ESS TARGET STATION AN OVERVIEW OF THE MONOLITH LAYOUT AND DESIGN R. Linander, S. Gallimore, M. Göhran, C. Kharoua, F. Mezei, P. Nilsson, E. Pitcher, F. Plewinski, P. Sabbagh, European Spallation Source ESS AB, Lund, Sweden M. Butzek, B. Laatsch, Forschungszentrum Jülich GmbH, Jülich, Germany Abstract The European Spallation Source (ESS), Lund, Sweden will be a 5 MW neutron spallation research facility. The spallation process, that converts high energy protons provided by the accelerator to cold and thermal neutron beams delivered to the neutron science instruments, takes place within the so called monolith. The main function of this monolith is to provide sufficient shielding of the high-energy particle and gamma radiation, in all situations and operational modes. It will house some of the main target station components; like the tungsten target, the moderators, the reflector and the beam extraction equipment. In the design process special attention is given to the physical and functional interfaces with the proton accelerator and with the neutron science systems. The engineering design of the monolith and the layout of its internal components shall also allow sufficient accessibility to facilitate planned maintenance and replacement as well as corrective repair. The design aims at on one hand to optimise the required time for scheduled component maintenance and on the other hand to assure robustness of the handling procedures. The monolith also provides a function of confinement of radioactive inventories. In particular, during operation of the facility, there are specific and challenging requirements for the components that separate the monolith atmosphere, (helium gas at a slight underpressure), from the accelerator ultra-high vacuum as well as the experimental hall ambient air atmosphere. The maintenance and replacement of the in-monolith equipment requires this confinement to be reasonably easy to open and restore within a given timeframe. Being in the middle of the on-going engineering work for the ESS target station monolith and its interfacing systems, the layout and design continues to evolve through modifications and improvement.. This paper presents an overview of the current engineering solutions that aim at satisfying stipulated requirements. REQUIREMENTS Interfaces The monolith and its internal plugs and components have several interfaces that need to be taken into account both for the functional and mechanical design and the elaboration of handling and maintenance procedures. The main interfaces are: The proton linear accelerator The neutron science systems, e.g. neutron imaging, SANS, etc. Handling systems for maintenance and repair, see [2] Radioactive waste and emissions management Building structures Integrated control systems, including target safety systems, see [3] Facility operations and decommissioning Main functions The overall function of the target station, i.e. its sole purpose, is to convert a high intensity proton beam to neutron beams of expected intensity and brilliance. This function is broken down into sub-functions like releasing neutrons by use of the high intensity proton beam, moderating the released neutrons to useful energies and directing moderated neutrons towards the neutron science instruments. These functions are allocated to the central target systems, namely the spallation target, the cold and thermal moderators and the reflectors. The spallation process will, in addition to useful neutron beams, produce undesired particle and gamma radiation and thus activated materials and fluids. Therefore it is essential to provide sufficient shielding and confinement in order to protect workers, users and public. The so-called monolith shall be designed, together with other systems, to fulfil these shielding and confinement functions. While these are the main functions of the monolith it shall also provide supporting functions and respect the constraints that are described in the following subsections. Supporting functions The spallation process will generate excess heat during normal power operation as well as decay heat during situations when the proton beam is off. This heat needs to be removed from the components and structures. This cooling function will be realised through connections to several fluid systems, such as the target helium cooling system, the cryogenic hydrogen system for the cold moderators, reflector water cooling system and shielding water cooling system, etc. The monolith shall provide the structural support for all intrinsic components, plugs and shielding blocks. Some of the plugs require precise alignment and adjustment features. The layout of the monolith shall ensure easy access to all regularly exchanged plugs. In addition it shall allow access for replacement or repair to any shielding block that might fail in a manner that prohibits the operation of the facility.
