By Mousom Singha:
Even with the failure of India making it to the prestigious and economically beneficial Nuclear Suppliers Group (NSG) for the time being, it is hopeful that our country will eventually be able to gain membership in the near future. With 47 out of the 48 members of NSG approving India’s bid, it is only a matter of time before the news breaks in. At this juncture, we should ask ourselves as a nation if we are really ready to assume such a responsible position in the league of nations. Do we have the economic backing? What about the human resources? What about technology? In this article, I shed some of the drawbacks and the advancements in a particularly troublesome area of being a nuclear state: nuclear waste management. Not only is this area one of the most cumbersome problems to handle, but also it requires meticulous planning for years ahead of starting operations.
Nuclear waste production is an especially large obstacle to the widespread acceptance of nuclear power. A commercial nuclear power plant produces about 20 metric tons of high level nuclear waste per year. This means that there are about 9000 metric tons produced worldwide each year. Spent nuclear fuel is irradiated fuel or targets comprising of uranium, plutonium, minor actinides and other fission products. There are three types of wastes given out by nuclear plants.
Low level wastes are those which have light radioactivity in them. Office materials used in nuclear plants, rags, tools, clothing etc fall under this category. These wastes consist of 90% of the waste volume and 1% of waste radioactivity. Storing these wastes for 10 to 50 years will cease the radioactivity present in these and thereafter can be disposed off as normal refuse.
Intermediate level wastes have higher radioactivity than the previous category. These wastes consist of resins, chemical sludge etc. These account for 7% of the waste volume and about 4% of the radioactivity volume (World Nuclear Association, 2014). These wastes are solidified in bitumen or concrete and are buried deep underground for disposal.
High level wastes mostly comprise of the spent fuel itself and the principal waste from reprocessing units. These wastes consist of 3% of the waste volume and about 95% of the waste radioactivity (World Nuclear Association, 2014). These are very difficult to dispose off as these have very long activity life. Table 2 shows the half lives of some of high level wastes to throw some light on their activity life.
The fuel pellets used by current reactors have a cladding of metal around the uranium fuel mixture. This cladding wears out after a certain period of use due to radiations and other causes. This in turn limits the life of the fuel pellets. The current life cycle of 4 years allows only 3% of the energy present to be extracted for practical use. The low efficiency causes the used fuel pellets to have about 97% of their energy intact and hence makes them very radioactive. These pellets are active for about a few hundred thousand years. In reprocessing, the idea is to use these same used waste pellets to generate more power. The worn out cladding is first removed and the used fuel pellets are then immersed in molten salt. This mixture is put into the core of the reactor to initiate the required reactions. Without the cladding, the fuel pellets can be used as long as they keep on supplying energy, thus removing the life cycle barrier of 4 years for a pellet. Hence, as a result, this produces just a little amount of waste materials. The final used fuel possesses a radioactivity period of just about a few hundred years, which falls under the solution set of current engineering advancement. Figure 1 shows a schematic flow diagram for usage of reprocessed material.
There have been rapid developments in nuclear technology over the past several years, many of which show promise for solving the above said problem. Standing out from the herd is an approach taken by Dr. Leslie Dewan and Dr. Mark Massie of MIT.
They have invented a new type of nuclear reactor, the Waste-Annihilating Molten Salt Reactor (henceforth, WAMSR), that can help solve the nuclear waste problem. The WAMSR consumes nuclear waste as it turns it into electricity, reducing the mass of the high-level actinide waste. Furthermore, it produces an enormous amount of electricity. The current high-level nuclear waste stored globally amounts to about 270,000 metric tons. If the existing high-level nuclear waste were put into WAMSR reactors, they could produce enough electricity to power the entire world for 72 years, even taking into account projected increases in worldwide energy demand.
In WAMSR, the idea is to use these same used waste pellets to generate more power. The worn out cladding is first removed and the used fuel pellets are then immersed in molten salt. This mixture is put into the core of the WAMSR to initiate the required reactions. Without the cladding, the fuel pellets can be used as long as they keep on supplying energy, thus removing the life cycle barrier of 4 years for a pellet. Hence, as a result, the WAMSR produces just about 3 kilograms of waste materials. The final used fuel possesses a radioactivity period of just about a few hundred years, which falls under the solution set of current engineering advancement.
Another issue which plagues the reputation of nuclear power is safety.
Though nuclear accidents are very rare in perspective to the usage of nuclear energy and its production, they are very disastrous in nature. A single nuclear accident can cause immense damage of unimaginable consequences to human life and the biodiversity in the surrounding areas, not to mention the sociological, economical and psychological disturbances. Three Mile Island, Chernobyl and in more recent times, Fukushima disasters stand testimonial to this harsh reality.
Keeping in line with such incidents, Dr. Dewan and Dr. Massie have designed their WAMSR to have a ‘walk-away’ safety feature.
Conventional nuclear power plants require a constant source of electric power to continuously supply coolants to the core to prevent a meltdown. In the WAMSR model, this problem is tackled very innovatively. The core which is filled with molten salt and the fuel pellets is the seat of the reactions. The core in addition has a plug at the bottom made of solidified salt – the very same material used in the molten state. Continuous electrical power supply is required to maintain the solid state of the plug. In case of a power failure, the heat of the reactions in the core melts the plug. Hence the whole reaction mixture flows into an auxiliary container, thus halting further heat production. In such cases, personnel are required to just walk-away as the molten mixture takes about 3 days to solidify.
In conclusion, it can be said that as the world moves towards cleaner and safer technologies for all its aspects, it is only fitting that nuclear industry moves in tandem. Hence the effort shown by the young and innovative scientists is awe-inspiring for budding engineers and scientists alike to follow in their footsteps.