TARGET OPERATION AT THE HIGH-POWER NEUTRON SPALLATION SOURCE SINQ SAFETY AND RELIABILITY ISSUES

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1 TARGET OPERATION AT THE HIGH-POWER NEUTRON SPALLATION SOURCE SINQ SAFETY AND RELIABILITY ISSUES W. Wagner Spallation Neutron Source Division, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland Abstract The Paul Scherrer Institut (PSI) is operating a 1 MW class research spallation neutron source named SINQ, driven by the PSI proton accelerator system. In terms of beam power, it is, by a large margin, the most powerful spallation neutron source currently in operation worldwide. As a consequence, target load levels prevail in SINQ which are beyond the realm of existing experience. Therefore, an extensive materials irradiation program (STIP) is currently underway which will help to select the proper structural material and make dependable life time estimates accounting for the real operating conditions that prevail in the facility. In parallel, both theoretical and experimental work is going on within the MEGAPIE (MEGAwatt Pilot Experiment) project, to develop a liquid lead-bismuth spallation target for a beam power level of 1MW. 1. INTRODUCTION The PSI proton accelerator complex consists of four accelerators, three of which, the Cockroft-Walton pre-accelerator (860 kev), the Injector Cyclotron 2 (72 MeV) and the Main Ring Cyclotron (590 MeV) are forming a cascade to provide protons for a wide range of uses. A schematic floor plan of these facilities is shown in Figure 1. The upgrade program took the proton current from its original design value of 100 µa through intermediate steps at 150 µa (the limit of the original injector, a Philips cyclotron) and 250 µa (the limit of the original rf-system on the main ring) to its present value of 1.8 ma. About 20 µa of the beam are peeled off in a splitter region for use in the PIREX proton irradiation area, a nuclear physics experiment, or for the isocentric gantry in which tumor treatment by proton irradiation is being undertaken. The main part of the beam continues through two pion production targets, M ("mince", thin) and E ("épais", thick), to produce pions and, from their decay, muons for a variety of applications in fundamental, nuclear and condensed matter physics. Condensed matter physics and materials science are also the main scientific disciplines at the spallation neutron source SINQ, although other uses are supported as well. They include isotope production with neutrons, neutron activation analysis, fission product physics and nuclear physics with polarized neutrons. 15

2 16 Fig. 1 Floor plan of the accelerator complex and associated research facilities at PSI.

3 2. THE SINQ FACILITY After passing through target E, a 4 cm (occasionally for certain beamtime periods 6 cm) thick graphite target, out of the primary 1800 µa about 1250 µa (69%) of protons with 570 MeV are transported to the SINQ target with a computed loss of as little as 10 ppm on the last 25 m of beam line. This low loss was confirmed by measurement during the early commissioning phase of SINQ [1]. Injection into the SINQ-target is from underneath (Figures 2 and 3), keeping the whole circumference of the target shielding block free for neutron utilization. Fig. 2 Partial cutaway view of the SINQ facility. For details of the target system and beam injection collimator see Figure 3. Being driven by a cyclotron that delivers an essentially continuous (cw)-beam (with a radio frequency (rf)-bunch structure of 51 MHz), SINQ is optimized for high time average neutron flux by surrounding the target with a large (2 m diameter) D 2 O-moderator (Figure 3) and avoiding neutron absorption in the inner regions of SINQ to the largest possible extent. Unlike a reactor, shielding of a spallation neutron source is not possible by a water pool alone. In order to absorb most of the thermal neutrons escaping the heavy water and to minimize activation and heat development in the inner shielding regions, the D 2 O tank is surrounded by a 10 cm thick layer of light water, but roughly 4.5 m of steel and 30 cm of borated concrete are needed laterally to bring the radiation levels at the outside of the target block to below legal standards. Despite the light water shell around the D 2 O tank, its immediate vicinity is made up from actively cooled steel plates. In particular in the forward region of the beam (upward direction), where most of the high energy cascade neutrons are concentrated, shielding requirements and cooling needs are substantial. On top of the steel shielding located in this direction, large amounts of concrete are stacked above the target head access cavity to prevent skyshine in the surrounding areas. Thermal neutron beams are extracted from the D 2 O moderator by beam tubes that end near the maximum of the thermal flux but avoid direct sight on the target. A well reflected volume of liquid D 2, at 25 K, serves to slow neutrons down below thermal energies and to supply "cold" neutrons for two beam tubes and seven supermirror coated neutron guides which house the majority of the neutron scattering instruments at SINQ. Cold neutrons are particularly well suited for many of the research fields of current interest, so much emphasis was put in the design of SINQ on an optimized cold neutron source [2] and neutron guide system [3, 4]. 17

