Verification of the TRACE code against the MEGAPIE transient data

Megawatt pilot target experiment (MEGAPIE) is an international project aimed at demonstrating the feasibility of a liquid lead–bismuth target for spallation facilities at a maximum beam power level of 1 MW. The thermal-hydraulics data measured during the MEGAPIE experiment was used for the TRACE code qualification for transient analysis of liquid metal cooled systems.     

Analysis of radioactivity releases in the MEGAPIE reference accident

A computer model for the MEGAwatt Pilot Target Experiment (MEGAPIE) was developed, verified and applied to the analysis of the reference accident, resulting from the hypothetical loss of a Pb–Bi inventory and subsequent cooling down of the target with free convection of the atmospheric air. The radioactivity releases caused by evaporation of activation products from the lead–bismuth films on the surfaces of the target inner structures are estimated. The maximum radioactivity releases were found to be from evaporation of the mercury and polonium estimated as 7.0 × 10¹¹ and 6.2 × 10⁸ Bq, respectively. Thermal conductivity and radiation heat transfer through the gas gaps were found to be more important mechanism of the target cooling down than the atmospheric air convection. The final conclusion made on the basis of the work is that MEGAPIE in the reference accident meets the 1 mSv criterion.     

Neutronic characterization of the MEGAPIE target

The MEGAPIE project aimed to design, build and operate a liquid metal spallation neutron target of about 1 MW beam power in the SINQ facility at the Paul Scherrer Institut (Villigen, Switzerland). This project is an important step in the roadmap towards the demonstration of the accelerator driven system (ADS) concept and high power liquid metal targets in general. Following the design phase, an experimental program was defined to provide a complete characterization of the facility by performing a “mapping” of the neutron flux at different points, from the center of the target to the beam lines. The neutronic performance of the target was studied using different experimental techniques with the goals of validating the Monte Carlo codes used in the design of the target; additionally, the performance was compared with the solid lead targets used before and after the MEGAPIE experiment.     

The MEGAPIE 1 MW target in support to ADS development: status of R&D and design

The MEGAPIE project is aimed at designing, building and operating a liquid metal spallation neutron target as a key experiment on the road to an experimental accelerator driven system and to improve the neutron flux at the PSI spallation source. The design of the target system has been completed. The target configuration and the operating conditions have been defined and the expected performance assessed. A preliminary safety analysis has been performed considering normal, off-normal and accident conditions and a corresponding report has been submitted to the authorities for licensing. The experience gained up to now shows that MEGAPIE may well be the first liquid metal target to be irradiated under high power beam conditions.     

Neutronic performance of the MEGAPIE spallation target under high power proton beam

The MEGAPIE project, aiming at the construction and operation of a megawatt liquid lead-bismuth spallation target, constitutes the first step in demonstrating the feasibility of liquid heavy metal target technologies as spallation neutron sources. In particular, MEGAPIE is meant to assess the coupling of a high power proton beam with a window-concept heavy liquid metal target. The experiment has been set at the Paul Scherrer Institute (PSI) in Switzerland and, after a 4-month long irradiation, has provided unique data for a better understanding of the behavior of such a target under realistic irradiation conditions. A complex neutron detector has been developed to provide an on-line measurement of the neutron fluency inside the target and close to the proton beam. The detector is based on micrometric fission chambers and activation foils. These two complementary detection techniques have provided a characterization of the neutron flux inside the target for different positions along its axis. Measurements and simulation results presented in this paper aim to provide important recommendations for future accelerator driven systems (ADS) and neutron source developments.     

Experience from the post-test analysis of MEGAPIE

The (MEGAWatt Pilot Experiment) MEGAPIE target was successfully irradiated in 2006 at the SINQ facility of the Paul Scherrer Institut. During the irradiation a series of measurements to monitor the operation of the target, the thermal hydraulics behavior and the neutronic and nuclear aspects, has been performed. In the post-test analysis phase of the project, the data were analyzed and important information relevant to accelerator-driven systems (ADS) was gained, in particular: (i) from the operation of the target several recommendations concern the simplification of the system and the improved reliability; (ii) data from the thermal hydraulic measurements have offered the opportunity to validate the codes used in the design phase; (iii) the neutronic analysis confirm the high performance of a liquid metal target and the importance of the delayed neutron measurements in an ADS target; (iv) the nuclear measurements of the gas released gave the opportunity to validate the codes used during the design phase and provided indications for the operation. From the results in these different domains recommendations to further development of ADS and heavy liquid metal targets are discussed.     

