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Methodology for accident analyses of fusion breeder blankets and its application to helium-cooled pebble bed blanket
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Methodology for accident analyses of fusion breeder blankets and its application to helium-cooled pebble bed blanket Dobromir Panayotov a,∗ , Andrew Grief b , Brad J. Merrill c , Paul Humrickhouse c , Martin Trow b , Michael Dillistone b , Julian T. Murgatroyd b , Simon Owen b , Yves Poitevin a , Karen Peers b , Alex Lyons b , Adam Heaton b , Richard Scott b a
Fusion for Energy (F4E), Josep Pla, 2; Torres Diagonal Litoral B3, Barcelona E-08019, Spain Amec Foster Wheeler, Booths Park, Chelford Road, Knutsford WA16 8QZ, Cheshire, United Kingdom c Idaho National Laboratory, PO Box 1625, Idaho Falls, ID, USA b
h i g h l i g h t s
g r a p h i c a l
a b s t r a c t
• Test Blanket Systems (TBS) DEMO •
•
•
• •
breeding blankets (BB) safety demonstration. Comprehensive methodology for fusion breeding blanket accident analysis that addresses the specificity of the breeding blanket designs, materials, and phenomena. Development of accident analysis specifications (AAS) via the use of phenomena identification and ranking tables (PIRT). PIRT application to identify required physical models for BB accidents analysis, code assessment and selection. Development of MELCOR and RELAP5 codes TBS models. Qualification of the models via comparison with finite element calculations, code-tocode comparisons, and sensitivity studies.
a r t i c l e
i n f o
Article history: Received 25 July 2015 Received in revised form 8 November 2015 Accepted 12 November 2015 Available online xxx Keywords: Fusion safety Fusion breeder blankets
a b s t r a c t ‘Fusion for Energy’ (F4E) is designing, developing, and implementing the European Helium-Cooled LeadLithium (HCLL) and Helium-Cooled Pebble-Bed (HCPB) Test Blanket Systems (TBSs) for ITER (Nuclear Facility INB-174). Safety demonstration is an essential element for the integration of these TBSs into ITER and accident analysis is one of its critical components. A systematic approach to accident analysis has been developed under the F4E contract on TBS safety analyses. F4E technical requirements, together with Amec Foster Wheeler and INL efforts, have resulted in a comprehensive methodology for fusion breeding blanket accident analysis that addresses the specificity of the breeding blanket designs, materials, and
∗ Corresponding author. Tel.: +34 93 3201149. E-mail address: [email protected] (D. Panayotov). http://dx.doi.org/10.1016/j.fusengdes.2015.11.019 0920-3796/© 2015 Published by Elsevier B.V.
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2 Accident analyses ITER DEMO Test blanket system
phenomena while remaining consistent with the approach already applied to ITER accident analyses. The methodology phases are illustrated in the paper by its application to the EU HCPB TBS using both MELCOR and RELAP5 codes. © 2015 Published by Elsevier B.V.
1. Introduction Testing the tritium breeding modules (TBM) in ITER [1], and the need to include them in the ITER licensing [2], offers a unique opportunity for further development and validation of the methodology for DEMO breeder blanket (BB) accident analysis [3–7]. The safety approach and accident analysis methodology for ITER is very well defined, as demonstrated by the successful construction licensing [8]. However, due to the later inclusion of the TBM testing program in the ITER agreement, the TBS safety demonstration was somewhat detached from the ITER machine safety at the beginning of the project. Safety studies for fusion facilities are commonly conducted using codes originally developed for fission reactor accident analysis. Some of these codes have been modified, and have additional physical models to treat fusion-relevant phenomena [9–13]. For example, fusion-adapted versions of MELCOR are widely applied for fusion accident analyses [3,12,14,15]. The underlying fission safety codes have undergone development and validation using extensive separate and integral effects experimental databases [16–18]. These huge international efforts (including the 2D/3D program [16] and many computational benchmark problems) using phenomena identification and ranking tables (PIRT) [19] and improved simulation models have led to development of a best estimate methodology for fission safety. The fusion-modified codes are validated against the limited available fusion experimental data or through benchmarking against validated code(s) or code version(s) [20–22]. Note that experimental data for many BB (accident) phenomena are not yet available. This work is devoted to establishing a systematic integrated ITER-TBS/DEMO-BB accident analysis methodology for simulating fault response of the breeding blanket and its interaction with the rest of the machine/plant. The methodology consists of several phases: (1) deterministic selection of accident scenarios supported by failure modes and effects analysis (FMEA) studies, (2) elaboration on these to develop accident analysis specifications (AAS) via the use of PIRT to identify required physical models to aid in code selection, (3) development of TBS models using the codes selected, and (4) qualification of the models via comparison with finite element calculations, code-to-code comparisons, and sensitivity studies. The paper continues the presentation of previously published methodology and its application to HCLL blanket [23]. The outlined methodology addresses the challenge in performing accident analysis for the EU TBS in an environment lacking experimental data on TBS phenomena. According to the French INB order 2012 [24] some TBS sub-systems and components are Protection Important Components and the application of the methodology provided in this paper to the ITER TBS accident analyses is a Protection Important Activity. For this reason the compliance with French INB order was a fundamental requirement for the work described hereafter.
2. Selection of reference accident scenarios As described in [5,7], the procedure developed for the selection of reference accident scenarios for ITER [25] has been used to identify scenarios for the EU TBS. The accidental conditions, or postulated initiating events, which might give rise to a release
of radioactivity were determined from a FMEA evaluation; a set of reference accidents was then identified, by grouping individual accident initiators that have similar consequences, which are outlined in [4]. 3. Development of the accident analysis specifications In the presented methodology, the reference scenarios are elaborated on to provide AAS. These are used, together with PIRT, to identify the requirements to be met by the analysis codes and TBS models. In this manner the limitations of individual analysis codes may be identified, and, where necessary, modelling approaches to overcome these limitations can be proposed. The definition of the accident analysis specifications for each scenario is performed in seven steps: (i) list the systems potentially engaged in the scenario; (ii) identify the phenomena that are likely to occur and list the required code models; (iii) select the most suitable code and version for analysis according to the predefined criteria; (iv) specify the input to the model development; (v) list the expected output of the accident analyses; (vi) define the accident sequence; (vii) prepare an accident flow chart. The key elements of these steps are described below, with reference to the HCPB TBS. 3.1. Phenomena identification tables and required code models The identification of the phenomena that potentially could occur within the reference accident scenarios is assessed in a two stage process. Initially, a review of existing analysis results is undertaken to obtain direct information on the more significant phenomena occurring in normal operation and the selected reference accident sequences. Second, a review of phenomena based on the physical processes imposed by the accident, system design, operating conditions, safety functions and materials of construction is performed to provide a more comprehensive basis for the assessment. This approach resembles that adopted for the PIRT procedure. For the HCPB TBS, the phenomena were grouped under eleven sub-headings: power source; flow; heat transfer; phase change; pebble bed modelling; chemical reactions; non-condensable gases; particle transport and tracking; numerical coupling (steady state setup), system modelling requirements, and material properties. The results from each of the reviews were compared and consolidated to produce a single set of phenomena that could potentially influence the progression of the accident sequences. An excerpt from this list for the pebble bed modelling is presented in Table 1. 3.2. Code assessment and selection The list of phenomena, described above, form the basis for an assessment of the code models within the code selection procedure. For the HCPB TBS, the code selection criteria included model availability, coverage of phenomena, verification status, and the ability to resolve local and 2D/3D effects. The code selection process has been limited to the assessment of different versions of the RELAP5 and MELCOR codes, as prescribed by F4E. The specific code versions that have been considered are RELAP5/MOD3.3, RELAP5-3D, fusion adapted MELCOR versions 1.8.2, 1.8.5
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D. Panayotov et al. / Fusion Engineering and Design xxx (2015) xxx–xxx Table 1 List of HCPB TBS phenomena for pebble bed modelling.