4 Fig. 3 The inner regions of SINQ and enlarged view of the target insert 3. THE SINQ-TARGET DEVELOPMENT PROGRAM 3.1 General target design concept Although the beam interaction region in the target is only about 30 cm long, the target unit is a 4 m long structure with 20 cm diameter in its lower 2 m, which widens to 40 cm in the upper half. In its current design the SINQ target is an array of D 2 O-cooled lead rods in steel cladding. In the first two SINQ targets the rods were from solid Zircaloy. The rod array is contained in a double walled Al-shell and suspended from a heavy shielding plug that is inserted into its operating position in the target shield block from above (Figure 3). The rods are mm in diameter and are in hexagonal close packing with a pitch of mm, i.e. the minimum width of the water flow path between the rods is 2mm. The target rods are mounted in an aluminum casing and are cooled in cross flow. The lowest layer is made of empty tubes in order to minimize the heat load in the region where the flow stabilizes. A picture of the rod bundle of the target Mark 3 is shown in Figure 4. 18

5 Fig. 4 The rod array of the target Mark 3 used in SINQ. In view of the continuous nature of SINQ operation neutron absorption must be minimized, at a maximum neutron yield, to keep the time average flux high. This makes lead and bismuth the best target materials. However, even when used in solid form, these materials must be contained in some cladding for structural rigidity as well as to prevent corrosion by the cooling water. For the solid target we concentrated on lead and considered Zircaloy and aluminum tubes of 0.75 mm or steel tubes of 0.5 mm thickness as cladding. In order to judge the neutronic effects of different materials and geometry choices, a very detailed calculational model of the moderator tank and the target system with its internal structure was set up [5] for use in conjunction with the LAHET- MCNP code system. Comparing the results of the flux calculations [6] for the different target types mentioned one finds, as expected, that Zircaloy tube cladding give the highest neutron flux because, while showing only little neutron absorption, Zircaloy generates neutrons of its own. However, being a hydride forming material, Zircaloy tubes might be particularly vulnerable to embrittlement by picking up hydrogen from the coolant and - at elevated temperatures - from the lead, in addition to that, which is generated in the Zircaloy itself. As for less critical solution, stainless steel tubes can be used with a tolerable penalty in neutron flux. In order to test this version, a few lead-filled T91 steel tubes were irradiated in the Target Mark 2 within the SINQ target irradiation program (STIP) [7] (see below). The tubes were still intact after an irradiation dose of about 11.5 dpa, giving sufficient confidence to construct the follow-up target Mark 3 of lead contained in austenitic stainless steel 316L tubes. This target finally received a proton charge of mah with peak dose levels of about 22 dpa in the 316L tubes. In December 2001 this target has been taken out of service, although it still was running without any problem, and is at present under investigation. 3.2 The SINQ target irradiation program (STIP) In the targets Mark 2 and 3 there were several (10 to 17) rods holding a large number of miniaturized test specimens (altogether about 1500 and 2000, respectively) from different materials and of different shapes (tensile test, bending fatigue, TEM-disks, Charpy test etc.), c.f. [7]. The sample rod locations of target Mark 3 are illustrated schematically in Figure 5. The specimens were enclosed between aluminum fillers and encapsulated in Zircaloy tubes. Dosimetry packages were placed with the test specimens. With the test capsules arranged along the central axis of the target, they are exposed to different proton energies and intensities, while the fast and thermal neutron spectrum is similar for all positions [8]. Detailed calculations on power deposition, proton energy distribution and other 19