Constraints The two most obvious and significant constrains for the design of the monolith are safety and cost. In order to avoid sub-optimisation it is essential to adopt a global perspective including interfacing systems, construction planning, operation schedule, maintenance procedures, etc. GENERAL LAYOUT OF COMPONENTS The flow of energy, delivered by the proton linear accelerator in the form of an incident pulsed proton beam, will interact with the components and structure of the monolith before it leaves as intense neutron beams as well as heat. The layout of the components marked in Fig. 1 are briefly described in the following subsections. The first component that the proton beam interacts with, after leaving the accelerator, is the proton beam window that separates the helium atmosphere of the monolith from the ultra high vacuum in the accelerator proton beam tube. It is located approximately 1.5 m into the steel shielding relative to the outer face of the monolith. Proton beam instrumentation plug A position between the proton beam window and the target front face, about 2.5 3 metres from the proton beam window, is suitable for proton beam instrumentation. It allows a good view for optical instruments of both the proton beam window surface and the target surface as well as providing the possibility to locate non-invasive proton beam diagnostics equipment in the beam path inside the monolith. The functions of this instrumentation plug is further addressed in [4].. The centre axis of the target wheel and its shaft will be placed 1.15 m downstream of the centre axis of the monolith. The target wheel, which is horizontally oriented, has a diameter of 2.5 m. Thus its front face, where the proton beam impacts, will be situated 0.1 m upstream the monolith centre while the neutronic hotspot will reside as central as possible. See [5] for further details on the rotating, helium cooled, tungsten target. Safety valve Target drive housing Target monitoring plug Shutter Proton beam instrumentation plug Neutron beam extraction Moderators and reflector plug Neutron beam window Figure 1: Section cut of the ESS monolith showing the layout of its intrinsic components and plugs.
Moderator and reflector (MR) plug The neutron moderators applied for ESS will provide thermal as well as cold neutron spectra. The moderator media are room temperature water and cryogenic hydrogen, respectively. One pair of thermal and cold moderator vessels is foreseen to be located above the target wheel and a similar moderator pair will be placed beneath. Surrounding the moderators is beryllium reflector material used in order to maximise the neutron flux to the neutron beam lines. The moderators and reflectors will be centred, in the horizontal plane to the monolith axis. The centre of gravity of the different moderator vessels will offset the proton beam axis by 180 mm upwards and downwards. The moderators and reflectors are assembled together with structural parts into one unit, called the moderators and reflector plug, which during the operation of the facility is fixed in the centre of the monolith cylindrical volume. Neutron beam extraction system In order for the scientific experimentalists to utilise the neutrons, they need to be able to view the surfaces of the moderator vessels. Therefore both the reflector material and the surrounding shielding will have cut outs. These openings are arranged in four sectors, 60 degrees each, of which two emanate from the upper moderators and the other two reveal the lower moderators. The layout and geometry of the monolith and its intrinsic components allows neutron beam guides to be positioned as close as 2 m from the monolith centre axis. 48 neutron beam port positions are distributed in a generic pattern with 5 degrees angular separation. The 22 planned scientific instruments will use the most optimal beam port positions which will be equipped with neutron beam guide sections, while the unused ports will be plugged by shielding material, awaiting future evolution of the ESS science program. The feasibility to locate a light-weight shutter component in the neutron beam guide insert has been confirmed [1]. Target monitoring plug On the downstream side of the target wheel a unit for condition monitoring of the wheel will be inserted. This location is suitable since it does not interfere with other equipment or the neutron beam extraction. The principal concept of this target monitoring plug is described in [4]. SHIELDING SOLUTIONS In addition to the useable thermalised and cold neutron beams the spallation processes will generate undesired particles and gamma radiation. Sufficient shielding is necessary to allow workers and users to access the experimental halls and adjacent parts of the facility as well as to reduce neutron background in the experimental halls. The most cost effective shielding solution found has been to use steel as the necessary high-density material for stopping high-energy particles and gamma rays. However, the steel also has to be complemented by concrete that through its hydrogen content efficiently shields thermal neutrons. The shielding requirement is directionally dependant both due to the anisotropic radiation field and that the acceptable radiation levels in the different adjacent halls and rooms are not the same. In order to achieve a simple and manageable design, the steel shielding, i.e. the monolith, will be