SOME CONSIDERATION IN THE TRITIUM CONTROL DESIGN OF THE SOLID BREEDER BLANKET CONCEPTS

Transcription

1 SOME CONSIDERATION IN THE TRITIUM CONTROL DESIGN OF THE SOLID BREEDER BLANKET CONCEPTS L.V. Boccaccini, N. Bekris, R. Meyder and the HCPB Design Team CBBI-13, Santa Barbara, 30th Nov.-2nd Dec L.V. Boccaccini slide # 1

2 HCPB Blanket 1) Box 2) Grid 3) Back Plate CBBI-13, Santa Barbara, 30th Nov.-2nd Dec L.V. Boccaccini slide # 2

3 Design of the Breeder Unit CBBI-13, Santa Barbara, 30th Nov.-2nd Dec L.V. Boccaccini slide # 3

4 Blanket Tritium control in HCPB DEMO hot leg H 2, HT, H 2 O, HTO H 2, HT, H 2 O, HTO TES SG CPS make-up unit cold leg H 2 H 2, H 2 O Compressor CBBI-13, Santa Barbara, 30th Nov.-2nd Dec L.V. Boccaccini slide # 4

5 1) Tritium Sources For a reactor of ~2400 MW (1000 MW el ) fusion power, in steady state operation mode, a Tritium production of 385 g/d is envisaged This tritium is produced mainly in the Ceramic Breeder; small quantity of tritium are produced also in Be pebbles (~10 g/day); this quantity is usually not accounted for the TBR but can be important, if cumulated, for the T Inventory. Impingement of T and D ions from the first wall. This can be a large sources for the T in the main Helium coolant system. Previous estimations gave a source of T of about 18 g/day (but with an uncertainties range of 60-2 g/day! ) in case of bare FM steel. In case of W coating the value should be reduced to less than 0.1 g/day.. FW T, D HCS CBBI-13, Santa Barbara, 30th Nov.-2nd Dec L.V. Boccaccini slide # 5

6 2) Tritium Purge (1/2) The extraction of tritium from the ceramic beds is made with a low pressure (0.11 MPa) helium flow. The Helium contains a concentration of H 2 of 0.1% (110 Pa) to facilitate the T extraction process. In steady state condition the amount of T production coincides with the T extracted from the pebbles. The tritium residence time (that is function of the temperature) of the material is used to estimate the inventory of T in the pebbles. In case of a pulsed reactor (or in ITER), the. time constant of the process becomes important. A model to describe the time dependent material release should be considered. Scheme of the purge flow in the BU. CBBI-13, Santa Barbara, 30th Nov.-2nd Dec L.V. Boccaccini slide # 6

7 2) Tritium Purge (2/2) The flow of He at the Blanket outlet, will contain (in addition to H 2 ) T - mainly in HT form - with traces of HTO and H2O due to the O that is liberated from the ceramic together with the T production. Tritium is extracted from the ceramic breeder in form of HTO in the first step. Because of the presence of a significant H2 partial pressure in the He purge gas, in a second step, HTO is subjected to the isotopic exchange reaction: HTO + H2 «H2O + HT which at equilibrium would lead to a very small amount, around 0.9%, of remaining HTO. In the past analyses, HTO/HT molar fraction was assumed to be 3.2%. In absence of further experimental data, this HTO/HT molar fraction in the He purge gas at the blanket outlet will be kept as reference in this study. CBBI-13, Santa Barbara, 30th Nov.-2nd Dec L.V. Boccaccini slide # 7

8 Purge gas at TES inlet for the HCPB reactor He flow-rate Pressure Temperature HT partial pressure H 2 partial pressure HTO partial pressure H 2 O partial pressure Other impurities (N 2, CO, CO 2, CH 4 ) ~0.4 kg/s K 1.6 Pa»110 Pa 0.05 Pa 1.6 Pa <0.1 Pa CBBI-13, Santa Barbara, 30th Nov.-2nd Dec L.V. Boccaccini slide # 8

9 3) Tritium Permeation in TBM. The permeation of tritium in the Helium coolant system is a major issue in the T control of a Fusion Reactor. The purge lay-out in the HCPB concept minimises this process. A mass flow of 0.4 kg/s is sufficient to reduce the partial pressure of HT lower than 1.6 Pa. The radial flow (from plasma to the back plate) causes a radial profile of T partial pressure from ~0 at the plasma side to the max at the end of plate. Drawback of a large mass flow is the large dilution of T in He and the huge quantity of H2 that should be added to the He purge. Plasma side CBBI-13, Santa Barbara, 30th Nov.-2nd Dec L.V. Boccaccini slide # 9

