any other integer or step or group of integers or steps further features of the present invention are more fully described in the following examples it is to be understood however that this detailed description is included solely for the purposes of exemplifying the present invention and should not be understood in any way as a restriction on the broad description of the invention as set out above high purity oxide components are batched in accordance with the following glass composition given in percentages by weight 72 sio 2 25 b 2 o 3 1 al 2 o 3 0 5 li 2 o 0 5 na 2 o 1 k 2 o a glass composition which has a specific gravity of 2 13 to this is added the required amount of yttria or other required radionuclides and the mixture of parent oxides is smelted in a contamination free crucible homogenised and then quenched in demineralised water to produce the frit the frit is then ground and sieved to yield a 20 to 50 micron size range fraction this sieved frit is then flame spheroidised by passing the powder from a feed hopper through a flame torch the resultant product is sieved into the 30 to 35 micron size range fraction if the microspheres are to be surface coated with a radionuclide such as yttria instead of incorporating it into the matrix of the microsphere then the exact same steps are taken with the exception that the radionuclide is not added to the components that form the matrix in this case a one wt suspension.