3
Phenomenon/Parameter
Location
Impact on faults
Power source (volumetric nuclear heating and decay heat) Heat transfer, thermal properties
Breeder unit (BU),
Release of stored thermal energy and decay heat
BU, Tritium extraction system (TES) BU, VV (ITER FW)
Temperature responses due to conduction and convection Hydrogen/BeO and heat production
assessments [3,15]. Although the standard version of MELCOR 1.8.6 has advantages over MELCOR 1.8.5 it is judged that the latter will provide an equally valid calculation of the conditions within the HCPB TBM as former. MELCOR 1.8.5 has the advantage that it has also been selected for the assessment of the HCLL TBS [23]. The RELAP5-3D version has some enhanced modelling features compared to RELAP5/MOD3.3 which includes the addition of the ATHENA multi-fluid package [21]. In addition, it has been selected to perform the RELAP5 assessment for the HCLL TBS [23] and therefore it is the logical choice for the HCPB TBS analyses to allow code consistency between the safety demonstration of the two TBSs.
VV
Rate and overall reaction of compounds.
3.3. Accident analysis specification
BU
Transport of reactants and reaction products
The overall method used for the definition of accident specifications was based on the following steps:
BU, VV (faulted)
Non-reactive, thermal conduction Chemical reaction: H2 , BeO, heat production, thermal conduction
• Definition of the accident analysis approach, • Purpose of scenario (definition of objectives and acceptance criteria), • Definition of accident initiating events and progression, • Identification of system operation/data.
Chemical reactions of beryllium with steam/air Flow and interaction of beryllium pebbles with VV atmosphere. Diffusion of steam/air/hydrogen within pebble bed Material properties of lithium ceramic bed Material properties of beryllium bed
BU, ITER FW
(multi-fluids version), and 1.8.6 and standard MELCOR versions 1.8.6 and 2.1. The individual versions of the MELCOR and RELAP5 codes have, in many respects, similar capabilities and attributes. In Table 2, the list of phenomena presented in Table 1 is repeated with statements indicating the ability of the codes to model specific phenomena; a more extensive evaluation was undertaken for key models and correlations. Thus the relevant similarities and differences between the codes (and where appropriate, between code versions) were identified. Fusion-adapted MELCOR, version 1.8.2 has undergone the most extensive verification and validation, including a line-by-line review of fusion modifications to the source code. Though not as extensive as for 1.8.2, MELCOR 1.8.5 has been subject to verification via comparison studies with standard (fission) MELCOR 1.8.5 and pedigreed MELCOR 1.8.2, and has been used in other fusion safety
Both the objectives and the definition of initiating events and aggravating failures used the TBS Preliminary Safety Report (and FMEA study) as starting points. Based on this information, the potential accident progression and key phenomena were identified and compared with those derived in Section 3.1. The acceptance criteria for each scenario have been selected based on the objectives defined. The objectives are accident specific and normally include (i) investigate the response to a given accident”; (ii) analyse the thermal hydraulics of the TBM and cooling systems; confirm that (iii) transients do not cause any damage to confinement barriers; (iv) post-accident cooling is established to remove decay heat; (v) radioactive and energy inventories can be kept under control; (vi) radioactive releases are within the plant limits. The specific safety design requirements given in system requirement documents were used as the basis for acceptance
Table 2 HCPB TBS code assessment for pebble bed modelling. Parameter/Phenomenon
MELCOR
Uncertainty
RELAP5
Uncertainty
Power source (volumetric nuclear heating and decay heat) Heat transfer, thermal properties
Modelled by the application of volumetric sources in heat structures within the BU. Sufficient detail in the modelling is required to capture the maximum temperatures of the bed material to ensure a conservative calculation Lithium ceramic bed thermal conductivity as a function of packing density and temperature is treated through a conservative estimate of the packing density and tabular input of conductivity versus temperature. Be bed thermal conductivity dependence on temperature and pressure is considered through a number of separate regions which exhibit different levels of strain and corresponding thermal conductivity Included in fusion adapted versions. Safety factors assigned in DBA assessments
M/L
*
M/L
M/L
*
M/L
Bounding with safety factors
Not dynamic can be represented by control functions
M
There is no mechanistic model of the discharge of beryllium pebbles from the BUs during accidents, treated in a conservative manner. The heat transfer from discharged pebbles will increase the pressurisation of the VV which needs to be captured in the analyses No mechanistic model of this process within the code, treated in a conservative manner by the model design
Hb
*
Hb
H
*
H
Material Property table input
L
*
L
Chemical reactions of Beryllium with steam/air Flow and interaction of beryllium pebbles with VV atmosphere
Diffusion of steam/hydrogen/air within pebble bed Material properties of lithium ceramic and beryllium beds
Notes: L—Low; M—Medium; H—High; Hb—High bounded. * The comment on the MELCOR code versions also applies to the RELAP5 codes.
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criteria where available. Third to sixth objectives (iii–vi) are used to define the most important acceptance criteria i.e. ones related with confinement integrity (iii and iv), control of hazardous inventories (v), and control of releases (vi). 4. Development of the breeding blanket and ancillary systems models
limited representation of the ITER building and systems within the TBS model. This is applicable only in cases when the TBS has a very weak impact on the ITER machine. The second and preferable option is coupling of the TBS and ITER models in order to simulate the interaction of both systems. These two approaches are discussed below. 5.1. Model of the ITER environment inside the TBS model
In the next step, accident analysis models were constructed using the selected MELCOR and RELAP5 codes. The TBS models cover all TBS systems and controls, and their relevant ITER environment. F4E requested flexible generic models able to handle a wide spectrum of accidents with minor adaptations, whilst maintaining consistency with ITER analysis models. During development of the TBS models, some following MELCOR code-specific issues were identified and addressed. These are common for HCLL and HCPB TBSs and were reported in [23]. Although the MELCOR model of the TBS includes all sub-systems and main components, we will limit our illustration below to the TBM.
