The reactor was normally stable, the power coefficient having a prompt positive component and a delayed negative component. Instabilities could occur if the power-to-flow ratio were high.

If the flow were decreased, the power would first rise and then later it would decrease to a new level. The first power changes were due to mechani-

cal variations and bowing of fuel rods. The second changes were not under­stood. A new restrained core composed of rods inside a hexagon can with a tightening rod in the center and a heavy structure with which to clamp all the assemblies was provided. This new core had only a prompt negative power coefficient, having lost both the prompt positive effect of bowing and its delayed negative effect.

The eventual explanation was that, while the bowing provided a prompt inward bowing, it also brought the upper end of the fuel rod into contact with a shield plate (see Fig. 2.38) that subsequently expanded, levering the fuel rod backward away from the core and thus giving the negative effect delayed by the thermal time constant of the massive shield plate.