Safety assessment of the lithium target of the international fusion materials irradiation facility

This article presents the international fusion materials irradiation facility lithium target safety and thermal transient analysis, to evaluate the most important risk factors related to the system operation and to verify the fulfillment of the safety criteria. Main conclusions are that target safety is accomplished: hazards associated with lithium operation are confined within the IFMIF security boundaries and environmental impact is negligible, and the plant well answers to the simulated transients, being able to reach steady conditions in a condition of safety.     

Unavailability analysis of a lithium target system for a test facility

For an experimental facility like the International Fusion Materials Irradiation Facility, safety, performance and reliability are outstanding attributes. The results of an analysis aimed at estimating the unavailability figure of the target system and evaluating the expected performance of the plant, in terms of safe and reliable operation, are presented in this article. Starting points of the analysis have been the identification of relevant plant functions and relationships between plant systems and functions and the definition of relationships among the systems themselves. Fault tree technique has been adopted to address the topic: with regard to the reference configuration, systems boundaries and interfaces have been identified as well as the dependent failures between systems and components. Due to the novelty of the plant and its prototypical character, reliability data introduced in the numerical simulation underwent an accurate screening process among the current available databases, sometimes requiring the expert judgment assessment. Finally an uncertainty, importance and sensitivity analysis has been performed in order to add credit to the model and to highlight reliability-critical systems and components: results are analyzed to assess the compliance with the plant availability requirements and design criteria. Risk Spectrum code has been utilized for the system unavailability quantification.     

Computational modelling of the IFMIF lithium target

The present work is devoted to the computational modelling of the process of beam action on a lithium target. The aim of the investigation is to determine the maximum values of temperature and pressure as well as general pattern of the process. The analysis is based on the compressible Euler equations with the stiffened gas equation of state with parameters corresponding to lithium. The energy influx allocation caused by the beam interaction with the target is described by the source term in the energy balance law. The formulated problem is solved numerically by a high-resolution Godunov-type method. The obtained results show a moderate rise in the lithium temperature and relatively large pressure variations.     

Concept of curved magnetically guided liquid lithium target without a back plate for IFMIF

Curved magnetically guided lithium target (MGLT) without a back plate was newly proposed in light of simplified structure, easy maintenance and enhanced availability and performance for international fusion materials irradiation facility (IFMIF). It can replace conventional lithium target with a curved material back plate under the most severe condition on neutron irradiation. Magnetic field suited for the curved MGLT is produced in combination of a couple of radiation-proof resistive coils and reduced activation ferritic/martensitic steel (F82H) parts (yokes, ducts/nozzles and high flux test module (HFTM)). Shape of the magnetic field becomes curved automatically in the target region by setting HFTM closely to MGLT. Characteristics of the lithium flow on MGLT was analyzed in detail by two dimensional equations of motion with the magnetic field calculated by the Poisson Superfish code. The necessary magnetic flux density at the target region was found to be about 0.5 T to fulfill the IFMIF target conditions, i.e., lithium flow speed of 15 m/s, curvature radius of 1–1.6 m and flow thickness of 0.025 m. A narrow gap (a few mm) between MGLT and HFTM could be controlled by adjusting the coil current. Future subjects for further development of this concept were identified.     

Status of engineering design of liquid lithium target in IFMIF-EVEDA

In International Fusion Materials Irradiation Facility (IFMIF), intense neutron flux (4.5 × 10¹⁷ n/m² s) with a peak energy of 14 MeV are produced by means of two deuteron beams with a total current of 250 mA and maximum energy of 40 MeV that strike a liquid Li target circulating in a Li loop. Major design requirement is to provide a stable Li jet at a speed of 10–20 m/s with a surface wave amplitude on the Li flow less than 1 mm for handling of an averaged heat flux of 1 GW/m² under a continuous 10 MW deuterium beam deposition. The target system consists of a target assembly, a replaceable back-plate, a Li main loop and a Li purification loop. In July 2007, Engineering Validation and Engineering Design Activities (EVEDA) started under Broader Approach. In this paper, status of the engineering design of the IFMIF Li target system performed in 2007/2008 is described. The future EVEDA tasks to develop the target system are also summarized.     