10 3) Tritium Permeation. In addition the permeation process is strong influenced by the surface state. The recombination factor can change of 3 orders of magnitude from a clean to an oxidised surface. The uncertainties about the surface status causes for this layout an uncertainty in the permeation rate from 0.1 to 10 g/d. The presence of small quantities of steams (in the right ratio with H2) in the HCS can build an oxidation layer at the channels wall that acts as T permeation barrier. The presence of large concentration of H2 in the purge flow causes (in the diffusion controlled regime) an inhibition of the T permeation. The calculation accounts for this effect, solving the contemporaneous flow of H and T due to the diffusion through the wall and recombination / adsorption at the two surfaces. CBBI-13, Santa Barbara, 30th Nov.-2nd Dec L.V. Boccaccini slide # 10

11 3) Tritium Permeation 1,0E-01 tritium permeation rate (g/d) 1,0E-02 1,0E-03 single HCPB DEMO blanket module tritium generation rate: 1,77 g/d thermal power: 13.1 MWth permeating surface: 61.5 m^2. 1,0E-04 1,00E-29 1,00E-28 1,00E-27 1,00E-26 recombination coefficient K rec (m 4 s-1) CBBI-13, Santa Barbara, 30th Nov.-2nd Dec L.V. Boccaccini slide # 11

12 4) Tritium Extraction System (1/2) The He coming from blanket is transported to the Tritium Extraction System. Different methods has been considered for the removal of the H isotopes from the gas mixture. The processed that have been examined were: 1. Hybrid Adsorption Process + Vacuum Pressure Swing Adsorption ( Canadian ) 2. Cold Traps + Temperature Swing Adsorption ( FZK ) 3. Oxidiser + TSA ( ENEA ) 4. TSA + Permeator ( ENEA ) An efficiency of the process of 0.9 is considered sufficient for the TES.. CBBI-13, Santa Barbara, 30th Nov.-2nd Dec L.V. Boccaccini slide # 12

13 4) Tritium Extraction System (2/2). CBBI-13, Santa Barbara, 30th Nov.-2nd Dec L.V. Boccaccini slide # 13

14 5) Coolant Purification System He coolant temperature at CPS inlet He coolant pressure at CPS inlet HT partial pressure in the CPS feed stream Tritium specific activity in the CPS feed stream H 2 partial pressure in the CPS feed stream (by external addition) H 2 O partial pressure in the CPS feed stream addition) CPS tritium removal rate Global CPS efficiency in tritium removal CPS feed flow-rate* (by external Fraction of the CPS flow-rate over total He coolant flow-rate (%)* 500 C 8 MPa 0.8 Pa 0.75 Ci/kg 1000 Pa 50 Pa g/d Nm 3 /h % 0.71% CBBI-13, Santa Barbara, 30th Nov.-2nd Dec L.V. Boccaccini slide # 14

15 Characteristics of the feed stream: high pressure (8 MPa); 5) Coolant Purification System (1/2) very low Q2O and Q2 molar fraction, although the partial pressure are of the same order of magnitude as in TES; presence of impurities, mainly N2, O2, CQ4, whose total molar fraction in the feed stream will be probably less than 1 vppm. The reference CPS solution was proposed by FZK in the past years for HCPB-DEMO 95. It is based on three main subsystems: a catalytic Q2 oxidiser; a cold trap to remove Q2O; a final step for the removal of the remaining impurities, mainly O2, N2, hydrocarbons and not oxidized Q2. This last system is based on microporous adsorbent beds operating at 77 K. CBBI-13, Santa Barbara, 30th Nov.-2nd Dec L.V. Boccaccini slide # 15

16 5) Coolant Purification System (2/2) 573 K Steam generator He + Q 2 + H 2 O + imp. 773 K O 2 oxidiser H 2, H 2 O HE1 423 K [Q 2 O]=131 vppm filter 2 HE2 250 K cooler/cold trap 2 Q 2 O to WGPS 200 K, [Q 2 O]= 13 vppm adsorbent beds 2 Q 2 + imp. to WPS 77 K CBBI-13, Santa Barbara, 30th Nov.-2nd Dec L.V. Boccaccini slide # 16

