The UK and Japan Science Media Centres wrapped up comment from scientists overnight on the nuclear situation in Japan.
Contact the NZ SMC for general queries on nuclear safety and radiation hazards. If you have questions you would like us to put to nuclear experts via the SMC network, please email them to us.
Latest Tweets from Japanese scientists:
Note: This content has been based on tweets gathered by Professor Ryugo Hayano (@hayano), School of Science, Tokyo University. A number of answers have come from experts other than Prof Hayano, including Tokyo University nuclear sciences graduates and volunteers. Click here to see more tweets from Japanese scientists as they come to hand and are translated by the SMCJ.
Q: There are reports saying that Fukushima is a level 4 nuclear accident on the International Nuclear and Radiological Event Scale (INES), how dangerous is that?
A: Level 4 describes an accident with no significantly large consequence outside of the installation. In general, a small amount of radioactive material may escape outside the installation, but the likelihood of it having an effect on human health is relatively small.
Q: I’ve heard reports that up to 190 people have been exposed to radiation, what are the chances that things will get worse in the future?
A: I cannot say for sure because there isn’t enough information available right now, but judging by the reports that I have seen, it looks unlikely that the health of those evacuees who were exposed to radiation will get worse. The reason behind this is because the hydrogen explosion at the Fukushima Dai-ichi Unit 1 was short and only released a small amount of material.
There’s a chance that more than 190 people will be found to have radioactive material on places like their clothes, but that’s because radiation detectors today are very sensitive and can detect even the slightest amount of radioactive material. Small amounts of radioactive material will most likely not affect a person’s health. Changing clothes or washing your face and hands can reduce the amount of radioactive material as well. Evacuation shelters in Fukushima prefecture have begun screening tests to see if people have been exposed to radiation, so those who are particularly worried can go there.
Q: What’s the likelihood of another reactor exploding in the future?
A: I can’t guarantee it, but looking at the information being released by the Tokyo Electric Power Company (TEPCO) and the Nuclear and Industrial Safety Agency (NISA), it looks unlikely that there’ll be another explosion at the Fukushima Dai-ichi nuclear power plant other than Unit 3 (as of 19:00 March 13). An explosion would happen when the fuel temperature goes over 1100 degrees celsius, causing the water and zirconium metal cladding material to react and create hydrogen. When a large amount of this hydrogen gas comes in contact with oxygen, it results in a hydrogen explosion.
In other words, the temperature inside needs to get hot enough to produce hydrogen gas, and then that gas needs to be passed through the vent and into the unit’s housing. Unless these conditions are met, it’s very unlikely that there’ll be another explosion like the one at Fukushima Dai-ichi Unit 1. The Fukushima Dai-ichi Unit 3 meets these two conditions, and an explosion could happen if the hydrogen concentration rises about a few to 10 per cent. (Updated 16:00 March 13)
* At 11:01 March 14 Unit 3 did have a hydrogen explosion, but we have decided to publish this answer for our records. However, we apologize for not being able to publish this answer before the explosion had occurred.
Q: By releasing pressure from the reactor, what effect does this have on the environment?
A: As long as it’s confined inside the containment, there’s no environmental danger, but a small amount of radioactive material could be released into the atmosphere during this process. I would imagine it would be about the same amount as that released by Fukushima Dai-ichi nuclear power plant’s Unit 1. Although the environmental impacts are small, it’s important that you follow the evacuation instructions given out by the government.
Q: Why did it take one-and-a-half days after the earthquake before they decided to inject in seawater?
A: Usually fresh water is used as the coolant. Using seawater would rust the reactor’s piping, making it unusable in the future. However, in the case of the Fukushima Dai-ichi’s Unit 1 reactor, either there wasn’t enough time to inject in sufficient fresh water or all of the fresh water supplies in the tank had already been used up. I can imagine that this would’ve lead them to give up on the idea of using the reactor in the future so they went ahead with injecting seawater.