The MEGAPIE-TEST project: Supporting research and lessons learned in first-of-a-kind spallation target technology

The megawatt pilot experiment (MEGAPIE) has been launched by six European institutions (PSI, FZK, CEA, SCK-CEN, ENEA and CNRS), JAEA (Japan), DOE (US) and KAERI (Korea) with the aim to carry out an experiment, in the SINQ target location at PSI (Switzerland), to demonstrate the safe operation of a liquid metal (lead–bismuth eutectic, LBE) spallation target hit by a 1 MW proton beam. The European Commission has joined the MEGAPIE project through the 5-year (2001–2006) project named MEGAPIE-TEST. This project has been formally concluded with an International Workshop, where the results and the lessons learned during the project have been summarised. This work presents a review of the outcome of that Workshop.     

SIMS investigation of the spallation and transmutation products production in lead

The production of spallation and activation products in liquid metals (Pb or Pb-Bi) is an important issue in the frame of the MEGAPIE and other ADS studies. Although it is usually evaluated by well established codes like the MCNPX, experimental validations of the calculations in real conditions are rare. This work is an attempt to deliver experimental data using the material irradiated in the Swiss Spallation source (SINQ). A target composed of SS316L cladding tubes filled with Pb was irradiated in SINQ with a total proton charge of 10.03 Ah. A cross-section of one rod has been prepared for analysis in the PSI fully shielded secondary ion mass spectrometer (SIMS). Mass scans have been realized on one irradiated specimen and compared to a reference un-irradiated sample. The SIMS measurement delivers no absolute quantitative measurements but allow the determination of the mass distribution of the long lived spallation and activation products in real materials. The analysis shows the production of a raw of isotopes in the mass range 50-140 amu in agreement with calculation realized in model cases. The measurements across the specimen demonstrate also the homogeneity of the spallation and activation products content in the rod cross-section. This analysis provides useful information on the long lived isotopes production in lead based spallation targets like MEGAPIE.     

Lead-alloy coolant technology and materials – technology readiness level evaluation

Lead and lead–bismuth eutectic heavy liquid metal coolants are under wide-ranging international investigation and development for advanced nuclear systems for energy production and waste transmutation (reactor-based or accelerator-driven). This report reviews the major supporting international R&D programs, the key advances in the main areas of coolant technology and materials, the state of technology, and the strategic directions for further development. Based on this review, we conservatively evaluate the technological readiness level (TRL) for programmatic and industrial applications in high-temperature advanced reactors to be 7, “one-dimensional engineering-scale demonstration”, or the first level in the proof-of-performance category. A 3-D engineering-scale integral test and demonstration facility should be the next step toward the realization of a test and demonstration nuclear system (reactor or accelerator-driven). The recent success of MEGAPIE, a 1 MW class lead–bismuth eutectic spallation target operating at the Paul Scherrer Institute signals that for such applications of short to intermediate durations at moderate temperatures, the TRL is close to 9, meaning the technology is nearly ready for deployment.     

ADS熔盐散裂靶中子学性能研究

与传统加速器驱动次临界系统（ADS）采用金属靶作为散裂中子靶的设计不同,加速器驱动次临界熔盐堆（AD-MSRs）采用靶堆一体的设计,直接使用燃料熔盐作为散裂中子靶。由于熔盐靶的中子学性能直接影响AD MSRs的能量放大系数、核废物的嬗变和核燃料增殖的效率,所以本研究基于MCNPX程序,详细计算了高能质子轰击氟盐和氯盐两种熔盐靶产生的散裂中子产额、散裂中子能谱、能量沉积分布以及散裂产物等中子学性能,并与液态Pb和铅铋共熔体（LBE）两种液态金属靶进行了对比。计算结果表明,熔盐靶在散裂中子产额上与液态金属靶有一定的差距,但熔盐靶内能量沉积分布的梯度较小,更有利于靶区的热量导出。与液态Pb和LBE靶相比,熔盐靶的散裂产物中包含更多的气体以及高质量数的α发射体核素。     

Effect of contact conditions on embrittlement of T91 steel by lead-bismuth

The T91 martensitic steel is a candidate structural material for the liquid lead-bismuth eutectic (LBE) MEGAPIE spallation target. This paper first reviews some results on Liquid Metal Embrittlement (LME) of martensitic steels by liquid metals. It appears that LME of steels can occur provided a few criteria are fulfilled simultaneously. Intimate contact between liquid metal and solid metal is the first one. Usually, it is impossible to avoid the oxide film formation on the steel surface even after short exposure to air. This explains the difficulty arising when one would like to determine the susceptibility to LME of T91 steel whilst put into contact with lead-bismuth. Later, we report on different methods of surface preparation in order to remove the oxide layer on the T91 steel (PVD, soft soldering fluxes) and the resulting susceptibility to LME.     

