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

Johan Braet, John Seghers, Kris Dylst

Final Meeting of contracts TW3 and TW417 January 2005

EFDA CSU, Garching

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:D4 Study on the possible detritiation of tritium

contaminated concrete with superheated steam. Final Report

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 detritiationTreatment of tritiated liquid organicsDecontamination 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

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.

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

When heated (tritiated) concrete* releases (tritiated) water

GroupGroup Temperature (°C)Temperature (°C) Percentage HPercentage H Percentage TPercentage T StateState

II < 200< 200 6565 7777 Liquid water (free, capillary Liquid water (free, capillary water)water)

IIII 200 – 400200 – 400 2020 1515 Water of crystallizationWater of crystallization

IIIIII 450 – 550450 – 550 1212 77 Decomposition of Ca(OH)Decomposition of Ca(OH)22

IVIV 550 – 850550 – 850 33 11 Dissociation of OH- base in Dissociation of OH- base in calcium silicate hydratescalcium silicate hydrates

Hydrogen/tritium distribution collected as water in hardened cement pastesHydrogen/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

Literature review on detritiation techniques

• Different detritiation techniques are available:

Chemical decontamination: Purging, oxidation, isotope exchangePurging with a noble gas at high temperatureOxidation in a two step process: tritium purging and

tritium/oxygen reactionAtmospheres promoting isotope exchange

• At SCK•CEN tests are done using steam, sometimes in the presence of an inert carrier gas (argon)

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 temperatureSteam flow rateRatio carrier gas/steam

An installation has been build that can be used for testing the detritiation

of concrete samples

Test installation

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

Detritiation tests with superheated steam

Test 1Test 1 Test 2Test 2 Test 3Test 3

DecontaminationDecontamination

³H activity concrete ³H activity concrete before before

Bq/gBq/g 80008000 50005000 50005000

³H activity concrete after ³H activity concrete after Bq/gBq/g 330330 7070 6060

DFDF 2424 7171 8383

Tritiated waste waterTritiated waste water

³H activity³H activity MBqMBq 1,371,37 1,031,03 0,220,22

volume waste watervolume waste water mLmL 12821282 26852685 31633163

mass per g concretemass per g concrete g/gg/g 7,27,2 15,215,2 67,167,1

average specific activity average specific activity Bq/mLBq/mL 10681068 325325 8282

³H activity first ³H activity first condensatecondensate

Bq/mLBq/mL 1570015700 16901690 1140011400

³H activity last ³H activity last condensate condensate

Bq/mLBq/mL 114114 2222 2,22,2

Test 1Test 1 Test 2Test 2 Test 3Test 3

ConcreteConcrete

weightweight gg 179179 208208 4040

crunched crunched -- -- √√

Gas flowsGas flows

carrier gas carrier gas L/hL/h -- 4343 --

steam flow steam flow raterate

g/hg/h 214214 452452 449449

Test conditionsTest conditions

durationduration minmin 360360 420420 360360

average average temperaturetemperature

°C°C 326326 343343 355355

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

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

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/gAverage surface contamination is 13 Bq/g at

JET• No appropriate (industrial) detritiation technique

available

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

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