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Conceptual design of Tritium Extraction System for the European HCPB Test Blanket Module
Fusion Engineering and Design 87 (2012) 620–624
Contents lists available at SciVerse ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Conceptual design of Tritium Extraction System for the European HCPB Test Blanket Module A. Ciampichetti a,g,∗ , F.S. Nitti a,g , A. Aiello a,g , I. Ricapito b , K. Liger c,g , D. Demange d,g , L. Sedano e,g , C. Moreno e,g , M. Succi f a
ENEA CR Brasimone, 40032 Camugnano (Bo), Italy Fusion for Energy, 08019 Barcelona, Spain c CEA, DEN, DTN/STPA/LIPC, Cadarache, 13108 St. Paul-lez-Durance, France d Karlsruhe Institute of Technology, ITEP-TLK, Postfach 36 40, 76021 Karlsruhe, Germany e EURATOM-CIEMAT Association, 28040 Madrid, Spain f SAES Getters Spa, 20020 Lainate (Mi), Italy g European TBM Consortium of Associates, Germany b
a r t i c l e
i n f o
Article history: Available online 3 March 2012 Keywords: ITER Ceramic breeding blanket Tritium extraction Tritium removal
a b s t r a c t The HCPB (Helium Cooled Pebble Bed) Test Blanket Module (TBM), developed in EU to be tested in ITER, adopts a ceramic containing lithium as breeder material, beryllium as neutron multiplier and helium at 80 bar as primary coolant. In HCPB-TBM the main function of Tritium Extraction System (TES) is to extract tritium from the breeder by gas purging, to remove it from the purge gas and to route it to the ITER Tritium Plant for the final tritium processing. In this paper, starting from a revision of the so far reference process considered for HCPB-TES and considering a new modeling activity aimed to evaluate tritium concentration in purge gas, an updated conceptual design of TES is reported. © 2012 Elsevier B.V. All rights reserved.
1. Introduction
2. Tritium extraction procedure
The Tritium Extraction System (TES) together with Helium Cooling System (HCS) and Coolant Purification System (CPS) are the principal auxiliary circuits of Helium Cooled Lithium Lead (HCLL) and Helium Cooled Pebble Bed Test Blanket Modules (HCPB TBMs) to be tested in ITER [1]. The HCS is the primary cooling circuit for the extraction of thermal power from the TBM first wall, TBM box and breeding region. The TES aims to extract tritium from the breeder while the CPS extracts the permeated tritium from the primary circuit, keeping the HCS chemistry controlled. The conceptual design of these systems was completed in Europe by the TBM Consortium of Associates (TBM-CA). This paper focuses on HCPB-TES.
2.1. Basic assumptions In HCPB-TBM, TES adopts a purge gas at low pressure to extract tritium from the ceramic pebble beds. In particular, TES has the following objectives: - to extract the bred tritium from the lithiated ceramic breeder as well as from the neutron multiplier using purge gas; - to remove the tritium compounds (T2 , HT, HTO) from the purge gas; - to route the extracted tritium to the Tritium Accountancy System (TAS) before the final processing in the ITER Tritium Plant; - to control the chemical composition and physical properties of the purge gas stream; - to remove oxygen containing impurities and particles from the purge gas. 2.2. Irradiation scenario and purge gas compositions
∗ Corresponding author. Tel.: +39 0534 801277; fax: +39 0534 801250. E-mail address: [email protected] (A. Ciampichetti). 0920-3796/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2012.01.047
Two TBM irradiation regimes are taken as operative hypothesis in this work: ITER low duty DT phase and high duty DT phase.
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Table 1 TES feed stream in the low duty DT phase during the tritium out-gassing phase. Purge gas flow-rate (Nm3 /h) Temperature (K) HT to be extracted (mg) HT mean molar fraction (vppm) HTO to be extracted (mg) HTO mean molar fraction (vppm) H2 O to be extracted (mg) H2 O mean molar fraction (vppm) H2 to be extracted (g) H2 mean molar fraction (vppm)
8 773 32.7 1.0 5.6 0.03 147 1.0 17.1 1000
Basically, during the low duty DT phase, HCPB-TBM is foreseen to be irradiated with isolated standard pulses, lasting 1800 s with 450 s of burning plasma (ramp-up and ramp-down included). Each standard pulse generates 0.52 mg of tritium in the HCPB-TBM. The objective of the HCPB-TBM testing campaign during the low duty DT phase will be mainly the determination of the tritium breeding performance and, especially, the neutronic code validation for the prediction of TBR (Tritium Breeding Ratio) in DEMO and Power Plant. On the contrary, in the high duty DT-phase long pulses (up to 3000 s) as well as sequences of standard pulses lasting several days without any interruption (back to back pulses) are foreseen. The main tritium related objectives in this phase are the investigations on the physics of tritium release from the ceramic breeder, tritium inventory in the breeder and permeation from the breeder into HCS (Helium Coolant System). 2.2.1. Low duty DT phase The following operational conditions were considered [2]: (a) the tritium out-gassing phase lasts 24 h and is carried out at 500 ◦ C after 50 isolated standard pulses; (b) the purge gas during the tritium out-gassing phase is the mixture He + H2 (0.1 vol.%); (c) the purge gas flow-rate is assumed to be 8 Nm3 /h.
