It is one of the mysteries of Japan's ongoing nuclear crisis: How much damage did the 11 March earthquake inflict on the Fukushima Daiichi reactors before the tsunami hit?

Fukushima Daiichi nuclear power plant after Japan's earthquake and tsunami in March. Photograph: Reuters The stakes are high: if the earthquake structurally compromised the plant and the safety of its nuclear fuel, then every similar reactor in Japan may have to be shut down. With almost all of Japan's 54 reactors either offline (in the case of 35) or scheduled for shutdown by next April, the issue of structural safety looms over any discussion about restarting them.

Plant operator Tokyo Electric Power Co (Tepco) and Japan's government are hardly reliable adjudicators in this controversy. "There has been no meltdown," government spokesman Yukio Edano repeated in the days after 11 March. "It was an unforeseeable disaster," Tepco's then president Masataka Shimizu famously and improbably said later. Five months since the disaster, we now know that meltdown was already occurring as Mr Edano spoke. And far from being unforeseeable, the disaster had been repeatedly forewarned by industry critics.

Throughout the months of lies and misinformation, one story has stuck: it was the earthquake that knocked out the plant's electric power, halting cooling to its six reactors. The tsunami then washed out the plant's back-up generators 40 minutes later, shutting down all cooling and starting the chain of events that would cause the world's first triple meltdown.

But what if recirculation pipes and cooling pipes burst after the earthquake – before the tidal wave reached the facilities; before the electricity went out? This would surprise few people familiar with the 40-year-old reactor one, the grandfather of the nuclear reactors still operating in Japan.

Problems with the fractured, deteriorating, poorly repaired pipes and the cooling system had been pointed out for years. In September 2002, Tepco admitted covering up data about cracks in critical circulation pipes. In their analysis of the cover-up, The Citizen's Nuclear Information Center writes: "The records that were covered up had to do with cracks in parts of the reactor known as recirculation pipes. These pipes are there to siphon off heat from the reactor. If these pipes were to fracture, it would result in a serious accident in which coolant leaks out."

On 2 March, nine days before the meltdown, government watchdog the Nuclear Industrial Safety Agency (NISA) warned Tepco on its failure to inspect critical pieces of equipment at the plant, including recirculation pumps. Tepco was ordered to make the inspections, perform repairs if needed and report to NISA on 2 June. It does not appear, as of now, that the report has been filed.

The Independent has spoken to several workers at the plant who recite the same story: serious damage, to piping and at least one of the reactors, occurred before the tsunami hit. All have requested anonymity because they are still working at or connected with the stricken plant. Worker A, a maintenance engineer who was at the Fukushima complex on the day of the disaster, recalls hissing, leaking pipes.

"I personally saw pipes that had come apart and I assume that there were many more that had been broken throughout the plant. There's no doubt that the earthquake did a lot of damage inside the plant... I also saw that part of the wall of the turbine building for reactor one had come away. That crack might have affected the reactor."

The reactor walls are quite fragile, he notes: "If the walls are too rigid, they can crack under the slightest pressure from inside so they have to be breakable because if the pressure is kept inside... it can damage the equipment inside so it needs to be allowed to escape. It's designed to give during a crisis, if not it could be worse – that might be shocking to others, but to us it's common sense." Worker B, a technician in his late 30s who was also on site at the time of the earthquake, recalls: "It felt like the earthquake hit in two waves, the first impact was so intense you could see the building shaking, the pipes buckling, and within minutes I saw pipes bursting. Some fell off the wall...

"Someone yelled that we all needed to evacuate. But I was severely alarmed because as I was leaving I was told and I could see that several pipes had cracked open, including what I believe were cold water supply pipes. That would mean that coolant couldn't get to the reactor core. If you can't sufficiently get the coolant to the core, it melts down. You don't have to have to be a nuclear scientist to figure that out." As he was heading to his car, he could see that the walls of the reactor one building had started to collapse. "There were holes in them. In the first few minutes, no one was thinking about a tsunami. We were thinking about survival."

The suspicion that the earthquake caused severe damage to the reactors is strengthened by reports that radiation leaked from the plant minutes later. The Bloomberg news agency has reported that a radiation alarm went off about a mile from the plant at 3.29pm, before the tsunami hit.

The reason for official reluctance to admit that the earthquake did direct structural damage to reactor one is obvious. Katsunobu Onda, author of Tepco: The Dark Empire, explains it this way: A government or industry admission "raises suspicions about the safety of every reactor they run. They are using a number of antiquated reactors that have the same systematic problems, the same wear and tear on the piping." Earthquakes, of course, are commonplace in Japan.

