Dressed in a hazardous materials suit, full-face mask and hard hat, Japan's prime minister, Shinzo Abe, left his audience in no doubt: "The future of Japan," he said, "rests on your shoulders. I am counting on you."

Abe's exhortation, delivered during a recent visit to the Fukushima Daiichi nuclear power plant, was only heard by a small group of men inside the plant's emergency control room. But it was directed at almost 6,000 more: the technicians and engineers, truck drivers and builders who, almost three years after the plant suffered a triple meltdown, remain on the frontline of the world's most dangerous industrial cleanup.

Yet as the scale of the challenge has become clearer with every new accident and radiation leak, the men working inside the plant are suffering from plummeting morale, health problems and anxiety about the future, according to insiders interviewed by the Guardian.

Even now, at the start of a decommissioning operation that is expected to last 40 years, the plant faces a shortage of workers qualified to manage the dangerous work that lies ahead.

The hazards faced by the nearly 900 employees of Tokyo Electric Power [Tepco] and about 5,000 workers hired by a network of contractors and sub-contractors were underlined this month when six men were doused with contaminated water at a desalination facility.

The men, who were wearing protective clothing, suffered no ill health effects in the incident, according to Tepco, but their brush with danger was a sign that the cleanup is entering its most precarious stage since the meltdown in March 2011.

Commenting on the leak, the head of Japan's nuclear regulator, Shunichi Tanaka, told reporters: "Mistakes are often linked to morale. People usually don't make silly, careless mistakes when they're motivated and working in a positive environment. The lack of it, I think, may be related to the recent problems."

Japan's prime minister, Shinzo Abe, wearing a red helmet, during a tour of the Fukushima Daiichi plant. Photograph: AP

The radiation spill was the latest in a string of serious water and radiation leaks, which have raised fears over the workers' state of mind – and Tepco's ability to continue the cleanup alone.

According to sources with knowledge of the plant and health professionals who make regular visits, the slew of bad news is sapping morale and causing concern, as the public and international community increase pressure on Japan to show demonstrable progress in cleaning up the world's worst nuclear accident since Chernobyl.

"Very little has changed at Fukushima Daiichi in the past six months," said Jun Shigemura, a lecturer in the psychiatry department at the National Defence Medical College who heads of a team of psychologists that counsels Fukushima plant workers. "Tepco is doing its best to improve matters, but you can see that the situation is severe."

Shigemura is most concerned about the 70% of Tepco workers at Fukushima Daiichi who were also forced to evacuate their homes by the meltdown. They have yet to come to terms with that loss and many live away from their families in makeshift accommodation near the plant.

"They were traumatised by the tsunami and the reactor explosions and had no idea how much they had been irradiated," Shigemura said. "That was the acute effect but now they are suffering from the chronic effects, such as depression, loss of motivation and issues with alcohol."

Their anxiety is compounded by uncertainty over the future of their embattled employer. Tepco is coming under mounting pressure to resolve the worsening water crisis at Fukushima Daiichi, which recently prompted the government to step in with half a billion dollars (£312m) to help contain the build-up of toxic water.

Its ability to stem the water leaks by the time Tokyo hosts the Olympics in 2020 – as promised by Abe – could be hampered by a looming labour shortage.

As Tepco was reducing costs and attempting to calm public anger over its handling of the crisis, it imposed a 20% pay cut for all employees in 2011. From a total workforce of 37,000, 1,286 people left the firm, between April 2011 and June this year. The firm did not hire any employees in fiscal 2012 and 2013.

The utility plans to take on 331 employees next April, according to Mayumi Yoshida, a Tepco spokeswoman. "[The employment] system will change so it will be easier for talented employees to gain promotion and for unproductive employees to be demoted," she said.

But there is little the firm can do about the departure of experienced workers, forced to leave after reaching their radiation exposure limit.

Tepco documents show that between March 2011 and July this year, 138 employees reached the 100-millisievert [mSv] threshold; another 331 had been exposed to between 75 mSv and 100 mSv, meaning their days at the plant are numbered. Those nearing their dose limit have reportedly been moved to other sites, or asked to take time off, so they can return to work at Fukushima Daiichi at a later date.

Some workers have left because of exhaustion and stress, while others have decided to find work closer to their displaced wives and children.

"They are less motivated and are worried about continuing to work for a firm that might not exist in a decade from now," Shigemura said.

Tepco employees wait for a bus at J Village, a football training complex now serving as an operation base for those battling Japan's nuclear disaster. Photograph: Reuters

Workers who have stayed on do so in the knowledge that they risk damaging their health through prolonged exposure to radiation and in accidents of the kind that occurred this week.

Earlier this year, Tepco said that 1,973 workers, including those employed by contractors and subcontractors, had estimated thyroid radiation doses in excess of 100 mSv, the level at which many physicians agree the risk of developing cancer begins to rise.

"These workers may show a tiny increased risk of cancer over their lifetimes," said Gerry Thomas, professor of molecular pathology at Imperial College, London University. "One hundred millisieverts is the dose we use as a cut-off to say we can see a significant effect on the cancer rate in very large epidemiology studies. The numbers have to be large because the individual increase is minuscule."

But she added: "I would be far more worried about these workers smoking or feeling under stress due to the fear of what radiation might do to them. That is much more likely to have an effect on any person's health."

While Thomas and other experts have cautioned against reaching hasty conclusions about a possible rise in thyroid cancer among Fukushima Daiichi workers, there is little doubt that their punishing work schedule, performed under the international spotlight, is taking a toll on their health.