AFM and MFM characterization of oxide layers grown on stainless steels in lead bismuth eutectic

Fast reactors and spallation neutron sources may use lead bismuth eutectic (LBE) as a coolant. Its thermal physical and neutronic properties make it a high performance nuclear coolant and spallation target. The main disadvantage of LBE is that it is corrosive to most steels and container materials. Active control of oxygen in LBE will allow the growth of protective oxides on steels to mitigate corrosion. To understand corrosion and oxidation of candidate materials in this environment and to establish a solid scientific basis the surface structure, composition, and properties should be investigated carefully at the smallest scale. Atomic force microscopy (AFM) is a powerful tool to map out properties and structure on surfaces of virtually any material. This paper is a summary of the results from AFM measurements on ferritic/martensitic (HT-9) and austenitic (D9) steels that are candidates for liquid metal cooled reactors.     

Oxidation of steels in liquid lead bismuth: Oxygen control to achieve efficient corrosion protection

Hybrid systems dedicated to waste transmutation are constituted of an accelerator generating a high energy proton flux, a spallation target on which the accelerated proton beam impinges to produce neutrons and a subcritical core. The Pb-Bi eutectic liquid alloy is considered as spallation target material due to its suitable nuclear and physical properties.However, liquid metals can be corrosive towards containment materials (austenitic and Fe9Cr alloys). In the case of liquid lead bismuth alloy, one of the protection means considered against the dissolution of the steels is the in situ protection by the formation of an oxide layer at the steels’ surface.However, in order to ensure the efficient protection of the steels by an oxide layer, the control and the monitoring of the oxygen content in the Pb-Bi alloy is a major issue. The paper recalls, first, the oxygen chemistry in a lead alloy system, in order to propose the oxygen operating window that complies with both the contamination by lead oxide of the coolant and the corrosion control by the promotion of an oxide film on the structure. Results of tests performed in stagnant lead bismuth at high oxygen concentrations are also presented showing the effect of various operating parameters on the oxidation kinetics and on the nature of the oxide layer. An oxidation mechanism and model are also proposed and compared with experimental data.     

Development of a gas layer to mitigate cavitation damage in liquid mercury spallation targets

Establishment of a gas layer between the flowing liquid and container wall is proposed for mitigating the effects of cavitation in mercury spallation targets. Previous work has shown an order of magnitude decrease in damage for a gas layer developed in a stagnant mercury target for an in-beam experiment. This work is aimed at extending these results to the more complex conditions introduced by a flowing mercury target system. A water-loop has been fabricated to provide initial insights on potential gas injection methods into a flowing liquid. An existing full-scale flow loop designed to simulate the Spallation Neutron Source target system will be used to extend these studies to mercury. A parallel analytical effort is being conducted using computational fluid dynamics (CFD) modeling to provide direction to the experimental effort. Some preliminary simulations of gas injection through a single hole have been completed and show behavior of the models that is qualitatively meaningful.     

Review of liquid metal corrosion issues for potential containment materials for liquid lead and lead–bismuth eutectic spallation targets as a neutron source

Lead (Pb) and lead–bismuth eutectic (44Pb–56Bi) have been the two primary candidate liquid metal target materials for the production of spallation neutrons. Selection of a container material for the liquid metal target will greatly affect the lifetime and safety of the target subsystem. For the liquid lead target, niobium–1 wt% zirconium (Nb–1Zr) is a candidate containment material for liquid lead, but its poor oxidation resistance has been a major concern. In this paper, the oxidation rate of Nb–1Zr was studied based on the calculations of thickness loss resulting from oxidation. According to these calculations, it appeared that uncoated Nb–1Zr may be used for a 1-year operation at 900°C at P_O2=1×10^–6 Torr, but the same material may not be used in argon with 5-ppm oxygen. Coating technologies to reduce the oxidation of Nb–1Zr are reviewed, as are other candidate refractory metals such as molybdenum, tantalum, and tungsten. For the liquid lead–bismuth eutectic target, three candidate containment materials are suggested, based on a literature survey of the materials’ compatibility and proton irradiation tests: Croloy 2-1/4, modified 9Cr–1Mo, and 12Cr–1Mo (HT-9) steel. These materials seem to be used only if the lead–bismuth is thoroughly deoxidized and treated with zirconium and magnesium.     

Coupled fluid/structure analyses of the MEGAPIE spallation source target during transients

