[en] A slit was machined through the cladding of an irradiated fuel pin and irradiation in the Experimental Breeder Reactor-II (EBR-II) was resumed. The condition of the fuel pin was continuously followed with delayed neutron (DN) monitors. When the DN signal increased to a previously established administrative limit, the test was terminated. Postirradiation examination showed the sodium-fuel reaction caused fuel swelling and extension of the machined slit. There was no evidence of fuel loss nor was there any indication of impending pin-to-pin failure propagation. This test supports an increase in DN signal for subsequent run-beyond-cladding-breach (RBCB) tests