The Mouvement mondial des partisans de la paix, or Comité permanent mondial des intellectuels pour la paix according to Touchard in Le Siècle des Excès, stood for "the absolute prohibition of atomic weapons" with the Stockholm Appeal on March, 19th 1950. Yet, according to the Wikipedia page of the Movement, it never condemned Soviet atomic experiments. I assume that this movement was of communist inspiration and that was the reason of justifying it. Considering the aura of its call, I wondered how it could assume and justify such a contradiction.
Because the movement was set up (or very quickly taken over) by communists:
Pacifism has always found great resonance in communist thought and action from its origins. Thus, the Amsterdam Pleyel movement in the 1930s foreshadowed the postwar communist pacifist movement.
Partly from the French Resistance (the future French Peace Movement ), partly from the gathering of intellectuals against the atomic weapon (the future World Peace Council ), the Global Movement of Peace Supporters (generic expression global) will be rapidly controlled by the international communist movement ( Kominform).
- google translate of the above
I placed the relevant text in bold
This was a common tactic used by the Soviet Union if they couldn't militarily match their opponents.