Image copyright Tokyo Electric Power Co

It's more than five years since the earthquake and tsunami off the coast of Japan caused a huge leak of radioactive material into the world's oceans.

Workers battled to prevent the Fukushima nuclear plant going into complete meltdown and radiation levels rose by a factor of tens of millions.

However, a new report by Australian scientists has revealed that radiation in the Pacific Ocean is rapidly returning to normal and should be at its previous level by 2020. So what does this say about radiation and us?

Time has stood still around the Fukushima nuclear plant, with homes and possessions abandoned - perhaps forever.

Efforts to curb further leaks of radioactive water are ongoing: an underground frozen wall of soil is being constructed to try to minimise the amount of radioactive material that seeps out into the sea.

Huge challenges remain in the future as decontamination efforts continue and it's going to take several decades before the plant is fully decommissioned.

Image copyright Reuters Image caption More than 18,000 people died or went missing in the tsunami

The seafloor and harbour near Fukushima are still highly contaminated, meaning monitoring of radioactivity levels and sea life in that area must continue for years to come.

But some sort of normal is returning to the wider ocean.

Nuclear energy is an emotive issue - besides the political, environmental and economic arguments, some believe radioactivity has a psychological dimension that prods at our inner fears.

In terms of human evolution, it's not that long ago since we were hunter gatherers facing dangers all around us - from poisonous plants to predators.

Media playback is unsupported on your device Media caption Watch: Aerial footage of a Fukushima radioactive waste dump

Because we're hardwired to react to the dangers we can see, smell or taste, radioactivity - which is an invisible threat - perhaps has a particular resonance.

Human beings are particularly useless about being able to assess risk but surprisingly, there is a bunch of academics who study this stuff.

Measuring up

•A becquerel (Bq), named after French physicist Henri Becquerel, is a measure of radioactivity

•A quantity of radioactive material has an activity of 1Bq if one nucleus decays per second - and 1kBq if 1,000 nuclei decay per second

•A sievert (Sv) is a measure of radiation absorbed by a person, named after Swedish medical physicist Rolf Sievert

And it seems our perceptions of risk from radiation are somewhat fickle.

Since Fukushima, polling internationally has shown large declines in support for nuclear power in countries including Germany, France and Japan. Indeed, the German government decided to close down its nuclear plants, as a result of Fukushima.

But in the UK and US, there remain as many people in favour as are opposed to nuclear power in such polls.

Of course, for those who experienced Fukushima first hand it's a different story.

Many died as a result of the earthquake and tsunami, but, according to the World Nuclear Association, nobody died or suffered radiation sickness from the radiation itself.

Image copyright Getty Images Image caption While fearful members of the public have shunned some plant workers, others have sent them messages of support

There were acts of altruism - a group of retired engineers and other pensioners volunteered to go into the plant, arguing they should be facing the dangers of radiation, not the young.

Hundreds of thousands of children from Fukushima are being monitored for cancer, but experts believe there will be few extra cases because of the radioactivity released.

However, government figures suggest more than a thousand evacuees have died from causes "related to the disaster".

A UN report in 2014 said the most important health effect was on mental and social well-being, from the enormous impact of the accident and the fear and stigma related to radiation and uncertainty about ever going home.

As one professor put it: "Nobody has died from the radiation, but it may actually have killed their souls."

Listen to this report from The World This Week via BBC iPlayer.