Download link: gentlespie-my.sharepoint.com/:u:/g/personal/gentlespie_gentlespie_onmicrosoft_com/EfbYz44QRNBFrzsU1f5vYwsBLxmQMkKz0FFUeT8ubkCvRg?download=1
Just extract the contents in your main War Thunder folder and it's required you own either of the KV-2's, the 1940 or 1939 one. Search "Nuclear" (without quotes) and you'll find the mission.