<p>They grapple with radioactive decay inside the reactor, massive volumes of contaminated water and soil, and hot spots found far from the restricted zone surrounding Fukushima Daiichi.<br><br>These are just a few of the major challenges Japanese officials face eight months after the world's second-worst nuclear accident.<br><br>A team of radiation experts dispatched by the <a href="http://www.iaea.org/">International Atomic Energy Agency</a> (IAEA), shown here donning protective suits as they examined reactor unit 3 on October 11, warned Japan to "<a href="http://www.iaea.org/newscenter/focus/fukushima/pre_report.pdf">avoid over-conservatism</a>" in its cleanup choices. Meanwhile, the government and <a href="http://www.tepco.co.jp/en/index-e.html">TEPCO</a> (the Tokyo Electric Power Company) face constant pressure from communities and citizen groups who worry remediation efforts are not aggressive enough.</p><p>(Related: "<a href="http://ngm.nationalgeographic.com/2011/12/japan-nuclear-zone/craft-text">Japan's Nuclear Refugees</a>")</p><p>The focus of the workers has changed substantially since the first weeks after the March 11 tsunami inundated the coastal power plant, crippling its &nbsp;crucial back-up cooling system, and in turn causing explosions and radioactive emissions. The initial releases of radioactive material amounted to <a href="http://www.iaea.org/newscenter/news/2011/fukushimafull.html">about 10 percent</a> of the fallout from Chernobyl; still, emissions were high enough that Fukushima joined that 1986 disaster in Ukraine as the only incident to be classified a "major accident" on the internationally accepted event scale.<br><br>(Related: "<a href="http://news.nationalgeographic.com/news/energy/2011/04/110426/chernobyl-25-years-liquidators-pictures/">Pictures: 'Liquidators' Endured Chernobyl 25 Years Ago</a>")<br><br>More than 80,000 households within a 12.4-mile (20-kilometer) radius were evacuated, and a frenzied effort to alleviate pressure and heat buildup in the facility consumed workers for weeks.<br><br>(Related: "<a href="http://news.nationalgeographic.com/news/energy/2011/03/pictures/110323-inside-fukushima-daiichi-japan/">Photos: Rare Look Inside Fukushima Daiichi</a>")<br><br>Now, outsiders agree that conditions inside the plant are relatively stable, but unknowns remain.<br><br>On November 2, for example, TEPCO workers detected radioactive elements that indicated <a href="http://www.nytimes.com/2011/11/03/world/asia/bursts-of-fission-detected-at-fukushima-reactor-in-japan.html?_r=2&amp;src=recg">bursts of nuclear fission</a> in the Unit 2 reactor. The team injected boron water into the reactor to slow what workers feared was a runaway reaction. But Japanese authorities later concluded that spontaneous fission as part of <a href="http://www.iaea.org/newscenter/news/2011/fukushimareport01.html">radioactive decay</a> had occurred, not the chain reaction of nuclear fission. <br><br>"It just shows there is still very limited knowledge of the state of the material within the reactors," says Edwin Lyman, senior scientist specializing in global security for the nonprofit <a href="http://www.ucsusa.org/nuclear_power/">Union of Concerned Scientists</a>.<br><br><em>-Jeff Smith</em><br><br>(Related: "<a href="http://news.nationalgeographic.com/news/energy/2011/03/pictures/110318-japan-reactors-fukushima-nuclear-power-plant-pictures-radiation/">Photos: Japan's Reactors Before And After</a>")<br><br>(Related: "<a href="http://news.nationalgeographic.com/news/energy/2011/04/pictures/110426-chernobyl-25th-anniversary-wildlife/">Animals Inherit a Mixed Legacy at Chernobyl</a>")</p><p><em>This story is part of a </em><a href="http://news.nationalgeographic.com/news/energy" target="_blank"><em>special series</em></a><em> that explores energy issues. For more, visit <a href="http://www.greatenergychallenge.com/" target="_blank">The Great Energy Challenge</a></em>.</p>

Surveying a Reactor’s Ruin

They grapple with radioactive decay inside the reactor, massive volumes of contaminated water and soil, and hot spots found far from the restricted zone surrounding Fukushima Daiichi.

These are just a few of the major challenges Japanese officials face eight months after the world's second-worst nuclear accident.

A team of radiation experts dispatched by the International Atomic Energy Agency (IAEA), shown here donning protective suits as they examined reactor unit 3 on October 11, warned Japan to "avoid over-conservatism" in its cleanup choices. Meanwhile, the government and TEPCO (the Tokyo Electric Power Company) face constant pressure from communities and citizen groups who worry remediation efforts are not aggressive enough.

(Related: "Japan's Nuclear Refugees")

The focus of the workers has changed substantially since the first weeks after the March 11 tsunami inundated the coastal power plant, crippling its  crucial back-up cooling system, and in turn causing explosions and radioactive emissions. The initial releases of radioactive material amounted to about 10 percent of the fallout from Chernobyl; still, emissions were high enough that Fukushima joined that 1986 disaster in Ukraine as the only incident to be classified a "major accident" on the internationally accepted event scale.

(Related: "Pictures: 'Liquidators' Endured Chernobyl 25 Years Ago")

More than 80,000 households within a 12.4-mile (20-kilometer) radius were evacuated, and a frenzied effort to alleviate pressure and heat buildup in the facility consumed workers for weeks.

(Related: "Photos: Rare Look Inside Fukushima Daiichi")

Now, outsiders agree that conditions inside the plant are relatively stable, but unknowns remain.

On November 2, for example, TEPCO workers detected radioactive elements that indicated bursts of nuclear fission in the Unit 2 reactor. The team injected boron water into the reactor to slow what workers feared was a runaway reaction. But Japanese authorities later concluded that spontaneous fission as part of radioactive decay had occurred, not the chain reaction of nuclear fission.

"It just shows there is still very limited knowledge of the state of the material within the reactors," says Edwin Lyman, senior scientist specializing in global security for the nonprofit Union of Concerned Scientists.

-Jeff Smith

(Related: "Photos: Japan's Reactors Before And After")

(Related: "Animals Inherit a Mixed Legacy at Chernobyl")

This story is part of a special series that explores energy issues. For more, visit The Great Energy Challenge.

Photograph courtesy Giovanni Verlini, IAEA

Pictures: The Nuclear Cleanup Struggle at Fukushima

Radioactive decay, contaminated water, soil, and hot spots pose challenges for Japan eight months after the world's second-worst nuclear accident, at Fukushima.

Read This Next

The science behind seasonal depression
These 3,000-year-old relics were torched and buried—but why?
How the Holocaust happened in plain sight

Go Further

Subscriber Exclusive Content

Why are people so dang obsessed with Mars?

How viruses shape our world

The era of greyhound racing in the U.S. is coming to an end

See how people have imagined life on Mars through history

See how NASA’s new Mars rover will explore the red planet

Why are people so dang obsessed with Mars?

How viruses shape our world

The era of greyhound racing in the U.S. is coming to an end

See how people have imagined life on Mars through history

See how NASA’s new Mars rover will explore the red planet

Why are people so dang obsessed with Mars?

How viruses shape our world

The era of greyhound racing in the U.S. is coming to an end

See how people have imagined life on Mars through history

See how NASA’s new Mars rover will explore the red planet