The analysis of the TBS accident sequences require the assessment of fluid leakages, possible chemical reactions, and the transport of radionuclides within the VV, Port Cell, Chemical and Volume Control System (CVCS) area, Tokamak Cooling Water System (TCWS) vault, and connecting shafts. Therefore, these areas are included but coarsely nodalised (e.g. with a single volume) in the TBS model. Models of ventilation flows, leakage flows and engineered pressure relief paths are provided between the nodes representing the buildings and environment, as well as heat structures modelling the tokamak building to represent the heat absorbed from gases and steam released during accidents.
4.1. TBM/breeder units nodalisation 5.2. Coupling of the TBS and ITER machine MELCOR model The representation of the HCPB TBM in MELCOR is significantly different to that of the HCLL TBM described in [23]. One reason for this is the much lower thermal conductivity of the pebble beds compared with that of PbLi eutectic, which results in significant thermal gradients within the BU. This temperature variation was resolved by dividing an individual BU into a number of distinct heat structures and control volumes. As a result, it is not practical to model all eight BUs on one side of the TBM Box individually because this would have required an unfeasibly large model with unacceptably long computation times. Therefore, only two BUs are modelled explicitly, one representing a single BU and the other representing the remaining fifteen BUs in the TBM. This allows the behaviour of the sixteen BUs in normal operation to be captured, as well as their behaviour in accident scenarios in which one of the BUs is assumed to have been breached. In one such scenario, the pebble bed in the breached BU is potentially exposed to steam and generates excess heat via the exothermic oxidation of the beryllium pebbles. The temperature map in a radial-poloidal plane through the centre of an ‘average’ BU in normal operation obtained in CFD design analyses was used to determine a suitable heat-structure and control volume nodalisation scheme for the BU models, ensuring that each node would encompass a reasonably uniform temperature gradient whilst keeping the number of nodes to a minimum. The HCPB model consists of a ‘Single BU’ model, a ‘Multi-BU’ model and a number of ‘common’ components that are shared by the two BU models (see Fig. 1). The common components consist of the FW and the coolant and purge gas manifolds at the back of the TBM box. The term ‘BU model’, therefore, refers to the MELCOR model of a BU plus its share of the TBM box, excluding the ‘common’ components. It therefore includes a representation of the Horizontal Stiffening Grids (HSGs), the Vertical Stiffening Grids (VSGs) and the side caps, in addition to the BU itself. The Multi-BU model is similar to the Single BU model but as mentioned above, it represents fifteen ‘average’ BUs and therefore the scalable parameters, such as volumes, flow areas and heat sources, are scaled up by a factor of fifteen compared to those of the Single BU model. 5. Representation of the ITER environment within TBS accident analyses Two approaches are proposed for the representation of the ITER environment of the TBS. The first consists of incorporation of a
In order to assess the impact of accidents originating in the TBS on the accident response of the ITER machine, the interaction of these systems must be captured. Examples of this include in-vessel leakage of helium that may cause a plasma disruption and ex-vessel leakage of helium that may lead to pressurisation of the port cell or triggering of the ITER Central Safety System (CSS). In general, when these interactions between the TBS and the ITER machine are sufficiently strong a coupled analysis is required, with the accident response of both systems represented. Direct integration of the TBS MELCOR model and the ITER machine MELCOR model into a single overall combined model is not possible since MELCOR limitations within a single simulation. Therefore, a loose coupling of two models is used to represent the transient interactions of the ITER machine and the TBS. The selected method for coupling the ITER and TBS models is to perform a sequence of analyses, with a coordinated exchange of data between the two models as described in [23]. Iteration between the two models will ensure that selfconsistent results are obtained. The MELCOR EDF (External Data File) package provides facilities to transfer leak flow rates, enthalpies and other data between models to support this form of coupled analysis. 6. Qualification of the breeding blanket and ancillary systems models The TBS accident analysis methodology includes qualification activities to assess the ability of the developed TBS models to represent the phenomena and transient responses associated with the accident sequences defined in the AAS. By analogy with HCLL TBS [23] the HCPB TBS models have been evaluated via a test matrix that includes (1) comparisons of the MELCOR and RELAP5 predictions with available finite-element analysis (FEA) results from the TBM design description documents (DDD) in steady state, pulsed plasma operations and MARFE (‘Multi-Faceted Radiation From the Edge’) power excursion; (2) further MELCOR and RELAP5 comparisons in normal operations and test transients designed to cover a representative range of accident-relevant phenomena, and (3) sensitivity and uncertainty studies in more complex accident scenarios. The test matrix shown in Table 3 consists of 10 cases executed in 15
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Fig. 1. MELCOR HCPB TBM nodalisation; Be—yellow, Li-ceramic—green, He coolant—blue, He purge—light blue. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
code runs. The final step of the qualification is code-to-code comparison for the 32 h LOOP accident analysis. We understand that the qualification of the BB and Ancillary System Models is limited at this stage to a generic models developed and whenever needed for the analyses of specific accident an additional effort will be undertaken.
6.1. Comparison with finite element design analyses Comparison of MELCOR and RELAP5 simulations of the TBM response to a 10 s MARFE power excursion (case 3 in Table 3) with design FEA results (labelled DDD in the figure) are shown in Fig. 2. Good agreement is seen between the plasma facing temperature
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6 Table 3 HCPB TBS qualification analysis summary. # 1
2 3
4
5
6 7 8 9 10
Case title
Scenario and runs executed
Comparison with the HCPB TBM design analyses Full power Steady-state TBM model analyses compared to design results. MELCOR and RELAP5 steady-state simulations Pulse TBM model analyse of a 400 s power pulse. MELCOR and RELAP5 simulations MARFE TBM model analyses of a MARFE transient occurring from steady-state full power conditions compared to design results. MELCOR and RELAP5 simulations Analysis of HCPB TBS normal operation Code to code comparison TBS pulse TBS model analysis of a 400 s plasma power pulse. Model includes Port Plug, HCS, TBM and TES ancillary system models. MELCOR and RELAP5 simulations Fault sequence test cases Code to code comparison Test transient In-vessel FW LOCA close to end of 400 s plasma S0 pulse followed by breach of the purge gas region of a BU to the VV. MELCOR and RELAP5 simulations Sensitivity study analysis cases MELCOR simulation S1: S0+ High beryllium temperature S2: S0+, S1+ High beryllium temperature (S1) and low beryllium conductivity S3: S0+ Reduced pebble bed friction S4: S0+ Few-node representation of BU purge gas regions S5: S0 + S1 + S4 Combined high beryllium temperature (S1) and few-node representation (S4)
Fig. 3. Test transient S0 VV and purge gas pressures with zoom at 5–30 s.
∼15 s duration. The pressure in the VV and TBM then follows the prescribed VV pressure variation. The depressurisation of the TES is arrested by the closure of the TES isolation valves at 14.7 s leaving a final TES pressure (outside the isolation valves) of ∼92 kPa. MELCOR and RELAP5 simulations demonstrated representative TBS response to the transient and good agreement between the codes results. 6.3. Sensitivity studies
for all three simulations. The TBM box pebble facing temperature is ∼20 ◦ C higher than the MELCOR and RELAP5 predictions. This is consistent with the conservative heat flux prescribed from the pebble bed on to the FW in the design simulations. The MELCOR and RELAP5 simulations appear to capture temperature transients associated with rapid changes in heat flux from the plasma.