The paper describes some R&D activities undertaken in support of the design and safe operation of MEGAPIE (MEGAwatt PIlot Experiment) spallation source target, which is scheduled to be irradiated by a proton beam in the SINQ facility at the Paul Scherrer Institute in 2006. The target material is lead bismuth eutectic (LBE), which also acts as the primary coolant. As a consequence of the spallation reactions, about 600 kW of heat would be deposited in the target during operation, and considerable R&D effort is being expended to demonstrate continuing coolability and structural integrity under a variety of operational and abnormal conditions. The paper gives three examples of transient analyses carried out as part of the safety assessment of the target: (1) a beamline trip and recovery; (2) failure of the primary electro-magnetic pump (EMP); (3) failure of the secondary EMP (used to cool the base of the target). The study involves the simultaneous application of a system-analysis code, in our case a version of RELAP5, a computational fluid dynamics (CFD) tool (CFX-4), and a structural analysis code (ABAQUS). The RELAP5 code is used to provide transient boundary conditions for a localized conjugate heat transfer analysis of the lower target region, undertaken using CFD, and includes the feed-back effects arising from the secondary cooling and control systems. A conjugate heat transfer problem is then solved using CFD, which provides time-dependent thermal and flow data within the LBE, together with the thermal and mechanical loads to the target structures. Finally, an in-house interface program is employed to transfer mesh geometry, model topology and (time-dependent) thermal/mechanical data to enable stress analysis of the principal lower-target structural components to be performed. It is demonstrated that none of the transients considered result in critical stress conditions occurring in the target components, but that further operation is not recommended unless both pumps are fully operational.     

Two-dimensional numerical simulation of single bubble rising behavior in liquid metal using moving particle semi-implicit method

Gas-lift pump in liquid metal cooling fast reactor (LMFR) is an innovative conceptual design to enhance the natural circulation ability of reactor core. The two phase flow characteristics of gas–liquid metal make significant improvement of the natural circulation capacity and reactor safety. It is important to study bubble flow in liquid metal. In present study, the rising behaviors of a single nitrogen bubble in 5 kinds of common stagnant liquid metals (lead bismuth alloy (LBE), liquid kalium (K), sodium (Na), potassium sodium alloy (Na–K) and lithium lead alloy (Li–Pb)) and in flowing lead bismuth alloy have been numerically simulated using two-dimensional moving particle semi-implicit (MPS) method. The whole bubble rising process in liquid was captured. The bubble shape, rising velocity and aspect ratio during rising process of single nitrogen bubble were studied. The computational results show that, in the stagnant liquid metals, the bubble rising shape can be described by the Grace's diagram, the terminal velocity is not beyond 0.3 m/s, the terminal aspect ratio is between 0.5 and 0.6. In the flowing lead bismuth alloy, as the liquid velocity increases, both the bubble aspect ratio and terminal velocity increase as well. This work is the fundamental research of two phase flow and will be important to the study of the natural circulation capability of Accelerator Driven System (ADS) by using gas-lift pump.     

Effect of wettability on bubble formation at gas nozzle under stagnant condition

Injection of gas microbubbles into mercury might be effective to mitigate pressure waves generating and propagating in liquid mercury target for MW-class spallation neutron sources. The effect of mitigation is very dependent on the bubble conditions; size, population, etc. It is important to make clear bubble formation behavior from a nozzle for development of making suitable gas microbubbles into mercury. Visualization of microbubbles in mercury was carried out by refraction-enhanced imaging with high-energy synchrotron radiation X-rays at SPring-8 to observe the bubble formation behavior at micro-gas nozzle of 100 μm in inner diameter and 200 μm in outer diameter. The bubble formation behavior in mercury was quite different from that in water. A constant taking account of the wettability and interfacial tension force between liquid and solid metals in an equation of force balance around bubble was identified by visualized bubble size. The bubble size under mercury flowing condition was estimated from the force balance equitation.     

MEGAPIE at SINQ - The first liquid metal target driven by a megawatt class proton beam

The lead-bismuth liquid metal target MEGAPIE (MEGAwatt Pilot Experiment) was operated at the Swiss Spallation Neutron Source SINQ starting mid-August 2006, for a scheduled irradiation period until 21st of December 2006. The continuous (51 MHz) 590 MeV proton beam hitting the target reaches routinely an average current of ∼1300 μA, corresponding to a beam power 0.77 MW. This article illustrates the main features of the target and the ancillary systems specially needed for the liquid metal target operation. Further, the operational experiences made with this target during start-up and routine operation are summarized, besides the general performance highlighting new beam and target safety devices, and last but not least the neutronic efficiency in relation to the previously operated solid lead target.     

基于水工质的有窗散裂靶流场分析及实验测量

散裂靶是加速器驱动的次临界系统(ADS)的重要组成部分,有窗散裂靶是唯一经实验验证、测量的液态金属高功率散裂靶,研究有窗靶内工质的流动对散裂靶的设计优化有重要意义。本文以水为工质对有窗靶件进行了可视化实验及数值模拟研究,实验采用粒子图像测速法对靶件可视化部分进行速度场测量,同时利用计算流体力学软件FLUENT对靶件流场进行数值模拟。通过5种湍流模型（标准k-ε模型、RNG k-ε模型、Realk-ε模型、SST k-ω模型、RSM模型）在不同流速下的模拟结果与实验结果的对比分析,表明采用RNG k-ε模型并结合相应的壁面函数能较准确模拟有窗靶内的流动。