Q: What does, “the reactor’s cooling mechanism has failed,” mean? (Updated 18:00 March 14)
A: It means that that the reactor, in particular the fuel part, has not been able to be cooled down enough. In a nuclear power plant, the heat (energy) generated by the nuclear fuel is turned into power by the circulating water. If the coolant (water) doesn’t circulate properly, then the fuel can’t be cooled down.
Q: What’ll happen to the water that’s being injected into the reactor when it’s thrown out? Will it pollute the environment? (Updated 18:00 March 14)
A. Waste water with low levels of radiation will be passed through a filter before before disposed of. Waste water with high levels of radiation will be boiled. This will turn the radioactive material into solids which can be picked out before the water is disposed of. But the amount of water being injected in this case is significantly large so right now I don’t know what method they’ll ultimately choose to treat the water.
Latest comments gathered by the Science Media Centre Rapid (released by the UK SMC at 7am NZT, Tues 15 March)
Expert reaction to latest concerns over nuclear reactors following Japanese earthquake:
Dr Jim Smith, Reader in Environmental Physics, School of Earth and Environmental Sciences, University of Portsmouth, said:
“In my opinion, a key risk at present is public panic in response to this incident. It is important to focus on the radiation risk, but experience from past nuclear incidents has shown that the stress and panic caused by these events can be as bad as, or worse than, the direct threat from radiation. Even after Chernobyl, although there were some severe health effects at the population level, the risk to individuals, except within the immediate vicinity of the plant, was very low. So: for people outside the immediate vicinity of the plant, even in meltdown on the scale of Chernobyl, the individual risk is likely to still be very low.”
Dr Bruce D. Malamud, Reader in Natural and Environmental Hazards, Department of Geography, King’s College London, said:
How frequent have earthquakes been over the last century and are they increasing?
“One of the questions that has been asked by many is whether there have been more frequent large earthquakes in the last few years. Let’s take as a ‘large’ earthquake one with moment magnitude 7. This would be The number of earthquakes per year with moment magnitude greater than or equal to 7 varies certainly, year to year, but the average from 1900 to present is about 17 magnitude 7 or greater earthquakes per year (compared to about 1 magnitude 8 or greater earthquake). If we just look at 1990 to 2010, then the average was about 15 magnitude 7 or greater earthquakes per year. And if we look at the last three years, then the average is also 15 of this size earthquake per year. So, no, the actual number of very large earthquakes is not increasing over time. It fluctuates year to year, with some years less, and some years more.”
How much energy is released in a magnitude 7 earthquake?
“Equivalent to the energy released in half a megaton nuclear bomb.”
How much energy is released in a magnitude 9 earthquake?
“Equivalent to 1000 times the energy released in a magnitude 7 earthquake, or one thousand half-megaton nuclear bombs. If we converted this to energy, this would be roughly enough to power every home in the USA for 50 days.”
How many and what size aftershocks might we expect?
“When an earthquake occurs, it releases stress that has built up over time, along a fault. However, in addition to releasing stress, it redistributes the stress along that fault, and sometimes these will be redistributed to other nearby faults. In the case of the Japan earthquake, approximately 400 km of fault was affected.
“With the redistribution of stress, aftershocks occur, for weeks, to months (and sometimes years) after the main shock. The magnitude 9 earthquake in Japan will result in aftershocks occurring all along the fault on which the original earthquake occurred. Some scientists say that one can expect aftershocks as much as 1 unit less than the original shock. So in this case, we might expect aftershocks of all sizes, but as big as a magnitude 8 (which would be in itself a concern of potentially triggering a tsunami).”
Are the earthquakes in New Zealand and Japan related, they occurred within weeks of each other?
“The main shocks that occurred in Christchurch on 22 February 2011 and off the coast of Japan on the 11 March 2011 are not related. They were on very different fault systems, almost ten thousand km apart.”
