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Steady state and transient thermal–hydraulic analyses on ITER divertor module
Fusion Engineering and Design 75–79 (2005) 457–461
Steady state and transient thermal–hydraulic analyses on ITER divertor module G. Dell’Orco a,∗ , A. Ancona b , A. Di Maio b , M. Merola c , V. Tomarchio b , G. Vella b , I. Zammuto b b c
a ENEA Brasimone, P.O. Box 1, 40032 Camugnano (Bo), Italy Universit`a di Palermo - DIN, V.le delle Scienze, 90128 Palermo, Italy EFDA CSU Garching, Boltzmannstr. 2, D-85748 Garching, Germany
Available online 12 September 2005
Abstract One of the most challenging components of ITER is the divertor devoted at controlling the characteristics of the plasma boundary, exhausting the ␣ particles and reducing the impurities in the plasma. The thermal–hydraulic design of the divertor is particularly, demanding because of the high heat loads and the cooling flow margin in the plasma-facing components (PFCs). The pressure drop is limited by the pumping power and also avoiding the risk of reaching critical heat flux (CHF). Furthermore, for maintenance operation foreseen, each single divertor cassette should be drained and dried before withdrawing it out from the vacuum vessel. To address these requirements, European Fusion Design Agreement (EFDA) has launched a research programme aimed at investigating the theoretical thermal–hydraulic behaviour of the whole divertor cassette, both in the operation and during the water discharge. The study has been carried out with the RELAP5 computer code and the results obtained are herewith presented. © 2005 Elsevier B.V. All rights reserved. Keywords: ITER; Divertor; RELAP code; Steady state; Transient thermal–hydraulic
1. Hydraulic behaviour of ITER divertor The total power extracted by ITER divertor is 127 MW. For the total nominal water flow rate of 934 kg/s (17.3 kg/s per each cassette assembly), the ∗ Corresponding author. Tel.: +39 0534 801129; fax: +39 0534 801244. E-mail address: [email protected] (G. Dell’Orco).
0920-3796/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2005.06.339
inlet/outlet nominal water temperatures and pressures are, respectively, 100–154 ◦ C and 4.2–2.8 MPa, with an overall pressure drop lower than 1.4 MPa. The divertor is divided into 54 cassettes [1] each provided with three plasma-facing components (PFCs) (Fig. 1): (i) the inner vertical target (IVT); (ii) the outer vertical target (OVT); (iii) the dome liner (DL) and (iv) the cassette body (CB). The PFCs are cooled in series, from the outer to the inner region, by routing the coolant via the cassette body. Each PFC consists of a number of
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G. Dell’Orco et al. / Fusion Engineering and Design 75–79 (2005) 457–461
Fig. 1. ITER divertor PFCs and CB.
plasma-facing units, cooled in parallel and assembled onto a supporting structure. The first wall (FW) in both OVT and IVT is provided with twisted tapes, inserted in the cooling tubes of the lower straight zones, to enhance its critical heat flux (CHF) limit. The DL is cooled by rectangular cooling channels with a hypervapotron heat transfer enhancement feature in the dome zone [1,2]. For each PFC, the margin to the CHF onset, at the nominal flow rate, should be 40% higher than the peak of the plasma heat flux, reaching up to 20 MW/m2 of the IVT and OVT.
2. The steady state theoretical analyses The steady state analyses were performed to determine the pressure drops in each PFC and in the overall divertor cassette with the RELAP5 Mod.3.3 thermal–hydraulic code [3]. A grid model was prepared to simulate the cooling path in the whole cassette, including the twisted tape inserts, the hypervapotron geometry, manifolds, changes in coolant cross sections and directions. An overview of the finite volume models is shown in Fig. 2. The pressure drop characteristic of each part and of the whole model is reported in Fig. 3. The interpolated parameters are listed in
Table 1 Pressure drop correlations and values Component
CB OVT IVT DL PFCs + CB
Coefficients of interpolation a
b
1.56E−03 1.96E−03 0.76E−03 5.02E−03
1.8830 1.9020 1.9880 1.9256
DP (MPa)
0.002 0.47 0.49 0.41 1.37
DP (MPa) = a Gb (kg/s).
Table 1 [4,5]. This analysis predicts for the reference flow rate 17.3 kg/s, a total pressure drop through the cassette 1.22 MPa, lower than the acceptable design limit 1.40 MPa.
