Pump in more helium and you get more reactivity. Remove helium and it moderates. Dump all the helium, and it stops reacting.
There are still issues in making the pebbles, but it is a very sound concept.
Not exactly..........the helium is the "working fluid". From the article: The pebble bed modular reactor uses helium gas, rather than water, as its working fluid. At a PBMR power output of 165 MW, helium gas flows at 190 kilograms per second down through the reactor in the interstices formed by the spherical carbon surfaced pebbles. The gas is heated by convection and radiation from the nuclear fission heat generated in the kernels and conducted to pebble surfaces. The PBMR design does not put the helium in direct contact with radioactive substances, since the fissile material and fission products are sealed in the kernels and pebbles. What's more, helium is inert, so neutron activity does not cause it to become radioactive. Thus, the helium may be used directly as the working fluid in gas turbine turbomachinery, without the need for an isolating heat exchanger and another, separate fluid loop. The part load characteristics of a closed-cycle gas turbine are remarkably good, quite unlike open-cycle operation. If the PBMR gas turbine is at the design output of 165 MW, load reduction is achieved by bleeding helium from the closed loop. This reduces the helium mass flow rate, reducing power output, and lowering the mean gas density, but maintaining constant gas velocities at constant rpm. In gas turbine designer terminology, the turbomachinery velocity triangles remain the same, so that the PBMR design efficiency of 41 percent will remain the same over a wide range of loads...