It must be from the explosion! Or from something TEPCO has done since the accident, whatever it is!
If TEPCO is to be believed, TEPCO has been hiding the damages for at least 10 years; the oldest damage was from 25 years ago.
According to Yomiuri Shinbun (11/12/2013), that's not clear, and you would be excused if you thought the damages were recent (after March 11, 2011).
From Yomiuri Shinbun (11/12/2013):
On November 12, TEPCO disclosed that there were three fuel assemblies out of 1533 fuel assemblies in the Spent Fuel Pool of Reactor 4 at Fukushima I Nuclear Power Plant that were deformed and would be difficult to remove. The removal of fuel assemblies from the Reactor 4 SFP will start this month.
The three fuel assemblies has slight deformation or damage caused by the past work [in the SFP]. No leak of radioactive materials has been observed. TEPCO will transport other fuel assemblies first, while they figure out how to remove and transport these three fuel assemblies. Nuclear Regulation Authority has already approved the fuel removal, and the work will start within this month as scheduled.
So, are these fuel assemblies new? used? How long ago was the "past work"?
I found the answer in Fukushima Minyu, one of the local newspapers in Fukushima Prefecture. It was long before the start of the March 11, 2011 nuclear accident, with the oldest "past work" 25 years ago...
From Fukushima Minyu (11/13/2013; part):
According to TEPCO, one of the damaged fuel assemblies is bent at a 90-degree angle [literal meaning: bent in the shape of a Japanese character "く"; actual angle could be less]. It was bent 25 years ago when a mistake occurred in handling the fuel. The other two were found to be damaged 10 years ago; there are small holes on the outside from foreign objects.
TEPCO is investigating whether the damages to the fuel assemblies were made public when they were discovered.
According to Fukushima Minyu, the occasion was a visit by the delegation of the Fukushima Prefectural Council for Decommission Safety Monitoring. The council members include officials from the Fukushima prefectural government, municipalities around Fukushima I Nuclear Power Plant, and experts.
How do you bend a fuel assembly at a 90-degree angle, handling it in the spent fuel pool?
And how could Yomiuri call a fuel assembly bent at a 90-degree angle "slight deformation"? Slight?
For your information, there are 1533 fuel assemblies (each of which contains 50 to 60 fuel rods in channels inside the casing) in the Spent Fuel Pool of Reactor 4 at Fukushima I Nuclear Power Plant.
There used to be 1535 fuel assemblies there, but two new fuel assemblies were taken out from the Reactor 4 SFP last year for examination.
Of 1533 fuel assemblies currently in the Reactor 4 SFP, 202 of them are never-used, brand-new fuel assemblies. The rest, 1331 are used fuel assemblies. Of 1331, 783 are spent fuel assemblies that have been stored in the SFP at least for several years; 548 are the fuel assemblies that had been used in the Reactor Pressure Vessel until Reactor 4 went into extended maintenance to replace the core shroud. These 548 "hot" fuel assemblies were taken out of the Reactor Pressure Vessel and placed temporarily in the Spent Fuel Pool.