1 ITR/P5-16 Progress in the EU Test Blanket Systems Safety Studies D. Panayotov 1, Y. Poitevin 1, J. Furlan 2, M. T. Porfiri 2,3, T. Pinna 3, M. Iseli 4 1 Fusion For Energy (F4E), Josep Pla, 2; Torres Diagonal Litoral B3, Barcelona, E-08019, Spain 2 Fusion For Energy (F4E) external expert 3 ENEA UTFUS-TEC Via E.Fermi 45, 00044 Frascati, Rome, Italy 4 ITER Organization, Route de Vinon sur Verdon, 13115 Saint Paul Lez Durance, France E-mail contact of main author: dobromir.panayotov@f4e.europa.eu Abstract. The European Joint Undertaking for ITER and the Development of Fusion Energy ('Fusion for Energy'- F4E) provides the European contributions to the ITER international fusion energy research project. Among others it includes also the development, design, technological demonstration and implementation of the European Test Blanket Systems (TBS) in ITER. An overview of the ITER TBS program has been presented recently at ISFNT-10. Currently two EU TBS designs are in the phase of conceptual design - Helium-Cooled Lithium-Lead (HCLL) and Helium-Cooled Pebble-Bed (HCPB). Safety demonstration is an important part of the work devoted to the achievement of the next key project milestone the Conceptual Design Review. The paper reveals the details of the work on EU TBS safety performed in the last couple of years in the fields of update of the TBS safety demonstration file; TBS Safety approach, design principles, requirements, features and safety functions; detailed TBS components classifications; Radiation Shielding and Protection; and Selection of reference accidents scenarios and Accidents analyses. Finally the authors share the planned future EU TBS safety activities. 1. Introduction The European Joint Undertaking for ITER and the Development of Fusion Energy ('Fusion for Energy'- F4E) provides the European contributions to the ITER international fusion energy research project. Among others it includes also the development, design, technological demonstration and implementation of the European Test Blanket Systems (TBS) in ITER [1]. Currently two EU TBS designs are in the phase of conceptual design - Helium-Cooled Lithium-Lead (HCLL) and Helium-Cooled Pebble-Bed (HCPB). Safety demonstration is an important part of the work devoted to the achievement of the next key project milestone the Conceptual Design Review. The purpose of this paper is to report the progress made in the safety studies and to reveal the details of the work on EU TBS safety performed in the last couple of years after the status reported in [2]. Drafting the safety strategy and approach [3] facilitated the significant progress made in the fields reported hereafter. Finally main planned EU TBS safety activities are briefly presented. 2. EU TBS safety demonstration files TBS safety demonstration files of both EU TBSs HCLL and HCPB have been updated in line with the TBSs 2011 baselines presented in [4]. The work covered the TBS design description on the level of sub-systems and main components; general operation states; expected maintenance activities and drafting of detailed TBS plant breakdown structures (PBS). It also facilitated the update of the TBS design principles, requirements, features and safety functions. The last has been achieved by circumstantial discussions both internally in the F4E Test Blanket Modules (TBM) team and externally with fusion safety experts.
2 ITR/P5-16 2.1. Safety approach and design principles Definition of the EU TBS objectives, strategy and approach presented in [3] laid the foundation of work on the TBS safety approach and design principles that are the main skeleton of the EU TBSs safety demonstration. When installed into the ITER machine, the TBS becomes part of the machine; hence, its design and operation are subject to the same safety and licensing regime as the machine. Therefore, it is essential the same safety design principles to be followed for the design of the TBS. Design, manufacturing and operation of these test blanket modules and related systems are performed according to the same safety objectives assigned to the ITER facility (confinement principle, doses minimization, wastes volumes minimization). The TBM experimental program will require additional safety assessments since some materials, fluids and parameters are different from the ITER basic machine. The safety analysis is performed accounting for the TBS potential failures, the consequences and the verification of the ITER safety objectives. TBS design basis accidents will have to be enveloped by the ITER design basis accidents and the consequences of TBS beyond design basis situations bounded by ITER beyond design basis situations. In the following, the approach will successively address: - TBS inventories (of hazardous products and risks) - Design arrangements to control and minimize possible releases and doses in normal and accidental situations. - Analysis of postulated design and beyond design accidents giving evidence that the safety objectives are met. The aim is to demonstrate that the TBM systems and ITER does not jeopardize safe operation mutually. In addition, according to the considerations of the ITER and TBM experimental programs, experience feedback, human factors, and common mode failure the TBM progressive experimental strategy is in favour of making safety arrangements more robust and validated. From the point of view of the TBM program, for each TBS type, it is planned to have up to four TBM versions, which will be tested in two main phases, a first phase that is called the learning phase and a second phase so called the DEMO-relevant data acquisition phase. It is assumed that all TBM versions will share the same basic architecture, in particular their structural part (including the attachment system), whose design will be qualified during the testing program in laboratory facilities before TBM commissioning and checked/monitored step-by-step during the different phases of ITER operation. This strategy ensures a relatively stable interface between the TBM and ITER during the whole operation time, with benefits for the availability and safety of the machine. It has to be noted that the safety analysis will take the most conservative case of the four TBM versions. According to a deterministic approach the safety analysis is performed accounting for the TBS potential failures, the consequences and the verification of the ITER safety objectives. The general safety requirements for the TBS are derived from the ITER safety requirements and two of them are as follows: Design, construction, operation, and decommissioning of TBS shall meet technologyindependent radiological dose and radioactivity release limits for the public and site personnel based on recommendations by international bodies such as IAEA and ICRP. All conventional (non-nuclear) safety and environmental impacts from construction, operation, and decommissioning shall meet common industrial standards for industrial practice. This includes chemical toxins and electromagnetic hazards.