The test transient sequence S0 (case 5 in Table 3) represents an in-vessel LOCA from the TBM FW followed by a subsequent through thickness breach of the TBM FW exposing the BU to the VV. It represents a complete rupture of the FW with total break area is 5.07 × 10−3 m2 equivalent to 24 times the FW channel crosssectional area corresponding to the broken ends of 12 up-flow and 12 down-flow channels. Fig. 3 shows the depressurisation of the TBM and Tritium Extraction System (TES) purge gas to the VV. Both the MELCOR and RELAP5 simulations show an initial blowdown of
The sensitivity study S5 (case 10 in Table 3) combines the S1 and S4 cases to assess the effect of the conservative regime of high beryllium temperature and ‘perfect’ steam mixing on hydrogen production in the MELCOR HCPB TBS model. The scenario is identical to S1, except that the coarse nodalisation purge gas model implemented in S4 is used. The mass of hydrogen present and produced in different regions of the TBM is shown in Fig. 4. In total, about 18 g of hydrogen is produced over the 100,000 s simulated compared to 1 g in S0, 2.8 g in S1, 3.2 g in S2, 1.6 g in S3 and 1.8 g in S4. The production is dominated by the Single BU and most of the hydrogen is discharged into the VV. The most rapid period of hydrogen production occurs in the first ∼100 s of the transient. The greater mass of hydrogen produced in S5 than in S1, which has the same initial beryllium temperatures, is due to the ‘perfect’ steam mixing allowing any steam entering the damaged Single BU to react with any heat structure in the BU. The initial steam ingress period caused by the VV pressurisation increases the hydrogen mass rapidly. The beryllium pebble in the Single BU
Fig. 2. MARFE transient MELCOR and RELAP5 FW surface temperatures comparison with FEA results.
Fig. 4. Mass of hydrogen produced in MELCOR simulation with zoom at first 2000 s.
6.2. Code-to-code comparison
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rise in temperature during this period is due to the exothermic nature of the reaction. The steam supply for the TBM after the initial steam ingress is a result of circulating flow through the breaches. The continuous steam supply allows the reaction to continue until the beryllium temperatures drop sufficiently to give a negligible reaction rate.
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sensitivity studies. We believe that the developed methodology is applicable to accident analyses of other TBSs to be tested in ITER and as well to DEMO breeding blankets. Acknowledgments
The impact of uncertainties associated with the accident analyses needs to be addressed to provide sufficient confidence in the level of conservatism in the results [23]. An expert review of areas of uncertainty (including an uncertainty PIRT) is planned and will be reported on in a subsequent dedicated paper.
The work reported in this paper has been performed by F4E, Amec Foster Wheeler and INL under F4E contract. The views expressed in this publication are the sole responsibility of the authors and do not necessarily reflect the views of the Fusion for Energy and the ITER Organization. Neither Fusion for Energy nor any person acting on behalf of Fusion for Energy is responsible for the use, which might be made of the information in this publication.
7. Applications of the methodology
References
6.4. Uncertainty analysis
Finally, the qualified models must be applied to analyse the selected accident scenarios defined in the TBS PrSR. The process is the same as the one reported in [23] for the HCLL TBS. The methodology has been applied to a 32 h loss of offsite power (LOOP) in both the HCLL and HCPB TBSs, using both MELCOR and RELAP in order to further qualify the models via code comparison. The analyses of loss of flow accidents in each TBS using MELCOR is on-going. These are TBS accidents with very limited impact on the ITER machine. In the next analyses, model coupling will be used to investigate TBS accidents that might have effect on the ITER machine and require the simulation of interacting phenomena and processes that take place in several systems. 8. Summary and conclusions A comprehensive methodology for fusion breeding blanket accident analyses that addresses the specificity of the designs, materials, and phenomena while remaining consistent with the approach already applied to ITER has been developed and applied to the EU HCLL and HCPB TBSs. The strong points of the methodology are the use of PIRT to identify requirements to be met by the analysis codes; development of high quality TBS models; the loose coupling of different codes or code versions in order to simulate multi-fluid flows and phenomena overcoming the codes’ limitations; qualification of the models by comparison with finite element analyses and code-to-code comparisons; and uncertainty analysis utilizing
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
[19] [20] [21] [22] [23] [24]
[25]
L. Giancarli, et al., Fusion Eng. Des. 87 (2012) 395–402. Y. Poitevin, et al., Fusion Eng. Des. 89 (2014) 1113–1118. B.J. Merrill, et al., Nucl. Fusion 47 (2007) S447–S452. T. Pinna, L.V. Boccaccini, J.F. Salavy, Fusion Eng. Des. 83 (2008) 1733–1737. D. Panayotov, et al., IAEA Fusion Energy Conference FEC-2012, 8–13 October 2012, USA, 2012. H.G. Jin, et al., IEEE Trans. Plasma Sci. 42 (3) (2014) 671–674. D. Panayotov, et al., Fusion Eng. Des. 98–99 (2015) 2201–2205. N. Taylor, P. Cortes, Fusion Eng. Des. 89 (2014) 1995–2000. B.J. Merrill, et al., Fusion Eng. Des. 51–52 (2000) 555–563. J. Andersson, et al., Fusion Eng. Des. 51–52 (2000) 453–460. B.J. Merrill, Fusion Eng. Des. 54 (2001) 485–493. B.J. Merrill, P.W. Humrickhouse, R.L. Moore, Fusion Eng. Des. 85 (2010) 1479–1483. C. Séropian, et al., Fusion Eng. Des. 88 (2013) 2698–2703. B.J. Merrill, et al., Fusion Eng. Des. 89 (2014) 1989–1994. P.W. Humrickhouse, B.J. Merrill, INL/CON-12-25015, August 2012. P.S. Damerell, J.W. Simons (Eds.), 2D/3D Program Work Summary Report, NUREG/IA-0126, GRS-100, MPR-1345, June 1993. N. Aksan, et al., Separate Effects Test Matrix for Thermal-Hydraulic Code Validation NEA/CSNI/R(93)14, OECD, Paris, July 1994. A. Annunziato, et al., CSNI Integral test facility validation matrix for the assessment of thermal-hydraulic codes for LWR LOCA and transients, NEA/CSNI/R(96)17, OECD, Paris, July 1996. D.J. Diamond, BNL-76750-2006-CP, September, 2006. P. Sardain, et al., Fusion Eng. Des. 54 (2001) 555–561. R.L. Moore, B.J. Merrill, Fusion Eng. Des. 63–64 (2002) 411–419. G. Geffraye, et al., Nucl. Eng. Des. 249 (2012) 237–247. D. Panayotov, A. Grief, B.J. Merrill, P. Humrickhouse, et al., SOFE 26 (2015), in press. French Official Gazette, Arrêté du 7 février 2012 fixant les règles générales relatives aux installations nucléaires de base, French Official Gazette (08 February 2012). T. Pinna, et al., Fusion Eng. Des. 75-79 (2005) 1103–1107.
