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Introduction to Nuclear Reactors and Radioactive Wastes
Nuclear energy relies on the controlled reactions of enriched uranium UO2. In a Pressurised Water Reactor, this UO2 in circular disc form is packed in fuel rods which are bundled together to form the fuel assembly.
The fuel assembly is placed in the core of the reactor where the reactions take place. The speed and the number of reactions that occur are achieved by the use of control rods and a moderator. In the core, uranium atoms are bombarded with free neutrons producing immense heat. This heat is transferred by cooling medium to a heat exchanger where it is used to convert process water to dry high pressure steam.
The steam is then used to drive steam turbine generator units, returning to the heat exchanger via the turbine condenser as in a normal thermal power station operating system.
The radioactive waste produced by this process of raising steam is a direct result of using uranium as a fuel in a nuclear reactor.
There are three categories of waste these being Low Level Waste (LLW), Intermediate Level Waste (ILW), and High Level Waste (HLW). Nuclear power plants are in operation all over the world, each county having its own method of dealing with the different levels of radioactive waste.
Normally LLW can be sent to hazardous landfill sites, but ILW and HLW are stored in various secure containers and mediums whilst a long-term method of storage is established.
This is another article in my series on nuclear energy, where we will examine the levels of radiation waste produced the process along with the current and proposed methods of its safe disposal and safe storage.
- slide 2 of 8
Radioactive Wastes Produced by a Nuclear Power Plant
As the Pressurised Water Reactor is the most popular nuclear reactor we shall examine the radioactive wastes produced by this reactor, which fall into three distinct categories.
- Low Level Waste.
- Intermediate Level Waste
- High Level Wastes
Low Level Waste
These wastes include some types of process equipment, protective clothing such as boiler suits and gloves along with rubble from decommissioned buildings. It is not considered as being dangerous to health.
Intermediate Level Waste
This waste is more radioactive than the low level waste and is made up from metal or alloy cladding fitted around fuel rods sludges from various processes, and resins used in coating components.
High Level Waste
These are prevalent in spent fuel and are highly contagious. Fuel rods are replaced periodically and the used ones are highly contagious containing uranium, plutonium along with minor actinides such as curium and neptunium isotopes.
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Disposal and Storage of Low and Intermediate Radioactive Waste
- Low Level Waste Disposal (LLW)
Low level radioactive waste can be disposed of by relatively shallow burying in hazardous landfill sites.
Some countries which produce low level waste only permit this type of disposal after the waste has been stored at the nuclear plant for a specified period.
- Intermediate Level Waste Disposal (ILW)
There are two sub-types in this category,
Short Lived Intermediate Level Waste
These types can be treated chemically, incinerated then compacted into manageable bales. The bales are then encases in concrete and either stored in underground concrete lined troughs or held in specially fabricated containers on the surface, being stored for 300 years.
Long Lived Intermediate Level Waste
These are treated similarly to the short lived ILW but due to their higher levels of radiation they have to be handled very carefully to avoid contamination.
Once the waste has been treated, compressed and encapsulated, it is stored underground in concrete or similar lined repositories.
Please read the next sections where we examine the treatment of High Level Wastes, along with their current short term storage measures and long term storage proposals.
- slide 4 of 8
Intermediate and high level radioactive waste requires to be vitrified before being stored in suitable repositories. The current proposal for long term storage of the vitrified wastes is to stack these cylinders in an underground cavern. Bentonite clay would be packed tightly around the containers, the whole evacuated area being backfilled with a further barrier. This repository will be designed last for 10,000 years being secure against possible earthquakes, world wars and terrorist intrusion or attacks. The fact that a number of European countries are proposing to construct underground repositories may be taken as proof that this method has been thoroughly investigated and represents a safe and secure method of long term storage.
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High Level Waste (HLW) Treatments and Short-term Storage
There are two methods of disposal of spent fuel rods these being reprocessing and vitrification with storage on site.
Reprocessing of High Level Waste
The fuel rods require replacing every four to five years, the spent fuel rods are very radioactive, requiring extreme caution in handling. The Thorpe Center at Sellafield in UK has been involved for some time in reprocessing used fuel both from here and abroad. The plutonium and uranium components are extracted from the irradiated fuel, the uranium being converted back to pellets for reuse. The plutonium was mixed with uranium producing MOX fuel, (this process has now been shelved) the remainder of the spent fuel being vitrified and stored on site.
Processing of High Level Wastes
The spent fuel rods as well as being highly radioactive are still very hot, and will continue to produce thermal energy for some time as they decay.
For this reason they are stored underwater in ponds specifically designed for this purpose which keep them cool and prevent the escape of radiation. They will remain in the ponds for four years, by which time the will have radioactively decayed and cooled sufficiently to proceed to the next operation.
All the following processes are carried out in a hot cell, a building constructed exclusively to contain radiation but enabling observation and operation of the process taking place within.
The liquid spent fuel is pumped from the cooling ponds to a liquor tank and from here into a rotating cylinder device which is suspended in a furnace. Here the liquor is calcined by subjection to high temperatures whilst continually rotating, thereby converting the liquid waste into a dry powdery substance.
This is then mixed with powdered borosilicate glass and fed into an induction furnace or a specialized melter such as a Cold Crucible Mixer (CCM) and heated to 1100C, causing both components to become molten. The molten mixture is held at this elevated temperature whilst being continuously stirred, both operations effectively binding the waste and glass in about eight hours.
The molten substance is then poured into a stainless steel container (resembling an old style milk container used by dairy farmers) and left to solidify. An air-tight lid is then welded on and the container subjected to high pressure water jetting to remove any contamination from outside of the container.
Sometimes the containers are covered in layers bentonite clay before being transported to an underground store where they can remain in safekeeping for up to fifty years.
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Proposals for Long-Term Storage of High Level Waste
At present there are three proposals for the storage of High Level Waste,
- Storage in Synroc
This is a synthetic crystalline rock incorporating minerals which effectively act as hosts and fix the major long lived elements of plutonium, strontium, caesium and barium contained in the HLW.
- Undersea Storage
The disposal of radioactive wastes into the sea was a legal and common practice carried out by some countries between the end of the second world war and as late as 1980's. This practice has now been banned by International Law. However, several suggestions have been proposed for storage of waste in specially constructed caverns under the seabed, Sweden currently researching one such location.
- Deep Geological Disposal of High Level waste
Most countries of the world have now agreed that this is the optimal method to be fully examined. An underground repository would consist of caverns, interconnected by tunnels and accessible by vertical lift shafts, enclosed by natural barriers of rock, clay and salt.
The locations for deep underground storage of HLW are limited to depths between 300 and 1000m, in stable rock structures free of groundwater, and where there is no evidence of volcanic or tectonic plate activities.
It is proposed that the stainless steel or copper cylinders containing vitrified HLW would be deposited in the repository being tightly packed with bentonite clay. The whole excavation area may then be backfilled with another barrier such as bitumen or concrete dependent on the type of host rock of the facility.
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