cylindrically shaped. The dimensions of this cylinder have been set to 12 m in diameter and 10 m in height [1]. These dimensions satisfies the agreed requirement that it will be possible to locate the first neutron beam shaping equipment as close as 6 m from the moderator surfaces. The chosen height will provide sufficient shielding in vertical direction for both the connection cells above the monolith and the ground water layer beneath the facility. Optimisation of the monolith dimensions will be explored during the design process in order to reach a compromise that satisfies all requirements at the same time, as it is cost effective for the entire facility. The complementary concrete shielding that is necessary will either be made up of building structures, like the monolith foundation below and the floor slab of the high bay above, or be part of the common removable shielding that surrounds the neutron beam lines in the experimental halls. The required thickness of this concrete is one to two metres, depending of the direction, to allow human access to adjacent rooms. For reduction of neutron background in the experimental halls the concrete layer needs to be considerably thicker. Determination of the dimensions and layout of this beam line shielding is an on-going activity at ESS. CONFINEMENT SOLUTIONS The target systems and the monolith structures will accumulate an inventory of radioactive isotopes as the facility is operated, i.e. through the spallation processes. In order to minimise the production of radioactive contaminants; e.g. gases, volatiles, dust; it is desirable to have a light and inert gas surrounding the equipment. Therefore the choice is to maintain a helium atmosphere in the free volume of the monolith, which prevents corrosion and minimises gas activation. Helium is also sufficiently transparent both to the proton beam and to the neutron beams, so that evacuated beam tubes and guides with additional windows can be avoided inside the monolith. The monolith will be designed with an outer liner that confines the helium atmosphere. The operating function of this liner is to keep the helium inside the monolith and prevent leakages between the monolith and the neighbouring rooms or connecting systems. In addition the liner system has the safety function to limit release of radioactivity in any operating mode, maintenance situations and accident scenarios. A slight under-pressure will be maintained in relation to the ambient air atmosphere.
The liner system consists of the vessel that surrounds the steel shielding blocks, the covers and lids for any necessary openings as well as all penetrations for electrical, fluid and optical systems. Special care needs to be taken to achieve sufficient leak tightness of the covers for openings in the top plate of the liner. Openings for access to frequently exchanged or serviced components will be equipped with a double seal with a monitored interstitial space. For other openings, that allow access to structures that do not require regular maintenance, a single seal with a complementary tightness weld will be considered. Specific parts of the monolith helium confinement are the proton beam window, the neutron beam windows and the housing that covers the parts of target wheel drive that reach above the top plate of the liner. These parts are addressed below. The sealing arrangement for the proton beam window, being a regularly replaced component, needs to be robustly designed, easily accessed and shall assure that the ultra high vacuum of the proton beam tube can be maintained, see Fig. 2. In order to mitigate any accidental case involving a proton beam window breach or leakage, a valve will be installed in the proton beam tube upstream the window and outbound the liner, see Fig. 1. This valve, actuated by the target safety system, will make up the confinement in such unexpected events.. openings. Also these openings will be equipped with double seals and interstitial space for leak detection. The target drive, seal and bearing system that extends above the top plate is contained in a housing, see Fig. 1, which is also part of the monolith helium confinement. The piping systems that supply the target helium coolant to and from the target wheel will penetrate this housing. The target coolant will be transferred via a rotary feedthrough in which a helium buffer gas system ensures that leakage of target coolant to the monolith helium atmosphere is kept below an acceptable threshold. This helium management system is further addressed in [6]. Figure 3: Detail showing a cut view of a neutron beam window Figure 2: assembly On the cylindrical surface of the liner towards the experimental halls there will be openings that correspond to each beam port. These openings will be covered by either a neutron beam window for each instrument in service or a blind cover for the unused port locations, see Fig. 3. The neutron beam guide sections that are inserted into the monolith will be accessed through these COOLING SOLUTIONS The large amount of heat deposited in the target wheel will be removed through a helium cooling system that is connected to the rotating wheel inside the target drive housing. The gas flows through the shaft of the wheel both to and from the wheel. The design of the target cooling is described in detail in [5] and [6]. Moderators and reflector plug The MR plug is an intricate and complex assembly with each part connected to one of four separate fluid systems. All fluid systems are connected in the connection cells and all the piping is routed down to the components of the plug. The cold moderators are fed by a cryogenic hydrogen system that keeps the condition of the fluid in the vessel at 20 K and 1.5 MPa. One dedicated system supplies water to pre-moderators and thermal moderators while another separate system cools the beryllium reflectors. Finally the rest of the plug structures are