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Contents lists available at ScienceDirect
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Methodology for accident analyses of fusion breeder blankets and its application to helium-cooled pebble bed blanket Dobromir Panayotov a,∗ , Andrew Grief b , Brad J. Merrill c , Paul Humrickhouse c , Martin Trow b , Michael Dillistone b , Julian T. Murgatroyd b , Simon Owen b , Yves Poitevin a , Karen Peers b , Alex Lyons b , Adam Heaton b , Richard Scott b a
Fusion for Energy (F4E), Josep Pla, 2; Torres Diagonal Litoral B3, Barcelona E-08019, Spain Amec Foster Wheeler, Booths Park, Chelford Road, Knutsford WA16 8QZ, Cheshire, United Kingdom c Idaho National Laboratory, PO Box 1625, Idaho Falls, ID, USA b
h i g h l i g h t s
g r a p h i c a l
a b s t r a c t
• Test Blanket Systems (TBS) DEMO •
•
•
• •
breeding blankets (BB) safety demonstration. Comprehensive methodology for fusion breeding blanket accident analysis that addresses the specificity of the breeding blanket designs, materials, and phenomena. Development of accident analysis specifications (AAS) via the use of phenomena identification and ranking tables (PIRT). PIRT application to identify required physical models for BB accidents analysis, code assessment and selection. Development of MELCOR and RELAP5 codes TBS models. Qualification of the models via comparison with finite element calculations, code-tocode comparisons, and sensitivity studies.
a r t i c l e
i n f o
Article history: Received 25 July 2015 Received in revised form 8 November 2015 Accepted 12 November 2015 Available online xxx Keywords: Fusion safety Fusion breeder blankets
a b s t r a c t ‘Fusion for Energy’ (F4E) is designing, developing, and implementing the European Helium-Cooled LeadLithium (HCLL) and Helium-Cooled Pebble-Bed (HCPB) Test Blanket Systems (TBSs) for ITER (Nuclear Facility INB-174). Safety demonstration is an essential element for the integration of these TBSs into ITER and accident analysis is one of its critical components. A systematic approach to accident analysis has been developed under the F4E contract on TBS safety analyses. F4E technical requirements, together with Amec Foster Wheeler and INL efforts, have resulted in a comprehensive methodology for fusion breeding blanket accident analysis that addresses the specificity of the breeding blanket designs, materials, and
∗ Corresponding author. Tel.: +34 93 3201149. E-mail address: [email protected] (D. Panayotov). http://dx.doi.org/10.1016/j.fusengdes.2015.11.019 0920-3796/© 2015 Published by Elsevier B.V.
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2 Accident analyses ITER DEMO Test blanket system
phenomena while remaining consistent with the approach already applied to ITER accident analyses. The methodology phases are illustrated in the paper by its application to the EU HCPB TBS using both MELCOR and RELAP5 codes. © 2015 Published by Elsevier B.V.
1. Introduction Testing the tritium breeding modules (TBM) in ITER [1], and the need to include them in the ITER licensing [2], offers a unique opportunity for further development and validation of the methodology for DEMO breeder blanket (BB) accident analysis [3–7]. The safety approach and accident analysis methodology for ITER is very well defined, as demonstrated by the successful construction licensing [8]. However, due to the later inclusion of the TBM testing program in the ITER agreement, the TBS safety demonstration was somewhat detached from the ITER machine safety at the beginning of the project. Safety studies for fusion facilities are commonly conducted using codes originally developed for fission reactor accident analysis. Some of these codes have been modified, and have additional physical models to treat fusion-relevant phenomena [9–13]. For example, fusion-adapted versions of MELCOR are widely applied for fusion accident analyses [3,12,14,15]. The underlying fission safety codes have undergone development and validation using extensive separate and integral effects experimental databases [16–18]. These huge international efforts (including the 2D/3D program [16] and many computational benchmark problems) using phenomena identification and ranking tables (PIRT) [19] and improved simulation models have led to development of a best estimate methodology for fission safety. The fusion-modified codes are validated against the limited available fusion experimental data or through benchmarking against validated code(s) or code version(s) [20–22]. Note that experimental data for many BB (accident) phenomena are not yet available. This work is devoted to establishing a systematic integrated ITER-TBS/DEMO-BB accident analysis methodology for simulating fault response of the breeding blanket and its interaction with the rest of the machine/plant. The methodology consists of several phases: (1) deterministic selection of accident scenarios supported by failure modes and effects analysis (FMEA) studies, (2) elaboration on these to develop accident analysis specifications (AAS) via the use of PIRT to identify required physical models to aid in code selection, (3) development of TBS models using the codes selected, and (4) qualification of the models via comparison with finite element calculations, code-to-code comparisons, and sensitivity studies. The paper continues the presentation of previously published methodology and its application to HCLL blanket [23]. The outlined methodology addresses the challenge in performing accident analysis for the EU TBS in an environment lacking experimental data on TBS phenomena. According to the French INB order 2012 [24] some TBS sub-systems and components are Protection Important Components and the application of the methodology provided in this paper to the ITER TBS accident analyses is a Protection Important Activity. For this reason the compliance with French INB order was a fundamental requirement for the work described hereafter.
2. Selection of reference accident scenarios As described in [5,7], the procedure developed for the selection of reference accident scenarios for ITER [25] has been used to identify scenarios for the EU TBS. The accidental conditions, or postulated initiating events, which might give rise to a release
of radioactivity were determined from a FMEA evaluation; a set of reference accidents was then identified, by grouping individual accident initiators that have similar consequences, which are outlined in [4]. 3. Development of the accident analysis specifications In the presented methodology, the reference scenarios are elaborated on to provide AAS. These are used, together with PIRT, to identify the requirements to be met by the analysis codes and TBS models. In this manner the limitations of individual analysis codes may be identified, and, where necessary, modelling approaches to overcome these limitations can be proposed. The definition of the accident analysis specifications for each scenario is performed in seven steps: (i) list the systems potentially engaged in the scenario; (ii) identify the phenomena that are likely to occur and list the required code models; (iii) select the most suitable code and version for analysis according to the predefined criteria; (iv) specify the input to the model development; (v) list the expected output of the accident analyses; (vi) define the accident sequence; (vii) prepare an accident flow chart. The key elements of these steps are described below, with reference to the HCPB TBS. 3.1. Phenomena identification tables and required code models The identification of the phenomena that potentially could occur within the reference accident scenarios is assessed in a two stage process. Initially, a review of existing analysis results is undertaken to obtain direct information on the more significant phenomena occurring in normal operation and the selected reference accident sequences. Second, a review of phenomena based on the physical processes imposed by the accident, system design, operating conditions, safety functions and materials of construction is performed to provide a more comprehensive basis for the assessment. This approach resembles that adopted for the PIRT procedure. For the HCPB TBS, the phenomena were grouped under eleven sub-headings: power source; flow; heat transfer; phase change; pebble bed modelling; chemical reactions; non-condensable gases; particle transport and tracking; numerical coupling (steady state setup), system modelling requirements, and material properties. The results from each of the reviews were compared and consolidated to produce a single set of phenomena that could potentially influence the progression of the accident sequences. An excerpt from this list for the pebble bed modelling is presented in Table 1. 3.2. Code assessment and selection The list of phenomena, described above, form the basis for an assessment of the code models within the code selection procedure. For the HCPB TBS, the code selection criteria included model availability, coverage of phenomena, verification status, and the ability to resolve local and 2D/3D effects. The code selection process has been limited to the assessment of different versions of the RELAP5 and MELCOR codes, as prescribed by F4E. The specific code versions that have been considered are RELAP5/MOD3.3, RELAP5-3D, fusion adapted MELCOR versions 1.8.2, 1.8.5
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3
Phenomenon/Parameter
Location
Impact on faults
Power source (volumetric nuclear heating and decay heat) Heat transfer, thermal properties
Breeder unit (BU),
Release of stored thermal energy and decay heat
BU, Tritium extraction system (TES) BU, VV (ITER FW)
Temperature responses due to conduction and convection Hydrogen/BeO and heat production
assessments [3,15]. Although the standard version of MELCOR 1.8.6 has advantages over MELCOR 1.8.5 it is judged that the latter will provide an equally valid calculation of the conditions within the HCPB TBM as former. MELCOR 1.8.5 has the advantage that it has also been selected for the assessment of the HCLL TBS [23]. The RELAP5-3D version has some enhanced modelling features compared to RELAP5/MOD3.3 which includes the addition of the ATHENA multi-fluid package [21]. In addition, it has been selected to perform the RELAP5 assessment for the HCLL TBS [23] and therefore it is the logical choice for the HCPB TBS analyses to allow code consistency between the safety demonstration of the two TBSs.