3 ITR/P5-16 Other general safety requirements are directly correlated to the safety principles that are as follow Defence-in-depth: in designing, constructing and operating the TBS all the five levels of the defence-in-depth principles have to be considered. Radiation protection and ALARA application. In TBS areas technical regulatory baselines regarding ionizing radiation protection shall be applied and observed. TBS specific safety design requirements: o Identification of safety important functions and components. o Specific requirements for TBM. The main practical measures undertaken to apply ALARA approach are listed below Design minimization of the amounts of radioactive and toxic materials contained in TBS and the hazards associated with their handling. Detritiation and filtering of TBS fluids (coolant and lithium-lead) shall be provided. TBS coolant purification system shall minimize impurities contained in the coolant. Use of appropriate fixed and temporary shielding during maintenance and repair activities in order to reduce worker doses. Use air detritiation systems during maintenance and use of worker protective device, whenever need, to protect workers from tritium contamination and tritium intake, from toxic and/or activated material contamination/in-taking. Surface contamination monitoring on TBS equipments shall be provided before maintenance activities. Operational dosimetry shall be implemented in TBS areas. Helium monitors shall be foreseen in TBS operating areas to immediately detect high pressure and temperature He releases. 2.2. TBS components classifications The detailed safety, seismic and quality classifications of the TBS systems and components is derived from the drafted TBS Product Breakdown Structure (PBS) and assign a Safety Importance Class (SIC) to components that are critical to maintain the TBS and ITER in safe state. Although the TBSs are still in the conceptual design phase the EU TBSs PBSs list more than 350 components for HCLL TBS and more than 300 components for HCPB TBS. The components names, functional category designators IDs and descriptions are derived according to the ITER naming conventions and guidelines. Components progress sequence and locations are also given in the PBSs tables when available. The safety classification was performed defining the safety important rational of the components that includes the safety function(s), class, and classification criteria. The safety, seismic and quality classifications conducted by the related IO guidelines specify the requirements for the further analyses and design update. The aim is to ensure that SIC components will receive adequate attention during the design, fabrication, installation, commissioning and operational stages. These TBS classifications are then submitted for discussion and approval by the IO. In parallel the work on the TBS classifications according the Pressure Equipment Directive (PED) [5] and the French Order concerning Nuclear Pressure Equipment (NPE) [6] is currently ongoing. It has to be noted that F4E TBM team very well recognize the importance of these classifications and considers them as crucial for the development, design and testing of the EU TBMs in ITER. The provisional classifications have been drafted in 2010 and have been extensively discussed with IO experts. Due to the particular configuration of the TBM TBS (e.g. TBM is a multi-chamber equipment, difficulty to implement pressure relief systems
4 ITR/P5-16 on TBM and ancillary systems) a two step approach has been selected. This includes a choice of a maximum admissible pressure and followed by the clarifications with the support of ITER IO and a Notified Body. 2.3. TBS Radiation Shielding and Protection Previous versions version of the EU TBS PrSRs made use of the neutronics studies performed in the EFDA time frame in and before the 2006. The neutronics analysis completed at the end of 2010 [7] pointed out severe deficiencies in the TBS shielding design, as follow Poor performance of the TBM rear shield; Underperformance of the Bioshield plug and of the PbLi loop shield Neutron (and gamma) streaming in the gap between the TBM and the Port Plug (PP) Frame. Applying the ALARA principle several Monte Carlo neutronics studies [8] have been performed in 2011-2012 in order to select the shielding materials and to confirm that the TBM rear shield performance will meet the radiation limits. We started by improvements of our and neighbours design by o Splitting the TBM shield into two