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TW4-TSW-002/D4: Studies on possible concrete detritiation

TW4-TSW-002/D4: Studies on possible concrete detritiation. Johan Braet , John Seghers, Kris Dylst Final Meeting of contracts TW3 and TW4 17 January 2005 EFDA CSU, Garching. Studies on possible concrete detritiation. Incentives to initiate the task at SCK • CEN : Possible waste problem:

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TW4-TSW-002/D4: Studies on possible concrete detritiation

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  1. TW4-TSW-002/D4: Studies on possible concrete detritiation Johan Braet, John Seghers, Kris Dylst Final Meeting of contracts TW3 and TW4 17 January 2005 EFDA CSU, Garching

  2. Studies on possible concrete detritiation • Incentives to initiate the task at SCK•CEN: • Possible waste problem: • Tritium can migrate deep into concrete => tritiated waste • Development of decontamination methods for concrete might be necessary. • SCK•CEN has experience in handling tritiated waste in general and in steam leaching detritiation processes for tritiated metals in particular. • The deliverable for SCK•CEN is:

  3. Tritiated waste streams at JET/ITER/CTR require specific solutions • The long term outcome is to have for each tritiated waste stream a route for its management and its eventual disposal or recycling. • SCK•CEN has been focusing for decades on the treatment of tritiated waste, amongst them: • Water detritiation • Treatment of tritiated liquid organics • Decontamination of tritiated solid materials • Recently SCK•CEN conducted lab scale experiments on the detritiation of tritiated metal using superheated steam. • Objectives are to obtain a good decontamination (high DF) and a production of low-tritiated water ready for discharge or a production of a limited volume of to-be-treated tritiated water

  4. Interaction with other associations • The diffusion and measurement of tritium in concrete in function of depth is studied by the French association within TW4-TSW-002. • Samples were to be made available by JET.

  5. H-atoms in concrete are present as water in pores/capillaries, water of crystallization and in actual cement molecules • Exposing concrete reactor walls for 20 years to air containing 0.02 Bq/cm³ results in a tritium concentration of 107 Bq/m³ in a region up to 0.65 cm deep. • Estimations for JET reveal that: • 2750 tonnes concrete would be above UK free release of 0.4 Bq/g (Belgian limit 100 Bq/g for T) • 70,000 tonnes of exempt concrete • T-measurements in torus hall JET show: • 13 Bq/g at surface • Few Bq/g for samples deeper in the wall

  6. When heated (tritiated) concrete* releases (tritiated) water Hydrogen/tritium distribution collected as water in hardened cement pastes *Cured concrete was exposed to air containing tritiated vapour. The tritiated water vapour mixes with and diffuses into the liquid water in the pores and capillaries. Some will interact further with hydrogen in the solid phase. • Free water is slightly enriched in tritium • Heating up to 400°C would result in a DF of 13. Observations

  7. Literature review on detritiation techniques • Different detritiation techniques are available: • Chemical decontamination: Purging, oxidation, isotope exchange • Purging with a noble gas at high temperature • Oxidation in a two step process: tritium purging and tritium/oxygen reaction • Atmospheres promoting isotope exchange • At SCK•CEN tests are done using steam, sometimes in the presence of an inert carrier gas (argon)

  8. In the frame of this limited task some tests are done to determine the feasibility • Three real objectives: • To obtain a good decontamination (high DF). • To obtain a low amount of tritiated water. • To avoid the production of secondary waste. • The influence of the following parameters is tested: • Working temperature • Steam flow rate • Ratio carrier gas/steam

  9. An installation has been build that can be used for testing the detritiation of concrete samples

  10. Test installation

  11. Specification of the samples • Since no JET samples were available it was agreed to use simulated samples • Origin: • Old concrete from an industrial installation • Based on Portland cement • Exposed to rain over several years => completely cured • Tritiation process: • 6 hours in a tritiated steam flow • Activity of the water 2.1 MBq/ml • No contact between concrete and the reflux • Dried at room temperature for at least 14 days • Activity of a sample: 4.9 kBq/g

  12. Detritiation tests with superheated steam

  13. Working with steam has certain advantages • An additional test was done by simply heating the concrete up to 850°C under a carrier gas flow • Advantage : • high temperature results in high DF (560) • Disadvantage: • Low dewpoint The concentration of the (entrained tritiated) water vapour is very low in the carrier gas leaving the furnace • Trapped on molecular sieves => secondary waste • Extra detritiation technique required

  14. Combining both seems an interesting option when high DF’s are required • Treatment using superheated steam (360°C, below triple point) • Supplementary treatment of ‘detritiated’ concrete at 850°C under carrier gas to remove final tritium residues • One experiment : • a DF of more than 1600 was reached • 15 g of water produced per g concrete • About 5 kBq tritium in water and 0.07 kBq in molecular sieves per g concrete

  15. Conclusions (1) • Tritiated concrete could be a waste issue during decommissioning: • Tritium can migrate deep into concrete. • 2750 tonnes of concrete at JET are above the UK free release limit of 0.4 Bq/g • Average surface contamination is 13 Bq/g at JET • No appropriate (industrial) detritiation technique available

  16. Conclusions (2) • Experiments were performed at SCK•CEN’s test installation for detritiation • Using concrete samples of 0.2 kg which were initially tritiated using steam of 2.1 MBq/ml. Initial activity was 5 kBq/g. • A decontamination factor between 20 and 80 was obtained using superheated steam of 360°C during 6 h • A DF of 13 is reached by simply heating the concrete up to 360°C. Heating up to 850°C gives a DF of 560 • A very high decontamination factor could be reached (1600) by further heating the sample after being treated with superheated steam

  17. Industrial applicability? • It seems that a treatment with superheated steam is feasible, since for free release of JET concrete a DF of only 33 is necessary. • However under current regulations very large amounts of concrete need to be treated • Free release: 0.4 Bq/g (UK) 100 Bq/g (BE) • Obviously not all the produced tritiated water can be treated in a WDS • During treatment segregation of the concrete is necessary in order to produce disposable water with very low tritium concentrations or as small as possible amounts of to-be-treated water

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