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.     

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.     

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.     

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.     

Scoping-level Probabilistic Safety Assessment of a complex experimental facility: Challenges and first results from the application to a neutron source facility (MEGAPIE)

This paper presents a scoping-level application of Probabilistic Safety Assessment (PSA) to selected systems of a complex experimental facility. In performing a PSA for this type of facility, a number of challenges arise, mainly due to the extensive use of electronic and programmable components and of one-of-a-kind components. The experimental facility is the Megawatt Pilot Target Experiment (MEGAPIE), which was hosted at the Paul Scherrer Institut (PSI). MEGAPIE demonstrated the feasibility of a liquid lead-bismuth target for spallation facilities at a proton beam power level of 1 MW.Given the challenges to estimate initiating event frequencies and failure event probabilities, emphasis is placed on the qualitative results obtainable from the PSA. Even though this does not allow a complete and appropriate characterization of the risk profile, some level of importance/significance evaluation was feasible, and practical and detailed recommendations on potential system improvements were derived.The second part of the work reports on a preliminary quantification of the facility risk. This provides more information on risk significance, which allows prioritizing the insights and recommendations obtained from the PSA. At the present stage, the limited knowledge on initiating and failure events is reflected in the uncertainties in their probabilities as well as in inputs quantified with bounding values. Detailed analyses to improve the quantification of these inputs, many of which turn out to be important contributors, were out of the scope of this study. Consequently, the reported results should be primarily considered as a demonstration of how quantification of the facility risk by a PSA can support risk-informed decisions, rather than precise figures of the facility risk.     

Vapour phase concentrations of volatile nuclear reaction products in the MEGAPIE cover gas

The licensing of the prototype liquid lead bismuth spallation target MEGAPIE required the assessment of the amount of radioactivity that can be released from the liquid metal to the gas phase under various scenarios. To estimate the radiological consequences of evaporation processes under normal operation conditions, the concentrations of hazardous volatile radionuclides in the gas phase of the MEGAPIE expansion volume were estimated using a simplified model based on an equilibrium state. In this report, we focus on those volatile impurities that do not show a strong retention in the liquid metal caused by chemical interactions, comprising mercury, cadmium and thallium. For mercury, temperature functions for the effective vapour pressure of mercury over liquid eutectic lead bismuth alloy were deduced from experimental data and compared to literature data available for the binary systems Hg-Pb and Hg-Bi. Conservative functions were selected from the data on hand for the evaluation of the maximum possible amounts of mercury, cadmium and thallium radioactivity in the gas phase of the MEGAPIE target. Substantial amounts of radioactive mercury are predicted to be released to the cover gas phase of MEGAPIE and liquid lead-alloy based spallation targets in general. The radioactivity resulting from evaporation of cadmium and thallium nuclides is expected to be low. Consequences for liquid metal spallation target systems 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.     

Analysis of fuel management in the KIPT neutron source facility

Argonne National Laboratory (ANL) of USA and Kharkov Institute of Physics and Technology (KIPT) of Ukraine have been collaborating on the conceptual design development of an experimental neutron source facility consisting of an electron accelerator driven sub-critical assembly. The neutron source driving the sub-critical assembly is generated from the interaction of 100 KW electron beam with a natural uranium target. The sub-critical assembly surrounding the target is fueled with low enriched WWR-M2 type hexagonal fuel assemblies. The U-235 enrichment of the fuel material is <20%. The facility will be utilized for basic and applied research, producing medical isotopes, and training young specialists. With the 100 KW electron beam power, the total thermal power of the facility is ∼360 kW including the fission power of ∼260 kW. The burnup of the fissile materials and the buildup of fission products continuously reduce the system reactivity during the operation, decrease the neutron flux level, and consequently impact the facility performance. To preserve the neutron flux level during the operation, the fuel assemblies should be added and shuffled for compensating the lost reactivity caused by burnup. Beryllium reflector could also be utilized to increase the fuel life time in the sub-critical core. This paper studies the fuel cycles and shuffling schemes of the fuel assemblies of the sub-critical assembly to preserve the system reactivity and the neutron flux level during the operation.     