VV
Rate and overall reaction of compounds.
3.3. Accident analysis specification
BU
Transport of reactants and reaction products
The overall method used for the definition of accident specifications was based on the following steps:
BU, VV (faulted)
Non-reactive, thermal conduction Chemical reaction: H2 , BeO, heat production, thermal conduction
• Definition of the accident analysis approach, • Purpose of scenario (definition of objectives and acceptance criteria), • Definition of accident initiating events and progression, • Identification of system operation/data.
Chemical reactions of beryllium with steam/air Flow and interaction of beryllium pebbles with VV atmosphere. Diffusion of steam/air/hydrogen within pebble bed Material properties of lithium ceramic bed Material properties of beryllium bed
BU, ITER FW
(multi-fluids version), and 1.8.6 and standard MELCOR versions 1.8.6 and 2.1. The individual versions of the MELCOR and RELAP5 codes have, in many respects, similar capabilities and attributes. In Table 2, the list of phenomena presented in Table 1 is repeated with statements indicating the ability of the codes to model specific phenomena; a more extensive evaluation was undertaken for key models and correlations. Thus the relevant similarities and differences between the codes (and where appropriate, between code versions) were identified. Fusion-adapted MELCOR, version 1.8.2 has undergone the most extensive verification and validation, including a line-by-line review of fusion modifications to the source code. Though not as extensive as for 1.8.2, MELCOR 1.8.5 has been subject to verification via comparison studies with standard (fission) MELCOR 1.8.5 and pedigreed MELCOR 1.8.2, and has been used in other fusion safety
Both the objectives and the definition of initiating events and aggravating failures used the TBS Preliminary Safety Report (and FMEA study) as starting points. Based on this information, the potential accident progression and key phenomena were identified and compared with those derived in Section 3.1. The acceptance criteria for each scenario have been selected based on the objectives defined. The objectives are accident specific and normally include (i) investigate the response to a given accident”; (ii) analyse the thermal hydraulics of the TBM and cooling systems; confirm that (iii) transients do not cause any damage to confinement barriers; (iv) post-accident cooling is established to remove decay heat; (v) radioactive and energy inventories can be kept under control; (vi) radioactive releases are within the plant limits. The specific safety design requirements given in system requirement documents were used as the basis for acceptance
Table 2 HCPB TBS code assessment for pebble bed modelling. Parameter/Phenomenon
MELCOR
Uncertainty
RELAP5
Uncertainty
Power source (volumetric nuclear heating and decay heat) Heat transfer, thermal properties
Modelled by the application of volumetric sources in heat structures within the BU. Sufficient detail in the modelling is required to capture the maximum temperatures of the bed material to ensure a conservative calculation Lithium ceramic bed thermal conductivity as a function of packing density and temperature is treated through a conservative estimate of the packing density and tabular input of conductivity versus temperature. Be bed thermal conductivity dependence on temperature and pressure is considered through a number of separate regions which exhibit different levels of strain and corresponding thermal conductivity Included in fusion adapted versions. Safety factors assigned in DBA assessments
M/L
*
M/L
M/L
*
M/L
Bounding with safety factors
Not dynamic can be represented by control functions
M
There is no mechanistic model of the discharge of beryllium pebbles from the BUs during accidents, treated in a conservative manner. The heat transfer from discharged pebbles will increase the pressurisation of the VV which needs to be captured in the analyses No mechanistic model of this process within the code, treated in a conservative manner by the model design
Hb
*
Hb
H
*
H
Material Property table input
L
*
L
Chemical reactions of Beryllium with steam/air Flow and interaction of beryllium pebbles with VV atmosphere
Diffusion of steam/hydrogen/air within pebble bed Material properties of lithium ceramic and beryllium beds
Notes: L—Low; M—Medium; H—High; Hb—High bounded. * The comment on the MELCOR code versions also applies to the RELAP5 codes.
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criteria where available. Third to sixth objectives (iii–vi) are used to define the most important acceptance criteria i.e. ones related with confinement integrity (iii and iv), control of hazardous inventories (v), and control of releases (vi). 4. Development of the breeding blanket and ancillary systems models
limited representation of the ITER building and systems within the TBS model. This is applicable only in cases when the TBS has a very weak impact on the ITER machine. The second and preferable option is coupling of the TBS and ITER models in order to simulate the interaction of both systems. These two approaches are discussed below. 5.1. Model of the ITER environment inside the TBS model
In the next step, accident analysis models were constructed using the selected MELCOR and RELAP5 codes. The TBS models cover all TBS systems and controls, and their relevant ITER environment. F4E requested flexible generic models able to handle a wide spectrum of accidents with minor adaptations, whilst maintaining consistency with ITER analysis models. During development of the TBS models, some following MELCOR code-specific issues were identified and addressed. These are common for HCLL and HCPB TBSs and were reported in [23]. Although the MELCOR model of the TBS includes all sub-systems and main components, we will limit our illustration below to the TBM.
The analysis of the TBS accident sequences require the assessment of fluid leakages, possible chemical reactions, and the transport of radionuclides within the VV, Port Cell, Chemical and Volume Control System (CVCS) area, Tokamak Cooling Water System (TCWS) vault, and connecting shafts. Therefore, these areas are included but coarsely nodalised (e.g. with a single volume) in the TBS model. Models of ventilation flows, leakage flows and engineered pressure relief paths are provided between the nodes representing the buildings and environment, as well as heat structures modelling the tokamak building to represent the heat absorbed from gases and steam released during accidents.
