Nuclear safety enhancement by structural changes of the control rods

This paper presents the modification of structure of the control rods which are intended to be done in order to improve the nuclear safety of the TRIGA-SSR-14 MW Pitesti, Romania. In 1996 and 1997 a number of 2 control rod got inoperable due to high corrosion of the welds, water penetration into the absorbent followed by the swelling generated by internal pressure. Although the safe operation of the reactor is not yet affected, it was decided that a more reliable control rod has to be designed and manufactured. Basically, the new control rod will use, as the old ones, boron carbide as absorbent, but boron carbide will be assembled in a different way. There will be manufactured a set of 16 incoloy tubes for each control rod filled with boron carbide pellets and leak tight sealed. These tubes will be mounted in a square array, on the perimeter of the control rod, 5 tubes on each side of the square. The preliminary design on which neutronic analysis on this structure has been performed using specific computation codes. The result of this analyze shows that new control rod reactivity is slightly lower (with about 10%) compared to the older one, but assures a full compensation capacity. Helium release analysis done here reveals that the Helium pressure in the tube at estimated end-of -life would not exceed 57 atm. The advantages of this new concept: it is avoided the problem of helium or radiolysis gases generated into the control rod, and which causes, as we shown, control rod swelling. (orig.)