Forschungszentrum Karlsruhe in der Helmholtz-Gemeinschaft September 2006, IAEA Headquarters, Vienna, AustriaTLKN. Bekris Slide # 1 FZK - EURATOM ASSOCIATIONHVT-TLK Gamma irradiation of flakes retrieved from the JET fusion machine N. Bekris, J.P. Coad, L. Doerr, W. Ngele Forschungszentrum Karlsruhe in der Helmholtz-Gemeinschaft September 2006, IAEA Headquarters, Vienna, AustriaTLKN. Bekris Slide # 2 FZK - EURATOM ASSOCIATIONHVT-TLK Introduction Forschungszentrum Karlsruhe in der Helmholtz-Gemeinschaft September 2006, IAEA Headquarters, Vienna, AustriaTLKN. Bekris Slide # 3 FZK - EURATOM ASSOCIATIONHVT-TLK Chemical composition of the flakes In JET two kinds of co-deposited films were found. Hard films Films co-deposited on hot surfaces (> 300C) having a D/C ~0.4 Soft films Films co-deposited on cold surfaces (~ 40C) having D/C ~0.7 to % Be 0.6% metals 96% Carbon 3% O 2 An elemental analysis of the latter gave Forschungszentrum Karlsruhe in der Helmholtz-Gemeinschaft September 2006, IAEA Headquarters, Vienna, AustriaTLKN. Bekris Slide # 4 FZK - EURATOM ASSOCIATIONHVT-TLK Flakes characterisation After the DTE1 campaign 150g flakes were collected in the louvres. Extensively analysed. They were found to have an average diameter of 400m and occasionally lengths up to 20mm. They are saturated with hydrogen isotopes with a D/T ratio ~ 50* and exhibit a layered structure archeologically deposited. ~ 1g was sent to TLK for analysis BET surface (4.7 0.3) m 2 g -1. Density 1.69 0.02 g cm -3 Average tritium activity (1.17 0.1) TBq g -1 or 3.3 mg tritium per g of flakes Strongly off-gassing S. Knipe, et al.Fusion Engineering and Design 5859 (2001) 383387 *assuming that at the divertor the flakes are highly hydrogenated i.e. Having the soft flakes composition (D/C~0.75) Forschungszentrum Karlsruhe in der Helmholtz-Gemeinschaft September 2006, IAEA Headquarters, Vienna, AustriaTLKN. Bekris Slide # 5 FZK - EURATOM ASSOCIATIONHVT-TLK Packing and shippment of ~1g flakes to TLK ~0.93 g flakes was packed and sent to TLK 41 g silicagel (flax bag), 71g Pd pellets, The tritium content of each part was measured 5.07 Ci inter. bag 0.43 Ci flax bag (silicagel) 5.78 Ci silicagel 16 mCi Pd 0.93 g ~700 times lower activity due to the alunised bag barrier From April 1999 (packed at JET) until to December 2000 first opening of the package there is 21 months Average tritium release 0.58 Ci g -1 m Ci 5.78 Ci 16 mCi Forschungszentrum Karlsruhe in der Helmholtz-Gemeinschaft September 2006, IAEA Headquarters, Vienna, AustriaTLKN. Bekris Slide # 6 FZK - EURATOM ASSOCIATIONHVT-TLK First Measured Tritium Release Tritium release rate 0.58 Ci/g/m0.43 Ci/g/m Forschungszentrum Karlsruhe in der Helmholtz-Gemeinschaft September 2006, IAEA Headquarters, Vienna, AustriaTLKN. Bekris Slide # 7 FZK - EURATOM ASSOCIATIONHVT-TLK Tritium released in the RBX box at TLK 3.35 x / x / x 10 9 / x 10 9 / x 10 9 / x 10 9 / x / x / Total release (Bq / Ci) ,5 to 1525 to 245N2N nd130.47N2N to to to 100* na Oxygen increase [ppm] to 2426N2N to N2N N2N Dry air nd1Dry air nd21Dry air Release rate Ci/g/m Moisture increase in glove box [ppm] Period of release [months] Atm.Date * Peak value 250 ppm Forschungszentrum Karlsruhe in der Helmholtz-Gemeinschaft September 2006, IAEA Headquarters, Vienna, AustriaTLKN. Bekris Slide # 8 FZK - EURATOM ASSOCIATIONHVT-TLK What is the driving force? The energy carried out by the beta particle is more than sufficient to produce the excitation and the ionisation of the surrounding molecules which it find along its path, delivering enough energy to break covalent chemical bonds such as T-C, and induce a series of all kind of chemical reactions between the constituents of carbon/hydrogen co- deposit. The neutralisation of the radicals and other transient species formed along the track, has as final result the production of tritiated hydrogen, oxygen and other molecular tritiated hydrocarbon species which are more or less volatile. The final and direct observable consequence is then the tritium chronic release. A radiation-induced mechanism taking place at room temperature and having a rate proportional to the tritium remaining in the flakes was proposed. R ray branch End of path -- Ionisations and excitations Forschungszentrum Karlsruhe in der