4.1. TBM/breeder units nodalisation 5.2. Coupling of the TBS and ITER machine MELCOR model The representation of the HCPB TBM in MELCOR is significantly different to that of the HCLL TBM described in [23]. One reason for this is the much lower thermal conductivity of the pebble beds compared with that of PbLi eutectic, which results in significant thermal gradients within the BU. This temperature variation was resolved by dividing an individual BU into a number of distinct heat structures and control volumes. As a result, it is not practical to model all eight BUs on one side of the TBM Box individually because this would have required an unfeasibly large model with unacceptably long computation times. Therefore, only two BUs are modelled explicitly, one representing a single BU and the other representing the remaining fifteen BUs in the TBM. This allows the behaviour of the sixteen BUs in normal operation to be captured, as well as their behaviour in accident scenarios in which one of the BUs is assumed to have been breached. In one such scenario, the pebble bed in the breached BU is potentially exposed to steam and generates excess heat via the exothermic oxidation of the beryllium pebbles. The temperature map in a radial-poloidal plane through the centre of an ‘average’ BU in normal operation obtained in CFD design analyses was used to determine a suitable heat-structure and control volume nodalisation scheme for the BU models, ensuring that each node would encompass a reasonably uniform temperature gradient whilst keeping the number of nodes to a minimum. The HCPB model consists of a ‘Single BU’ model, a ‘Multi-BU’ model and a number of ‘common’ components that are shared by the two BU models (see Fig. 1). The common components consist of the FW and the coolant and purge gas manifolds at the back of the TBM box. The term ‘BU model’, therefore, refers to the MELCOR model of a BU plus its share of the TBM box, excluding the ‘common’ components. It therefore includes a representation of the Horizontal Stiffening Grids (HSGs), the Vertical Stiffening Grids (VSGs) and the side caps, in addition to the BU itself. The Multi-BU model is similar to the Single BU model but as mentioned above, it represents fifteen ‘average’ BUs and therefore the scalable parameters, such as volumes, flow areas and heat sources, are scaled up by a factor of fifteen compared to those of the Single BU model. 5. Representation of the ITER environment within TBS accident analyses Two approaches are proposed for the representation of the ITER environment of the TBS. The first consists of incorporation of a
In order to assess the impact of accidents originating in the TBS on the accident response of the ITER machine, the interaction of these systems must be captured. Examples of this include in-vessel leakage of helium that may cause a plasma disruption and ex-vessel leakage of helium that may lead to pressurisation of the port cell or triggering of the ITER Central Safety System (CSS). In general, when these interactions between the TBS and the ITER machine are sufficiently strong a coupled analysis is required, with the accident response of both systems represented. Direct integration of the TBS MELCOR model and the ITER machine MELCOR model into a single overall combined model is not possible since MELCOR limitations within a single simulation. Therefore, a loose coupling of two models is used to represent the transient interactions of the ITER machine and the TBS. The selected method for coupling the ITER and TBS models is to perform a sequence of analyses, with a coordinated exchange of data between the two models as described in [23]. Iteration between the two models will ensure that selfconsistent results are obtained. The MELCOR EDF (External Data File) package provides facilities to transfer leak flow rates, enthalpies and other data between models to support this form of coupled analysis. 6. Qualification of the breeding blanket and ancillary systems models The TBS accident analysis methodology includes qualification activities to assess the ability of the developed TBS models to represent the phenomena and transient responses associated with the accident sequences defined in the AAS. By analogy with HCLL TBS [23] the HCPB TBS models have been evaluated via a test matrix that includes (1) comparisons of the MELCOR and RELAP5 predictions with available finite-element analysis (FEA) results from the TBM design description documents (DDD) in steady state, pulsed plasma operations and MARFE (‘Multi-Faceted Radiation From the Edge’) power excursion; (2) further MELCOR and RELAP5 comparisons in normal operations and test transients designed to cover a representative range of accident-relevant phenomena, and (3) sensitivity and uncertainty studies in more complex accident scenarios. The test matrix shown in Table 3 consists of 10 cases executed in 15
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Fig. 1. MELCOR HCPB TBM nodalisation; Be—yellow, Li-ceramic—green, He coolant—blue, He purge—light blue. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
code runs. The final step of the qualification is code-to-code comparison for the 32 h LOOP accident analysis. We understand that the qualification of the BB and Ancillary System Models is limited at this stage to a generic models developed and whenever needed for the analyses of specific accident an additional effort will be undertaken.
6.1. Comparison with finite element design analyses Comparison of MELCOR and RELAP5 simulations of the TBM response to a 10 s MARFE power excursion (case 3 in Table 3) with design FEA results (labelled DDD in the figure) are shown in Fig. 2. Good agreement is seen between the plasma facing temperature
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6 Table 3 HCPB TBS qualification analysis summary. # 1
2 3
4
5
6 7 8 9 10
Case title
Scenario and runs executed
Comparison with the HCPB TBM design analyses Full power Steady-state TBM model analyses compared to design results. MELCOR and RELAP5 steady-state simulations Pulse TBM model analyse of a 400 s power pulse. MELCOR and RELAP5 simulations MARFE TBM model analyses of a MARFE transient occurring from steady-state full power conditions compared to design results. MELCOR and RELAP5 simulations Analysis of HCPB TBS normal operation Code to code comparison TBS pulse TBS model analysis of a 400 s plasma power pulse. Model includes Port Plug, HCS, TBM and TES ancillary system models. MELCOR and RELAP5 simulations Fault sequence test cases Code to code comparison Test transient In-vessel FW LOCA close to end of 400 s plasma S0 pulse followed by breach of the purge gas region of a BU to the VV. MELCOR and RELAP5 simulations Sensitivity study analysis cases MELCOR simulation S1: S0+ High beryllium temperature S2: S0+, S1+ High beryllium temperature (S1) and low beryllium conductivity S3: S0+ Reduced pebble bed friction S4: S0+ Few-node representation of BU purge gas regions S5: S0 + S1 + S4 Combined high beryllium temperature (S1) and few-node representation (S4)
Fig. 3. Test transient S0 VV and purge gas pressures with zoom at 5–30 s.
∼15 s duration. The pressure in the VV and TBM then follows the prescribed VV pressure variation. The depressurisation of the TES is arrested by the closure of the TES isolation valves at 14.7 s leaving a final TES pressure (outside the isolation valves) of ∼92 kPa. MELCOR and RELAP5 simulations demonstrated representative TBS response to the transient and good agreement between the codes results. 6.3. Sensitivity studies
for all three simulations. The TBM box pebble facing temperature is ∼20 ◦ C higher than the MELCOR and RELAP5 predictions. This is consistent with the conservative heat flux prescribed from the pebble bed on to the FW in the design simulations. The MELCOR and RELAP5 simulations appear to capture temperature transients associated with rapid changes in heat flux from the plasma.
