The conventional commercial fission reactors use uranium-235 as fuel. Its cross section of (n, fission) reaction at thermal neutron is about 585 barns. My question is: Is there a known threshold of the cross section that makes a nuclear reaction not sustainable if the minimal cross section is not reached? For example, perhaps if the U-235(n, fission) = 580 barns, I guess it still works, but if only 100 barns, still work? It is only a must-have condition of sustainable chained reaction that the new generated neutrons should be more than used one, but it is not the full-fill condition. Big enough cross section is the another must-have condition, just wondering how big is narrowly big enough. For cancer therapy, the cross section of Gd-157 neutron capture is 253366 barns. Obviously it is too much, even it decreases to 585 barns, i.e. same with U-235, it should still work.