6 quantities were performed with the computer model mentioned above. The results were used to examine thermo-mechanical aspects of the test sample rods [9] DY, C A B Proton Beam Rod Including samples A B Hg Capsules Pb-Bi Capsules C Pb-Bi Capsules 1 Tensile, Fatigue, TEM Tensile, Fatigue, TEM Tensile, Fatigue, TEM Bend bar, Tensile, TEM Tensile, TEM 6 CT, Tal-clad W, SiC SANS 7 8 Bend bar, Tensile, TEM Fatigue, SANS, TAP 9 Carbon fibre samples 10 Bend bar, Tensile, TEM 11 Charpy samples Charpy samples Charpy samples 14 Charpy samples Note: The materials of the samples are Steel, Inconel Zircaloy mainly. Thermocouple Fig. 5 A sketch illustrating the arrangement of the 17 specimen rods in the target Mark 3. Temperatures during irradiation are monitored with a total of 10 thermocouples. In order to monitor the temperatures of the test specimens and experimental rods during the irradiation, thermocouples were placed in several of the rods. Examples of the temperature recordings are shown in Figure 6 for a period with two short beam trips. It can be seen that the center line temperatures reach the level of the cooling water temperature within 25 seconds after a sudden beam trip. The measured temperatures agreed well with calculated data [9], showing that the assumptions made on heat transfer coefficients were reasonable. The temperatures of most of the test specimens were in the range of 250 to 400 C, for target Mark 3, operated with the 4 cm target E, up to 480 C. These are temperatures at which beam windows of future liquid metal targets are likely to run and for which presently almost no data are available. Also, the frequent thermal cycling the samples are subjected to makes for a realistic simulation of the situation also in future spallation neutron sources. 20

7 Temp ( C) Time (s) Beam (microamp) Lead; layer 6a, center Experim. rod: Tear, TMP/SP; layer 4b, center Zircaloy; layer 1a, center Experim. rod: Tensile, Tear, TEM/SP; layer 1b, center Beam (microamp) Fig. 6 Measured temperatures in some of the target rods of the SINQ target Mark 2 during a period with two short beam trips (Recording intervals are 5 seconds). 3.3 Analysis of irradiated test specimens The so-called ATEC area in the PSI Experiment Hall immediately adjacent to the SINQ Neutron Guide Hall ( Hot Cell in Figure 1), is designed and equipped to handle large radioactive components and is generally used to service, repair and prepare for disposal all kinds of radioactive parts used in the operation of the facility. The hot cell is equipped with a power manipulator, master-slave manipulators and a movable heavy duty working table as well as several other items such as radiation monitoring, video cameras etc. Although access to the hot cell is possible through its roof, a specially designed port for horizontal access is used to insert the SINQ target, because limitations in height prevent vertical insertion with sufficient clearance for the manipulations required. In ATEC the test specimen rods were removed from the spent targets. These rods were then transported to the standard hot cells at PSI. Some of the irradiated samples where shipped to CEA, FZJ and JAERI. Most of the post-irradiation examinations (PIE) is done at PSI. The analysis of the Al-Mg(3) safety-hull has been performed on several 40 mm discs cut from the beam window and side wall (Figure 7). γ-mapping revealed the proton beam center. Tensile test specimens were cut from all the discs, but only those at or near the beam center have been tested at room temperature. The engineering strain-stress curves shown in Figure 8 demonstrate: a) significant hardening has been introduced by the irradiation; and b) the material is ductile at the highest fluence of p/m 2 although irradiation embrittlement effects exist [10]. In addition to the rods holding test specimen, others were made either of different types of steel as bulk material or contain steel capsules filled with stationary liquid metal (PbBi and mercury). Further, 5 solid Zircaloy rods are available for non-destructive and destructive testing. For this examination, among other techniques three neutron instruments at SINQ are equipped with hot sample handling capabilities, which allow to investigate whole target rods or sections thereof. The instruments are the thermal neutron transmission radiography facility NEUTRA and NEURAP [11], which is used to determine position dependent hydrogen concentrations in materials of low or moderate neutron attenuation 21