Professor Theodosios Korakianitis, Chair of Energy Engineering, Queen Mary, University of London, said:
“Today nuclear power provides about 7% of the global energy consumption, and about 17% of global electricity production. The combined Fukushima power plant total power output is about 4.7 GW. This is a very small fraction of the total power consumed by the energy industry today (15 TW or 15,000 GW globally). Therefore, on the global scale, closure of the Fukushima or any other power plant will not affect electricity supply in the long term. However, this generalization is misleading as nuclear power plants provide a significant portion of electric energy in some countries (e.g. 70% in France).
“The half life of Uranium 238 is 4.47 billion years, of Uranium 235 is 704 million years, of Plutonium 239 is 24,000 years and of Cesium 137 is about 30 years. Half life means the initial radioactivity is reduced to half its initial value, while the radioactivity decay after that is very slow. These numbers mean that the half life of Plutonium 239 is 10 times the age of the Parthenon on the Acropolis of Athens, while Cesium will only affect the next 2-3 generations of our children. The half life of Uranium is too long to sensibly make similar analogies in terms of human life-time scales.
“Without regeneration of spent nuclear fuel, known resources of Uranium can power the nuclear stations at current levels till about 2040; and if we also account for resources yet to be found this will take us to about 2050. Thus nuclear power is at best a stop-gap measure in the global energy supply. Current-technology wind turbines can provide over 70 TW globally, solar energy over 50 TW globally, and geothermal well over 40 TW globally. Given that each one of the above renewable sources can fully meet our energy demand (15 TW, or 500 EJ per year, approximately), the only reason to continue using nuclear power plants is one of using inexpensive power, and not lack of renewable energy resources.”
Professor Barry Marsden, Professorial Fellow of Science in Nuclear Graphite Technology, University of Manchester, said:
“These are boiling water reactors in which the cooling circuit goes straight through to the generator and back to the reactor in one loop. This is different from a pressurised water reactor, which has a primary cooling circuit which transfers heat via a steam generator (heat exchanger) to a second circuit in which the turbine is located.
“For any nuclear reactor the main safety imperatives are: firstly under all normal and fault conditions the reactor should be capable of being shut and held down (that is the nuclear reaction stopped) and secondly the nuclear fuel needs to be cooled to remove decay heat. In this case as a response to the earthquake the reactors were immediately shut down. However, the tsunami appears to have knocked out the main cooling loop, and compromised the emergency backup systems.
“Also the power supplies from outside the power plant was obviously lost and emergency power systems such as diesel generators and batteries damaged. Thus the fuel could not be properly cooled and as water levels in the core dropped the fuel would have become exposed and the fuel element zirconium cladding would have overheated and reacted with water to produce potentially explosive hydrogen. When the pressure was released from the core by the operators, to prevent damage to the steel pressure vessel, it is this hydrogen that must have exploded and damaged the outer containment.
“However, it is clear from the limited amount of irradiation released that the main reactor containment (the steel pressure vessels) are still intact. The reactor operators appear to be now using sea water to cool the core in some way but the details of this are still unclear.
“In the UK there are no Boiling Water Reactors, there are graphite moderated reactors, 14 advanced gas-cooled reactors (AGRs) [across seven sites] and 4 Magnox and one Pressurised Water Reactor.”
Professor Bob Cywinski F.Inst.P, Dean, School of Applied Sciences, University of Huddersfield, said:
“The international atomic energy agency (IAEA) have rated the seriousness of the nuclear situation at the Fukushima reactors as 4 on a scale of 7. Three Mile Island was rated as 5/7 while Chernobyl was rated 7/7. Each additional point on this scale represents a factor of ten, so the situation at Fukushima is 1/10th as serious as that at Three Mile Island, and 1/1000 that at Chernobyl.”
Professor Mike Fitzpatrick, Lloyd’s Register Educational Trust Chair in Materials Fabrication and Engineering at The Open University, said:
“The plant will have been designed to withstand earthquakes in terms of both structure and safety devices. The fact that the plant experienced a major explosion following the earthquake, during the operation to cool the core, shows just how robust the main pressure vessel containment system is.
