DOE OSTI2017
In this paper we present modelling and design of compact radiation shielding required for operation of the EMU instrument, located at Australia's OPAL nuclear reactor. The acceptance criteria for the shielding design were (i) safety - dose rate ≤ 3 μSv/h at external surfaces of the instrument's experimental area, (ii) floor load limit < 20 tons/m{sup 2}, (iii) low cost and (iv) for maintenance purposes 'easy access' to components of the instruments. The EMU instrument is a cold neutron backscattering spectrometer designed to measure inelastic neutron scattering. Its distinctive feature is that it has a tight elastic energy transfer resolution, of about 1 μeV. The instrument provides unique opportunities to study diffusion in complex materials, e.g. water molecules in confinement, polymers or biological molecules, particularly in the Asia-Oceania region. EMU is the first instrument located on the CG3 neutron beam line of the OPAL Neutron Guide Hall (NGH). A pre-existing evacuated neutron guide of dimensions 200 mm (height) x 50 mm (width) feeds into the shared EMU/KOOKABURRA pre-monochromator bunker shielding assembly, and its bottom cross section will be intercepted ∼200 mm further in air by the highly orientated pyrolytic graphite (HOPG) EMU pre-monochromator (take-off angles from 125.4 deg. to 148 deg.). The bunker shielding assembly for EMU also contains a beryllium filter and Focusing Guide Section D (FGD, 170 mm (height) x 50 mm (width)). To accommodate the range of take-off angles Pb cylindrical shielding blocks of radius 140 mm and 500 mm height have been designed. These blocks will be located at the section of the bunker where the neutron beam will exit towards the experimental area and they will reside on top of a hollow steel base (sides 10 mm thick), on which they move by sliding and are integrated with the tertiary shutter. The experimental area is enclosed by a concrete wall of 2.5 m height and 300 mm thickness, shared with the KOOKABURRA instrument, Gyprock Plasterboard panels with viewing windows, a wall from the Out-of-Pile Bunker and the CG3 shielding. The neutron beam enters the experimental area via the tertiary shutter, into the Focusing Guide Section C (FGC), which has an entrance guide cross-section of 158 mm (height) x 79 mm (width) and exit guide cross-section of 149 mm (height) x 85 mm (width) and is of length 816 mm. The vacuum chamber of FGC will be connected to the vacuum chamber of the Background Chopper (BC, disc of outer radius 350 mm). Focusing Guide Section B (FGB) is aligned with FGC and its vacuum shared with the BC vacuum. The FGB has an exit guide cross section of 50 mm (height) x 49 mm (width) and is of length 4046 mm, which will focus the neutron beam vertically and horizontally towards the Al window entrance (0.82 mm thickness, labelled N16) of the Scattering Tank assembly (constructed from 20 mm - 40 mm thickness stainless steel and will be under vacuum < 1 x 10{sup -3} mbar). Inside the Scattering Tank there is another section of Focusing Guide (FGA), with an exit guide cross-section of 30 mm (height) 30 mm (width) and of length 417 mm. The neutron beam exiting the FGA will be reflected by the HOPG crystal pieces of the Graphite Chopper (GC) towards the Backscattering Monochromator (BM). The reflected neutron beam will first pass through the other Al window (0.82 mm thickness, labelled N17) of the Scattering Tank and then enter into a flight tube (steel of 10 mm thickness). The BM is formed in an 1800 mm spherical radius array of Si crystal elements (0.87 mm thickness) and is oscillated by the linear motor Doppler Drive. The neutron beam will be reflected back at 180 deg. from, and focussed by the BM to reenter the Scattering Tank through the N17 Al window. It will then pass through the GC (HOPG crystals are rotated out of the way) and will reach the scattering sample of interest to the instrument end user. Also, inside the Scattering Tank there will be {sup 3}He linear position-sensitive detectors, and Analyser Arrays formed in an 1800 mm radius spherical arrays of Si crystal elements (0.87 mm thickness) glued on aluminium support frames (10 mm thickness) and span ∼160 deg. arc.
21 SPECIFIC NUCLEAR REACTORS AND ASSOCIATED PLANTS↗