parts: front SS/water cooled and rear without cooling o Selecting the optimal SS/water ratio of 50%/50% for the front TBM shield and Boron carbide as material for additional rear TBM shield, both done by scoping calculations o Proposing the Bioshield plug thickening from 450mm to 600 mm on plasma side and 50 mm cast Iron plate on the PC side again by scoping calculations o Improving the PbLi loop shield by changing the material from cast Iron to Lead. We run the new model with completely closed gap between the TBM (and the shield) and the PP Frame to see (in June 2011) marginal reduction of the neutron flux behind the PP and to realize that there is significant neutron streaming in the gap between the PP Frame and the Port extension. We couldn t go further with the current contract and we decided to amend it implementing additional modifications in the model. Taking advantage of the ongoing studies on other ports we implemented doglegs and mid-stoppers in the all mentioned above gaps. After resolving successfully all arisen contractual and computational resources issues, adjusting the plan and the schedule, we arrived in December 2011 to the point to observe once again that our efforts wasn t productive enough and we were still one order of magnitude away from the target. At that moment we decided to focus on the TBMs neutronics and shielding only. This has been achieved by closing the radiation contributions from the lower and upper ports to our equatorial port # 16. Finally the value of the neutron flux obtained behind the port plug was around the target of 5.10 7 n/(cm 2 s) [8]. Table I summarizes the TBS neutronics models studied in 2010 2012. TABLE I SUMMARY OF THE TBS NEUTRONICS MODELS STUDIED IN 2010 2012 Label Shield Model TBM/PP gap PP/VV gap Upper & Lower Ports Reference Dec 2010 96 cm SS/H 2 0 (70/30%) straight open straight open open [7] 61 cm SS/H June 2011 2 0 (50/50%) + completely straight open open [8] 141 cm B 4 C closed 61 cm SS/H 2 0 (50/50%) + doglegs and doglegs and Aug 2012 100 cm B 4 C enclosed in 5 closed [8] mid-stoppers mid-stoppers cm thick SS box FIG. 1 presents the comparison of neutron flux radial profiles for three selected points. The units are n/cm 2 per src due to the neutron flux normalisation to per neutron source of
5 ITR/P5-16 1.9717 10 19 n/sec. In general, the neutron fluxes are lowest in the current working model when the three cases are compared. In the position along the PbLi cooling pipes (position A), the attenuation at the rear of the shields is about one order of magnitude better when compared to Dec 2010 results and about two orders of magnitude better compared to the June 2011. These differences are caused by the various shielding combinations in different models. Regarding the corner position D the reduction of the neutron fluxes in the current model is a bit more than a order of magnitude when compared to both previous shielding options. This difference is clearly associated with the streaming gaps inside the equatorial port. Since the position H is located through the solid shielding materials, the effectiveness of the shielding improvement in current design is similar, i.e. about 20 times better than June 2011, and roughly 100 times better than the Dec 2010 model. 1.00E-04 1 2 3 D C A H B G E F Neutron Flux (n/cm2 per src) Neutron Flux (n/cm2 per src) Neutron Flux (n/cm2 per src) 1.00E-05 1.00E-06 1.00E-07 1.00E-08 1.00E-09 1.00E-10 1.00E-11 ST2.1 Aug 2012 ST2 June option 2011 2 previous Dec 2010 results 1.00E-12 820 870 920 970 1020 1070 1120 1170 1.00E-04 1.00E-05 1.00E-06 1.00E-07 1.00E-08 1.00E-09 1.00E-10 1.00E-11 1.00E-12 1.00E-13 ST2.1 Aug 2012 ST2 June option 2011 2 previous Dec 2010 results Radial Distance (cm) 1 2 3 1.00E-14 820 870 920 970 1020 1070 1120 1170 1.00E-04 1.00E-05 1.00E-06 1.00E-07 1.00E-08 1.00E-09 1.00E-10 Radial Distance (cm) 1 2 3 1.00E-11 1.00E-12 ST2.1 Aug 2012 ST2 June option 2011 2 1.00E-13 previous Dec 2010 results 1.00E-14 820 870 920 970 1020 1070 1120 1170 Radial Distance (cm) FIG. 1. Radial neutron flux profile comparison: D- top corner of HCLL side (up); A - PbLi cooling pipe (middle); H - solid inside PP frame (down); 1: X=1045 cm; 2: X=1117 cm; 3: X=1158 cm [8]. Finally in May 2012 we confirmed the performance of our newly designed TBM shield by obtaining a dose rate in the port interspace of 74 µsv/h after 12 days cooling time [8]. The approach and efforts so far made on the improvement of shielding capability, reduced dose rates and activation inventories.