8 the small angle scattering facility SANS I [12], which shall be used to search for changes in the material's homogeneity and for precipitations or large defect clusters a specially designed internal stress diffractometer POLDI [13], which allows to measure strain distributions that could result from diffusion of gaseous spallation products under the thermal gradient that builds up in the rods during operation and to detect new phases that may have formed in the material. 50 mm Fig. 7 The window of the aluminium safety-hull of target Mark 2 after cutting several discs from it in ATEC. Stress [MPa] x10 25 p/m 2 3.1x10 25 p/m 2 1.0x10 25 p/m 2 2.9x10 25 p/m 2 Unirr Strain (%) Fig. 8 Tensile test results of samples cut from the centre and edge area of the proton beam and unirradiated material. 22

9 There were several rods inspected with neutron radiography. One interesting result is shown in Figure 9, which illustrates the picture taken from a Zircaloy tube containing a martensitic steel (F82H) rod. The dark spots seen in the picture are believed to be hydrides formed in the Zircaloy tube. However, such dark spots were not seen in the other Zircaloy tubes containing austenitic steel 316L rods. Zy tube }steel Zy tube Fig. 9 A picture of neutron radiography showing the middle part of a Zircaloy clad martensitic steel (F82H) sample. The black spots are believed to be hydrides formed in the Zircaloy cladding. Radiographs taken after rotating the sample prove that the dark spots are not located in the bulk steel but in the cladding near the interface. As an example for collected data from the STIP II program, investigating the samples irradiated in target Mark 3, Figure 10 shows the ductile-to-brittle transition temperatures (DBTT) of T91 and F82H evaluated from small punch tests [14]. These ferritic-martensitc steels are possible candidates for the enclosure hull and beam window of future high-power spallation targets. The data reveal that the DBTT of T91 increases to ~250 C after irradiation at 275 C to 9.4 dpa/770 appm He. This temperature is uncomfortably close to the operation temperature of a beam window in a spallation target, although the received dose must be rated still rather low. This result shows that in these materials the radiation embrittlement is more severe than anticipated T91 F82H DBTT ( C) Displacement (dpa) Fig. 10 Ductile to Brittle Transition Temperature (DBTT) diagram of the ferritic-martensitic steels T91 and F82H, after irradiation in SINQ target Mark 3 in the frame of the STIP program. 23

10 4. WORK TOWARDS A LIQUID METAL TARGET FOR SINQ It is now generally acknowledged that, for high beam power and in particular for high beam power density as required for efficient neutron flux generation, liquid metal targets are the concept of choice for several reasons. Implementing a liquid metal target in SINQ has been a goal from the early days of the project on [15]. The original concept was to use natural convection only to drive the circulation of the liquid metal, which is one of the reasons for the vertical injection into the target. 4.1 The MEGAPIE project As is well known, the early concept of a liquid metal target for SINQ was abundant in favor of a solid target. Nevertheless, meanwhile a new initiative was launched by Commissariat à l Energie Atomique, Cadarache (France) and Forschungszentrum Karlsruhe (Germany) in collaboration with PSI to develop a liquid metal target for SINQ, the Megawatt Pilot Target Experiment, MEGAPIE [16]. The aim of this initiative is to demonstrate, in an international collaboration, the feasibility of a liquid leadbismuth target for spallation facilities at a beam power level of 1 MW. It is the goal of this experiment to explore the conditions under which such a target system can be licensed, to accrue relevant materials data for a design data base for liquid lead-bismuth targets and to gain experience in operating such a system under the conditions of present day accelerator performance. Furthermore, design validation by extensive monitoring of its operational behaviour and post irradiation examination of its components are integral parts of the project. An extensive preirradiation materials R&D program will be carried out in order to maximize the safety of the target and to optimize its layout. At present (Dec. 2002), the MEGAPIE project is at the end of the detail design phase, ready for purchasing, envisaging a target date of January 2005 to be installed at SINQ, followed by an approximately 9 months operational period in the year CONCLUDING REMARKS The Paul Scherrer Institut is currently operating the world s most powerful spallation neutron source. Owing to the nature of the accelerator used, this is a continuous source. Concerns have been voiced during the planning and construction phase that background problems due to the high energy neutrons generated in the spallation process might seriously affect the performance of its neutron scattering instruments. This turned out not to be the case and ever since routine operation was started in June 1998, SINQ has performed as well, or better than many MTR-type research reactors currently in use as neutron sources. While there is a potential for further increase of the neutron flux in SINQ, which will be exploited to improve the facility s competitiveness, running a Megawatt class spallation facility in itself is an important cornerstone for many development projects in the field. Examples are the participation of PSI in the MEGAPIE project [16], and PSI s input to the target concepts of the next generation spallation neutron sources. The SINQ target development program will, intentionally or inadvertently through a broader data base that accrues from this work, also benefit most other projects in the field of accelerator application in nuclear technology. Although research centers that operate high current accelerators are natural points of focus for this kind of work, the tasks to be accomplished are often of large enough scale to justify international collaboration of all parties interested. As a user-oriented laboratory, PSI is willing to make its facilities available for such collaborations. 24

