Our simulation results show that the differences between the CFC wall and ILW cases are due to the positive feedback effect between (1) the different plasma parameter profiles due to the different wall, (2) the difference in the injected/mixed argon amount, and (3) the different radiation characteristics of beryllium and carbon. Variations in the argon content in these simulations have a considerable effect on the runaway generation. The Dreicer fraction is reduced by the presence of beryllium, but is almost unaffected by the presence of carbon. This results in a lower Dreicer current generation in the ILW case compared with the CFC wall case. The runaway population in the ILW case consists mostly of slowly growing avalanche runaways and they are effectively transported out from the plasma by a low level of magnetic perturbations or other losses. Note, that the presence of beryllium is beneficial only if the amount of argon is not too large. Above~ 50% argon content the radiation of the argon takes over and the differences due to the wall material eventually vanish. In view of the results of this paper, upcoming massive gas injection experiments with the ILW on JET will most probably have to face with the reoccurance of runaways for the scenarios that produced runaways using MGI with the carbon wall. Dedicated runaway experiments with the ILW on JET are necessary to be able to better estimate the runaway behaviour in ITER.