Diamond windows have an important role in fusion power applications, as this material only currently enables both tritium confinement and transmission of mm-wave power in the MW range. Diamond window units are safety related components and therefore their integrity has to be guaranteed both during normal and accidental operation, like, for example, in absence of coolant flow. In this work, a beam power of 1 MW is applied to a FEM model of a diamond window unit to assess the maximum time allowed until critical temperatures and thermal stresses are reached during such accidental scenario.