Tensions are rising in Japan over radioactive water leaking into the Pacific Ocean from Japan's crippled Fukushima Daiichi nuclear plant, a breach that has defied the plant operator's effort to gain control.

Prime Minister Shinzo Abe on Wednesday called the matter “an urgent issue” and ordered the government to step in and help in the clean-up, following an admission by Tokyo Electric Power Company that water is seeping past an underground barrier it attempted to create in the soil. The head of a Nuclear Regulatory Authority task force told Reuters the situation was an "emergency." (See Pictures: The Nuclear Cleanup Struggle at Fukushima.”)

It marked a significant escalation in pressure for TEPCO, which has come under severe criticism since what many view as its belated acknowledgement July 22 that contaminated water has been leaking for some time. The government now says it is clear that 300 tons (71,895 gallons/272,152 liters) are pouring into the sea each day, enough to fill an Olympic-size swimming pool every eight days. (See related, “One Year After Fukushima, Japan Faces Shortages of Energy, Trust.”) While Japan grapples with the problem, here are some answers to basic questions about the leaks:

Q: How long has contaminated water been leaking from the plant into the Pacific?

But even after the immediate crisis eased, scientists have continued to find radioactive contamination in the waters off the plant. Ken Buesseler, a senior scientist with the Woods Hole Oceanographic Institution who has analyzed thousands of samples of fish from the area, said he’s continued to find the high levels of cesium-134, a radioactive isotope that decays rapidly. That indicates it’s still being released. "It’s getting into the ocean, no doubt about it," he said. "The only news was that they finally admitted to this." (See related: "Photos: Japan's Reactors Before And After.")

Q: How much and what sort of radiation is leaking from the plant into the Pacific?

TEPCO said Monday that radiation levels in its groundwater observation hole on the east side of the turbine buildings had reached 310 becquerels per liter for cesium-134 and 650 becquerels per liter for cesium-137. That marked nearly a 15-fold increase from readings five days earlier, and exceeded Japan’s provisional emergency standard of 60 becquerels per liter for cesium radiation levels in drinking water. (Drinking water at 300 becquerels per liter would be approximately equivalent to one year’s exposure to natural background radiation, or 10 to 15 chest X-rays, according to the World Health Organization. And it is far in excess of WHO’s guideline advised maximum level of radioactivity in drinking water, 10 becquerels per liter.) Readings fell somewhat on Tuesday. A similar spike and fall preceded TEPCO’s July admission that it was grappling with leakage of the radioactive water. (See related: "Would a New Nuclear Plant Fare Better than Fukushima?")

Scientists who have been studying the situation were not surprised by the revelation, since radiation levels in the sea around Japan have been holding steady and not falling as they would if the situation were under control. In a 2012 study, Jota Kanda, an oceanographer at Toyko University of Marine Science and Technology, calculated that the plant is leaking 0.3 terabecquerels (trillion becquerels) of cesium-137 per month and a similar amount of cesium-134. While that number sounds mind-boggling, it’s actually thousands of times less than the level of radioactive contamination that the plant was spewing in the immediate aftermath of the disaster, estimated to be from 5,000 to 15,000 terabecquerels, according to Buesseler. For a comparison, the atomic bomb dropped on Hiroshima released 89 terabecquerels of cesium-137 when it exploded. (See related: "Animals Inherit a Mixed Legacy at Chernobyl.")

Another potential worry: The makeup of the radioactive material being leaked by the plant has changed. Buesseler said the initial leak had a high concentration of cesium isotopes, but the water flowing from the plant into the ocean now is likely to be proportionally much higher in strontium-90, another radioactive substance that is absorbed differently by the human body and has different risks. The tanks (on the plant site) have 100 times more strontium than cesium, Buesseler said. He believes that the cesium is retained in the soil under the plant, while strontium and tritium, another radioactive substance, are continuing to escape. (Related: "Japan's Nuclear Refugees")

Q: Why is the plant continuing to leak?

There are at least a couple of possibilities. In an effort to cool and control the damaged reactors, TEPCO has pumped enormous amounts of water in and out. But that water is contaminated with radioactive material, and it has to go someplace. According to a recent report issued by the International Atomic Energy Agency, the plant operator has been storing highly contaminated water in seven underground storage ponds, which have a total of 60,000 tons (14.4 million gallons/54.5 million liters) of capacity. In April, TEPCO workers discovered that at least three of the ponds were leaking. The IAEA concluded that the company’s monitoring system, which hadn’t spotted the breach, was insufficient to spot such outflow. So it could be that the faulty containments, which are now being replaced, are the source of at least some of the contaminated water that’s gotten into the ocean.

But most experts seem to think that ordinary movement of groundwater probably is the real culprit. An estimated 400 tons (95,860 gallons/ 362,870 liters) of water streams into the basements of the damaged reactors each day. Keeping that water from continuing to flow into the ocean is crucial. As the IAEA noted in its report, "the accumulation of enormous amounts of liquids due to the continuous intrusion of underground water into the reactor and turbine buildings is influencing the stability of the situation."

"Big surprise—water does flow downhill," said Dr. Janette Sherman, a medical expert on radiation and toxic exposure who once worked as a chemist for the Atomic Energy Commission, the forerunner of today’s U.S. Nuclear Regulatory Commission. "If you’ve ever had a leak in your house during a storm, you know how hard it is to contain water. There’s a lot of water going into the plant, and it’s got to go someplace. It’s very hard to stop this."

