SummaryThe spread of radioactive material from the damaged reactors at Fukushima is not necessary. It is far more cost effective to control the emissions at source than to try to deal with the effects of radiation over thousands of square kilometres of land in Japan and neighbouring countries. The technology required is not new, although it is large in scale. The problem seems to be that the collective mind of Tepco (Tokyo Electric Power, the company that owns the site) is understandably focussed solely on cooling, to the exclusion of containment. The measures outlined here are to give an overview of the measures needed, in order to open people’s minds to the possibility of Emergency Containment.
Overview – both cooling and containment
The crisis at Fukushima in March 2011 has resulted in widespread airborne radioactive contamination of Japan, spreading as far as the USA and Europe in low levels.
In the summer months, prevailing winds will change from NW to SE, carrying radiation to mainland China and Korea.
Water contamination is also present in the sea, and meltdown would result in contamination of ground water.
The efforts of the nuclear authorities for the first four weeks have concentrated solely on cooling the three affected reactors.
There is no reason that efforts to contain air pollution should not be started while emergency cooling operations continue. Cooling efforts may have to continue for months or years.
The only requirement is that containment efforts should not interfere with cooling efforts. This can be met by carefully coordinated planning on site.
Overall, the aim should be to prevent as far as possible the spread of radiation in air, water and soil, both by cooling in order to try to avoid meltdown, and also by containing emissions.CoolingAt present, there is highly radioactive water in the basement of the reactors, and vast quantities of used cooling water are accumulating in reservoirs.
It is desirable to produce a closed circuit loop for this cooling water, pumping it out, filtering it, cooling it to increase the efficiency of the operation, it and passing it back into the reactor. It is not necessary to filter the water to perfection if this is not possible in the early stages, as contaminated water will be nearly as effective as pure water for cooling. Progressive passes through filters will have an effect over time, as more filtration and separation modules are added to the circuit.
An alternative method of cooling would be to fill the whole reactors with boron sand or pebbles and run water over the Reactor Pressure Vessels (RPVs). This is proposed by
Devon Horntvedt. The advantage of this would be the neutron absorbing properties of the boron. The sand would also damp the effects of any explosion, and could be complementary to netting or funnelling (see below).
Whether the cooling would be as effective as the present situation is a matter for calculation. The presence of sand would pose a problem if it were decided that access to the RPV was needed at a later date.
Cooling the spent fuel rodsAt present the rods are being cooled by water.
A complication on site is that spent fuel rods are stored in containers within the reactor buildings, and one is leaking. This latter should if possible be lifted out and placed in a non leaking container.
Since levels of radiation are too high to allow humans to apply slings to the fuel rod container, the work would have to be conducted by robots. Normal robots are unable to work in high radiation environments, but a French robots is available. Alternatively endoscope technology could be used to carry out this work.
There are two phases to the work: first in passing two slings under the fuel rod container, and linking them to a crane, and secondly, if necessary, in cutting the attachment of the container. Neither aspect is insuperable.
Once the leaking container has been removed, depending on the difficulty of the operation, it might be possible to remove the other rods in order to gain easier access to control their temperatures.
Emissions to AirUnit 2 is the place to start in reducing air emissions, since it still has an intact reactor building.
The picture shows steam escaping from a panel in Unit 2 that was deliberately removed to release gases to prevent a hydrogen explosion. If that panel is covered and a duct inserted, the air inside the unit can be extracted and treated.
Experience gained with Unit 2 can then be repeated with the other reactors, once they are covered. This plan is currently under consideration: “Officials at the stricken plant are also planning to cover three badly damaged outer reactor buildings with special fabric caps and fit air filters to limit the release of radiation.”
SourceAir pumped out of the reactors will then be under control, no longer released to the environment, and can be filtered and treated on site as outlined below.
There are several methods of dealing with the polllution, which can be used in combination or separately:
1. First, the gases will need to be purged of hydrogen because of the explosion risk.
A palladium filter would absorb the hydrogen in a reversible way (the palladium can be de-hydrogenated and used again) but the condition of the air would probably forbid this as other compounds might poison the palladium.
2. Instead, simple bubbling (washing) of the gases through an oxidant solution such as chlorine should be sufficient to remove the hydrogen.
H2+2Cl- => 2HCl
3. a simple particle filter should be put in place.
4. the gases should be passed through water to dissolve the radionuclides. This would also cool them as they need to be close to 40*C for ion exchange filters to work.
6. Electrolysis will attract positively charged ions to the anode, where they will be fixed.
7. Zeolites can be used to absorb radionuclides
8. ion-exchange resins would be used for entrapment of anything left. The exact form of resin would depend on the mix of ions left in solution.
The ducts containing the contaminated fluids may be provided with manifolds, so that more treatment modules can be attached as they become available.
The filters would be housed in such a way as to allow them to be changed without interrupting the process or release of radioactivity. Details of this design can be shown on request.
All filters and precipitates would have to be stored as radioactive waste.
Once experience has been gained with Reactor 2 gases, reactors 1 and 3 will need to be clad in fabric and have ducts inserted in order to carry out the same process with them.
All of this is standard industrial technology. Normally it is carried out to very high specifications to avoid the smallest possibility of leakage. In the circumstances at Fukushima, specifications can be less perfect, since leakage is occurring in any case.
At the end of the treatment, the cleaned water can be cooled further, and merged with the cooling circuit.
In this way, the releases to air which are happening at the moment can be contained.
However, air releases may get far worse if meltdown occurs.
Meltdown
To some extent, the fuel and spend fuel rods in reactors 1, 2 and 3 have melted. This hot, lava-like melted fuel is called corium.
