Japan's idle nuclear reactors will not be allowed to restart unless it is proven they can survive giant tsunamis and other extreme events, the country's government has said.

But with no timetable decided for two rounds of "stress tests" and electricity demand soon to reach its summer peak, concern is growing that Japan may experience power shortages at the hottest time of year.

The first round of tests will focus on reactors that have completed routine safety checks and would have already restarted had the accident at Fukushima Daiichi power plant not plunged Japan's nuclear industry into crisis.

The tests will examine the reactors' ability to withstand natural onslaughts of far greater force than previously factored into safety checks.

They will, for example, examine their ability to withstand an unusually severe event similar to the magnitude-9 earthquake and 15m tsunami that knocked out back-up generators at Fukushima Daiichi, leading to core meltdowns in three reactors.

In the second stage, all of Japan's nuclear power facilities will undergo a general safety assessment.

Only 19 of the country's 54 nuclear reactors are in operation due to delays in restarting those that have undergone maintenance checks or were shut down after the 11 March tsunami.

In addition, reactors that are operating could be forced to close immediately if they fail the first round of tests, the government has said in a statement.

As it attempts to calm public fears over safety, the government must contemplate a worst-case scenario in which every last nuclear reactor will be idle by next April as more come offline for regular inspections.

That would seriously compromise the country's ability to produce enough electricity and could result in power shortages lasting into the summer of 2012.

Major users of electricity have already been told they must cut peak power use by 15% from this month to avoid blackouts this summer.

Nuclear provided about 30% of Japan's energy before the crisis. Plans to increase supply to more than 50% by 2030 were ditched after the prime minister, Naoto Kan, ordered a review of energy policy and promised a much bigger post-Fukushima role for renewables.

"Safety and a sense of security are the top priority," said Yukio Edano, the chief cabinet secretary. "One the other hand the government must fulfil its responsibility to provide a stable supply of electricity … we will make every effort to secure that in the medium and long term."

Edano could not say when the first round of tests would begin, only that they would happen "soon" and be overseen by Japan's nuclear safety commission, which is considered more independent of the government than the nuclear safety agency.

Last week's surprise decision to conduct stress tests has caused confusion among the public and angered industry officials who want idle reactors restarted as soon as possible.

In June government officials, citing reviews by the nuclear safety agency, said reactors that had been shut down for regular checks were safe to restart.

At the time the economy minister, Banri Kaieda, said: "There is no problem regarding safety concerning the continued operation and restart of nuclear power stations."

On Friday Kan apologised for dithering over the tests, which will be modelled on those being conducted on reactors in European Union member states. "My instructions were inadequate and came too late," he said. "I take responsibility for that."

Officials in Fukushima prefecture said they had detected high levels of radioactive caesium in straw fed to cattle at a farm in Minamisoma, a town near the Fukushima Daiichi power plant.

Tests were carried out on feed and water samples from the farm after caesium at levels three to six times higher than the provisional legal limit of 500 becquerels per kilogram were found in 11 cows, according to the Tokyo metropolitan government.

The cows had been shipped to a meat packing plant in Tokyo and slaughtered but their meat did not go on sale, Kyodo news agency said.