Fig. 1. Q2 and Q2 O partial pressure in purge gas.
by means of a Tritium Transfer Modelling Tool based on TMAP7. The following scenario has been considered for this calculation: 40 Nm3 /h as purge gas flow-rate 300 Pa of H2 in the purge gas mol H2 O = mol HT for the HT/HTO ratio an experimental correlation for LiO2 is assumed [4] - minimum permeation rate toward HCS - back to back series of standard pulses (450 s burn time, 1350 s dwell time, 1800 s repetition time).
-
The results in terms of Q2 and Q2 O concentration in purge gas are reported in Fig. 1. During the burn time these concentrations are within the ranges presented in Table 2. In Fig. 2 the HTO ratio on the total tritium in purge gas is reported. The results show that the mean value of HTO/(HT + HTO) is around 2.6%. This value is close to 3.2% which is the value so far considered [3,5]. However, it is important to underline that recent experiments performed in Japan on tritium release from Li4 SiO4 pebbles show that tritium is mainly released as HTO [6]. 3. Candidate technologies for TES
In Table 1 the feed stream composition at the TES inlet during the tritium out-gassing phase is reported [3]: 2.2.2. High duty DT phase In order to fix the gas stream composition to be processed at the TES inlet during a generic back to back pulse series, the following assumptions were considered: (a) tritium is extracted from the ceramic breeder by purging with He doped with H2 to accelerate the kinetics of tritium desorption; (b) the selected ranges of H2 partial pressure and He flow-rate are 10–300 Pa and 8–40 Nm3 /h, respectively.
The so far proposed TES configuration consists of the following main components [3]: - a cooler to cool down the He stream from the TBM outlet up to RT; - an adsorption column for Q2 O removal operated at RT in adsorption phase coupled with a reducing bed placed in the column regeneration line; - a two bed TSA (Temperature Swing Adsorption) for Q2 removal; - a heater to raise the He stream temperature at the TBM inlet;
In Table 2 the feed stream composition at the TES inlet during a back to back pulse series is reported [2]: The values indicated in Table 2 were evaluated with a simplified model. A more accurate calculation has been developed in CIEMAT Table 2 TES feed stream in the high duty DT phase. Purge gas flow-rate (Nm3 /h) Temperature (K) HT mean molar fraction (vppm) HTO mean molar fraction (vppm) H2 O mean molar fraction (vppm) H2 mean molar fraction (vppm)
8–40 773 0.8–3.9 0.03–0.13 0.8–3.9 100–3000
Fig. 2. Ratio between HTO and total tritium in purge gas.
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- a blower to circulate the He purge gas. During a review process of this configuration it has been decided to compare the TSA and the reducing bed with alternative technologies.
others the proposed regeneration procedures are too demanding in terms of operative conditions) and this component is placed in the port cell of ITER where the operators cannot entry, it has been concluded to consider a PERMCAT reactor as reference solution. 4. System description
3.1. Q2 removal from purge gas The so far reference solution considered for Q2 removal from He purge gas is based on an adsorption process performed on zeolites. In particular, a two bed TSA, operated at LN2 temperature during the adsorption phase and at 100 ◦ C during the regeneration step performed with counter-current purge gas, was selected. It is hereafter proposed an alternative solution based on the adoption of getter beds, whose main advantages are compactness and simple operation procedures. Some Zr-based alloys are attractive for tritium handling because the equilibrium pressure is sufficiently low at room temperature and adequate temperatures for tritium release are not too high so that parasitic leaks by permeation through the containing walls can be avoided. Among the different Zr-based alloys, ZrCo seems to be the most promising. In fact, the intermetallic compound ZrCo is under consideration to be used in ITER for tritium handling instead of Uranium. Thus, different studies have been carried out in order to use ZrCo in ITER fuel gas storage and delivery system (SDS) as reported in [7]. The advantages of using a ZrCo based getter can be summarised as follows: • Low hydrogen absorption pressure at room temperature; • Low hydrogen release temperatures; • Not-radioactive material and low pyrophoricity (in comparison to Uranium); • No need of operation at liquid nitrogen temperature (in comparison to TSA). Among the drawbacks of this technology, the hydrogen-induced disproportionation is probably the most serious one. During disproportionation the dynamic equilibrium between hydrogen in gas phase and in the solid generates a complete rearrangement of the material with an almost total loss of its original absorptiondesorption capability. However, it was shown that ZrCo keeps fully its reversible hydrogen absorption/desorption capability in the temperature range 25–350 ◦ C even when submitted to a large number of thermal cycles [8]. Since a maximum temperature of 300 ◦ C is foreseen for ZrCo in HCPB TES, it is possible