3. The transient theoretical analyses For the foreseen maintenance, each cassette will be drained and dried from the residual internal water. Due to the complex pipe geometry, the discharge by gravity is incomplete. Therefore, a compressed gas, supplied through the cooling channels at adequate input pressure and flow conditions, should provide a rapid evacuation, without bubbling in vertical channels and separated
G. Dell’Orco et al. / Fusion Engineering and Design 75–79 (2005) 457–461
459
Fig. 2. RELAP5 geometrical model.
streams in the horizontal channels. During the drain phase the water coolant at atmospheric pressure, is blown out by N2 at 4.5 MPa injected in the inlet manifolds. During the drying phase, the cassette is evacuated
by a vacuum pump and heated for evaporating the residual water. A simulation of the draining phase has been carried out by the RELAP5 code, considering the critical flow model active in all the junctions of the
Fig. 3. Divertor hydraulic characteristic.
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G. Dell’Orco et al. / Fusion Engineering and Design 75–79 (2005) 457–461
Fig. 4. Gas inlet mass flow rates.
amount (indicating probably a longer duration of the draining sequence than the desired slug flow reference pattern). The N2 mass blowing through the cassette amounts to 54 kg corresponding to 43 m3 (at standard pressure and temperature condition), whilst the total mass of 534 kg of water–nitrogen mixture is discharged into the outlet tank (Fig. 6). The mass of discharged water is 480 kg while 90 kg of water remains inside the cassette. This value is conservative as a consequence of the flow domain modelling in some particular regions with an unstable equilibrium between a mass of water pushed from below by gas at higher pressure. These results confirm the possibility to evacuate the divertor cassette by gas blowing. Experimental comparisons will contribute both to assess this procedure and to qualify the RELAP5 code in specific fusionrelevant applications. 4. Conclusions
Fig. 5. Water/gas outlet mass flow rates.
finite-volume model [4,5]. The initial draining by gravity has been neglected. The calculations demonstrate that the whole cassette is almost completely drained in little more than 32 s, whilst the OVT, IVT and DL are cleared off in about 9, 21 and 27 s, respectively (Figs. 4–6). At the end, a discrepancy remains between the computed inlet and outlet mass flow rates, suggesting that the draining sequence requires few more than 32 s to be completed (about 35 s could be reasonably extrapolated for the whole draining sequence). The analysis of the flow distribution in the channels indicates the presence of bubbles and stratified horizontal flows in a tolerable
The hydraulic behaviour of the full-scale divertor cassette has been investigated in both steady state and operational transient conditions, by means of the RELAP5 Mod.3.3 code. The hydraulic characteristics of each PFC and of the whole divertor cassette have been determined and the overall pressure drop is within the design limit. Although the evaluation of the cooling of the whole cassette, with its serial flow organization, its different and complicated PFC geometries, is challenging, the code predicts that the adopted solution fulfills all the design requirements: (i) pressure drop lower than the design value (1.4 MPa); (ii) adequate margins to the CHF occurrence (>40%) and (iii) acceptable flow misdistribution in the DL tubes (<20%). The draining operational procedure, by using N2 pressurized at 4.5 MPa, has been studied, determining the time evolution of the hydraulic parameters of both the coolant and insufflated compressed gas. The water discharge transient will have an estimated duration of 32–35 s with a corresponding N2 gas consumption of 43 m3 at standard pressure and temperature condition. References
Fig. 6. Total outlet water/gas mass.
[1] ITER Final Design Report, 2001. [2] M. Merola, Technical Specification for the Contract 02-682: Preparation of Integration and Hydraulic Tests of Full-Scale
G. Dell’Orco et al. / Fusion Engineering and Design 75–79 (2005) 457–461 Divertor Components, EFDA Close Support Unit, Garching, 2003. [3] RELAP5 Code, INEL Nuclear Safety Analysis Division. [4] G. Dell’Orco, A. Ancona, P.A. Di Maio, V. Tomarchio, G. Vella, I. Zammuto, Final Report on the Contract EFDA 682: Preparation of Integration and Hydraulic Tests of Full-Scale Divertor Com-
461
ponents, ENEA Internal Report, DC-G-R-003, Rev. 0, February 27, 2004. [5] V. Tomarchio, I. Zammuto, Caratterizzazione Idraulica e studio delle sequenze di draining and drying di un simulacro sperimentale di una cassetta del divertore del reattore a Fusione ITER, Tesi di Laurea, DIN Palermo, Anno Accademico 2002–2003, Maggio, 2004.