17 6) Tritium losses in the Steam Generator The two main paths for the tritium environmental release are: tritium permeation into the secondary circuit through the steam generator walls: in this evaluation it is considered a max value of 20 Ci/d. tritium leakage into the water steam circuit because of the positive ΔP between HCS loop (at 8 MPa) and water-cooling system (at 7.5 MPa) in the present HCPB-DEMO 2003 design; Therefore, the maximum HT partial pressure in HCS loop compatible with a tritium permeation rate of 20 Ci/d can be determined having assumed a value of H-T recombination coefficient for oxidised INCOLOY 800 equal to m4 s-1 and Ks equal to mol m-3 Pa-0.5. The maximum admissible HT partial pressure in HCS loop is 1.1 Pa. Generally, under normal operation a leak rate for these systems of 3.78 kg/d is considered as the normal baseline. If such leak rate is still assumed for the case under study, supposing a tritium partial pressure in the primary coolant of 1.1 Pa, the tritium release because of leaks into the secondary circuit through the steam generator pipe bundle should be 3.8 Ci/d. A max. value of 0.8 Pa for the T partial pressure in HCS has been assumed in the DEMO 2003 lay-out CBBI-13, Santa Barbara, 30th Nov.-2nd Dec L.V. Boccaccini slide # 17

18 7) Fuel Cycle CBBI-13, Santa Barbara, 30th Nov.-2nd Dec L.V. Boccaccini slide # 18

19 Working Point (an example) In the following a recent parametric calculation based on the scheme presented in slide 4 and on the parameters in slide 20 is presented as example for the determination of a suitable working point for the T control (the data can differ slightly from the DEMO-2003 lay-out). Slide 21 shows T permeation in the steam generator as a function of the T partial pressure in the HCS for different assumptions of the permeation reduction factor (PRF) caused by an oxide layer. The effect of the layer is strong: only a PRF of 10 justifies a value of 0.8 Pa for the max allowable T partial pressure. Slide 22 shows the additional effect of oxide layer in EUROFER in the TBM. For two values of the design allowable T partial pressure in HCS, two different values of the PRF (0 and 10) are considered. The CPS recirculation ratio is presented as a function of the TES mass flow. Slide 23 presents for the 4 cases analysed in slide 22 the values of the required addition of H2 in TES and HCS CBBI-13, Santa Barbara, 30th Nov.-2nd Dec L.V. Boccaccini slide # 19

20 Assumptions used in the example TBM EUROFER HCS loop permeation area: m2 mass flow 2500 kg/s temperatures 500 C H2 pressure (HCS) 500 Pa cooling plate thickness m CPS efficiency 0.9 Steam Generator: INCOLOY-800 TES loop surface m2 mass flow 0.4 kg/s temperatures (pipe, surf- HCS, surf-steam) pipe thickness 500, 400, 300 C m H2 pressure TES efficiency 110 Pa 0.9 T Sources T production in ceramics 380 g/d T from W-coated FW ~0 g/d CBBI-13, Santa Barbara, 30th Nov.-2nd Dec L.V. Boccaccini slide # 20

21 Permeation in the Steam Generator safety limit of 20 Ci/d CBBI-13, Santa Barbara, 30th Nov.-2nd Dec L.V. Boccaccini slide # 21

22 CPS/HCS Re-circulation ratio CBBI-13, Santa Barbara, 30th Nov.-2nd Dec L.V. Boccaccini slide # 22

23 H2 addition in TES and HCS CBBI-13, Santa Barbara, 30th Nov.-2nd Dec L.V. Boccaccini slide # 23

24 Working points T-pressure in HCS [Pa] PRF in TBM TES mass flow [kg/s] CPS/HCS recirculation ratio [%] H2 addition in TES [kg/d] H2 addition in HCS [kg/d] < CBBI-13, Santa Barbara, 30th Nov.-2nd Dec L.V. Boccaccini slide # 24

25 Conclusions The Table in slide 24 summarised the results of the calculations starting from the base case in which a PRF = 0 in the steam generator and in the TBM is assumed. The other cases shows possible working points assuming a reduction of the permeation rate due to the oxide layers. Only factor 10 in the PRF in TBM and/or in the Steam generator gives relatively comfortable working points that allow a reduction of the TES mass flow (in order to minimise the H2 addition) without to increase to much the CPS re-circulation factor. If confirmed, these results shows that suitable working points can be found controlling the T permeation only with the mass flow and chemistry of the coolant and purge flow. A stable and self-healing oxide film can assure PRF larger than 10 in TBM and Steam Generator. CBBI-13, Santa Barbara, 30th Nov.-2nd Dec L.V. Boccaccini slide # 25