The fuel in reactors 2 and 3 is hot with “decay heat”, and needs to be cooled. It is believed that in Reactor 2 nuclear fission has restarted itself – “re-criticality”. This will generate much more heat, overcoming attempts to cool. In this case, covering with sand and allowing water to flow through the sand is an option that should be considered.
Once re-criticality restarts, there is a very high likelihood of its simply burning its way out of the Reactor Pressure Vessel (RPV), since steel loses 90% of its strength at 800ยบ Centigrade, and these temperatures can easily be exceeded in fission reactions. The corium will then collect on the concrete floor of the reactor. It can react with the concrete, or find a crack, and then burrow its way down through the soil, eventually reaching the water table – the so-called China Syndrome.
In contact with ground water, there will be an immediate, explosive generation of steam, most of which will pass upwards around the RPV, possibly disrupting the cooling efforts in the reactor which has melted down, and blowing off the fabric cover if one has already been put in place
This explosion can be mitigated (see below).
The initial explosion is likely to scatter the corium, diminishing further re-criticality, but the decay heat will continue to generate steam, which will seep up through the soil, and will need to be contained by capping the area, and extracting the gases for treatment.
The likelihood of groundwater contact can be reduced by lowering the groundwater and pumping out the radioactive slurry. (see below)
Containing the explosion
Contact with ground water will result in a geyser of steam of greater or lesser magnitude.
This would result in a serious injection of radioactive steam, metallic debris and soil into the atmosphere, resulting in levels of contamination of the work area that would be very prejudicial to the health of the workers, making operations even more difficult.
Devon Horntvedt suggests attempting to lower the groundwater by pumping. It is not certain that this will be possible, since Fukushima is by the coast, so groundwater will be replenished rapidly by infiltration of seawater. An extensive
silt barrier, effectively pushing the sea further away from the reactor, might reduce seawater infiltration. Whether or not it effectively reduces the level of groundwater, pumping in any case will produce a supply of cold cooling water.
The force of the initial explosion can be contained to some extent, lessening the local and distant spread of radioactivity.
There are two ways to contain the initial explosion: netting and funnelling.
Netting Containment of Explosion
A series of nets can be placed over a reactor that is in meltdown.
Figures 1 shows an elevation, and 2 shows a plan view of a net laid over a reactor. The net material would be anchored to elastic guys to absorb the shock of the explosion. Nets with wide mesh size would be overlaid with nets with progressively smaller mesh size. The outermost layer could be of hot air balloon fabric of large capacity, so that the steam would inflate the balloon in the initial stages of the explosion. Cooling of the steam would cause the balloon to deflate. The radioactive balloon fabric would then be rolled up and disposed as radioactive waste, but the overall effect of these measures would be to prevent wider spread of the results of the explosion.
When in place, the balloon fabric would enable the gases inside to be drawn off and filtered prior to the explosion, as shown above.
Funnel Containment of Explosion
An alternative containment method, requiring more engineering, is to cap each reactor with a metal funnel.
Figures 3 and 4 show a reactor building capped with a prefabricated metal roof topped with a duct. The walls would also be faced with metal plate, enabling negative pressure to be applied to the building, drawing off the radioactive gases inside.
The mesh is in place to prevent clogging of the duct with debris.
As in the section on emissions to air, above, the gases would first encounter a hydrogen filter would be first in series, and then a pump followed by the series of filters.
During the initial explosive phase, the pump would be run at maximum power. Subsidiary ducts and pumps could be installed in roof and walls to deal with the volumes produced at this stage. The high volume of emissions would mean that the filters might have to be bypassed, and the gases and debris pumped straight to containment, for treatment later on.
After the initial explosion, emissions will amount to steam seepage, which can be drawn off and filtered.
Figure 4 shows a plan view of the funnel arrangement.
Meltdown - Boreholes
When the initial steam explosion has passed, the corium will be lying in subsoil, generating steam that will carry radioactive materials into the soil, groundwater, and seeping up into the air, possibly in sites rather remote from the reactor, depending on what passages it finds in the soil.
This problem can be mitigated by drilling boreholes under the melted down reactor and pumping out the slurry. In order to continue, water should continue to be poured over the affected reactor, even though its use as an effective control on heat will have passed, since it will allow the slurry to be pumped.
The radioactive slurry will have to be placed in storage, or purification can be carried out on site at a later stage.
Sea Pollution
Dr Sasaki, a nuclear physicist,
has a page here showing that seawater in the lagoon by the power plant is highly contaminated. He proposes containing this seawater and also re-siting the inlet pipe of the cooling units which are using this contaminated water. Alternatively a new cooling unit could be constructed.
Mounding
Concrete has been suggested for final capping of the site, but has many disadvantages in terms of cost and later access. The preferred method would be through simply covering the sides of the site with earth banking, filling the core with boron sand, and capping the whole mound with an impermeable membrane. Ducts would be inserted inside the membrane to collect such warm contaminated gases that were percolating upwards.
Administration
Tepco have not distinguished themselves in their response to the disaster, not least in their blindness to trying to contain the pollution. They have a record of dubious practices, and have not been transparent and open. Particularly scandalous has been their treatment of the "responders" - workers who are doing the remedial work. They have had only two meals a day in the past, and there are only 300 of them, working in groups of 50 in order to limit their radiation exposure. At Chernobyl there were about 8000 "liquidators" of whom some 50 died. Tepco should clearly expand the pool of responders in order to reduce their exposure.
It would be reasonable for the IAEA to take over the operation, particularly since it is an international affair. They would be able to raise funds from the nuclear industry, who would be keen to limit the spread of radiation in order to lessen the damage to the image of their industry.
Conclusion
Emissions from the Fukushima site can and must be captured and processed at source.
R Lawson