to conclude that disproportionation is not a concern. Other issues concerning the adoption of a ZrCo getter, as poisoning by impurity gases and isotope effects, were addressed in [2] and it was concluded that they not limit this choice. However, tests are necessary to confirm the suitability of ZrCo for this specific application and in particular to check the absorption kinetic in the operating conditions foreseen in HCPB-TES. 3.2. Regeneration line of Q2 O adsorption column It is foreseen to reduce the desorbed Q2 O to Q2 before to send it to the tritium plant. For this purpose it is possible to use either a reducing bed based on the adoption of hot metals, so far considered as reference option, or a PERMCAT reactor. A PERMCAT is a catalytic membrane reactor combining a Pd/Ag membrane exclusively permeable to hydrogen isotopes (Q2 ) and a catalyst bed promoting isotope exchanges to liberate tritium from molecules (e.g. H2 + Q2 O ↔ H2 O + Q2 ) [9]. Since the regeneration of reducing beds based on hot metals is a critical issue (many candidate metals are unregenerable and for
The system under consideration is composed by the following components arranged as showed in the P&ID of Fig. 3: -
Filter Economizer Heat exchanger Q2 O adsorption column PERMCAT reactor Q2 getter beds Compressor Heater
The process gas coming from the breeder units is cooled up to room temperature by means an economizer (A) and a water heat exchanger (I), and driven to the adsorption column (C). The adsorption column removes the tritiated water and the gas moves to the getter bed (E and F) for tritium removal. A getter bed constituted by a single column (E) will be used during low duty DT phase. A compressor (H) at the getter outlet pushes the gas in the breeder unit assuring the head for the circulation in the entire loop. The adsorption column is regenerated flushing pure helium and using a PERMCAT reactor (D) to recover tritium from Q2 O in the Q2 form. Filters (B) are used to remove solid impurities and an electrical heater (G) adjust the purge gas temperature at the TBM inlet. Components (A), (B), (C), (D) and (H) will be placed in the port cell, while (E) and (F) in the Tritium Building. The system and its components were designed, in conservative calculation, considering the maximum flow and maximum tritium concentration. Fig. 3 contains also a preliminary identification of tools for process control and tritium monitoring. However, a detailed study of the Tritium Measurement and Accountancy Systems for both HCLL and HCPB TBMs is ongoing (see D. Demange, this conference). The piping of the circuit will be made of austenitic steel (grade AISI 304 or 316) and will have an internal diameter of 0.02337 m. 4.1. Components description 4.1.1. Economizer and water exchanger The two main processes of purge gas purification, Q2 O and Q2 removal, will work at room temperature. Therefore, the purge gas coming out from the breeder at high temperature, 500 ◦ C, needs to be cooled. The cooling of the purge gas will be realized with two heat exchangers in line: a gas/gas economizer and a gas/water heat exchanger. The main function of the economizer is to cool the gas coming from the TBM transferring the heat to the gas flowing toward the TBM. The two exchangers will be assembled in one compact packaging. They will be realized with longitudinal finned double-pipes and counter-current flow scheme, with the hot gas on inner pipe and the cold fluid flowing in the annulus. The design of these components is reported in [2]. 4.1.2. Q2 O adsorption column An adsorption column based on molecular sieve is considered as the reference solution for Q2 O removal. It is foreseen to use a single column.
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Fig. 3. TES piping and instrumentation diagram.
The column will allow the regeneration of the adsorbent material. It is foreseen to regenerate the column with a counter-flow of Helium at 300 ◦ C. Considering 1 vppm for H2 O + HTO in the feed stream and a back to back series of standard pulses lasting 11 days, about 2 g of water will be adsorbed in the column. However, the size of the column will be overestimated to take into account the uncertainties on the percentage of HTO in the stream. The regeneration flow-rate will be in the range 1–10% of the stream flowing during the adsorption phase. However, the regeneration scheme will be optimized by suitable tests. At the end of the series of pulses lasting 11 days, 3 days are available for the regeneration of the column.
4.1.3. PERMCAT reactor The PERMCAT unit will be placed in the regeneration line of the Q2 O adsorption column and will be operated at typically 400 ◦ C in the counter-current isotope swamping technique allowing high tritium recover. The Helium flow coming from the adsorption column will be maintained at about 1 bar, while the hydrogen purge gas at about 50 mbar. Impurity outlet constituted by He and H2 O can be discharged to Tritium Plant Detritiation System (DS) without accountancy, while purge outlet has to be routed to Tritium Accountancy System before discharged to Tokamak Exhaust Processing (see Fig. 3).
