Recent reports from Japan have revealed that there is radioactive contamination in the groundwater and that it’s headed seawards. To keep it from flowing into the sea the Japanese have tried to stop it in its tracks – in response the water table is rising and the contaminated groundwater is rising towards the surface. Not only that, but contamination levels in the groundwater have gone up dramatically – by a factor of over 100 in some cases. The questions are whether or not these changes are expected, why they are happening, and what they portend – in particular, whether or not they bode ill for the Japanese and the environment around the reactor plants.
To answer the first question, none of these changes are really unexpected. We know that tons of water have been poured into the reactor plants for the last few years to try to keep what remains of the reactor cores cool. We also know that there was severe damage – enough to drain water from the cores in the first place. And remember that in the early days following the accident the basements of the reactor plants were filled with contaminated water. Unless every single leak has been plugged (which is almost certainly not the case) we can expect that contaminated cooling water will be continuing to leak from the reactor plant into the basement and thence into the groundwater beneath the reactor plant. This might reflect an increase in the release of radionuclides from the reactor if, for example, there has been further damage opening another path for water to be released from the reactor plant. It could also reflect the dynamics of the local groundwater flow, or even changes in the local groundwater flow patterns. But I think it’s safe to say that this is NOT the result of some sort of re-criticality – first, because it’s implausible that the remnants of the reactor fuel can even achieve criticality again (think of all the design and precision engineering that goes into making a reactor that can achieve criticality by design, let alone an accidental start-up of the remnants of a ruined reactor core), and also because, if there was a miraculous criticality then the groundwater would also hold a number of short-lived radionuclides (e.g. I-131) that don’t seem to be there.
It’s also no surprise that groundwater levels seem to be rising towards the surface. While the notion of “underground rivers” is not quite accurate, water does flow through the interstices of the sediments of which soil is made. Near the ocean the water is most likely to be flowing towards the sea – if a barrier is put in its way then the groundwater will back up, just as it backs up behind a dam to form reservoirs and lakes. The only way to keep the groundwater from backing up and rising towards the surface is to pump it out at the same rate it’s accumulating behind the barrier – the longer the barrier the more volume will have to be pumped out (and treated or stored since it’s contaminated with radioactivity).
So the bottom line is that neither the increase in radioactivity concentrations nor the rising groundwater levels should be a surprise. But what about safety?
An easy answer is that this probably doesn’t affect human safety since nobody is likely to be drinking the groundwater anywhere in the area. But that’s a little too facile, so let’s try to dig a little deeper.
One way to look at it is to look at a concept called the Allowable Limit for Intake (abbreviated ALI). The ALI is the amount of a radionuclide that, if inhaled or ingested, will give a person a radiation dose of 5 rem (50 mSv). The ALI for Sr-90 is 30 µCi (about 1 MBq) – don’t worry too much about exactly what the units mean, they’re various methods of determining how much radiation is emitted by a radioactive object. TEPCO has reported that the groundwater in this area now has about 0.056 MBq of Sr-90 and other beta-emitting radionuclides – if we assume that all of the radioactivity in the groundwater is from Sr-90 then drinking about 20 liters of groundwater would give one an uptake of 1 ALI, and a radiation dose of 5 rem (50 mSv) to the body. On average, we drink about 4 liters of water daily, so it would take 5 days to drink enough water to reach a dose of 5 rem, or about 1 rem (10 mSv) per day.
So taking this a little further, drinking this water for an entire year would expose a person to a dose of over 300 rem (3 Sv). This is a high exposure – more than is safe. Again, this assumes that all of the radioactivity is from Sr-90 – in reality there are other nuclides present, but even so this water should not be consumed. But for those living in the real world (i.e. not on the site and not drinking the groundwater), it looks as though the health effects will be minimal since the radiation dose outside of the immediate area is very low (this is consistent with the readings I got when I was in the area a month or so after the accident, and is also consistent with what we’ve seen in the Ukraine and Byelorussia in the aftermath of the Chernobyl accident).
It’s also worth wondering about the impact on the environment. This one’s a little harder to work through because it really depends on the exact levels of the various nuclides, the types of sediment they encounter underground and in the harbor, the rate of groundwater flow into the sea, and the characteristics of the flow of seawater from the harbor into the ocean (and onwards).
Consider the importance of the sediments alone (and here I should confess that, until I took graduate classes in soil mineralogy and clay mineralogy I thought there was likely nothing more boring than studying sediments – I was wrong). Some types of clay minerals latch onto some radionuclides and never let them go – radioactivity never spread all that far from the Kursk or from the sunken American subs (Thresher and Scorpion) while other clays don’t hold onto nearly as much; while some chemical elements are much more likely to be immobilized than others. So without knowing these details about the nuclides and the sediments we really can’t make more than an educated guess as to the mobility of these nuclides in the environment – without knowing that, we also can’t really determine the exact environmental effects. But we can think of some extremes and figure that reality is somewhere in between.
One extreme would be to assume that all of the radioactivity stays in the local area – that it isn’t very environmentally mobile. In this case, radionuclide concentrations in the immediate vicinity of the reactor site would be fairly high – possibly high enough to harm the local organisms – but radiation dose further afield would be virtually nil.
The other extreme would be that the radioactivity disperses into the open ocean and doesn’t stick around the local area. In this case we’d see radioactivity spreading far and wide, but nowhere would it be concentrated to level that could cause harm. And this is actually not impossible – airborne radionuclides from Fukushima (and Chernobyl too, for that matter) were detected throughout the Northern Hemisphere, but in such trace amounts that they posed no harm to anyone. Part of the reason for this is that our radioactivity detection technologies are so advanced that we can detect the natural radioactivity in the human body (I have a detector that can get noticeable counts from a pack of cigarettes or from a bunch of bananas) – but at such low levels that there is no risk. By analogy, I can measure the speed of a snail inching across the floor and I can calculate its kinetic energy – but the snail is far from deadly.
Reality is likely somewhere between these extremes. If we think about the Chernobyl accident we can see some similarities. Trace amounts of radioactivity dispersed into the atmosphere and had a global reach, even if there was no discernable health or environmental impact outside the immediate vicinity of Chernobyl. Similarly, it is not unreasonable to expect that there may well be local environmental effects – likely confined to the vicinity of the reactors – but the impact is unlikely to be global.
So there’s mixed news here. First is that the radiological conditions – at least with respect to radioactivity in the groundwater – seem to have worsened recently. On the other hand, this worsening can be explained without resorting to extraordinary explanations (such as the reactor somehow achieving criticality). The concentrations of radionuclides in the groundwater could be dangerous to people who drank only the groundwater but, given the realities of the situation (e.g. that the area has been evacuated), it’s not likely that anyone will be drinking the groundwater. And, while the exact environmental consequences remain to be seen, the worst case would be localized damage to the harbor ecosystem, but ill effects under any scenario are unlikely to extend outside the immediate area of the reactor plants.
The bottom line is that this was (obviously) a serious accident and it’s very possible that there will be environmental consequences. But we have to be realistic and acknowledge that these consequences – whatever they are – will be limited in scope and extent. Fukushima was a bad accident – let’s not let our misconceptions make it appear any worse than it actually was.