The Cold Neutron Moderator for the Continuous Spallation Neutron Source SINQ
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NEACRP-A-1052 Topic 1.4 E The cold neutron moderator for the continuous spallation neutron source SINQ F. Atchison, G. Bauer, W. Fischer, K. Skala, H. Spitter PAUL SCHERRER INSTITUTE CH-5232 Villigen-PSI, Switzerland Abstract The paper describes the boundary conditions and the resulting design concept of the cold D+uoderator for the continuous high power spallatidn neutron source SINQ. Initial thermal-hydraulic calculations performed have shown that the system can be operated safely under the expected load conditions and that also accidental situations such as loss of insulating vacuum do not.cause serious safety hazards. Paper presented at the International Workshop on Cold Neutron Sources March 5 - 8, 1990 Las Alamos NW, USA 17110001 *. I. INTRODUCTION SINQ is a spallation neutron source under construction at the Paul-Scherrer- Institute in Switzerland. It will be driven by the proton beam of a high current isochronous cyclotron with a radio-frequency of 51 MHz. This means that SIN&, like most research reactors, will be a cw-neutron source in contrast to most other spallation neutron sources in operation world wide. This has several consequences on the design of the source itself and of its cold neutron moderator. II. THE SINQ GENERAL CONCEPT AND BOUNDARY CONDITIONS FOR THE COLD MODERATOR DESIGN Given the fact that SINQ will be a cw-neutron source, the decision was made to make 0 it resemble as closely as possible a research reactor in terms of neutron scattering oppor- tunities. This means that the prime optimisation criterion was a high thermal and cold neutron flux and the maximum useful number of horizontal beam ports. This lead to four significant decisions: 1. A large DsO-tank should be used as neutron moderator 2. A vertical beam injection from underneath into the target was selected to max- imise the useful space around the target shield 3. A target of low absorption cross section for thermal neutrons was favoured (liquid Pb-Bi eutectic mixture) 4. Much emphasis was placed on an efficient cold moderator and associated neutron guides. A vertical cut through the SINQ target station along the direction of the incident proton 0 beam on the left hand side and along the direction of the main axis of the neutron guide system on the right is shown in Fig. la. Fig. lb shows a somewhat enlarged cut through the plane containing the cold moderator inserts. 1 17110002 Fig. 2 gives a horizontal section through thr target block .I5 Iowing XII bca~~n port~s ~vhich in fact are on three different levels as indicated by thr corresponding ~~umbcrs. The neutron production target in the center of the target block will lx cooled I>y nn~tnr;rl convection of the Pb-Bi eutectic and hence has a height of almost 4 m to provide enough buoyancy force for the hot liquid. As a consequence, the target brcomes wry massive and gives good self-shielding for the high energy neutron component in the forward dirrction of the proton beam. This can be seen from Fig. 3 which gives a p&r plot of the nrutrou intensity in various energy regimes leaving the target. It can also bc seen from this plot, that it significant component of high energy neutrons is present which leads to a, biologicnl shield much thicker than e.g. with a reactor. This should be as compact as possible. This has two important consequences for the choice of cold moderator concept: 1. Shielding in this direction should remain a,s compact a.s possible 2. Significant heating rates and activation of structural cwuponcnts will pc~-sis~. out to quite large distances from thr center of the SOIIKC. Design provisions have been made on the SINQ D,O- modrrntclr lank to 1lil~VC 110111, a II2 and a Dz-cold moderator availnblc as a long term opt.iou. Ilrsults of calculations performed for the spectral intensity available at the outer edgr of the rnn,i~l shirltl for thr three types of moderators at their projected positions (no effect of neutron guidrs incllldcd) are shown in Fig. 4. In view of these findings, the final decision on the 112-cold ~nodcrnto~ was delayed until operational experience on the source itself and instruments Iocn.~.ctl at its beam tubes are available. 2 Service Target bulk neutron target hall w maintenance room I \ Target catcher o 5m IO I L- - I I Figure 1: a) Vertical section through the S’INQ target station. Section is along the proton I injection line on the left and lower half of the right hand side. Upper half of the right side is a cut along the main axis of the neutron guide system. , Figure 1: b) Section through part of the SlNQ target block along the plane of the coId moderator inserts. For detailed parts designation see Figs. 721) and i’b). 