11 REFERENCES [1] G.S. Bauer, W.E. Fischer, U. Rohrer and U. Schryber, Commissioning of the 1 MW spallation neutron source SINQ, Proc. Particle Accelerator Conference, Vancouver, BC, American Physical Society (1997) [2] H. Spitzer, G.S. Bauer, T. Hofmann, First Operation Experience with the Cryogenic Moderator at the SINQ Spallation Neutron Source, Proc. Int. Workshop on Cold Moderators for Pulsed Sources, Argonne, III. (1997) [3] W. Wagner, J. Duppich, Proc. ICANS XII, Rutherford Appleton Laboratory, Report No (1994) 368 [4] W. Wagner, G.S. Bauer, J. Duppich, S. Janssen, E. Lehmann and M. Lüthy, Flux measurements at the New Swiss Spallation Neutron Source SINQ, J. Neutron Research, 6 (1998) 249 [5] A. Dementjev and E. Lehmann, Calculations of Spallation target properties by the code system LAHET, PSI TM (1997) [6] G.S. Bauer, A. Dementyev, E. Lehmann, Target Options for SINQ - A Neutronic Assessment, in Proceedings ICANS XIV, ANL-98/33 (1998) 703 [7] Y. Dai and G.S. Bauer, Status of the first SINQ irradiation experiment, STIP-I, J. Nucl. Mater. 296 (2001) 43 [8] A. Dementyev, G.S. Bauer, Y. Dai, E. Lehmann, Neutronic Aspects of the SINQ Mark 2 Target with Irradiation Test Samples, in Proceedings ICANS XIV, ANL-98/33, (1998) 717 [9] L. Ni, G S. Bauer, Heat Transfer and Thermomechanic Analyses on the Test Rods in the SINQ-Target Mark 2, in Proceedings ICANS XIV, ANL-98/33 (1998) 396 [10] Y. Dai, H. Kaiser, K. Geissmann, G.S. Bauer, R. Zumsteg, H.P. Linder, F. Groeschel, Preliminary examination of the safety hull of SINQ target Mark-II, PSI Scientific and Technical Report, volume VI (2000) 33 [11] E. Lehmann, H. Pleinert and L. Wiezel, Design of a neutron radiography facility at the spallation source SINQ, Nucl. Inst. Meth. Phys. Res. A 377 (1996) 11 [12] J. Kohlbrecher and W. Wagner, J. Appl. Cryst. 32 (1990) 804 [13] U. Stuhr, Concept for a High Resolution-High Intensity Diffractometer, ICNS'97, Physica B (1998) 224 [14] Y. Dai, X.J. Jia, K. Farrell, submitted to Journal of Nuclear Materials (2002) [15] C. Tschalär, Spallation Neutron Source at SIN, Proc. ICANS IV, KENS-report II, Tsukuba (1981) 56 [16] G.S. Bauer, M. Salvatores, G. Heusener, J. Nucl. Mater. 296 (2001) 17 25