The JNN/TBS cam has displayed the demolition of walls in unit 4. A large amount of dust was generated by this operation. The concrete in these walls have been subject to neutron bombardment for the past 15 months. This makes the chemical elements in this material radioactive. The main isotopes to be found in this dust, among others, would include cobalt-60, europium-152 and -154, cesium-134, scandium-46, zinc-65, barium-133, and manganese-54.
Majia Nadesan has speculated that this operation was taped and shown at a later time. The Tepco cam showed no such activity at the time. So it is possible that the huge radiation spike in Fukushima prefecture on the 24th was due to dust from this operation. It is also possible that there was a criticality involved. Unit 2, which has been having rising hydrogen levels, has shown an increase in emissions of krypton-85. Professor Koide from Kyoto University warned about a re-criticality in the unit 2 corium on the 21st.