Steady state and transient thermal–hydraulic analyses on ITER divertor module G. Dell’Orco a,∗ , A. Ancona b , A. Di Maio b , M. Merola c , V. Tomarchio b , G. Vella b , I. Zammuto b b c
a ENEA Brasimone, P.O. Box 1, 40032 Camugnano (Bo), Italy Universit`a di Palermo - DIN, V.le delle Scienze, 90128 Palermo, Italy EFDA CSU Garching, Boltzmannstr. 2, D-85748 Garching, Germany
Available online 12 September 2005
Abstract One of the most challenging components of ITER is the divertor devoted at controlling the characteristics of the plasma boundary, exhausting the ␣ particles and reducing the impurities in the plasma. The thermal–hydraulic design of the divertor is particularly, demanding because of the high heat loads and the cooling flow margin in the plasma-facing components (PFCs). The pressure drop is limited by the pumping power and also avoiding the risk of reaching critical heat flux (CHF). Furthermore, for maintenance operation foreseen, each single divertor cassette should be drained and dried before withdrawing it out from the vacuum vessel. To address these requirements, European Fusion Design Agreement (EFDA) has launched a research programme aimed at investigating the theoretical thermal–hydraulic behaviour of the whole divertor cassette, both in the operation and during the water discharge. The study has been carried out with the RELAP5 computer code and the results obtained are herewith presented. © 2005 Elsevier B.V. All rights reserved. Keywords: ITER; Divertor; RELAP code; Steady state; Transient thermal–hydraulic
1. Hydraulic behaviour of ITER divertor The total power extracted by ITER divertor is 127 MW. For the total nominal water flow rate of 934 kg/s (17.3 kg/s per each cassette assembly), the ∗ Corresponding author. Tel.: +39 0534 801129; fax: +39 0534 801244. E-mail address: [email protected] (G. Dell’Orco).
0920-3796/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.fusengdes.2005.06.339
inlet/outlet nominal water temperatures and pressures are, respectively, 100–154 ◦ C and 4.2–2.8 MPa, with an overall pressure drop lower than 1.4 MPa. The divertor is divided into 54 cassettes [1] each provided with three plasma-facing components (PFCs) (Fig. 1): (i) the inner vertical target (IVT); (ii) the outer vertical target (OVT); (iii) the dome liner (DL) and (iv) the cassette body (CB). The PFCs are cooled in series, from the outer to the inner region, by routing the coolant via the cassette body. Each PFC consists of a number of
458
G. Dell’Orco et al. / Fusion Engineering and Design 75–79 (2005) 457–461
Fig. 1. ITER divertor PFCs and CB.
plasma-facing units, cooled in parallel and assembled onto a supporting structure. The first wall (FW) in both OVT and IVT is provided with twisted tapes, inserted in the cooling tubes of the lower straight zones, to enhance its critical heat flux (CHF) limit. The DL is cooled by rectangular cooling channels with a hypervapotron heat transfer enhancement feature in the dome zone [1,2]. For each PFC, the margin to the CHF onset, at the nominal flow rate, should be 40% higher than the peak of the plasma heat flux, reaching up to 20 MW/m2 of the IVT and OVT.
2. The steady state theoretical analyses The steady state analyses were performed to determine the pressure drops in each PFC and in the overall divertor cassette with the RELAP5 Mod.3.3 thermal–hydraulic code [3]. A grid model was prepared to simulate the cooling path in the whole cassette, including the twisted tape inserts, the hypervapotron geometry, manifolds, changes in coolant cross sections and directions. An overview of the finite volume models is shown in Fig. 2. The pressure drop characteristic of each part and of the whole model is reported in Fig. 3. The interpolated parameters are listed in
Table 1 Pressure drop correlations and values Component
CB OVT IVT DL PFCs + CB
Coefficients of interpolation a
b
1.56E−03 1.96E−03 0.76E−03 5.02E−03
1.8830 1.9020 1.9880 1.9256
DP (MPa)
0.002 0.47 0.49 0.41 1.37
DP (MPa) = a Gb (kg/s).
Table 1 [4,5]. This analysis predicts for the reference flow rate 17.3 kg/s, a total pressure drop through the cassette 1.22 MPa, lower than the acceptable design limit 1.40 MPa.
