Zircaloy-clad elements of bulk B/sub 4/C and B/sub 4/C--SiC were irradiated in pressurized hot water loops and in the Materials Testing Reactor (MTR) process water to depletions as high as 97% burnup of the boron-10 atoms present. The B/sub 4/C showed a linear volume swelling rate of approximately 0.15% DELTA V per 10/sup 20/ depletions/cc up to about 90 x 10/sup 20/ depletions/cc after which a more rapid swelling occurred as a result of B/sub 4/C cracking. Elements containing B/sub 6/C- SiC showed greater thickness changes as a result of SiC cracking. A severe decrease in hot water corrosion resistance occurs during irradiation of the bulk B/sub 6/C and B/sub 4/C-SiC poison cores, resulting in disintegration of these poison cores when exposed to the water environment in-pile. B/sub 4/C platetype poison elements show greater thickness increases than would be expected from poison core volume swelling because of the formation of poison core-cladding gaps in the elements. The element swelling was extremely sensitive to fabrication variables. Helium gas released from B/sub 4/C during irradiation was slight and not related to gas diffusion. Lattice parameter values obtained on irradiated natural B/sub 4/C indicated that the M direction of the hexagonal B/sub 4/Cmore » remained essentially constant while the c/sub o/ direction decreased with increasing boron-10 depletion. Evidence of plasticity of B/sub 4/C in-pile was noted. The irradiation behavior of the ceramic materials studied in this investigation and the wrought alloy of H-10 austenitic stainless steel poisons are compared. (auth) « less