There is another worker who's been tweeting from Fukushima I Nuclear Power Plant besides "Happy". He is known as "Sunny", and despite the cheerful twitter handle, he is often more sarcastic and more critical of TEPCO than "Happy". Still, just like "Happy", "Sunny" is doing what he's been doing with the sense of responsibility and mission. (Both of which are sorely lacking in TEPCO's management, I have to say.)
In short, he says the leak of RO waste water from the storage tank(s) happened because TEPCO counted on ALPS, multi-nuclide removal system, too much.
Oops. The hot run of the system has been stopped when TEPCO found the leak and corrosion in the stainless steel tanks used in the system.
The following is "Sunny"'s take on the RO waste water leak:
"TEPCO's Bad but Cheap Strategy" [i.e. "You Get What You Pay For"]
Tanks are assembled with bolts and linings, as opposed to being welded using automatic welding machines like MIG and MAG. Shipbuilding uses automatic welding machines. Why didn't TEPCO use them? It's not the matter of speed [that's TEPCO's excuse] but cost. But if you build a large enough number of tanks you have extra capacities and the cost becomes lower. Why didn't TEPCO do that? It's because all their plans depended on ALPS.
If ALPS becomes operational, there will be excess tanks. So TEPCO didn't build tanks more than enough, because it would be waste of money. Welding the tanks would cost more money. However, the incident this time will cost yet more money for decontamination of the tank(s), investigation, and countermeasures. If the tanks were welded from the beginning, TEPCO wouldn't have needed to empty the water, decontaminate the tank(s) and weld.
The incident was bound to happen, because of TEPCO's cost cutting measures, their "our plan is always perfect" mentality, their risk management and their lack of knowledge in the field it is unfamiliar with. In other words, it is exactly the same as the Fukushima I Nuclear Power Plant accident. Besides, the incident this time was easily preventable. It is like riding a car without any seat belt, because it is "annoying".