4.1.4. Q2 getter bed A preliminary design of a Q2 getter bed based on ZrCo has been carried out in collaboration with SAES getters spa. It has been
considered a Helium flow-rate in the range 8.0–40 Nm3 /h and a Q2 concentration up to 3000 ppmv. The getter will be constituted by: • a thin layer of a high surface getter to remove O2 containing gases, like oxygen, carbon monoxide and carbon dioxide; • ZrCo alloy, capable to remove H2 at room temperature and to release it at temperatures in the range 250–300 ◦ C. The proposed purifier will be made with two vessels, one in absorption phase and the other in regeneration. SAES company indicates a range of 1–10% of the stream flowing during absorption phase for the regeneration flow-rate. The best conditions for the H2 sorption, the ZrCo H2 capacity, the regeneration cycle and the stability of the ZrCo versus the number of cycles will be verified by experimental dedicated tests. In the by-pass line will be placed a getter contained in a single vessel to be used during low duty DT phase. 4.1.5. Compressor The function of the compressor is to supply the appropriate head to guarantee the gas circulation inside the TBM and the TES circuit. The proposed solution is a reciprocating oil-less compressor equipped with a control valve on a by-pass line to guarantee the lowest flow rate (8 Nm3 /h) required by TES. 4.1.6. Filter The purge gas that flows through the pebble bed in the breeder units and through the Q2 getter matrix can detach and drive solid
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radioactive particles that have to be removed and collected with filters. They are placed in the cold part of the loop after the heat exchanger and after the getter beds. Commercial filters with a mesh of 0.01 m will be used.
Measurement and Accountancy Systems and with a more accurate study of tritium concentration in purge gas in several irradiation scenarios.
5. Conclusions
This paper is partly based on the outcomes from the grant F4E2008-GRT-09, funded by Fusion For Energy.
In HCPB-TBM, TES adopts a purge gas at low pressure to extract tritium from the ceramic breeder and from the neutron multiplier. The composition of TES feed stream was evaluated both in high duty DT phase, assuming a back to back series of standard pulses, and in low duty DT phase assuming an ad hoc tritium out-gassing phase after 50 standard pulses. A revision of the reference configuration of TES was carried out together with a preliminary design of the system. The TES scheme foresees to cool the purge gas coming from the TBM up to room temperature by means of an economizer and a water heat exchanger and to remove the tritiated water with an adsorption column based on zeolites. Then, the gas moves to the ZrCo getter beds for tritium removal. Filters are used to remove solid impurities while an electrical heater adjust the purge gas temperature at the TBM inlet. The adsorption column is regenerated flushing pure helium and using a PERMCAT reactor to recover tritium from Q2 O in the Q2 form. TES components will be placed in the ITER port cell n.16, except for the getter beds that will be located in the Tritium Building (level L2). Other activities concerning TES are ongoing in the frame of the European TBM-CA. They deal with the design of Tritium
Acknowledgments
References [1] I. Ricapito, O. Bede, L.V. Boccaccini, A. Ciampichetti, B. Ghidersa, L. Guerrini, et al., The ancillary systems of the European Test Blanket Modules: configuration and integration in ITER, Fusion Eng. Des. 85 (2010) 1154–1161. [2] F.S. Nitti, A. Ciampichetti, HCPB TES preliminary engineering design and analyses, TBM-CA technical report D3-HPS.2009.002. [3] I. Ricapito, A. Ciampichetti, G. Benamati, M. Zucchetti, Tritium Extraction Systems for the European HCLL/HCPB TBMs, Fusion Sci. Technol. 54 (2008) 107–112. [4] M. Nishikawa, A. Baba, S. Odoi, Y. Kawamura, Tritium inventory estimation in solid blanket system, Fusion Eng. Des. 39-40 (1998) 615–625. [5] I. Ricapito, A. Ciampichetti, P. Agostini, G. Benamati, Tritium processing systems for the helium cooled pebble bed test blanket module, Fusion Eng. Des. 83 (2008) 1461–1465. [6] K. Munakata, T. Shinozaki, K. Inoue, S. Kajii, Y. Shinozaki, R. Knitter, et al., Tritium release from lithium silicate pebbles produced from lithium hydroxide, Fusion Eng. Des. 83 (2008) 1317–1320. [7] S. Konishi, T. Nagasaki, N. Yokokawa, Y. Naruse, Development of Zirconium–Cobalt beds for recovery, storage and supply of Tritium, Fusion Eng. Des. 10 (1989) 355–358. [8] N. Bekris, M. Sirch, U. Besserer, R.D. Penzhorn, Disproportionation of ZrCo and its reproportionation, in: Proceedings of 20th SOFT, vol. 2, Marseille, France, 1998, pp. 1021–1024. [9] D. Demange, M. Glugla, K. Günther, T.L. Le, K.H. Simon, R. Wagner, S. Welte, Counter-current isotope swamping in a membrane reactor: The PERMCAT process and its applications in fusion technology, Catal. Today 156 (2010) 140–145.
