April 6, 2011 | 8
During the morning of April 6, our colleagues at Nature ran a live, online question-and-answer event about the Fukushima Daiichi nuclear crisis. Visitors posted questions for Jim Smith, an environmental physicist from the University of Portsmouth, U.K., and Geoff Brumfiel, Nature’s senior physical sciences correspondent. Brian Owen served as moderator.
To start off, we’ve had a couple of people asking about the safety of traveling to the region, including someone whose daughter is studying in Japan, and someone on their way to Korea. Jim, could you tell us a bit about the risks from radiation to travelers?
I guessed this would come up, so here’s one I prepared earlier:
There is no significant risk for travel to countries outside Japan – you’re likely to get a much higher radiation dose from cosmic rays on the flight over than you would from the radiation. Even in Japan, the risks of radiation are very, very low. This is because (1) contaminated products are being kept out of the food supply (2) apart from in the area within, say 40-50km there isn’t a big risk from deposited radioactivity. (3) the radioactive iodine which is causing all the concern at the moment is decaying away. All of it will be gone within a few weeks. I’d be more worried about the disruption to services and anxiety caused by the earthquake, tsunami and nuclear accident.
Although there are high levels of Cesium 137 and Iodine 131 in the water around Japan many people believe that it is not cause for concern because of the ocean’s ability to vastly dilute what is put in it. Do you believe other countries should be concerned or do you agree that the ocean can fix it?
Yes, there is enormous dilution of radioactivity in the marine system. By the time any contamination got across the ocean to other countries (if any did), the concentrations would be tiny and wouldn’t present any risk. The concern for the marine system is mainly in the local area to Fukushima.
That’s a good question. Not sure what the fish species are in the region and how far they might migrate. If they were in the contaminated region for a long time (it takes time for radioactivity to get up the aquatic food chain into fish) they might get contaminated. During migration they would lose some of that as they expel it as well (I-131 would all go by decay) but some Cs-137 could remain. Guess they’ll have to monitor migratory species if there are any.
How long do you expect the Fukushima situation to continue to be in play? The mainstream media has played it out like a disaster movie and now seemed to be lost as to what to do now that there isn’t a foreseeable conclusion.
To be fair to the media Magdeline, I think everyone’s at a loss right now. The situation at the reactors is unpredictable and the next steps are far from clear. What we can say is this: the situation at Fukushima is going to be with us for years. First, the reactors will need to be brought under control, but then they’ll need to be cleaned up, and that is going to be very difficult…
I was looking at the British Windscale accident earlier today, which occurred in 1957. Cleanup is still underway. Three Mile Island took 14 years. I’m not sure where Fukushima fits, but it’s going to be over a decade. Probably many decades.
We’ve also had several questions here and on the blog about the safety of food and milk in Japan. Jim, could you give us your thoughts on that?
As I said in my answer about travel above – I don’t think there will be significant risks from ingesting contaminated food/milk/water in Japan. Levels in water are now very low, and the Japanese authorities are working hard to keep contaminated products out of the food supply chain. That will be a difficult job given the that they are also dealing with an earthquake and tsunami. I couldn’t say for sure that some “contaminated” products won’t slip through the net, but I would emphasize that the risks from consuming some products above limits are still very low. The limits are set so that products can be consumed at that level for a long time. So if somebody were to inadvertently eat products above the level, it doesn’t mean that they would get an unsafe dose.
Moving forward, how can nuclear power facilities (particularly in earthquake prone areas) be designed differently to withstand seismic activity?
Many of the newer designs are supposed to be "passively safe" meaning that, even if the control room is completely destroyed or incapacitated, the reactor will cool itself down on its own.
I think a much more difficult question is how to make existing reactors safer. In the case of Fukushima Dai-Ichi, the position of the diesel backup generators relative to the coast was clearly a problem. I imagine regulators world wide will be looking at the number and state of various back up systems to see if they can be improved.
Is there any great difference in the effect of having ingested radiation (fish, kelp) versus having been externally exposed to it?
The main difference is that it stays in your body for some time. This is taken account of in the risk models. For example Cs-137 is excreted from the body over a period of months, I-131 much faster. Some people also think there is a bigger risk because the radioactivity is inside you. I don’t agree. The distinction between “internal” and “external” emitters is a bit false. An X-ray or external gamma ray causes the production of high speed electrons in the body in the same way as internal emitters do. It’s the high speed electrons which cause ionization, potentially leading to DNA damage and cancer – it doesn’t matter where they originate.
If it takes over decades, does it mean that the spreading of radioactive materials continue during that time?
Jim may have some thoughts on this too, but no, I don’t think we need to be worried about the continuous spreading of radiation. For one thing, the fuel will continue to cool down in the coming years, and will be far less dangerous than it is now. For another, engineers should be able to come up with some measures to reduce the spread of any radiation that does escape. I’ve already seen that TEPCO is spraying resin on the site to keep radioactive dust down, for example.
