Hydrogen gas retention in nuclear waste sludge is of interest on many nuclear licensed sites. The risk of hydrogen hold-up in either man-made voids or within fracture zones inside a sludge bed and a sudden release of gas following bed disturbance is clear. Despite this, much remains to be understood about the likelihood of large gas pockets forming within sludge and the features of nuclear waste sludge that may increase or decrease the risk of this happening.