Recent progress in nuclear data measurement for ADS at IMP

Recent progress in nuclear data measurement for ADS at Institute of Modern Physics is reviewed briefly.Based on the cooler storage ring of the Heavy Ion Research Facility in Lanzhou, nuclear data terminal was established.The nuclear data measurement facility for the ADS spallation target has been constructed, which provides a very important platform for the experimental measurements of spallation reactions. A number of experiments have been conducted in the nuclear data terminal. A Neutron Time-of-Flight(NTOF)spectrometer was developed for the study of neutron production from spallation reactions related to the ADS project.The experiments of 400 MeV/u ~(16)O bombarded on a tungsten target were presented using a NTOF spectrometer.Neutron yields for 250 MeV protons incident on a thick grain-made tungsten target and a thick solid lead target have been measured using the water-bath neutron activation method. Spallation residual productions were studied by bombarding W and Pb targets with a 250 MeV proton beam using the neutron activation method. Benchmarking of evaluated nuclear data libraries was performed for D-T neutrons on ADS relevant materials by using the benchmark experimental facility at the China Institute of Atomic Energy.     

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。     

Verification of Monte Carlo transport codes FLUKA, GEANT4 and SHIELD for radiation protection purposes at relativistic heavy ion accelerators

The crucial problem for radiation shielding design at heavy ion accelerator facilities with beam energies of several GeV/n is the source term problem. Experimental data on double differential neutron yields from thick targets irradiated with high-energy uranium nuclei are lacking. At present there are not many Monte Carlo multipurpose codes that can work with primary high-energy uranium nuclei. These codes use different physical models for simulating nucleus–nucleus reactions. Therefore, verification of the codes with available experimental data is very important for selection of the most reliable code for practical tasks. This paper presents comparisons of the FLUKA, GEANT4 and SHIELD code simulations with experimental data on neutron production at 1 GeV/n ²³⁸U beam interaction with a thick Fe target.     

回旋加速器快中子治癌装置的研究

就30MeV回旋加速器上建立快中子治癌装置的几个主要方面,如中子束特性参数、中子屏蔽准直器、中子靶、中子剂量监测及中子治癌控制等进行了可行性的研究。     

Design study on an accelerator-based facility for BNCT and low energy neutron source

We have challenged to reduce an accelerator beam power for an accelerator-based BNCT facility. The required neutron source strength at the target has been estimated so as to make the epithermal neutron flux in the patient irradiation field exceed 1.7 × 10⁹ n/cm²s. The energy of the incident proton and the arrangement of the moderator assemblies are optimized. The beam current and the accelerating voltage are determined so that the accelerator power becomes minimum. The beam power required for the treatment in one hour is 62.5 kW. The proposed facility is equipped with a 2.5 MeV proton accelerator of 25 mA. a lithium target, and a heavy water moderator contained in an aluminum tank.     

Turbulent heat mixing of a heavy liquid metal flow in the MEGAPIE target geometry—The heated jet experiment

The MEGAPIE target installed at the Paul–Scherrer Institute is an example of a spallation target using eutectic liquid lead–bismuth (Pb⁴⁵Bi⁵⁵) both as coolant and neutron source. An adequate cooling of the target requires a conditioning of the flow, which is realized by a main flow transported in an annular gap downwards, u-turned at a hemispherical shell into a cylindrical riser tube. In order to avoid a stagnation point close to the lowest part of the shell a jet flow is superimposed to the main flow, which is directed towards to the stagnation point and flows tangentially along the shell.The heated jet experiment conducted in the THEADES loop of the KALLA laboratory is nearly 1:1 representation of the lower part of the MEGAPIE target. It is aimed to study the cooling capability of this specific geometry in dependence on the flow rate ratio (Q_main/Q_jet) of the main flow (Q_main) to the jet flow (Q_jet). Here, a heated jet is injected into a cold main flow at MEGAPIE relevant flow rate ratios. The liquid metal experiment is accompanied by a water experiment in almost the same geometry to study the momentum field as well as a three-dimensional turbulent numerical fluid dynamic simulation (CFD). Besides a detailed study of the envisaged nominal operation of the MEGAPIE target with Q_main/Q_jet = 15 deviations from this mode are investigated in the range from 7.5 ≤ Q_main/Q_jet ≤ 20 in order to give an estimate on the safe operational threshold of the target.The experiment shows that, the flow pattern establishing in this specific design and the turbulence intensity distribution essentially depends on the flow rate ratio (Q_main/Q_jet). All Q_main/Q_jet-ratios investigated exhibit an unstable time dependent behavior. The MEGAPIE design is highly sensitive against changes of this ratio.Mainly three completely different flow patterns were identified. A sufficient cooling of the lower target shell, however, is only ensured if Q_main/Q_jet ≤ 12.5. In this case the jet flow covers the whole lower shell. Although for Q_main/Q_jet ≤ 12.5 the flow is more unstable compared to the other patterns most of the fluctuations close to the centerline are in the high frequency range (>1 Hz), so that they will not lead to severe temperature fluctuations in the lower shell material. In this case the thermal mixing occurs on large scales and is excellent.For flow rate ratios Q_main/Q_jet > 12.5 complex flow patterns consisting of several fluid streaks and vortices were identified. Since in these cases the jet flow does not fully cover the lower shell an adequate cooling of the MEGAPIE target cannot be guaranteed and thus temperatures may appear exceeding material acceptable limits.All conducted experiments show a high sensitivity to asymmetries even far upstream. A comparison of the numerical simulation, which assumed a symmetric flow, with the experimental data was due to the experimentally found asymmetry only partially possible.     