The test transient sequence S0 (case 5 in Table 3) represents an in-vessel LOCA from the TBM FW followed by a subsequent through thickness breach of the TBM FW exposing the BU to the VV. It represents a complete rupture of the FW with total break area is 5.07 × 10−3 m2 equivalent to 24 times the FW channel crosssectional area corresponding to the broken ends of 12 up-flow and 12 down-flow channels. Fig. 3 shows the depressurisation of the TBM and Tritium Extraction System (TES) purge gas to the VV. Both the MELCOR and RELAP5 simulations show an initial blowdown of
The sensitivity study S5 (case 10 in Table 3) combines the S1 and S4 cases to assess the effect of the conservative regime of high beryllium temperature and ‘perfect’ steam mixing on hydrogen production in the MELCOR HCPB TBS model. The scenario is identical to S1, except that the coarse nodalisation purge gas model implemented in S4 is used. The mass of hydrogen present and produced in different regions of the TBM is shown in Fig. 4. In total, about 18 g of hydrogen is produced over the 100,000 s simulated compared to 1 g in S0, 2.8 g in S1, 3.2 g in S2, 1.6 g in S3 and 1.8 g in S4. The production is dominated by the Single BU and most of the hydrogen is discharged into the VV. The most rapid period of hydrogen production occurs in the first ∼100 s of the transient. The greater mass of hydrogen produced in S5 than in S1, which has the same initial beryllium temperatures, is due to the ‘perfect’ steam mixing allowing any steam entering the damaged Single BU to react with any heat structure in the BU. The initial steam ingress period caused by the VV pressurisation increases the hydrogen mass rapidly. The beryllium pebble in the Single BU
Fig. 2. MARFE transient MELCOR and RELAP5 FW surface temperatures comparison with FEA results.
Fig. 4. Mass of hydrogen produced in MELCOR simulation with zoom at first 2000 s.
6.2. Code-to-code comparison
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rise in temperature during this period is due to the exothermic nature of the reaction. The steam supply for the TBM after the initial steam ingress is a result of circulating flow through the breaches. The continuous steam supply allows the reaction to continue until the beryllium temperatures drop sufficiently to give a negligible reaction rate.
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sensitivity studies. We believe that the developed methodology is applicable to accident analyses of other TBSs to be tested in ITER and as well to DEMO breeding blankets. Acknowledgments
The impact of uncertainties associated with the accident analyses needs to be addressed to provide sufficient confidence in the level of conservatism in the results [23]. An expert review of areas of uncertainty (including an uncertainty PIRT) is planned and will be reported on in a subsequent dedicated paper.
The work reported in this paper has been performed by F4E, Amec Foster Wheeler and INL under F4E contract. The views expressed in this publication are the sole responsibility of the authors and do not necessarily reflect the views of the Fusion for Energy and the ITER Organization. Neither Fusion for Energy nor any person acting on behalf of Fusion for Energy is responsible for the use, which might be made of the information in this publication.
7. Applications of the methodology
References
6.4. Uncertainty analysis
Finally, the qualified models must be applied to analyse the selected accident scenarios defined in the TBS PrSR. The process is the same as the one reported in [23] for the HCLL TBS. The methodology has been applied to a 32 h loss of offsite power (LOOP) in both the HCLL and HCPB TBSs, using both MELCOR and RELAP in order to further qualify the models via code comparison. The analyses of loss of flow accidents in each TBS using MELCOR is on-going. These are TBS accidents with very limited impact on the ITER machine. In the next analyses, model coupling will be used to investigate TBS accidents that might have effect on the ITER machine and require the simulation of interacting phenomena and processes that take place in several systems. 8. Summary and conclusions A comprehensive methodology for fusion breeding blanket accident analyses that addresses the specificity of the designs, materials, and phenomena while remaining consistent with the approach already applied to ITER has been developed and applied to the EU HCLL and HCPB TBSs. The strong points of the methodology are the use of PIRT to identify requirements to be met by the analysis codes; development of high quality TBS models; the loose coupling of different codes or code versions in order to simulate multi-fluid flows and phenomena overcoming the codes’ limitations; qualification of the models by comparison with finite element analyses and code-to-code comparisons; and uncertainty analysis utilizing
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
[19] [20] [21] [22] [23] [24]
[25]
L. Giancarli, et al., Fusion Eng. Des. 87 (2012) 395–402. Y. Poitevin, et al., Fusion Eng. Des. 89 (2014) 1113–1118. B.J. Merrill, et al., Nucl. Fusion 47 (2007) S447–S452. T. Pinna, L.V. Boccaccini, J.F. Salavy, Fusion Eng. Des. 83 (2008) 1733–1737. D. Panayotov, et al., IAEA Fusion Energy Conference FEC-2012, 8–13 October 2012, USA, 2012. H.G. Jin, et al., IEEE Trans. Plasma Sci. 42 (3) (2014) 671–674. D. Panayotov, et al., Fusion Eng. Des. 98–99 (2015) 2201–2205. N. Taylor, P. Cortes, Fusion Eng. Des. 89 (2014) 1995–2000. B.J. Merrill, et al., Fusion Eng. Des. 51–52 (2000) 555–563. J. Andersson, et al., Fusion Eng. Des. 51–52 (2000) 453–460. B.J. Merrill, Fusion Eng. Des. 54 (2001) 485–493. B.J. Merrill, P.W. Humrickhouse, R.L. Moore, Fusion Eng. Des. 85 (2010) 1479–1483. C. Séropian, et al., Fusion Eng. Des. 88 (2013) 2698–2703. B.J. Merrill, et al., Fusion Eng. Des. 89 (2014) 1989–1994. P.W. Humrickhouse, B.J. Merrill, INL/CON-12-25015, August 2012. P.S. Damerell, J.W. Simons (Eds.), 2D/3D Program Work Summary Report, NUREG/IA-0126, GRS-100, MPR-1345, June 1993. N. Aksan, et al., Separate Effects Test Matrix for Thermal-Hydraulic Code Validation NEA/CSNI/R(93)14, OECD, Paris, July 1994. A. Annunziato, et al., CSNI Integral test facility validation matrix for the assessment of thermal-hydraulic codes for LWR LOCA and transients, NEA/CSNI/R(96)17, OECD, Paris, July 1996. D.J. Diamond, BNL-76750-2006-CP, September, 2006. P. Sardain, et al., Fusion Eng. Des. 54 (2001) 555–561. R.L. Moore, B.J. Merrill, Fusion Eng. Des. 63–64 (2002) 411–419. G. Geffraye, et al., Nucl. Eng. Des. 249 (2012) 237–247. D. Panayotov, A. Grief, B.J. Merrill, P. Humrickhouse, et al., SOFE 26 (2015), in press. French Official Gazette, Arrêté du 7 février 2012 fixant les règles générales relatives aux installations nucléaires de base, French Official Gazette (08 February 2012). T. Pinna, et al., Fusion Eng. Des. 75-79 (2005) 1103–1107.
Please cite this article in press as: D. Panayotov, et al., Methodology for accident analyses of fusion breeder blankets and its application to helium-cooled pebble bed blanket, Fusion Eng. Des. (2015), http://dx.doi.org/10.1016/j.fusengdes.2015.11.019