Contents lists available at SciVerse ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Conceptual design of Tritium Extraction System for the European HCPB Test Blanket Module A. Ciampichetti a,g,∗ , F.S. Nitti a,g , A. Aiello a,g , I. Ricapito b , K. Liger c,g , D. Demange d,g , L. Sedano e,g , C. Moreno e,g , M. Succi f a
ENEA CR Brasimone, 40032 Camugnano (Bo), Italy Fusion for Energy, 08019 Barcelona, Spain c CEA, DEN, DTN/STPA/LIPC, Cadarache, 13108 St. Paul-lez-Durance, France d Karlsruhe Institute of Technology, ITEP-TLK, Postfach 36 40, 76021 Karlsruhe, Germany e EURATOM-CIEMAT Association, 28040 Madrid, Spain f SAES Getters Spa, 20020 Lainate (Mi), Italy g European TBM Consortium of Associates, Germany b
a r t i c l e
i n f o
Article history: Available online 3 March 2012 Keywords: ITER Ceramic breeding blanket Tritium extraction Tritium removal
a b s t r a c t The HCPB (Helium Cooled Pebble Bed) Test Blanket Module (TBM), developed in EU to be tested in ITER, adopts a ceramic containing lithium as breeder material, beryllium as neutron multiplier and helium at 80 bar as primary coolant. In HCPB-TBM the main function of Tritium Extraction System (TES) is to extract tritium from the breeder by gas purging, to remove it from the purge gas and to route it to the ITER Tritium Plant for the final tritium processing. In this paper, starting from a revision of the so far reference process considered for HCPB-TES and considering a new modeling activity aimed to evaluate tritium concentration in purge gas, an updated conceptual design of TES is reported. © 2012 Elsevier B.V. All rights reserved.
1. Introduction
2. Tritium extraction procedure
The Tritium Extraction System (TES) together with Helium Cooling System (HCS) and Coolant Purification System (CPS) are the principal auxiliary circuits of Helium Cooled Lithium Lead (HCLL) and Helium Cooled Pebble Bed Test Blanket Modules (HCPB TBMs) to be tested in ITER [1]. The HCS is the primary cooling circuit for the extraction of thermal power from the TBM first wall, TBM box and breeding region. The TES aims to extract tritium from the breeder while the CPS extracts the permeated tritium from the primary circuit, keeping the HCS chemistry controlled. The conceptual design of these systems was completed in Europe by the TBM Consortium of Associates (TBM-CA). This paper focuses on HCPB-TES.
2.1. Basic assumptions In HCPB-TBM, TES adopts a purge gas at low pressure to extract tritium from the ceramic pebble beds. In particular, TES has the following objectives: - to extract the bred tritium from the lithiated ceramic breeder as well as from the neutron multiplier using purge gas; - to remove the tritium compounds (T2 , HT, HTO) from the purge gas; - to route the extracted tritium to the Tritium Accountancy System (TAS) before the final processing in the ITER Tritium Plant; - to control the chemical composition and physical properties of the purge gas stream; - to remove oxygen containing impurities and particles from the purge gas. 2.2. Irradiation scenario and purge gas compositions
∗ Corresponding author. Tel.: +39 0534 801277; fax: +39 0534 801250. E-mail address: [email protected] (A. Ciampichetti). 0920-3796/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2012.01.047
Two TBM irradiation regimes are taken as operative hypothesis in this work: ITER low duty DT phase and high duty DT phase.
A. Ciampichetti et al. / Fusion Engineering and Design 87 (2012) 620–624
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Table 1 TES feed stream in the low duty DT phase during the tritium out-gassing phase. Purge gas flow-rate (Nm3 /h) Temperature (K) HT to be extracted (mg) HT mean molar fraction (vppm) HTO to be extracted (mg) HTO mean molar fraction (vppm) H2 O to be extracted (mg) H2 O mean molar fraction (vppm) H2 to be extracted (g) H2 mean molar fraction (vppm)
8 773 32.7 1.0 5.6 0.03 147 1.0 17.1 1000
Basically, during the low duty DT phase, HCPB-TBM is foreseen to be irradiated with isolated standard pulses, lasting 1800 s with 450 s of burning plasma (ramp-up and ramp-down included). Each standard pulse generates 0.52 mg of tritium in the HCPB-TBM. The objective of the HCPB-TBM testing campaign during the low duty DT phase will be mainly the determination of the tritium breeding performance and, especially, the neutronic code validation for the prediction of TBR (Tritium Breeding Ratio) in DEMO and Power Plant. On the contrary, in the high duty DT-phase long pulses (up to 3000 s) as well as sequences of standard pulses lasting several days without any interruption (back to back pulses) are foreseen. The main tritium related objectives in this phase are the investigations on the physics of tritium release from the ceramic breeder, tritium inventory in the breeder and permeation from the breeder into HCS (Helium Coolant System). 2.2.1. Low duty DT phase The following operational conditions were considered [2]: (a) the tritium out-gassing phase lasts 24 h and is carried out at 500 ◦ C after 50 isolated standard pulses; (b) the purge gas during the tritium out-gassing phase is the mixture He + H2 (0.1 vol.%); (c) the purge gas flow-rate is assumed to be 8 Nm3 /h.
