LONDON — An Iranian ship captured by Somali pirates carried sealed containers of a powdery substance believed to be nuclear or chemical weapons agents. Western intelligence agencies have been monitoring the capture of the Iran Deyanat, seized by Somali pirates on Aug. 21. The cargo ship, owned and operated by the state-owned Islamic Republic of Iran Shipping Lines, or IRISL, contains sealed cargo thought to be linked to the death of 16 pirates.