Mitsuhiko Tanaka, a former nuclear plant designer, describes what occurred on 11 March as a loss-of-coolant accident. "The data that Tepco has made public shows a huge loss of coolant within the first few hours of the earthquake. It can't be accounted for by the loss of electrical power. There was already so much damage to the cooling system that a meltdown was inevitable long before the tsunami came."

He says the released data shows that at 2.52pm, just after the quake, the emergency circulation equipment of both the A and B systems automatically started up. "This only happens when there is a loss of coolant." Between 3.04 and 3.11pm, the water sprayer inside the containment vessel was turned on. Mr Tanaka says that it is an emergency measure only done when other cooling systems have failed. By the time the tsunami arrived and knocked out all the electrical systems, at about 3.37pm, the plant was already on its way to melting down.

Kei Sugaoka, who conducted on-site inspections at the plant and was the first to blow the whistle on Tepco's data tampering, says he was not surprised by what happened. In a letter to the Japanese government, dated 28 June 2000, he warned that Tepco continued to operate a severely damaged steam dryer in the plant 10 years after he pointed out the problem. The government sat on the warning for two years.

"I always thought it was just a matter of time," he says of the disaster. "This is one of those times in my life when I'm not happy I was right."

During his research, Mr Onda spoke with several engineers who worked at the Tepco plants. One told him that often piping would not match up to the blueprints. In that case, the only solution was to use heavy machinery to pull the pipes close enough together to weld them shut. Inspection of piping was often cursory and the backs of the pipes, which were hard to reach, were often ignored. Repair jobs were rushed; no one wanted to be exposed to nuclear radiation longer than necessary.

Mr Onda adds: "When I first visited the Fukushima Power Plant it was a web of pipes. Pipes on the wall, on the ceiling, on the ground. You'd have to walk over them, duck under them – sometimes you'd bump your head on them. The pipes, which regulate the heat of the reactor and carry coolant are the veins and arteries of a nuclear power plant; the core is the heart. If the pipes burst, vital components don't reach the heart and thus you have a heart attack, in nuclear terms: meltdown. In simpler terms, you can't cool a reactor core if the pipes carrying the coolant and regulating the heat rupture – it doesn't get to the core."

Tooru Hasuike, a Tepco employee from 1977 until 2009 and former general safety manager of the Fukushima plant, says: "The emergency plans for a nuclear disaster at the Fukushima plant had no mention of using seawater to cool the core. To pump seawater into the core is to destroy the reactor. The only reason you'd do that is no other water or coolant was available."

Before dawn on 12 March, the water levels at the reactor began to plummet and the radiation began rising. The Tepco press release published just past 4am that day states: "The pressure within the containment vessel is high but stable." There was one note buried in the release that many people missed: "The emergency water circulation system was cooling the steam within the core; it has ceased to function."

At 9.51pm, under the chief executive's orders, the inside of the reactor building was declared a no-entry zone. At around 11pm, radiation levels for the inside of the turbine building, which was next door to reactor reached levels of 0.5 to 1.2 mSv per hour. In other words, the meltdown was already underway. At those levels, if you spent 20 minutes exposed to those radiation levels you would exceed the five-year limit for a nuclear reactor worker in Japan.

Sometime between 4 and 6am, on 12 March, Masao Yoshida, the plant manager decided it was time to pump seawater into the reactor core and notified Tepco. Seawater was not pumped in until hours after a hydrogen explosion occurred, at roughly 8pm. By then, it was probably already too late.

Later that month, Tepco went some way toward admitting at least some of these claims in a report called "Reactor Core Status of Fukushima Daiichi Nuclear Power Station Unit One". The report said there was pre-tsunami damage to key facilities, including pipes.

"This means that assurances from the industry in Japan and overseas that the reactors were robust is now blown apart," said Shaun Burnie, an independent nuclear waste consultant who works with Greenpeace. "It raises fundamental questions on all reactors in high seismic risk areas."

As Mr Burnie points out, Tepco also admitted massive fuel melt 16 hours after loss of coolant, and seven or eight hours before the explosion in Unit One. "Since they must have known all this, their decision to flood with massive water volumes would guarantee massive additional contamination – including leaks to the ocean."

No one knows how much damage was done to the plant by the earthquake, or if this damage alone would account for the meltdown. But certainly Tepco's data and eyewitness testimony indicates that the damage was significant.

As Mr Hasuike says: "Tepco and the government of Japan have provided many explanations. They don't make sense. The one thing they haven't provided is the truth. It's time they did."