Fig. 1. Q2 and Q2 O partial pressure in purge gas.
by means of a Tritium Transfer Modelling Tool based on TMAP7. The following scenario has been considered for this calculation: 40 Nm3 /h as purge gas flow-rate 300 Pa of H2 in the purge gas mol H2 O = mol HT for the HT/HTO ratio an experimental correlation for LiO2 is assumed [4] - minimum permeation rate toward HCS - back to back series of standard pulses (450 s burn time, 1350 s dwell time, 1800 s repetition time).
-
The results in terms of Q2 and Q2 O concentration in purge gas are reported in Fig. 1. During the burn time these concentrations are within the ranges presented in Table 2. In Fig. 2 the HTO ratio on the total tritium in purge gas is reported. The results show that the mean value of HTO/(HT + HTO) is around 2.6%. This value is close to 3.2% which is the value so far considered [3,5]. However, it is important to underline that recent experiments performed in Japan on tritium release from Li4 SiO4 pebbles show that tritium is mainly released as HTO [6]. 3. Candidate technologies for TES
In Table 1 the feed stream composition at the TES inlet during the tritium out-gassing phase is reported [3]: 2.2.2. High duty DT phase In order to fix the gas stream composition to be processed at the TES inlet during a generic back to back pulse series, the following assumptions were considered: (a) tritium is extracted from the ceramic breeder by purging with He doped with H2 to accelerate the kinetics of tritium desorption; (b) the selected ranges of H2 partial pressure and He flow-rate are 10–300 Pa and 8–40 Nm3 /h, respectively.
The so far proposed TES configuration consists of the following main components [3]: - a cooler to cool down the He stream from the TBM outlet up to RT; - an adsorption column for Q2 O removal operated at RT in adsorption phase coupled with a reducing bed placed in the column regeneration line; - a two bed TSA (Temperature Swing Adsorption) for Q2 removal; - a heater to raise the He stream temperature at the TBM inlet;
In Table 2 the feed stream composition at the TES inlet during a back to back pulse series is reported [2]: The values indicated in Table 2 were evaluated with a simplified model. A more accurate calculation has been developed in CIEMAT Table 2 TES feed stream in the high duty DT phase. Purge gas flow-rate (Nm3 /h) Temperature (K) HT mean molar fraction (vppm) HTO mean molar fraction (vppm) H2 O mean molar fraction (vppm) H2 mean molar fraction (vppm)
8–40 773 0.8–3.9 0.03–0.13 0.8–3.9 100–3000
Fig. 2. Ratio between HTO and total tritium in purge gas.
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- a blower to circulate the He purge gas. During a review process of this configuration it has been decided to compare the TSA and the reducing bed with alternative technologies.
others the proposed regeneration procedures are too demanding in terms of operative conditions) and this component is placed in the port cell of ITER where the operators cannot entry, it has been concluded to consider a PERMCAT reactor as reference solution. 4. System description
3.1. Q2 removal from purge gas The so far reference solution considered for Q2 removal from He purge gas is based on an adsorption process performed on zeolites. In particular, a two bed TSA, operated at LN2 temperature during the adsorption phase and at 100 ◦ C during the regeneration step performed with counter-current purge gas, was selected. It is hereafter proposed an alternative solution based on the adoption of getter beds, whose main advantages are compactness and simple operation procedures. Some Zr-based alloys are attractive for tritium handling because the equilibrium pressure is sufficiently low at room temperature and adequate temperatures for tritium release are not too high so that parasitic leaks by permeation through the containing walls can be avoided. Among the different Zr-based alloys, ZrCo seems to be the most promising. In fact, the intermetallic compound ZrCo is under consideration to be used in ITER for tritium handling instead of Uranium. Thus, different studies have been carried out in order to use ZrCo in ITER fuel gas storage and delivery system (SDS) as reported in [7]. The advantages of using a ZrCo based getter can be summarised as follows: • Low hydrogen absorption pressure at room temperature; • Low hydrogen release temperatures; • Not-radioactive material and low pyrophoricity (in comparison to Uranium); • No need of operation at liquid nitrogen temperature (in comparison to TSA). Among the drawbacks of this technology, the hydrogen-induced disproportionation is probably the most serious one. During disproportionation the dynamic equilibrium between hydrogen in gas phase and in the solid generates a complete rearrangement of the material with an almost total loss of its original absorptiondesorption capability. However, it was shown that ZrCo keeps fully its reversible hydrogen absorption/desorption capability in the temperature range 25–350 ◦ C even when submitted to a large number of thermal cycles [8]. Since a maximum temperature of 300 ◦ C is foreseen for ZrCo in HCPB TES, it is possible