According to our current understanding of physics, an object with mass, such as a piece of radioactive material, cannot travel at the speed of light in a vacuum. As an object with mass accelerates towards the speed of light, its relativistic mass increases, requiring an infinite amount of energy to reach or exceed the speed of light. This is described by Einstein's theory of special relativity.
However, let's assume for the sake of hypothetical discussion that the radioactive material could somehow travel at the speed of light. In that case, time dilation effects would come into play. According to special relativity, time slows down for objects in motion relative to an observer at rest.
For an outside observer, time would appear to stop for the object traveling at the speed of light. From the perspective of the moving object, time would appear to pass normally. However, due to the extreme time dilation, the concept of "decay" as we know it might not be applicable.
Decay of radioactive material is a probabilistic process governed by the half-life of the material. The half-life represents the time it takes for half of the radioactive atoms in a sample to decay. In the case of an object moving at the speed of light, time would essentially stand still from an outside observer's perspective, so it is unclear how the decay process would manifest.
It's important to note that this hypothetical scenario of an object moving at the speed of light goes beyond the limits of our current understanding and the laws of physics as we know them.