A fuel poison bar(22) is made up of 2 concentric tubes (30,24) of zirclay that have an annulus between them. The bottom of the tubes is sealed by a plug(26) that has a central hole(28). At the top the inner and outer tubes are sealed by another cap(32) which also has an orifice(46) that communicates with the inner bore(44). This allows the reactor coolant to circulate through the inner tube and increase the moderator proportion of the fuel poison bar. At the lower end of the annulus between the tubes there is a spring(38) that rests on the lower plug. This spring supports pellects of the fuel poison, e.g. B4C-Al2O3, ZrB2 or Gd2O3, that fill the annulus.
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