to conclude that disproportionation is not a concern. Other issues concerning the adoption of a ZrCo getter, as poisoning by impurity gases and isotope effects, were addressed in [2] and it was concluded that they not limit this choice. However, tests are necessary to confirm the suitability of ZrCo for this specific application and in particular to check the absorption kinetic in the operating conditions foreseen in HCPB-TES. 3.2. Regeneration line of Q2 O adsorption column It is foreseen to reduce the desorbed Q2 O to Q2 before to send it to the tritium plant. For this purpose it is possible to use either a reducing bed based on the adoption of hot metals, so far considered as reference option, or a PERMCAT reactor. A PERMCAT is a catalytic membrane reactor combining a Pd/Ag membrane exclusively permeable to hydrogen isotopes (Q2 ) and a catalyst bed promoting isotope exchanges to liberate tritium from molecules (e.g. H2 + Q2 O ↔ H2 O + Q2 ) [9]. Since the regeneration of reducing beds based on hot metals is a critical issue (many candidate metals are unregenerable and for
The system under consideration is composed by the following components arranged as showed in the P&ID of Fig. 3: -
Filter Economizer Heat exchanger Q2 O adsorption column PERMCAT reactor Q2 getter beds Compressor Heater
The process gas coming from the breeder units is cooled up to room temperature by means an economizer (A) and a water heat exchanger (I), and driven to the adsorption column (C). The adsorption column removes the tritiated water and the gas moves to the getter bed (E and F) for tritium removal. A getter bed constituted by a single column (E) will be used during low duty DT phase. A compressor (H) at the getter outlet pushes the gas in the breeder unit assuring the head for the circulation in the entire loop. The adsorption column is regenerated flushing pure helium and using a PERMCAT reactor (D) to recover tritium from Q2 O in the Q2 form. Filters (B) are used to remove solid impurities and an electrical heater (G) adjust the purge gas temperature at the TBM inlet. Components (A), (B), (C), (D) and (H) will be placed in the port cell, while (E) and (F) in the Tritium Building. The system and its components were designed, in conservative calculation, considering the maximum flow and maximum tritium concentration. Fig. 3 contains also a preliminary identification of tools for process control and tritium monitoring. However, a detailed study of the Tritium Measurement and Accountancy Systems for both HCLL and HCPB TBMs is ongoing (see D. Demange, this conference). The piping of the circuit will be made of austenitic steel (grade AISI 304 or 316) and will have an internal diameter of 0.02337 m. 4.1. Components description 4.1.1. Economizer and water exchanger The two main processes of purge gas purification, Q2 O and Q2 removal, will work at room temperature. Therefore, the purge gas coming out from the breeder at high temperature, 500 ◦ C, needs to be cooled. The cooling of the purge gas will be realized with two heat exchangers in line: a gas/gas economizer and a gas/water heat exchanger. The main function of the economizer is to cool the gas coming from the TBM transferring the heat to the gas flowing toward the TBM. The two exchangers will be assembled in one compact packaging. They will be realized with longitudinal finned double-pipes and counter-current flow scheme, with the hot gas on inner pipe and the cold fluid flowing in the annulus. The design of these components is reported in [2]. 4.1.2. Q2 O adsorption column An adsorption column based on molecular sieve is considered as the reference solution for Q2 O removal. It is foreseen to use a single column.
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Fig. 3. TES piping and instrumentation diagram.
The column will allow the regeneration of the adsorbent material. It is foreseen to regenerate the column with a counter-flow of Helium at 300 ◦ C. Considering 1 vppm for H2 O + HTO in the feed stream and a back to back series of standard pulses lasting 11 days, about 2 g of water will be adsorbed in the column. However, the size of the column will be overestimated to take into account the uncertainties on the percentage of HTO in the stream. The regeneration flow-rate will be in the range 1–10% of the stream flowing during the adsorption phase. However, the regeneration scheme will be optimized by suitable tests. At the end of the series of pulses lasting 11 days, 3 days are available for the regeneration of the column.
4.1.3. PERMCAT reactor The PERMCAT unit will be placed in the regeneration line of the Q2 O adsorption column and will be operated at typically 400 ◦ C in the counter-current isotope swamping technique allowing high tritium recover. The Helium flow coming from the adsorption column will be maintained at about 1 bar, while the hydrogen purge gas at about 50 mbar. Impurity outlet constituted by He and H2 O can be discharged to Tritium Plant Detritiation System (DS) without accountancy, while purge outlet has to be routed to Tritium Accountancy System before discharged to Tokamak Exhaust Processing (see Fig. 3).