6 ITR/P5-16 2.4. Selection of TBS reference accidents scenarios and Accidents analyses The new design baseline triggered also the systematic review and update of the list of selected reference accidents scenarios. Discussed below HCLL TBS selection process is based on the performed revision of the existing list of postulated initiating event (PIE) reported in [9]. The PIEs frequency has been derived using the similarities with the US Dual-Coolant Lithium- Lead TBS recently published FMEA study [10] (see Table II). Three PIEs were already pointed out by the FMEA on other ITER systems and documented: LFP2, LFV2, and VVA2 (see Table I for the labels of PIEs). The event LVV2 Small rupture in the internal VV shell was excluded because the consequences are enveloped by the ITER event "Multiple ITER FW pipe break". From the remaining list few accidents have been selected in [9] as reference because they have the highest expected consequences i.e., those that establish the system safety design requirements, or because they present some peculiarities from the design point of view. They are FB1, LBB1, LBO3, LBP1, LBV1, and TBP2. TABLE III. TOTAL LIST OF PIEs IDENTIFIED BY THE HCLL TBS FMEA [9] AND [10] PIEs Description Frequency/yr Category FB1 Loss of flow (LOFA) in a TBM HCS caused by HCS circulator (rotor) 2.8*10-2 II FB2 Partial flow blockage (LOFA) in a TBM HCS caused by filter clogging 1.9*10-3 III HB1 Loss of heat sink in TBS HCS 1.46*10-1 II LBB1 In-TBM box Large break LOCA due to a sealing weld rupture 1.8*10-4 III LBB2 In-TBM box Small break LOCA due to a sealing weld leak 1.8*10-3 III LBO1 Ex-vessel Large break LOCA due to of TBS HCS pipe break inside CVCS 2.4*10-3 III LBO2 Ex- vessel Small break LOCA due to of TBS HCS pipe leak inside CVCS 8.9*10-2 II LBO3 Ex- vessel LOCA due to tubes break in a primary TBM HCS heat 4.2*10-2 II LBP1 Ex- vessel LOCA due to a large break of TBM HCS pipe inside the Port 8.0*10-4 III LBP2 Ex- vessel LOCA due to a small break of TBM HCS pipe inside Port Cell 8.0*10-3 III LBV1 In- vessel Large break LOCA: one side break of the TBM FW 1.2*10-2 II LBV2 In- vessel Small Break LOCA: one side leak TBM FW 2.94*10-3 III LFP2 Ex- vessel LOCA due to a small break of ITER FW/BLK cooling circuit pipe inside Port Cell - III LFV2 ITER FW/BLK small in-vessel LOCA - II LMP2 Loss of liquid metal into Port Cell due to small break in liquid metal TBS 4.4*10-2 II LVP2 Small rupture of VV cooling circuit pipe inside Port Cell 7.3*10-2 II LVV2 Small rupture in the internal VV shell - III TBL1 TBS TRS process line large leak inside the Glove Box - II TBL2 TBS TRS process line small leak inside the Glove Box - II TBP2 TBS TRS process line leak inside the Port Cell 5.0*10-3 III VMM1 Loss of vacuum in VV: large ingress of liquid metal into VV due to TBM 1.0*10-3 III VMM2 Loss of vacuum in VV: small ingress of liquid metal into VV due to TBM 1.0*10-2 II VVA2 Ingress of air into the VV - small leak 1.9*10-3 III In addition to the previous reference accidents selected by HCLL FMEA [9] two other PIEs must be included in the list that are: LMP2 (Small break of liquid metal TBS loop PC) should not be significant in terms of containments challenging and risks for releases because the amount of H 2 produced will be limited. Nevertheless, it could be of particular concern in terms of worker safety and plant availability because difficulties to drain, clean and decontaminate PC surfaces.