Nuclear analysis of the IFMIF European lithium target assembly system

In the framework of the Engineering Validation and Engineering Design Activities (EVEDA) phase of the International Fusion Materials Irradiation Facility (IFMIF) project, ENEA was in charge of the design of the European version of the target assembly (TA) system which employs a removable bayonet backplate (BP) concept. With the aim of assessing the nuclear behaviour of the system and supplying the necessary input data to the thermomechanical analysis, coupled neutron-gamma transport calculations have been carried out for the whole TA + BP system, using the MCNP5 1.6 Monte Carlo transport code integrated with the McDeLicious-11 neutron source code provided by KIT. Neutron activation calculations have been performed by means of the EASY-2010 activation system in order to provide radioactive inventories useful for thermomechanical analysis and safety purposes. This paper summarizes the results obtained by the neutronic and activation calculations for the most irradiated components of the TA, such as backplate, frame, nozzle and target chamber.     

Thickness distribution of high-speed free-surface lithium flow simulating IFMIF target

International Fusion Materials Irradiation Facility (IFMIF) employs liquid lithium as D⁺ beams target. The liquid Li target is formed as flat plane free-surface flow by a nozzle and flows at high speed around 15 m/s. This paper focuses on flatness of the liquid Li target. The Li flow experiment was conducted in Osaka University Li Loop with a test section which was 1/2.5 scaled model of IFMIF. The thickness of the Li flow was measured and obtained by a contact method which was developed for the measurement. Analytical study on Kelvin wake and numerical calculation on the wake near the side wall of the flow channel were also conducted and compared with the experimental results. As a result, the positions of the wake crest obtained from both of the experiment and numerical calculation assuming contact angle 140° agreed well with the iso-phase line of the analytical model. The generation of the wake likely depends on wettability between Li and the structural material which is 304SS in the present study.     

Safety assessment of the NET Predesign

Work carried out by European Associations on Safety and Environment inside the EC Fusion Technology Programme and by NET has been concentrated on safety-related guidance and on safety assessment of the NET Predesign. Emphasis has been put on analysis of accident sequences of regulatory concern up to worst case accidents, including quantification of consequences to components, systems, and plant, and of doses to the public. Probabilistic safety studies for the most important systems and broadly for the entire plant have been performed to supplement the calculated accidental doses by expectation values for their occurrence rate, and to check the relevance of the reference accident sequences selected initially by judgment. Waste masses per year of operation and for decommissioning have been quantified. For two countries and according to the practices and regulations of these countries, volumes of radioactive waste packaged for final disposal have been determined.Presented at the IAEA Technical Committee Meeting on Developments in Fusion Safety, Toronto, June 7–11, 1993.     

Start-up and shutdown thermomechanical transient analyses of the IFMIF European lithium target system

In the framework of the current IFMIF Engineering Validation and Engineering Design Activities (IFMIF/EVEDA) phase, ENEA is responsible for the design of the European concept of the IFMIF lithium target system which foresees the possibility to periodically replace only the most irradiated and thus critical component (i.e., the backplate) while continuing to operate the rest of the target for a longer period (bayonet backplate concept). With the objective of evaluating the performances of the system in terms of temperature, stress and displacement fields evolution during start-up and shutdown phases, an uncoupled thermomechanical transient analysis has been performed in close collaboration with the University of Palermo by means of a qualified finite element (FE) thermomechanical code. The calculations employed a realistic 3D time-dependent FE model which takes into account all the mechanical and thermal loads including the nuclear heating due to neutron and prompt gamma fields during start-up and decay power of activated products during shutdown. The nuclear data have been calculated by ENEA as part of a parallel extensive neutronic analysis carried out through the MCNP transport code and the EASY-2010 activation code package and then passed as input to the thermomechanical FE model. In this paper, the results of the above transient analyses are reported, highlighting the relevant indications obtained with respect to the fulfillment of the design requirements and possible hints for improving the system design. In particular, it is found that ITER design rules taking into account secondary loads are not always fulfilled during the transient, whereas the predicted displacements allow to exclude any contact of the target system with the surrounding structures.     