cooled via connection to the general water-cooling system. For insulation purposes the plug also includes a vacuum jacket, made up of the space between the cold moderator vessels including their feed pipes and the surrounding water moderator volumes. The heat deposition in the proton beam window and its frame structures, when the proton beam passes through, will not be insignificant. For the window itself helium cooling will be adopted through connection to a separate cooling system. Helium cooling is chosen in order to avoid water activation as well as the risk of overpressurisation due to boiling. The frame structures and the surrounding shielding blocks will be connected to the general water-cooling system for shielding. All piping connection will be located on top of the monolith in the so called connection cells. Monolith shielding blocks Up to a radius of about 1.5 m from the centre of the monolith the steel shielding and around all irradiated components will need active cooling. A water-cooling system will be connected on the top of monolith to pipes routed down to the individual shielding blocks. Sufficiently distributed water channels will ensure the removal of the heat that is deposited by the radiation from the target systems. For shielding blocks that will not be regularly handled during maintenance (those with less heat load) redundant cooling-channels might be considered in order to avoid unscheduled downtime for corrective maintenance. Inlet water condition will be about 25 C at 1.0 MPa and the objective of the cooling is to keep the block bulk temperature below 50 C although locally slightly higher temperatures could be accepted.. Shielding blocks in the outer regimes of the monolith will be only passively cooled through internal conduction and convection. The helium atmosphere system will not be assigned a cooling function explicitly, but will enhance the distribution of the heat internally. MAINTENANCE AND HANDLING SOLUTIONS The handling of components internal to the monolith, during exchange or maintenance, is described in [1] and further elaborated in [2]. Monolith shielding blocks Besides the shielding function, the monolith needs to be designed to allow handling of components located inside the monolith and insertion and removal of neutron beam guides for the instruments. Access to the different components should be facilitated in accordance with their expected lifetime and reliability. This means that shielding blocks that needs to be moved for replacement of target components, as well as the regularly handled components themselves, shall be easily accessed and handled. Other parts of the monolith that do not require frequent maintenance can benefit from longer and more complicated handling procedures with the gain of a simpler and more cost effective design. Another pre-requisite for the design of the monolith and the elaboration of handling procedures is that alignment of critical components must be achievable in a repeatable and robust manner. Internal plugs and components The proton beam window is expected to be the most frequently replaced component due to limited lifetime. Thus particular attention is paid to optimise the vertical extraction and insertion of this plug and the shielding blocks above. The moderator and reflector plug, foreseen to be replaced as one single unit, will need to be shifted horizontally before it is lifted vertically passing the target wheel. The two axis motion of the plug will be facilitated by a specific handling equipment. The lifetime of the MR plug is expected to be longer than for the proton beam window. The proton beam instrumentation plug will be an integral part of a shielding block that is located upstream the MR plug, and will therefore be possible to replace each time the moderators are maintained. The target wheel is designed to be vertically lifted, which requires the MR plug to be removed prior to access. This handling procedure is foreseen to be less frequent since the expected lifetime of the target is at least five years. The target monitoring plug, situated on the downstream side of the wheel, will be vertically replaced independent of the wheel. Finally, the neutron beam guide inserts in the monolith will be designed to be horizontally handled which requires access from the experimental halls. REFERENCES [1] S. Peggs, ESS Technical Design Report, ESS-doc- 274. [2] M. Göhran et al., ESS target station hot cells and logistics, AccApp 13, Bruges, MOPTA04 [3] A. Sadeghzadeh et al., An overview of safety control system for ESS target station, AccApp 13, Bruges, MOPTA11 [4] S. Gallimore et al., An overview of the in-monolith monitoring and instrumentation at ESS, AccApp 13, Bruges, WEOTA07 [5] C. Kharoua et al., The ROtating Tungsten HElium cooled Target (ROTHETA) concept, AccApp 13, Bruges, WEOTA08 [6] P. Nilsson et al., Helium management of the ESS target and monolith systems, AccApp 13, Bruges, MOPTA03