4 Subthermal neutron 1 _ Cooled iron shielding, Cold neutron beam ports Outer concrete shielding layer ,. Massive iron shieldin Thermal neutron --=-!f+Y +btherq.a; neutron beam ports Y oeam po Plug position for 0, cold source Figure 2: Horizontal section through the SIiVQ target block showing aJJ horizontal pene- 0 trations, although on different levels (levels above Aoor indicated). 0 0 or Figure 3: Angular distribution ofneutrons in different energy bins emerging from the SINQ Pb-Bi liquid metal target. Note the forward self shielding effect! III. THE GENERAL DESIGN OF THE SINQ COLD D,-MODERATOR Given the high energy neutron distribution shown in Fig. 3, it was decided not to place any components of the cold moderator system other than the moderator vessel itself into areas of high radiation fields of fast neutrons. This means that a horizontal insertion port for 0 the cold moderator was foreseen and the heat exchanger of the natural convection system was moved back into a well shielded region. This results in fairly long tubes between the moderator vessel (main heat source) and the heat exchanger (heat sink) and hence requires a large vertical separation of the two to overcome the frictional forces in the tube system by the buoyancy force of the liquid-gas mixture flowing back from the moderator vessel. Experiments carried out by Hoffmann 111 in conjunction with the development of the second cold moderator at the ILL-reactor 121 showed that such a system could be operated over a large range of heat input. In our case we decided to place the vertical leg partway to the outside of the shielding block for three reasons: 1. The need for shielding behind the horizontal leg would have blocked space valu- able for neutron scattering experiments 2. Placing the heat exchanger in the shielding block automatically provides good mechanical protection 6 17170007 . log- 1 I I I I 1 I I 1 _ .a Beam Tubes 6m long Cross Section 15x8cm2 - Neutron Wavelength A CA1 Figure 4: Anticipated neutron current at the nominal monochromator positions (6m from the start of the beam tube) for the different moderators of SINQ. The effect of neutron guides is not included. 3. The length of the tube and hence the amount of liquid Ds as well as the frictional forces could be reduced. This design implies that the horizontal and vertical leg of the cold source cannot Abe installed as one unit, but have to be put in place separately with a connection made before the 0 shielding plug behind the horizontal leg can be mounted. Cryogenic equipment (cold box) and the control system for the source will be placed next to the main shield at a level of 9.5m above ground on top of the neutron guides with a connecting channel to the service areas and to the Ds-storage tank and parts of the gas handling system, located outside the neutron target hall on the roof of the equipment access building. A block diagram of the various sub-systems is shown in Fig. 5. IV. THE COLD MODERATOR SYSTEM The design of the cold moderator part was governed by the neutronics considerations published elsewhere 131. In order to avoid rethermalizationof cold neutrons upon extraction from the moderator, the cold moderator insertion port will be connected to the neutron extraction port resulting in a T-shaped structure inside the moderator tank, Fig. 6. 19110008 Chimney Chimney /////I 4 2Om3. 2.8 bar tleuterium supply system vscuum supply system Nz-bsrrler' cryogenic---+a--- plant 3OOOW. 19K --i>ot -! Nfll l4! DC Deuterium compressor OB Burst disc Y".DM.HM Gas detector Moderatorts<k Shielding 020.circuit Figure 5: Simplified diagram of the various subsystems of the Dz cold moderator for SINQ. Helium is used for the protective barrier of the D2-System only inside the target block. Outside the target block N2 is used. cold neutron ----- I---.-____. -7 --.-._____ Helium atmosphere Nitrogen atmosphere 2 C 0 Figure 6: Layout of the SINQ-DZO-Tank with T-shaped structures for cold moderators 3 insertion and neutron extraction to avoid rethermalising D20 layers. c? The mechanical stability of this structure under thermal loads, buoyancy forces and various differential pressure conditions is presently under investigation by computational methods. The horizontal cold moderator insert is completely independent of this T-structure and has the following main sub-units: (see Fig. ?‘a) l a double-walled vacuum jacket with a helium barrier between the outer and the inner tube. The part of the vacuum jacket intruding into the DzO-tank consists of an outer 2 mm thick AlMg,-tube and an inner zircalloy tube, 3 to 4 mm thick which is the pressure safety tube and will withstand an internal pressure of 30 bar.