[panick21_]

You are right, I thought Shippingport reactor 3 did but it also was a blanket design.

I don't really think this matters that much. The neutronics is pretty well understood and the principle has been proven. Its just a matter of investing the money to do it.

[Schroedingersat]

Putting fertile material near fissile material and some sodium or lead or beryllium has never been the hard part. Every attempt has sunk at the filthy and expensive reloading part and the making something that doesn't catch fire, leak everywhere or melt down when used with the blanket in part.