wall from dust: 1.40MBq.m-2 (280MBq total)‏

• Mass activities (Bq/g): aerosols 2.1 104 , dust 2.5 106, debris 2.7 103

• Accumulation of dust in lower part of outer wall

• Kinetics of T desorption faster for small particles

Summary of dust/flake analysis 1986/1992

Dust and flakes from JET MkIIa in 1996

Dust and flakes from JET MkIIa in 1996

• Vacuuming: negligible dust <1mg/m2

• Smearing: Divertor and first wall, average 1.2g/m2

• Flakes collected from inner louvres and bottom of tile 3

• Flakes found to have a high D/C ratio

• Flaking of deposit at louvres at venting, spallation of thick deposits

• Mechanisms:

• Dust from bombardment of deposits by ions and charge exchange neutrals in areas exposed to the plasma

Analysis of dust and flakes removed in 1998 after DTE1 (MkIIa)‏

• Dust and flakes collected in 2 cyclone pots from vacuuming divertor

• 56.8g total collected in pot 1
• 97.5g total collected in pot 2

• Analysed at JET

• Samples sent to tritium laboratory, Karslruhe and AEA technology, Winfrith

Analysis of dust and flakes removed in 1998 after DTE1

• Dust:

• Samples from 2 cyclone pots + meshes analysed by AEA Technology. Total mass ~0.12g (no.1) & 0.09g (no.2)‏
• A few inner corner flakes were collected in Pot1 – to exclude these from analysis, the sample was sieved to limited particle size to 90m

• Particle sizes peak (by number) at 0.6-0.8 and 5-6 microns

• Flakes

• 89% released by heating to 800C (peak release rate at 500C)‏
• There was also a BET value of 675 m2/g (believed to be incorrect)‏

Analysis of dust and flakes removed in 1998 after DTE1

Flakes on louvres in 2001

Analysis on JET 2004 dust by JET Health Physics Group

• Airborne dust – CMD 0.2-0.3 µm, VMD 0.3-4.4 µm, MMAD 0.56-7.5 µm

• Vacuumed dust – all sizes sub-µm to flakes, T 3.43GBq/g, SSA 20-29m2/g, ~9% Be in C

• Tile scrapings – T 0.36GBq/g, SSA 3-16m2/g, ~13% Be in C

• Dissolution tests (tile scrapings) – For ~1GBq/g activity levels 10s of milligram of dust could results in a dose of up to 3.7 mSv dose but would not be detected in urine tests.

• For higher tritium activity levels (i.e. 1TBq/g as observed after DTE) dust can be categorised as “highly radiotoxic.”

• CMD: count median diameter ALI: annual limit of intake (20mSv)‏

• VMD: volume median diameter SSA: specific surface area

• MMAD: mass median aerodynamic diameter

Summary of historic JET dust catalogue

• Characterisation of a range of dust/flake samples from JET, including T content, particle sizes, specific surface area and composition.

• Potential for large doses from dust. Tritiated dust could be classified as highly radiotoxic.

• Some work has been done on mobilisation. Resuspension fraction ~10-5.

• Of the order of 1kg/campaign of material is deposited in JET that may lead to flakes and dust

• Further evaluation of the last two points form the basis of CRP proposal

Dust production

• Dust formation is linked to the erosion and re-deposition of material from plasma facing components.

• Re-deposited material is found to be a major source of dust

• Amongst open issues for assessment of dust production for ITER is the percentage of eroded and re-deposited material contributing to dust production

• Safety considerations in ITER assumes the most pessimistic case

• All co-deposited material is available for dust production.

• By measuring deposition and erosion on JET tiles an estimate of the “worse case” quantity of dust production can be determined.

• Aim of CRP proposal:-
• Correlate the amount of dust produced in JET with the eroded and re-deposited of material from plasma facing components
• Estimate of the fraction of eroded/re-deposited material contributing to dust formation.

Dust collection plan

• Formulate a plan for collecting dust from JET during ITER-like wall (ILW) shutdown (2008/2009)‏

• Collect loose flakes and dust samples from different areas of the JET vessel (particularly the divertor) during ILW shutdown in 2009-10.

• Weigh the amount of dust collected from different areas of the JET vessel and estimate dust production

• Difficulties:

• Quantifying the period over which the dust has accumulated as various tiles have been exchanged over the operating lifetime of JET and dust sampling has been made previously.
• Mechanical abrasion during maintenance may also provide a source of flakes and debris.

• Determine the composition and quantity of deposits on JET tile surfaces which constitutes a potential source of dust in tokamak devices.

• Ion beam analysis, cross sectional microscopy, SIMS

• Evaluate erosion from tiles in specified areas of the JET vessel

• Profile of eroded samples

Vacuuming dust in JET

• Dust collection will be via remote handling vacuuming

• Dust will be collected is a series of cyclone pots according to a dust collection plan

• Pots will be weighed before and after vacuuming

Other dust studies in JET

• Collaboration with Laboratoire de Physique et de Métrologie des Aérosols, Instiut de Radioprotection et de Sureté Nucléaire

• Assessment the aerosols in JET
• Average mass concentration,
• Size distribution with cascade impactor, APS (Aerodynamic Particle Sizer) and by EEPS (Engine Exhaust Particle Sizer),
• Measurement of the number concentration in real time,
• Specific sampling for an analysis with a Transmission Electronic Microscope (TEM).

• Perform dust sampling during the shutdown phase, such as smears of the JET vessel walls and air sampling in-vessel.

• From dust sampling the amount and distribution of dust collecting at the vessel wall and the mobilisation of dust during the shutdown phase can be determined

• Dust collection plan is mainly foreseen as a collection programme as extensive analysis facilities are not available at JET.

• Dust samples will be made available to interested associations for analysis.

Current status of CRP proposal

• Phase 1: (2008-09)‏

• Plan collection of dust from JET tokamak.

• Identify tiles to be removed from vessel for analysis of deposits.

• Plan other dust sampling procedures.

• Plan the measurement of tile surfaces to determine erosion from specified areas.

• Phase 2: (2009-10)‏

• Collect dust from JET vessel during 2009-2010 shutdown phase.

• Complete other dust sampling techniques during 2009-2010 shutdown phase of JET vessel.

• Remove tiles from JET vessel for both analysis of deposits and surface erosion.

• Phase 3: (2010-2013)‏

• Quantify the amount of dust collected from the JET vessel.

• Analyse deposits on tile surfaces. Quantify the potential for dust creation in-vessel and compare with actual dust quantities collected.

• Quantify the amount of erosion from specified tile surfaces and correlate with deposits.

• Determine other dust quantities from dust sampling techniques.

The End