"I'm particularly worried about depression and alcoholism," said Takeshi Tanigawa, a professor in the department of public health at Ehime University in western Japan. "I've seen high levels of physical distress and symptoms of post-traumatic stress disorder."

Many of the casual labourers employed by subcontractors live in cheap accommodation in places such as Yumoto, a hot-spring resort south of the exclusion zone around the plant. The number of workers has declined in the past year amid complaints from hoteliers and inn-keepers about drink-fuelled fights. These days, more seem to prefer the bars and commercial sex establishments of nearby Onahama port.

A 42-year-old contract worker, who asked not be named, confirmed that alcohol abuse had become a problem among workers. "Lots of men I know drink heavily in the evening and come to work with the shakes the next day. I know of several who worked with hangovers during the summer and collapsed with heatstroke."

"There isn't much communication between workers. People want to look after number one. Newcomers are looked down on by their colleagues and some don't really know how to do their jobs."

Another worker, who spoke on condition of anonymity, said he had seen hungover colleagues collapse with heatstroke just minutes after beginning work.

Tepco's logo at its headquarters in Tokyo. From a workforce of 37,000, 1,286 people left the firm between April 2011 and June this year. Photograph: Yuriko Nakao/Reuters

In the long term, Tepco and its partner companies will struggle to find enough people with specialist knowledge to see decommissioning through to the end, according to Yukiteru Naka, a retired engineer with General Electric who helped build some of Fukushima Daiichi's reactors.

"There aren't enough trained people at Fukushima Daiichi even now," he said. "For Tepco, money is the top priority – nuclear technology and safety come second and third. That's why the accident happened. The management insists on keeping the company going. They think about shareholders, bank lenders and the government, but not the people of Fukushima."

Naka, who runs a firm in Iwaki, just south of Fukushima Daiichi, that provides technical assistance to Tepco, said the lack of expertise afflicts the utility and general contractors with a pivotal role in the cleanup.

"Most of their employees have no experience of working in conditions like these, and all the time their exposure to radiation is increasing," he said. "I suggested to Tepco that it bring in retired workers who said they were willing to help, but the management refused."

Faced with labour shortages and a string of accidents, Tepco has in recent weeks come under pressure to accept more specialist help from overseas. At the start of this month, Shinzo Abe, told an international science conference in Kyoto: "My country needs your knowledge and expertise."

But this apparent spirit of openness is unlikely to turn the decommissioning operation into a genuinely international effort, said Ian Fairlie, a London-based independent consultant on radioactivity in the environment. "Japanese officials ask for help, but Tepco and the government are not in the business of saying: 'This is serious, please come and help us,'" he said.

Tepco's unshakable belief in its ability to complete the decommissioning operation rules out any meaningful co-operation, even with Japanese government officials. "Tepco has always wanted to do its own thing," said Akihiro Yoshikawa, a Tepco employee of 14 years who recently left the company. "It doesn't want the government stepping in and telling it what to do; it just wants the government's money."

Yoshikawa said the spirit of resilience his former colleagues had displayed in the aftermath of the accident had turned to despondency amid mounting criticism at home and abroad, forcing younger workers to leave and older ones to take early retirement. "They felt like they were being bullied, even though they were putting their lives at risk," he said.

"Tepco is spending its money on fixing the technical problems, but it also needs people to carry out that work. I'm very worried about the labour shortage. If they don't do something about it soon, the employment system at Fukushima Daiichi will collapse first, not the plant."

For the thousands of non-Tepco employees hired across Japan to perform backbreaking, dangerous work for contractors and subcontractors, the lure of earning decent money in return for working close to lethal levels of radiation has proved an illusion.

Once money for accommodation has been subtracted from their wages, labourers are typically left with a few thousand yen at the end of each day. In some cases, smaller companies withhold danger money, which can amount to more than half of a worker's daily wage because, they say, they need the extra cash to keep their business afloat.

The poor pay has forced growing numbers of men to quit and take up jobs decontaminating the area around the plant, for which they can earn similar momey but with much less exposure to radiation.

"The real work at Fukushima Daiichi is being done by the general contractors, with the smaller companies picking up the crumbs," Yoshikawa said. "They outbid each other for contracts and so end up with less money to pay their workers. They have no choice but to hire cheap labour."

Conditions for Tepco workers living in J Village – a football training complex just south of Fukushima Daiichi – have only recently improved.

For two years after the disaster, those living in prefabricated units at J Village had to walk hundreds of metres to use communal toilets at night. Tepco belatedly installed private toilets earlier this year after the firm's incoming president, Naomi Hirose, heeded health experts' warnings that the lack of facilities was compromising employees' health.

"The managers at Tepco headquarters have little idea of how their Fukushima Daiichi employees live," said Tanigawa, the public health professor. "The company's management is focused on the compensation problem and doesn't want to be accused of only looking after its own when there are still evacuees who haven't been compensated."

But as concern grows over Tepco's ability to address the myriad technical challenges facing Fukushima Daiichi – starting next month with the removal of 1,300 spent fuel assemblies from the top of reactor No 4 – the unfolding human crisis is being largely ignored.

There is still no full-time mental health counselling available at the plant, said Shigemura, whose team visits about once a month to talk to workers and administer pharmacological treatments. "That amazes me," he said.

"Tepco workers worry about their health, but also about whether Tepco will take care of them if they fall ill in the future. They put their lives and their health on the line, but in the years to come, they wonder if they will just be discarded."