3. The transient theoretical analyses For the foreseen maintenance, each cassette will be drained and dried from the residual internal water. Due to the complex pipe geometry, the discharge by gravity is incomplete. Therefore, a compressed gas, supplied through the cooling channels at adequate input pressure and flow conditions, should provide a rapid evacuation, without bubbling in vertical channels and separated
G. Dell’Orco et al. / Fusion Engineering and Design 75–79 (2005) 457–461
459
Fig. 2. RELAP5 geometrical model.
streams in the horizontal channels. During the drain phase the water coolant at atmospheric pressure, is blown out by N2 at 4.5 MPa injected in the inlet manifolds. During the drying phase, the cassette is evacuated
by a vacuum pump and heated for evaporating the residual water. A simulation of the draining phase has been carried out by the RELAP5 code, considering the critical flow model active in all the junctions of the
Fig. 3. Divertor hydraulic characteristic.
460
G. Dell’Orco et al. / Fusion Engineering and Design 75–79 (2005) 457–461
Fig. 4. Gas inlet mass flow rates.
amount (indicating probably a longer duration of the draining sequence than the desired slug flow reference pattern). The N2 mass blowing through the cassette amounts to 54 kg corresponding to 43 m3 (at standard pressure and temperature condition), whilst the total mass of 534 kg of water–nitrogen mixture is discharged into the outlet tank (Fig. 6). The mass of discharged water is 480 kg while 90 kg of water remains inside the cassette. This value is conservative as a consequence of the flow domain modelling in some particular regions with an unstable equilibrium between a mass of water pushed from below by gas at higher pressure. These results confirm the possibility to evacuate the divertor cassette by gas blowing. Experimental comparisons will contribute both to assess this procedure and to qualify the RELAP5 code in specific fusionrelevant applications. 4. Conclusions
Fig. 5. Water/gas outlet mass flow rates.
finite-volume model [4,5]. The initial draining by gravity has been neglected. The calculations demonstrate that the whole cassette is almost completely drained in little more than 32 s, whilst the OVT, IVT and DL are cleared off in about 9, 21 and 27 s, respectively (Figs. 4–6). At the end, a discrepancy remains between the computed inlet and outlet mass flow rates, suggesting that the draining sequence requires few more than 32 s to be completed (about 35 s could be reasonably extrapolated for the whole draining sequence). The analysis of the flow distribution in the channels indicates the presence of bubbles and stratified horizontal flows in a tolerable
The hydraulic behaviour of the full-scale divertor cassette has been investigated in both steady state and operational transient conditions, by means of the RELAP5 Mod.3.3 code. The hydraulic characteristics of each PFC and of the whole divertor cassette have been determined and the overall pressure drop is within the design limit. Although the evaluation of the cooling of the whole cassette, with its serial flow organization, its different and complicated PFC geometries, is challenging, the code predicts that the adopted solution fulfills all the design requirements: (i) pressure drop lower than the design value (1.4 MPa); (ii) adequate margins to the CHF occurrence (>40%) and (iii) acceptable flow misdistribution in the DL tubes (<20%). The draining operational procedure, by using N2 pressurized at 4.5 MPa, has been studied, determining the time evolution of the hydraulic parameters of both the coolant and insufflated compressed gas. The water discharge transient will have an estimated duration of 32–35 s with a corresponding N2 gas consumption of 43 m3 at standard pressure and temperature condition. References
Fig. 6. Total outlet water/gas mass.
[1] ITER Final Design Report, 2001. [2] M. Merola, Technical Specification for the Contract 02-682: Preparation of Integration and Hydraulic Tests of Full-Scale
G. Dell’Orco et al. / Fusion Engineering and Design 75–79 (2005) 457–461 Divertor Components, EFDA Close Support Unit, Garching, 2003. [3] RELAP5 Code, INEL Nuclear Safety Analysis Division. [4] G. Dell’Orco, A. Ancona, P.A. Di Maio, V. Tomarchio, G. Vella, I. Zammuto, Final Report on the Contract EFDA 682: Preparation of Integration and Hydraulic Tests of Full-Scale Divertor Com-
461
ponents, ENEA Internal Report, DC-G-R-003, Rev. 0, February 27, 2004. [5] V. Tomarchio, I. Zammuto, Caratterizzazione Idraulica e studio delle sequenze di draining and drying di un simulacro sperimentale di una cassetta del divertore del reattore a Fusione ITER, Tesi di Laurea, DIN Palermo, Anno Accademico 2002–2003, Maggio, 2004.