Patent Number: 040574687
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENT The liquid metal cooled nuclear reactor core shown in FIG. 3 comprises a plurality of upstanding fuel element sub-assemblies 21 closely arranged side-by-side and secured at their lower ends to fuel support means 22. The support means 22 comprises a diagrid 23 and a plurality of fuel sub-assembly carriers 15 each of which is adapted to carry a group of fuel element sub-assemblies. The diagrid 23 also serves as a distributor for coolant flow to the fuel element sub-assemblies the coolant being delivered to the diagrid 23 by way of pipes 25. At the centre of each group of fuel element sub-assemblies 21 there is a control rod guide tube 24 and the entire assembly of fuel element sub-assemblies and control rod guide tubes is surrounded by a plurality of neutron shield rods 27. A fuel charge chute and fuel storage rotor are shown at 28 and 29. On assembly of the core the fuel element sub-assemblies 21 are free standing with small clearances between neighbouring assemblies. The fuel element sub-assemblies 21 are arranged in groups of sub-assemblies and the upstanding sub-assemblies of each group are disposed about a central member 14 (such as a control rod guide tube 24) of the group and are arranged to lean on the central member. The arrangement is shown in FIG. 2 and the sub-assemblies and central members, designated 13 and 14 in FIG. 2, plug into the carrier 15 associated with the core supporting diagrid. Contact between each sub-assembly and the central member is through a pad 16 attached to the sub-assembly. The inertia damper is shown attached to the top of a wrapper 17 of the sub-assembly and the upper sleeves 2 of the sub-assemblies, in combination, provide an upper shield for the reactor core. The inertia damper shown in detail in FIG. 1 comprises a cylindrical tubular spine 1 having upper and lower sleeves 2, 3. The lower sleeve 3 is welded to the spine 1 at 4 and is of hexagonal shape to provide an end plug for the hexagonal wrapper of a fast breeder reactor sub-assembly; the lower sleeve 3 has a flange 5 for abutting the end of the wrapper. The upper sleeve 2 is of massive steel to provide neutron shielding of hexagonal cross-section and has a cylindrical bore 6 there being a radial spacing 7 between the spine 1 and the upper sleeve 2. The upper and lower sleeves 2, 3 are resiliently coupled together by a bellows unit 8 and the lower end of the upper sleeve 2 has six spaced radial ducts 9 for enabling liquid metal coolant to flood the radial spacing 7. The spine 1 has a diverging tubular extension 10 attached to its upper end and the upper sleeve 2 has a complementary diverging bore 11. The sleeve 2, being captive, is retained on the spine by means of six radially extending retaining pegs 12. In operation, the reactor core is submerged in a pool of liquid metal coolant which is flowed upwardly through the sub-assembly wrappers in heat exchange with fuel pins therein. The radial spacings between the sleeves and the spines are flooded with liquid sodium so that when the upper sleeves vibrate the sodium in each radial spacing is forced from one side of the radial spacing to the other and its resistance to this motion provides a damping force. The damping force changes with the amplitude at a slow rate initially but finally at a very rapid rate so that, for average amplitudes, the damping rate is higher than that predicted for zero amplitude. Thus as a result of hardening of the damping rate with displacement of the upper sleeve relative to the spine, the final damping rate is so high that metal-to-metal contact between spine and upper sleeve is very unlikely. There is also a tendency for the upper sleeve to pump itself into concentricity with the spine even though concentricity may not be achieved in manufacture. The expected reduction in sub-assembly vibration amplitudes using such a device is in the region of 3 to 10 times. In inertia damping means for a fuel element sub-assembly wherein the upper sleeve has a bore of 90 mm nominal and parallel length 350 mm, a radial clearance between spine and upper sleeve of 1.20 mm is considered to be satisfactory.