Neutronic design of the ESS targetmoderatorreflector. Luca Zanini For the ESS target division and in-kind collaborators

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1 Neutronic design of the ESS targetmoderatorreflector system Luca Zanini For the ESS target division and in-kind collaborators Nordic-Gen4 seminar, Risoe, October 2012

2 ESS timeline On schedule for the basic milestones: TDR by end The baseline for TDR is based on best state-of-the-art knowledge. The TDR focuses on feasibility, reference performance and basis for costing Further optimization Jan July Design frozen Q Manufacturing and installation for start of tests without beam: Q Commissioning with proton beam starts: April 2019, completed by Q2 2020

3 ESS target station baseline Outer reflector Inner reflector protons target moderator 12 m 60 top 60 bottom 60 bottom 60 top 3

4 Beam Extraction Baseline Angular spacing between the neutron beam lines 5 degrees 48 useful beam lines Arrangement of beam lines in four 60-degree sectors, whereof two facing the upper moderator and the other two facing the lower moderator. From 2 m radius and outwards the allocated spaces for the neutron beam guide inserts are exclusively reserved. To place beam extraction guide parts closer to the centre requires agreement with the instrument teams of the neighbouring beam lines. No choppers within the boundaries of the target shielding monolith. The use of individual light shutters is prescribed.

5 MCNPX model

6 Moderator brightness [a.u.] Innovative beam extraction concept Brightness can be further increased by reducing the beam extraction angular range (e.g. reactors: about per moderator). Innovative ESS beam extraction layout: instead of fixed beam line positions covering uniformly about 240 (current practice on pulsed spallation sources) a flexible grid of possible beam line positions every 5 for the whole lifetime of the facility. This allows grouping the beam-lines depending of the instrumental needs in minimal angular spread. Flux gain e.g. with 22 operational beamlines: % above baseline. Brightness depends on the total angle of the opening (i.e. brightness with one 120 opening is about the same as with two 60 openings) Beam extraction baseline Sum of extraction sector angular widths for one moderator [ ]

7 Moderator design strategy Rely on existing technologies Best state-of-the-art technology Changed about every year, possible upgrades Consider for ESS startup only proven recent concepts such as: Bi-spectral extraction Be filter/reflector Grooved (thermal) moderators Participate actively to development program of new concepts (i.e. directional extraction) for future upgrades 7

8 COLD moderator 8

9 Moderator concept selection Baseline hoice cylindrical, 99% para- H 2, coupled, with optional Be filter and bi-spectral extraction. 9

10 2 Instantaneous brightness [n/cm /s/str/å] Why is volume moderator better Characteristics of scattering cross section of para-h 2 Lower below 50 mev favors extraction diminishes captures by H (less collisions) Pulse shaping is defined by choppers 1,6x ,4x ,2x ,0x ,0x kj/pulse (SNS,1 MW) 300 kj/pulse (ESS, 5 MW) 6,0x ,0x ,0x , Time [ m s]

11 Moderator/reflector baseline parameters parameter Moderator shape Moderator diameter (inner vessel dimensions) Moderator height (inner vessel dimension) Nominal value cylindrical 0.16 m 0.13 m Moderator fluid Para-H 2 Moderator temperature Pre moderator fluid Pre moderator temperature Thickness of optional cooled Be filter/reflector Inner reflector material Outer reflector material Thickness of pre moderator water layers facing the target wheel 20 K H 2 O 330 K 0.1 m Be steel 20 mm 11

12 Optimization of moderator diameter

13 Absolute brightness from cold moderator Comparison of calculated ESS neutron brightness to the analytical reference Preliminary results based on ESS baseline cold moderator Proton beam parameters: 2.86 ms, 14 Hz, 125 MW/pulse Baseline cold neutron brightness: neutron yield improved by about a factor of 2 compared to earlier practice (volume vs box mods) covering 240 beam extraction range with two moderators 13

14 Systematic uncertainties

15 Bi-spectral moderator 15

16 Design of thermal moderator In order to approach a good design, we need To know how much we can achieve To give a penalty to the cold spectrum as small as possible. This penalty is given by Reduction of reflector material Positioning of the moderator 16

17 60% of best theoretical stand alone thermal moderator (no cold, other side closed by reflector) Other options considered

18 Design of bi-spectral moderator Wide wavelength range increases scientific applications Many options considered Top view Cold neutrons Thermal neutrons

19 Engineering Design by FZJ Fluid: Light Water Structure material : Al 6061-T6 Inlet Pressure: 3 bar Inlet Temperature: 20 C Average Volumetric Heat H 2 O: 6.5 W/cm³ Average Volumetric Heat Al: 9.9 W/cm³ Max. Local Temperature: 46 C

20 Iterative refinement of the MCNPX model

21 Bi-spectral extraction results HZB Berlin ESS Thermal neutrons will be available at all beam line positions for thermal or bi-spectral thermal and cold extraction. Favorable performance offered already by the Be reflector without extra adjustment (as in Berlin). The peak ratio thermal/cold at HZB is about 4. Now we have about 2 times the cold spectrum, so the blue curve at HZB would go up a factor of 2, bringing the ratio to a similar value found at ESS. 21

22 Summary Target design status: on schedule for the basic milestones Baseline cold neutron brightness: neutron yield improved by about a factor of 2 compared to earlier practice (volume vs. box moderator) covering 240 beam extraction range with two moderators Brightness can be further increased by reducing the beam extraction angular range Thermal neutrons will be available at all beam line positions for thermal or bi-spectral thermal and cold extraction. 22