Helmholtz-Gemeinschaft September 2006, IAEA Headquarters, Vienna, AustriaTLKN. Bekris Slide # 9 FZK - EURATOM ASSOCIATIONHVT-TLK Why use gamma rays as possible detritiation method Among the possible interactions between the electromagnetic radiation and the matter the released after the tritium decay can interact with graphite only via the photoelectric effect. Could be the process enhanced? -irradiation could be used as a possible in-situ detritiation technique Forschungszentrum Karlsruhe in der Helmholtz-Gemeinschaft September 2006, IAEA Headquarters, Vienna, AustriaTLKN. Bekris Slide # 10 FZK - EURATOM ASSOCIATIONHVT-TLK Absorber thickness (g/cm) particles transmitted RmRm Background Electron maximum range in matter As an electron passes through matter it loses its energy in ionising and radiative collisions. Its range R (E,d) is an experiemental concept and it is related to the thickness of an absorber needed to stop the electron. A typical experimental number- distance curve for a continuous beta- ray spectrum is illustrated. The maximum range R m, is taken as the point where the absorption curve merges with the background. As the electron is Continusly Slowing Down along its path, R is also called CSDA range. is the apparent absorption coefficient (cm -1 ) x the thickness of the absorbing material (cm) The transmission curve is nearly exponential over the majority of its length Forschungszentrum Karlsruhe in der Helmholtz-Gemeinschaft September 2006, IAEA Headquarters, Vienna, AustriaTLKN. Bekris Slide # 11 FZK - EURATOM ASSOCIATIONHVT-TLK Continuous Slowing Down Approximation range (CSDA) Based on a compilation of a large number of Range-Energy data, for energies from 10keV to ~3MeV, Katz and Penfold proposed the following empirical relantionship: where n= lnE and E in MeV * Calculated using ESTAR, Materials Science and Engineering Laboratory NIST, 100 Bureau Drive, Stop 8500, Gaithersburg, MD , USA. In a similar way the mass-absorption coefficient can be calculated based on Bethe Bremstrahlungs theory using ESTAR. Using experimental data or Bethe`s theory, we may estimate an average range for the electron in graphite of ~0.3m (max range ~3.4m) Target: Graphite d=1.7g/cm) Forschungszentrum Karlsruhe in der Helmholtz-Gemeinschaft September 2006, IAEA Headquarters, Vienna, AustriaTLKN. Bekris Slide # 12 FZK - EURATOM ASSOCIATIONHVT-TLK The G -value In radiation chemistry, the energy of the photon or the particles exceeds by far the binding energy of an electron (normally i.e. 285 tritons are generated after each tritium decay (assuming that we are forming HT) For the 0.93g flakes, calorimetry (10/04/01) estimated a total tritium activity of ~ Bq or decays / month x 285 tritons / decay = tritons / month therefore = tritons/month/g flakes. As A= N and = s -1 we obtain an average release rate of Bq/month/g or Ci/month/g flakes. However, the assumption that we are producing HT is overestimating the tritium amount present in the sample. Indeed, the D/T=50. That means that the average tritium release the 10 th of April 2001 is = 0.46 Ci/g/month !! Forschungszentrum Karlsruhe in der Helmholtz-Gemeinschaft September 2006, IAEA Headquarters, Vienna, AustriaTLKN. Bekris Slide # 14 FZK - EURATOM ASSOCIATIONHVT-TLK Possible Reactions Tritium decay Homolytic rupture Homolytic rupture and trap Hydrogen abstraction Hydrogen substitution Peroxide formation Oxygen formation Polymerisation Hydrogen formation Forschungszentrum Karlsruhe in der Helmholtz-Gemeinschaft September 2006, IAEA Headquarters, Vienna, AustriaTLKN. Bekris Slide # 15 FZK - EURATOM ASSOCIATIONHVT-TLK Experimental set-up Four 60 Co were used having an activity going from 1.4 to 62 Ci. The sources were places in the corners of a square having a side of 6 cm while the flakes were placed in the centre. The dose delivered by the 4 sources at the centre of the square was measured using the Fricke dosimetry. The irradiation was performed inside a lead box at the Hot- Cells of the FZK. The method is based on the oxidation of a solution of ammonium sulphate followed by a spectrophotometric analysis at 305 nm. Dose rate = 6.06 krad h Co Sources 0.9g Flakes Forschungszentrum Karlsruhe in der Helmholtz-Gemeinschaft September 2006, IAEA Headquarters, Vienna, AustriaTLKN. Bekris