[DBrow]

I didn't see in the article how they modulate reactivity.

Otherwise, what a great idea!

Too bad a president got rid of the National helium Reserve, though. Who needs Zeppelin in the 21st century?

[LesbianThespianGymnasticMidget]

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.

[Red Badger]

I didn't see in the article how they modulate reactivity.

It is constant. The uranium and the "moderators" are mixed together in the pebbles, kinda like ingredients in a cake. You can't see the eggs, but they are there................

[PapaBear3625]

I didn't see in the article how they modulate reactivity.

It's at the point in the article where they mention Doppler broadening

At the higher temperature, the more plentiful uranium-238 nuclei absorb more neutrons (due to an effect called Doppler broadening) and the reactor output decreases, lowering the reactor temperature until an equilibrium is reached.

If the temperature goes up, the fission rate decreases, which is a nice negative-feedback mechanism. So you modulate reactivity by modulating core temperature, and you modulate core temperature my modulating coolant flow

[Wonder Warthog]

"I didn't see in the article how they modulate reactivity."

Control rods of neutron absorbing material---just like any other nuke plant.

[B-Chan]

The reactor is reactivity-limited by a phenomenon called Doppler broadenining. Basically, the hotter the reactor runs, the more the U-235 atoms of the fuel vibrate around, thus “broadening” their virtual capture cross-section to the thermalized neutrons being emitted by other,surrounding fuel that sustain the chain reaction. As the reactor heats up, the rate of neutron capture goes up, and the total flux of thermalized neutrons needed to sustain criticality falls. Once a certain temperature is reached, the fuel is soaking up enough of the excess t-neutrons to take the core subcritical, and the heating stops. Of course, once the heating stops, the capture x-section of the fuel drops again, allowing more neutron flux, and the core goes critical again. To control reactivity, therefore, one need only control the temperature inside the core by manipulating coolant flow. If the flow of helium coolant running through the core is decreased, the core heats up, capture x-section increases, and reactivity decreases. If coolant flow is increased, core temp drops, capture x-section decreases, and reactivity increases.

It really is a clever system.