Q: What can be done to stop the leaking?

According to TEPCO’s latest full status report on the cleanup of Fukushima Daiichi, issued in October 2012, the utility company already had put in place an array of measures to try to control the radioactive water. It built a groundwater bypass system, which tries to siphon off and reroute groundwater flowing down from the mountain side of the complex, before it can get into the basements of the reactor buildings and be contaminated. But that doesn’t seem to have made much of a dent in the problem. (See related: "Pictures: 'Liquidators' Endured Chernobyl 25 Years Ago.")

Plant workers also tried to create an underground barrier by injecting chemicals into the soil to solidify the ground along the shoreline of the Unit 1 reactor building. But TEPCO officials Tuesday said the water was seeping under or past this barrier. Officials also believe the water is rising to the surface, which is a troubling development because it could hasten leakage into the sea.

The company also continues to add to a massive tank farm on the site, with capacity to store about 400,000 tons (95 million gallons/360 million liters) of contaminated water, and is planning to add an additional 300,000 tons of capacity over the next three years. Unfortunately, TEPCO must deal with an ever-increasing amount of contaminated water—nearly 150,000 tons (35.9 million gallons/136 million liters) a year—so it’s inevitable that the company is going to run out of storage space.

That’s why TEPCO seems to be betting heavily on another solution—an elaborate state-of-the art system for filtering the accumulated water and removing radioactive materials from it. According to New Scientist, the new system supposedly can filter out 62 different radioactive substances. However, the April IAEA report noted that the filtering system is still a work in progress, and that in tests so far, "it has not accomplished the expected result" in terms of removing radioactive material from the water. Additionally, the system doesn’t remove tritium, which isn’t as radioactive as other materials in the water, but which still is a health hazard if it is inhaled or ingested. The Wall Street Journal recently reported that TEPCO hopes eventually to be able to discharge the cleansed water into the ocean, though that plan would likely meet intense opposition from local fishermen. Sherman, who has a chemistry background, said she’s skeptical that such a process could work on the enormous scale required. "You can precipitate these things out in the laboratory, but you’re talking about millions of gallons here," she explained.

In a July 26 press release, TEPCO also said it would continue construction of a shielding wall along the waterline, but that structure will not be finished until September 2014. Marine scientist Buesseler isn’t sure that will work, either. "You can build a dam, but eventually the water goes around it," he explained.

Q: How far is the radiation spreading, and how fast does it travel?

The initial gigantic deluge of contaminated water dispersed through the immediate Fukushima coastal area very quickly, according to a 2012 report by the American Nuclear Society. But it takes years for the contamination to spread over a wider area. A mathematical model developed by Changsheng Chen of the University of Massachusetts at Dartmouth and Robert Beardsley of the Woods Hole Oceanographic Institute found that radioactive particles disperse through the ocean differently at different depths. The scientists estimated that in some cases, contaminated seawater could reach the western coast of the United States in as little as five years. Buesseler thinks the process occurs a bit more rapidly, and estimates it might take three years for contamination to reach the U.S. coastline.

Q: What are the potential risks to humans, and who might be affected by the contamination?

This is a murky question, because it’s not that easy to determine whether health problems that may not show up for decades are caused by exposure to radioactive contamination. A report released in February by the World Health Organization, which was based upon estimates of radiation exposure in the immediate wake of the accident, concluded that it probably would cause "somewhat elevated" lifetime cancer rates among the local population. But figuring out the effect of years of exposure to lower levels of radioactive contamination leaking into the ocean is an even more complicated matter.

Minoru Takata, director of the Radiation Biology Center at Kyoto University, told the Wall Street Journal that the radioactive water doesn’t pose an immediate health threat unless a person goes near the damaged reactors. But over the longer term, he’s concerned that the leakage could cause higher rates of cancer in Japan.

Marine scientist Buesseler believes that the leaks pose little threat to Americans, however. Radioactive contamination, he says, quickly is reduced "by many orders of magnitude" after it moves just a few miles from the original source, so that by the time it would reach the U.S. coast, the levels would be extremely low. (See related, “Rare Video: Japan Tsunami.”)

Q: Will seafood be contaminated by the leaks?

As Buesseler’s research has shown, tests of local fish in the Fukushima area still show high enough levels of radiation that the Japanese government won’t allow them to be caught and sold for human consumption—a restriction that is costing Japanese fishermen billions of dollars a year in lost income. (But while flounder, sea bass, and other fish remained banned for radiation risk, in 2012 the Japanese government did begin allowing sales of octopus and whelk, a type of marine snail, after tests showed no detectable amount of cesium contamination.)

Buesseler thinks the risk is mostly confined to local fish that dwell mostly at the sea bottom, where radioactive material settles. He says bigger fish that range over long distances in the ocean quickly lose whatever cesium contamination they’ve picked up. However, the higher concentration of strontium-90 that is now in the outflow poses a trickier problem, because it is a bone-seeking isotope. "Cesium is like salt—it goes in and out of your body quickly," he explains. "Strontium gets into your bones." While he’s still not too concerned that fish caught off the U.S. coast will be affected, "strontium changes the equation for Japanese fisheries, as to when their fish will be safe to eat." (See related blog, “Safety Question on Fukushima Anniversary: Should Plants of the Same Design Have Filtered Vents?”)