Hydraulics and heat transfer in the IFMIF liquid lithium target: CFD calculations

CFD (Computational fluid dynamics) calculation turns out to be a good approximation to the real behavior of the lithium (Li) flow of the target of the international fusion materials irradiation facility (IFMIF). A three-dimensional (3D) modelling of the IFMIF design Li target assembly, made with the CFD commercial code ANSYS-FLUENT has been carried out. The simulation by a structural mesh is focused on the thermal-hydraulic analysis inside the Li jet flow. For, this purpose, the two deuteron beams energy deposition profile is modelled as an energy source term inside the volume of liquid affected. Turbulence is estimated using the RNG k– model, and a surface-tracking technique applied to a fixed Eulerian mesh called volume of fluid (VOF) is used to determine the position of the free surface. Calculations varying the jet velocity from a range of 10–20 m/s, show that maximum calculated temperatures are still below the lithium's boiling point, due to the increase of the pressure induced by centrifugal force.     

Safety assessment of liquid radioactive wastes buried for a long time in deep repositories

The concepts of specific and absolute toxicity permit distinguishing in radioactive wastes belonging to different categories the particular components that present the greatest ecological danger at different stages of their disposal in repositories. It is shown that transuranium elements determine the toxicity parameter of high-and medium-level wastes only 350–400 years after initial disposal. At the present stage of operation of deep repositories, the behavior of the long-lived fission products of ⁹⁰Sr and ¹³⁷Cs is most important. For low-level wastes, the chemical and not the radioactive component is most dangerous. __________ Translated from Atomnaya énergiya,Vol. 100, No. 2, pp. 86–92, February, 2006.     

Incorporation of lithium lead eutectic as a working fluid in RELAP5 and preliminary safety assessment of LLCS

The current work involves thermal hydraulic calculation of Lithium Lead Cooling System (LLCS) for the Indian test blanket module (TBM) for testing in International Thermonuclear Experimental reactor (ITER). It uses the RELAP portion of RELAP/SCDAPSIM/MOD4.0. Lithium-lead eutectic (LLE) has been used as multiplier, breeder and coolant in TBM. Thermodynamic and transport properties of the LLE have been incorporated into the code. The main focus of this study is to check the heat transfer capability of LLE as coolant for TBM system for steady state and the considered anticipated operational occurrences (AOO's), namely, loss of heat source, loss of primary flow and loss of secondary flow. The six heat transfer correlation (reported for liquid metals in the literature) has been tested for steady state analysis of LLCS loop and results are roughly same for all of them. A good agreement has been observed between the operating conditions of LLCS with those of RELAP5 calculations. Results from transient calculations show that a maximum temperature of 875 K is attained during a 300 s loss of primary flow (LLE).     

人类入侵景象下的处置库安全评价分析

利用剂量计算和风险评估模型,基于国际原子能机构(IAEA)推荐的标准数据库和日本核燃料循环开发机构(JNC)的玻璃固化体源项数据,利用程序计算了人类入侵处置库景象下的受照剂量和风险,计算结果表明:在人类入侵景象下,最大外照射剂量在10-4数量级,贡献较大的核素有126 Sn、243 Am;最大内照射剂量在101数量级,贡献较大的核素有240 Pu、241 Am、237 Np和243 Am;内照射和外照射的最大风险值在10-12数量级,远远低于其他类型风险。     

电热膜电磁辐射安全性评估

从急性和累积效应两方面对采暖季中长时间使用远红外辐射电热膜引发电磁辐射的安全性问题进行实验分析。急性效应方面,实测电磁辐射场瞬时强度与标准限值进行比较;累积效应方面则通过构建量子物理模型进行分析。结果表明,电热膜产生的电磁辐射场中最大电磁辐射仅为225.4 n T,仅为国际非电离辐射防护委员会(International commission for non-ionizing radiation protection,ICNIRP)规定限值的1/444,中国国家标准限值的1/100,其对人体神经微管有一定影响,但无累积效应。     

Cavitation damage prediction for spallation target vessels by assessment of acoustic vibration

Liquid-mercury target systems for MW-class spallation neutron sources are being developed around the world. Proton beams are used to induce the spallation reaction. At the moment the proton beam hits the target, pressure waves are generated in the mercury because of the abrupt heat deposition. The pressure waves interact with the target vessel leading to negative pressure that may cause cavitation along the vessel wall. In order to estimate the cavitation erosion, i.e. the pitting damage formed by the collapse of cavitation bubbles, off-beam tests were performed by using an electric magnetic impact testing machine (MIMTM), which can impose equivalent pressure pulses in mercury. The damage potential was defined based on the relationship between the pitting damage and the time-integrated acoustic vibration induced by impact due to the bubble collapses. Additionally, the damage potential was measured in on-beam tests carried out by using the proton beam at WNR (Weapons Neutron Research) facility in Los Alamos Neutron Science Center (LANSCE). In this paper, the concept of the damage potential, the relationship between the pitting damage formation and the damage potential both in off-beam and on-beam tests is shown.     

