To date,performance assessments of the consequences of a volcanic eruption at Yucca Mountain have assumed that all fuel that comes into contact with erupting magma disintegrates as a result of physical abrasion and forms particles of respirable size that are entrained within larger droplets of magma which erupt into the air.It has been assumed that the spent fuel remains a distinct solid phase at all times,but this assumption requires re-examination.Uranium oxides do not melt at the temperatures of basaltic magmas,but the solubility of UO_2 in basaltic magma is approximately 20%by weight.For small particles,the rate-limiting process in dissolution will likely be diffusive mass transfer away from the surface of the solid particle.We calculated the dissolution time of the largest respirable particle,with diameter of 10μm,for a range of reasonable diffusion constants and obtained a value between 2.1 sec and 21 sec.Consequently,small respirable particles of spent fuel that are formed when magma intrudes into a repository can be expected to dissolve into the magma before reaching the surface.Because dissolution time varies with the square of the particle radius,1-mm particles will have a dissolution time of 6 hours to 2 days,and larger particles will dissolve even more slowly.Thus,respirable particles of spent fuel will dissolve into the magma,but larger pieces of fuel will remain solid.The net effect is that respirable ash particles will be formed entirely by solidification of small droplets of magma and will be depleted in refractory spent fuel components compared to the average composition of the erupted material.
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