c$)$ If we apply the same logic for the Mo diffusion inside $U{O}_2-5\%$ Mo fuel $($microcell fuel pellet$),$can you estimate whether Mo would be rich at the grain boundaries or poor at the grain boundaries? When you answer this, consider the following: $1)$ should we use atomic radii or ionic radii for Mo$?2)$ Based on the radii, would Mo be interstitial defects or substitutional defects? $3)$ Would Mo be rich or poor at the grain boundaries?