心脏起搏器无线充电电磁暴露安全评估

为准确地评估心脏起搏器无线充电工作过程中产生的电磁暴露安全性,使用基于有限元的COMSOL软件进行建模及仿真计算。构建人体头部组织、简易胸腔及心脏的模型,并且在无线充电系统工作频率设置为251 kHz时,得出了以上3个部位中产生的磁感应强度（B）、感应电场强度（E）分布,以及人体头部组织比吸收率（Specific absorption rate, SAR）值的分布,将结果与国际非电离辐射防护委员会（ICNIRP）制定的限值进行比较。结果显示：人体头部中B值最大为0.337μT,占ICNIRP阈值的9.26%;E值最大为0.393 V/m,占ICNIRP阈值的0.45%;心脏部位B值最大为3.118μT,占ICNIRP阈值的85.01%;E值最大为0.278 V/m,占ICNIRP阈值的0.32%;人体头部脑组织中SAR值为3.009×10-7 W/kg,远低于ICNIRP制定的公众电磁暴露推荐值。提示心脏起搏器无线充电过程对使用者的健康无明显影响。     

Development of remote handling technology of liquid lithium target and replaceable back plate with lip seal in IFMIF-EVEDA

The IFMIF is an accelerator-based intense neutron source for testing candidate fusion materials. Intense neutrons equivalent to neutron irradiation damage of about 50 dPa/y are emitted inside the Li flow through a back plate. Around the back plate, a lip seal made of 316 L is welded by laser-welding system for replacement by remote handling. The back plate will be designed for replacement at least every year. According to material tests of the lip seal weld joint (316 L/316 L) at room temperature, significant deterioration was not observed. Further investigation of the welding process of the lip seal such as a welding direction and a welding joint shape is in progress. Remote handling procedure of the back plate is examined. At first, three lip seal joints of connection piping will be cut by the laser cutting/welding device and then the target assembly with the back plate will be moved to a hot cell. The back plate lip seal will be cut by the laser arm in the hot cell. After machining and Li cleaning of the lip seal, a new back plate will be welded and moved to test cell/target room.     

Approach to the lifetime assessment of the bayonet back plate for IFMIF target

The IFMIF facility is aimed at the production of high flux (10¹⁸ n/m²/s) of 14 MeV neutrons to test the candidate Fusion materials under significant neutron damage, up to 50 dpa/year. The conceptual configuration of the IFMIF target, based on the bayonet back plate (BP), has been developed in the past years by several authors. The appropriate engineering design of the back plate, to be developed in the EVEDA (Engineering Validation and Engineering Design Activities) phase, would require a very high level of knowledge on the materials behaviour under irradiation, that will be acquired only after some years of IFMIF experimental activities. For this reason the back plate, which is primarily invested by the highest IFMIF neutron flux, has to be considered a sacrificial component. In spite of its systematic replacement, the engineering design has to be optimised and the lifetime analysis has to be made carefully, in order to credibly estimate the expected replacement frequency. Since the replacement time interval must be conservatively shorter than the back plate lifetime and, at each replacement, the facility has to be stopped for, at least, one week and subjected to risky and uncomfortable operations, it is necessary to perform a trustworthy analysis of the lifetime. To this purpose the various interconnections between the main damaging causes are discussed in order to evidence the most plausible reasons of back plate malfunctioning. Due to the lack of knowledge in some fields and the early stage of design, the analysis is only semi-quantitative. The analysis, which accounts for erosion/corrosion, hydraulic stability, neutron damage and thermo-mechanical stress as the main damaging causes, evidences also the research areas which deserve foremost attention during the EVEDA phase. The considered malfunctions are: lithium boiling, burning/piercing of the back plate, non-sufficient neutron flux, brittle rupture of the back plate, creep rupture, loss of tightness of the back plate sealing.