Design of the thermal neutron beam for neutron radiography at the Syrian MNSR

Neutron beam design was studied at the Syrian reactor (MNSR, 30 kW) with a view to generating thermal neutron beam in the vertical irradiation sites for neutron radiography. The design of the neutron collimator was performed using MCNP4C and the ENDF/B-V cross-section library. Thermal, epithermal and fast neutron energy ranges were selected as <0.4 eV, 0.4 eV–10 keV, >10 keV, respectively. To produce a good neutron beam quality, bismuth was used as photon filter. In this design, the L/D ratio of this facility had the value of 125. The thermal neutron flux at the beam exit was about 2.548 × 10⁵ n/cm² s. If such neutron beam were built into the Syrian MNSR many scientific applications would be available using the neutron radiography.     

Feasibility of uranium spallation target in accelerator-driven system

A feasibility study on natural uranium spallation target in accelerator-driven system (ADS) for minor actinide (MA) transmutation was performed. As a result of comparative study of uranium and lead-bismuth (PbBi) targets in the bare case without blanket surrounding, it was found that uranium target had better neutron generation performance, but limited by the geometrical size due to high neutron absorption in ²³⁸U. In ADS for MA transmutation, uranium used as target instead of PbBi also absorbs neutrons passing the target area.More realistic concept of pin type uranium spallation target cooled by liquid PbBi was considered aiming at enhancing spallation target performance in terms of neutron generation efficiency and operation temperature. The uranium pin target design had nothing better effects on neutron balance of such system than a conventional PbBi target in ADS and it was concluded that uranium target was not suitable for the full-scale ADS.     

准直D-T中子束穿过大块板状聚乙烯样品积分实验

本文介绍了中国原子能科学研究院建立的准直中子束积分实验装置。该装置利用T(d,n)⁴He反应产生14.8 MeV脉冲中子束,经1.1 m厚重水泥屏蔽墙上的准直孔道后与样品作用,用飞行时间法测量样品不同方向的泄漏中子谱。首次测量了样品厚度分别为4.5、9、18和27 cm的大块板状聚乙烯样品在30°和50°方向的泄漏中子谱;考虑靶结构、源中子能谱和角分布、脉冲束宽度及探测器效率,利用MCNP程序模拟计算了相同实验条件下的泄漏中子飞行时间谱。实验结果与模拟结果符合较好。     

Concept of electron accelerator-driven system based on subcritical cascade reactor

A concept of accelerator-driven system consisting of electron accelerator, neutron-generating target and two-core subcritical blanket with fast and thermal neutron spectrum has been presented. Some general features of the cascade neutron multiplication in the two-core subcritical blanket are discussed. Calculation results are demonstrated for the processes of electron–photon–neutron interactions of electron beam with the target material. Neutronic and heat engineering characteristics of the facility are also described. It has been shown that with the system's subcritical level of 2% and electron beam power of 4 MW, the facility power is as high as 50 MW.     

Thermo-siphon mock-up test for the cold neutron source of HANARO

Due to the national demand for a cold neutron beam utilization, a cold neutron research facility project has been carried out since July 2003 to install a cold neutron source (CNS) in HANARO. The CNS adopts a two-phase thermo-siphon of liquid hydrogen as a working fluid to remove a heat load. The CNS consists of an in-pool assembly (IPA) and other components including a hydrogen buffer tank. The liquid level in the moderator cell is measured by a gamma-ray densitometer under a normal cold operation, and the cool down time to fill the required liquid hydrogen into the moderator cell is numerically simulated and compared with the experimental data from a thermo-siphon mock-up test. The self-regulating characteristic of the two-phase thermo-siphon loop is also confirmed by a comparison of a model simulation’s results, which use a thermodynamic model, with the experiment’s results. From these results, the HANARO-CNS is found to maintain the required conditions for a stable cold neutron production against a heat load variation along with the reactor’s power. Furthermore, the thermodynamic behavior in the IPA is observed to determine whether or not the integrity of the IPA is maintained under an abnormal condition of the helium refrigeration system.