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"   The headlines are dire: Two and a half years after the nuclear disaster at Fukushima Dai-ichi, contaminated water is leaking from the plant at a faster rate than most people realized, getting into the groundwater and the Pacific Ocean. The Japanese government is getting involved, saying it will no longer leave the issue with Tepco, the utility that has been trying to stem the leaks. But how did this happen, and what does it mean? The great earthquake that caused the tsunami also rattled the reactor plants, subjecting them to far more stress than they were designed to withstand. The shock opened up seams in the reactors and water drained from the cores. In the opening days of the disaster Japan&amp;#039;s focus was on pouring water into the reactors to keep them cool. But it should have been obvious even then that water had to be leaking from the damaged reactors at a prodigious rate. In fact, the only radiation injuries reported to date from Fukushima were water-related: Two workers were wading through a flooded basement area when radioactivity-laced water filled their boots, giving them radiation burns to their lower legs. In those early days there were concerns about contaminated water entering the sea, and, in fact, urgent efforts were made to stem this problem. All of this took place about two years ago. And until recently the water problems at the Fukushima Dai-ichi site passed from the world&amp;#039;s attention. But within the last few weeks the problem has come to the fore again, as contaminated groundwater continues to reach the ocean, and as a bevy of radioactive water holding tanks have been found to be leaking. To add to Fukushima&amp;#039;s woes, there have also been reports of contaminated groundwater, blocked from its path to the ocean, rising toward the surface and threatening to swamp the site. The Spread Groundwater everywhere does pretty much the same thing. It flows from areas of high pressure toward areas of low pressure. In some cases this can mean water taking a convoluted path to get to where it&amp;#039;s going. Near the sea, the path is usually simple.  The water that leaked out of the Fukushima Dai-ichi reactors sank through the ground until it reached the water table, and then merged with the water there and began flowing to the sea. (This is in addition to manmade flow paths\u2014pipes, culverts, etc.\u2014through which some water has flowed). This groundwater brings with it any of the dissolved radionuclides it carries. Not only that, but any attempt to block its path can only be temporary\u2014eventually the water will back up, as water will behind any dam, until it flows around the obstruction or overtops it. This is why contaminated groundwater is rising toward the surface. It has effectively been dammed and it&amp;#039;s acting like any dammed stream. There has been one recent suggestion to stop the contaminated groundwater: freezing the soil from the surface to the bedrock. Exotic as this sounds (it would involve perforating the ground with liquid nitrogen\u2013carrying pipes), soil freezing is an established technology. I used a similar technology in the Navy in the 1980s when we used liquid nitrogen jackets to freeze ice plugs in reactor-plant piping to enable downstream work. These ice plugs\u2014or ice fences in the case of soil freezing\u2014will last as long as liquid nitrogen continues to flow. Even if the flow of nitrogen is interrupted it will take the soil some time to thaw. The other problem is contaminated water leaking from storage tanks. Some of this water is highly radioactive. One report notes that radiation dose rates are up to 100 mSv/hr (10 rem/hr) in close proximity to the tanks, mostly from beta radiation (which only penetrates about 1 cm (1/2 inch) into the body). This is a high dose rate compared with natural radiation levels, and from a regulatory standpoint, it&amp;#039;s substantial. The annual allowable radiation dose to radiation workers in Japan is about 20 mSv (2 rem) to the whole body, and skin dose is limited to 500 mSv (50 rem) annually. So a person close to the tanks could receive their annual skin dose limit in about 5 hours. On the other hand, radiation sickness begins to appear at a dose of about 1 Sv (100 rem), and skin burns start to occur at about 3 Sv (300 rem) of skin exposure. Since the beta radiation given off by the water won&amp;#039;t penetrate the body, it can&amp;#039;t give a person radiation sickness, but a person who splashed some water onto their skin and forgot to clean it off can get burned\u2014just what happened to the workers in the early days after the accident. It might not hurt someone instantly, but the water in these tanks is a serious source of radioactivity and a potential problem. The Impact The environmental impact of all of this is hard to quantify without extensive and long-term environmental sampling. But there are a few extreme examples that might be worth considering. For example, what would happen if all of the radioactivity remains in the local area? And what would happen if it flowed unimpeded into the ocean? The first case is not as implausible as it might seem. Clay minerals are very good at latching on to contaminants and holding them tight. As the contaminated water flows through the soil, it is continually losing contaminants to the soil. Sediments on the seafloor also latch on to contaminants. So it is plausible to believe that a substantial amount of contamination will remain in the immediate vicinity of the reactor site, with minor amounts of radioactivity traveling somewhat farther, carried by organisms that disturb the sediments or migrate farther afield. If this were to happen, it could well be damaging to the ecosystem in that area. But in this case, the damage would extend only as far as the nuclides spread. If they remain limited in extent, then so will their harmful effects. It&amp;#039;s also worth keeping in mind that there is a finite amount of radioactivity from Fukushima, and the farther it spreads, the lower the risk it poses. As the radius doubles, the dose rate drops by a factor of four or more. If, on the other hand, the radioactivity is all well-dissolved and disperses into the wider ocean, then the dose rate will be quite low because there won&amp;#039;t be a large concentration of radioactivity anywhere. In this case we might be able to detect trace concentrations of radioactivity at great distances from the reactor site but not enough to cause harm. This is the case with tuna: A number of tuna have been analyzed and found to contain measurable amounts of radioactive cesium, but the concentrations are far too low to cause harm (just as I can measure naturally occurring radioactive potassium in a bunch of bananas\u2014or in a person\u2014but it&amp;#039;s not harmful). The Bottom Line \u2028It&amp;#039;s not surprising that radioactive water from the damaged Fukushima Dai-ichi nuclear reactors has entered the environment, both from leaking collection tanks as well as from contaminated groundwater. And while it might be possible to create a barrier to slow the progress of contaminated water, in the meantime radioactivity has entered and will continue to enter the environment. If this radioactivity remains local, then the local marine environs might be damaged, but the larger environment should remain unscathed. On the other hand, if the radioactivity spreads far and wide, the local environment should emerge relatively intact, and even the wider environment should not fare badly, due to the dispersion of a finite amount of radioactivity into a very large ocean. "^^<http://www.w3.org/2001/XMLSchema#string>
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