Slide # 16 FZK - EURATOM ASSOCIATIONHVT-TLK Fricke dosimetry Forschungszentrum Karlsruhe in der Helmholtz-Gemeinschaft September 2006, IAEA Headquarters, Vienna, AustriaTLKN. Bekris Slide # 17 FZK - EURATOM ASSOCIATIONHVT-TLK = extinction coeff. ( cm -1 ) = length of the optical path (cm) = density, (1.025g/cm) G-value (Fe 3+ ) = 15,6 Dose-concentration correlation The mean absorbed dose (D A ) for the dosimeter solution is given by Forschungszentrum Karlsruhe in der Helmholtz-Gemeinschaft September 2006, IAEA Headquarters, Vienna, AustriaTLKN. Bekris Slide # 18 FZK - EURATOM ASSOCIATIONHVT-TLK Purging flakes with He+0.1%H 2 before irradiation + CO Forschungszentrum Karlsruhe in der Helmholtz-Gemeinschaft September 2006, IAEA Headquarters, Vienna, AustriaTLKN. Bekris Slide # 19 FZK - EURATOM ASSOCIATIONHVT-TLK Purging flakes with He+0.1%H 2 after 3.5 Mrads Forschungszentrum Karlsruhe in der Helmholtz-Gemeinschaft September 2006, IAEA Headquarters, Vienna, AustriaTLKN. Bekris Slide # 20 FZK - EURATOM ASSOCIATIONHVT-TLK Comparing the behaviour of the more representative masses before and after 3.5 Mrads + C 2 H 4 / + CO Forschungszentrum Karlsruhe in der Helmholtz-Gemeinschaft September 2006, IAEA Headquarters, Vienna, AustriaTLKN. Bekris Slide # 21 FZK - EURATOM ASSOCIATIONHVT-TLK Comparison with irradiation of sulfates Gamma irradiation seems ineffective. Even though the amount of the released O 2 seems to be constant, the amount of the released water after irradiation seems to be reduced indicating that water is consumed after tritium decay. On the other hand, it was already demonstrated that the presence of water and heavy metals is essential for an effective absorption of the gamma rays. Indeed, Fe 2+ sulfates submitted to a 60 Co irradiation are oxidised to Fe 3+. The oxidation rate increases with the dose and with the number of water molecules. The radiolytic yield expressed by: = Fe 3+ / Fe tot follows a first order exponential law = 1-e (-kD) D = absorbed dose (Mrads) k = rate constant (Mrads -1 ). Forschungszentrum Karlsruhe in der Helmholtz-Gemeinschaft September 2006, IAEA Headquarters, Vienna, AustriaTLKN. Bekris Slide # 22 FZK - EURATOM ASSOCIATIONHVT-TLK Comparison with irradiation of sulfates (2) The reaction taking place in the sulfate system are very similar to the ones evoked earlier for saturated hydrocarbons H 2 O H. + OH. The OH. radicals are directly bonded to the Fe ++ and therefore can easily oxidise the iron, On the other hand the H. radicals are much more mobile (than. OH) and therefore they can easily move across the crystal network and give the same type of reactions observed during the irradiation of alcanes, i.e. hydrogen abstraction, substitution or addition and finally recombination. H. + HOH-Fe ++ H 2 +. OH-Fe ++. OH-Fe ++ - OH-Fe +++ H. + H. H 2 Forschungszentrum Karlsruhe in der Helmholtz-Gemeinschaft September 2006, IAEA Headquarters, Vienna, AustriaTLKN. Bekris Slide # 23 FZK - EURATOM ASSOCIATIONHVT-TLK The influence of water and the Z number FeSO 4.xH 2 O I M 2 (FeSO 4 ) 2.6H 2 O K NH 4 Rb CsCs Forschungszentrum Karlsruhe in der Helmholtz-Gemeinschaft September 2006, IAEA Headquarters, Vienna, AustriaTLKN. Bekris Slide # 24 FZK - EURATOM ASSOCIATIONHVT-TLK Summary Flakes are very likely to be present in any fusion machine unless a mitigation or detritiation process, delay or destroy their formation. Flakes are characterised by a chronic tritium release which is probably initiated by the tritium -decay (radio-induced process). The tritium release experiments performed at TLK gave an average tritium release rate of ~0.58Ci/month/g flakes (21m after packing). This rate is slowly decreasing with the time. The release experiment also showed the release of O 2 and small water amounts. The presence of O 2, H 2 (HT) and other volatile compounds (identified by mass spectrometry) can be explained by a series of radiochemical reactions induced by the tritium decay. During such reactions the water contained in the samples is consumed. Gamma irradiation does not enhance the tritium release rate. As it was already demonstrated (sulfates) only the presence of water or heavy elements can increase the efficiency of the -irradiation. There is still a need for an efficient detritiation method.