4.1.4. Q2 getter bed A preliminary design of a Q2 getter bed based on ZrCo has been carried out in collaboration with SAES getters spa. It has been
considered a Helium flow-rate in the range 8.0–40 Nm3 /h and a Q2 concentration up to 3000 ppmv. The getter will be constituted by: • a thin layer of a high surface getter to remove O2 containing gases, like oxygen, carbon monoxide and carbon dioxide; • ZrCo alloy, capable to remove H2 at room temperature and to release it at temperatures in the range 250–300 ◦ C. The proposed purifier will be made with two vessels, one in absorption phase and the other in regeneration. SAES company indicates a range of 1–10% of the stream flowing during absorption phase for the regeneration flow-rate. The best conditions for the H2 sorption, the ZrCo H2 capacity, the regeneration cycle and the stability of the ZrCo versus the number of cycles will be verified by experimental dedicated tests. In the by-pass line will be placed a getter contained in a single vessel to be used during low duty DT phase. 4.1.5. Compressor The function of the compressor is to supply the appropriate head to guarantee the gas circulation inside the TBM and the TES circuit. The proposed solution is a reciprocating oil-less compressor equipped with a control valve on a by-pass line to guarantee the lowest flow rate (8 Nm3 /h) required by TES. 4.1.6. Filter The purge gas that flows through the pebble bed in the breeder units and through the Q2 getter matrix can detach and drive solid
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radioactive particles that have to be removed and collected with filters. They are placed in the cold part of the loop after the heat exchanger and after the getter beds. Commercial filters with a mesh of 0.01 m will be used.
Measurement and Accountancy Systems and with a more accurate study of tritium concentration in purge gas in several irradiation scenarios.
5. Conclusions
This paper is partly based on the outcomes from the grant F4E2008-GRT-09, funded by Fusion For Energy.
In HCPB-TBM, TES adopts a purge gas at low pressure to extract tritium from the ceramic breeder and from the neutron multiplier. The composition of TES feed stream was evaluated both in high duty DT phase, assuming a back to back series of standard pulses, and in low duty DT phase assuming an ad hoc tritium out-gassing phase after 50 standard pulses. A revision of the reference configuration of TES was carried out together with a preliminary design of the system. The TES scheme foresees to cool the purge gas coming from the TBM up to room temperature by means of an economizer and a water heat exchanger and to remove the tritiated water with an adsorption column based on zeolites. Then, the gas moves to the ZrCo getter beds for tritium removal. Filters are used to remove solid impurities while an electrical heater adjust the purge gas temperature at the TBM inlet. The adsorption column is regenerated flushing pure helium and using a PERMCAT reactor to recover tritium from Q2 O in the Q2 form. TES components will be placed in the ITER port cell n.16, except for the getter beds that will be located in the Tritium Building (level L2). Other activities concerning TES are ongoing in the frame of the European TBM-CA. They deal with the design of Tritium
Acknowledgments
References [1] I. Ricapito, O. Bede, L.V. Boccaccini, A. Ciampichetti, B. Ghidersa, L. Guerrini, et al., The ancillary systems of the European Test Blanket Modules: configuration and integration in ITER, Fusion Eng. Des. 85 (2010) 1154–1161. [2] F.S. Nitti, A. Ciampichetti, HCPB TES preliminary engineering design and analyses, TBM-CA technical report D3-HPS.2009.002. [3] I. Ricapito, A. Ciampichetti, G. Benamati, M. Zucchetti, Tritium Extraction Systems for the European HCLL/HCPB TBMs, Fusion Sci. Technol. 54 (2008) 107–112. [4] M. Nishikawa, A. Baba, S. Odoi, Y. Kawamura, Tritium inventory estimation in solid blanket system, Fusion Eng. Des. 39-40 (1998) 615–625. [5] I. Ricapito, A. Ciampichetti, P. Agostini, G. Benamati, Tritium processing systems for the helium cooled pebble bed test blanket module, Fusion Eng. Des. 83 (2008) 1461–1465. [6] K. Munakata, T. Shinozaki, K. Inoue, S. Kajii, Y. Shinozaki, R. Knitter, et al., Tritium release from lithium silicate pebbles produced from lithium hydroxide, Fusion Eng. Des. 83 (2008) 1317–1320. [7] S. Konishi, T. Nagasaki, N. Yokokawa, Y. Naruse, Development of Zirconium–Cobalt beds for recovery, storage and supply of Tritium, Fusion Eng. Des. 10 (1989) 355–358. [8] N. Bekris, M. Sirch, U. Besserer, R.D. Penzhorn, Disproportionation of ZrCo and its reproportionation, in: Proceedings of 20th SOFT, vol. 2, Marseille, France, 1998, pp. 1021–1024. [9] D. Demange, M. Glugla, K. Günther, T.L. Le, K.H. Simon, R. Wagner, S. Welte, Counter-current isotope swamping in a membrane reactor: The PERMCAT process and its applications in fusion technology, Catal. Today 156 (2010) 140–145.