That doesn’t mean that there couldn’t be a significant release further down the road, but overall, I expect that the radiation will decline at the site.
Radioactive Cs-137 is held in soil for decades (the amount halves every 30 years). It doesn’t spread too far – tiny amounts are spread by the wind and water. As the reactors are now (hopefully) much more stable, the problem for the land system is the radioactivity that is already out there in the soil.
The fuel rods in Unit 1 has been reported to have exposed up to 70% and they are trying to dilute the hydrogen concentration by incubating Nitrogen. This means that the cooling of unit 1 has been failing… possibly by build up of salt (no water circulation). In your opinion(s) what would be the worst case scenario for this?
We talked a little about that in a story I wrote earlier this week (http://www.nature.com/news/2011/110405/full/news.2011.211.html). Basically if cooling has failed, then there’s a chance that the core has melted partially or entirely through the reactor and ended up in the concrete drywell below. That would mean a longer, costlier and more dangerous cleanup.
Another danger is that there is some transient power-producing fission inside the reactor, or that the reactor could inadvertently restart. That could be a real problem (obviously) both in terms of radiation release and managing the accident.
But the evidence that this has happened is slim for now. As I said earlier up, things appear stable.
Actually, I should qualify that: we don’t really know what’s happening in the reactors, but for now it looks stable…
At Chernobyl they encased the reactor in a concrete tomb. What are the advantages and disadvantages of this sort of technique. Would it do more harm than good at Fukushima?
I was speaking to somebody about this the other day. The risk of a big concrete tomb is that it would have to be there for decades, even a century. Given the seismic and other environmental risks in the region, I think it would be extremely difficult to go down the sarcophagus route. Jim may have other thoughts.
That makes sense to me, though it makes it more difficult to decommission when that starts.
Is the neutron beam that has been reported a sign of fission on the site? What is the best and worst case situation concerning that release of neutrons?
Ah yes, the mysterious "neutron beam". First of all, it’s almost certainly a translation problem: they more likely mean a neutron "burst". Second, most of the physicists I’ve spoken to have disregarded the reports of neutron beams near the site. There are (roughly) three reasons:
1. Neutrons are extremely difficult to detect, so it would be a tough measurement.
2. The readings are fairly low, and could be caused by noise from the much higher levels of gamma radiation on the site.
3. It just doesn’t make sense. The readings are far from the buildings. It’s hard to understand why neutrons would be making it out of the containment vessel, even if they are being released.
That said, there’s definitely neutron activity in these reactors as a result of other fission products in the fuel and it’s certainly dangerous.
If there was fission going on, you would also see some evidence in very short-lived fission products appearing in air and water monitoring data – I haven’t seen any clear evidence of this yet, though I haven’t seen much air monitoring data at the site.
Most of the radiation readings around the 19km zone have been made in a somewhat less than scientific manner, often driving around from point to point, not including critical details such as wind direction at the time of reading. Can we trust these readings to be accurate when large swings from high readings to low readings could be attributed to things like wind direction or the large areas between reading sites? It would seem we only have a vague idea at best of the actual dumping of radioactive particles over this vast area.
In the early stages (i.e. when the contaminated material is in the air) the readings will be very dependent on things like wind direction, and will vary a lot. But once the radioactivity is deposited on the ground, the wind direction won’t affect the readings. Soil moisture will, and I notice that the readings say whether it is raining or not, but it doesn’t make too much difference. Generally I’m happy with the data, but sometimes it’s been difficult to interpret because, as you say, there needs to be more accompanying information on methods etc. This may be a problem of translation – maybe the Japanese version is more complete.
The US Department of Energy has done some surveying with aircraft that provide a pretty good picture of where the radiation has spread: http://blog.energy.gov/content/situation-japan/
Beyond that, I’d add that, to be fair, this is a really tough situation. It’s easy to second-guess the Japanese, and indeed it’s worthwhile to do so in many cases. But we shouldn’t forget that, especially in the immediate aftermath of the tsunami, instrumentation was limited, as was access to the region.
How does any increase in cancer risk from exposure to Fukushima radioactive contamination compare to other cancer risks such as smoking?
Certainly much lower. As an example, the radiation dose to the Chernobyl clean up workers was about 100 mSv on average. That translates to about a 0.5-1 % additional risk of fatal cancer in later life. Continual smokers have about a 50% chance of dying from a smoking related illness.
Is there an international watchdog that inspects nuclear power stations or are such inspections left to national regulatory control? Clearly there seems to have been a lack or insight into risks that perhaps could have been foreseen.
The International Atomic Energy Agency has some authority to inspect civilian nuclear reactors, but mainly it’s just to ensure that they are not being used for nuclear weapons development or other covert purposes. I imagine that some people will ask whether their authority should be expanded in the aftermath of this accident.
Ultimately, however, safety is likely to remain squarely in the hand of domestic regulators, for better or worse.
12 Digital Issues + 4 Years of Archive Access just $19.99X