“Once the reactor was shut down, it was still necessary to remove residual heat from the core. The failure of the cooling pumps meant that this was not happening effectively, hence there was an increase in temperature and pressure. Venting steam from the core, which was initially proposed, would release some radioactivity but would not cause long-term contamination. What must be avoided is the release of parts of the fuel assemblies, which is what happened at Chernobyl, although the sequence of events was very different there, with a lack of coolant whilst the reactor was generating power.
“The reactors that are being proposed for construction in the UK are much more advanced in terms of design and safety systems, and can cool passively following a shutdown which makes them inherently safer.”
Prof Neil Hyatt, Department of Materials Science & Engineering, University of Sheffield, said:
“The current Japanese nuclear emergency should not cast a shadow over building new nuclear stations in the UK, if the stricken reactors can be brought under control without release of substantial radioactivity. The UK is not in an earthquake prone location and the proposed new fleet of reactors do not require a diesel generators to keep coolant flowing through the core, which failed in the Fukushima reactors. Instead, the proposed new reactors use a passive gravity and convection driven cooling circuit, which do not require a power source. In the UK, much more of concern is the safe and rapid decommissioning of our existing radioactive waste storage facilities, some of which are in a highly hazardous state.”
Dr Shaun Fitzgerald, University of Cambridge, said:
“The challenge of providing sufficient energy to society has been heightened in a horrible way through the events in Japan. Nuclear is viewed as a necessary part of the worldwide supply base in order to reduce our carbon emissions. If we were to apply the same gigantic effort we expend in providing energy to trying to find ways of needing less in the first place, the number of nuclear plants we would need in the future would be lower. This in turn would make the world a safer place. Clearly our immediate thoughts are with the people of Japan. However, once the immediate tragedy has passed, we should use the disaster for good and reflect on how we can seriously reduce our energy consumption and hence reliance on nuclear.”
Dr Richard Wakeford, Visiting Professor, Dalton Nuclear Institute, University of Manchester, said:
“Nuclear reactors rely on fission chain reactions in uranium for their power. The systems are designed to detect earthquakes and the Japanese reactors immediately shut-down when the earthquake occurred by the insertion of neutron absorbing rods (e.g. made of boron). Boric acid is being added to the emergency cooling water to mop up neutrons. There is no possibility of a nuclear explosion, even if there was no neutron absorbing materials present because the uranium in these reactors is “low enrichment” (i.e. low uranium-235 content) whereas a bomb needs high enrichment uranium. So, the suggestions of a nuclear explosion are ill-founded and not based on scientific fact.
“The problems being faced at the moment in the Japanese reactors are the hot fuel in the reactor cores that need to be cooled to keep pressure in the reactor vessels at appropriately low levels. If this requires venting gases from the reactor vessel, then this is being done. It would appear that the explosions that have been seen are hydrogen explosions caused by hydrogen produced from the cooling water and vented into the outer building – these are not nuclear explosions.”
Dr Stephen Edwards, Aon Benfield UCL Hazard Research Centre, said:
“As we watch the terrible disaster in Japan unfold, it is worth reflecting on vulnerability and risk. The Developed North often views the Developing South as being most vulnerable to the impact of disasters, and often this is true. However, the images coming from Japan clearly show that big natural events, such as earthquakes and tsunamis, do not respect any socio-economic background.
“The nuclear facilities in Japan are regarded by many as a sign of advanced development. But, as we are seeing, the very technologies that reduce certain vulnerabilities – in this case nuclear power to reduce energy insecurity – present other risks. The seismic-nuclear situation in Japan demonstrates how a chain reaction of disasters might evolve. It also exemplifies the complex interlinkages that exist in disasters and risk, which are likely to become more commonplace in our increasingly densely populated and developed world.
“Let us hope for at least two positive outcomes from this disaster: that the nuclear technology employed in Japan is robust enough to mitigate the risk of a major nuclear disaster; and, that out of all of the horrific devastation and destruction we take away much learning and apply it rapidly, responsibly and effectively.”
Professor John Gittus, a Fellow of the Royal Academy of Engineering, said:
“So far, it seems as though the engineers at Fukushima have the situation in hand and the radiation risk to the public is still relatively small.”