7 ITR/P5-16 VMM1 (LOVA: large ingress of liquid metal into VV due to liquid Metal TBM leak) has been considered to take into account a significant rupture inside the vessel of the TBM structure containing the LiPb liquid metal. For the selection of reference accidents the results of the FMEA had been further complemented in order to link the TBS events (identified by the HCLL TBS FMEA) to those 7 accidents defined as references for ITER. The next step taken in the identification of the reference was to make sure that the selected above events envelope the consequences of all conceivable events. This process establishes a correspondence, when it exists, between FMEA PIEs choice and ITER accident list. Some parametric studies are included in the accidents to be analyzed in order to cover the aggravating failures evidenced in [9]. Thus the original ITER seven accidents inferred a list of 14 reference accident. Table III reports the summary of the selected for the analyses accidents providing their categorization into incidents, accidents, DBA and DBDA. TABLE II. SUMMARY OF THE SELECTED HCLL INCIDENTS AND ACCIDENTS ID# Event Event category Corresponding PIE 1 Loss off-site power DBA LOOP 4 LOFA + aggravating failure Accident FB1 5 LOCA in VV Incident LBV1 6 LOVA + aggravating failure Accident LBV1 8 LOCA in PC Accident LBP1 11 LOCA in CVCS vault Accident LBO1 12 LOCA in CVCS vault + aggravating failure BDBA LBO1 14 Cask failure Incident in maintenance Thus the needs for TBSs accidents analyses have been defined and an approach for their completion has been developed. It includes along with the possible grouping and envelopments of the scenarios also the synergies between HCLL and HCPB TBS scenarios and optimization of the analyses, TBSs accidents enveloped by ITER accidents, ranking of the TBS accident scenarios (for the purpose of accident analyses only). 3. Future EU TBS safety activities The following main TBS safety activities has been so far planned for the next couple of years in the preparation for the TBS conceptual design reviews categorization of the TBS Radwaste according to the French regulation and drafting of the waste management plan update of the design of the TBS rear shields and new neutronics studies using of the latest ITER MCNP model available and accounting for the contribution of other ports, assessment of the biological potential hazard from inhalation and digestion, update of the operational radiation exposure assessment development and qualification of the TBS models and several accidents analyses (BDA and BDBA) of each of the TBSs, run the experimental campaign on transient behaviour of the TBS. The HCLL safety demonstration is planned to be submitted to IO review later this year, while the HCPB file will be completed early in 2013. Planned activities will support the following up update of the safety demonstration files.
8 ITR/P5-16 4. Conclusions The work progress on EU TBS safety made in the fields of update of the safety demonstration file, safety approach and design principles, detailed TBS components classifications, radiation shielding and protection, selection of reference accidents scenarios and planned future EU TBS safety activities has been reported demonstrating the approach and efforts devoted in the last couple of years. Acknowledgements The TBS neutronics analysis reported in this paper has been performed by UK AEA CCFE under F4E contract No F4E-2008-OPE-02-01-04. The authors would like to express their gratitude to the CCFE team for its efforts and indispensable contribution in resolving the TBS shielding issues. 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. References [1] L. Giancarli, et al., Overview of ITER TBM Program, Fusion Eng. Des. 87, 395-402, (2012). [2] T. Pinna, Studies for the preparation of the Preliminary Safety Reports for the European TBS, Fusion Eng. Des. 86, 2204-2207, (2011). [3] D. Panayotov, et al., Safety Approach in the EU Test Blanket Systems Design, Fusion Eng. Des. 87, 1035-1039, (2012). [4] L.V. Boccaccini, et al., Present status of the conceptual design of the EU test blanket systems, Fusion Eng. Des. 86 (2011) 478 483. [5] French Decree No. 99-1046 dated 13th December 1999 concerning pressure equipment (amended by further Decrees in 2003 and 2007) and Order dated 21st December 1999 concerning classification and conformity assessment of pressure equipment. [6] Order dated 12th December 2005 concerning nuclear pressure equipment (NPE). [7] R. Pampin, et al., European ITER TBS neutronics, activation and shutdown dose preliminary mapping, F4E-2008-OPE-02-01-02 final report, December 2010. [8] S. Zheng, et al., Neutronics and activation analysis for the European TBS with improved nuclear shielding, Deliverables 4&5, F4E-2008-OPE-02-01-04 Subtasks 4&5, August 2012. [9] T. Pinna, L.V. Boccaccini, J.F. Salavy, Failure mode and effect analysis for the European test blanket modules, Fusion Eng. Des. 83 (2008) 1733 1737 [10] L. C. Cadwallader, Preliminary Failure Modes and Effects Analysis of the US DCLL Test Blanket Module, INL/EXT-07-13115 Rev. 1, June 2010