Solution
We are facing the Nankai Trough Earthquake of M8.4 in the south part of Japan.
After the huge quakes, the colossal tsunami would come along immediately. Japanese government is still running 12 nuclear reactors in that area. The radioactive contamination can not be undone. In order to save our future and our lives we need to seek for the solution of this crisis by ourselves. Currently we have the following dangers:
1. 19,000 tons of the spent nuclear fuel
2. 12 active nuclear reactors
3. High-level radioactive waste in Tokai and plutonium in Monju
We are going to think about the solution of these problems.
1. 19,000 tons of the spent nuclear fuel
Nuclear power plants produce the spent nuclear fuel whose radioactivity is billion-times more intense than the natural uranium. After one year of the operation 100 tons of spent nuclear fuel is moved from the nuclear reactor to the cooling pool. Currently most of 19,000 tons of the spent nuclear fuel is stored in the cooling pool and the loss of electricity will cause the meltdown of the fuel within three days. All nuclear power plants are located in the coast line and are going to be exposed to both extremely fierce quakes and tsunami in the next earthquakes. Therefore we need to move the spent fuel to safe places beyond the reach of the tsunami. The best solution to this problem is to use “dry casks” which will be explained below.
Dry casks are sealed steel cylinders and store the spent fuel rods surrounded by inert gas.(Wikipedia (Dry cask storage) [1]) They can keep the spent fuel cool without electricity. The lifetime of the dry casks is several decades up to 100 years. The spent nuclear fuel retrieved from the nuclear reactor is most dangerous. However, in the cooling pool the radioactivity of the spent fuel decreases rapidly and after a few years it can be stored in the dry casks. The minimum cooling period in the pool before the dry cask storage is one year for the normal uranium fuel and three years for the MOX fuel (the uranium-plutonium mixed fuel). (United States Nuclear Regulatory Commission [2], Satoshi Ishikawa et al.[3]) Most of the spent nuclear fuel in Japan except in the active nuclear power plants is cooled enough for the dry cask storage. Therefore we should put them in the dry casks and move to safe places. That is the simple and best solution.
For instance, in Hamaoka nuclear power plant 1100 tons of spent nuclear fuel is still stored in the cooling pool. According to the homepage of Chubu Electric Power, they increased the earthquake resistance of the facilities up to 1200 gal and added the breakwater.(Chubu Electoric Power [4]) But those are completely useless. Hamaoka nuclear plant is located on the top of the three tectonic plates boundaries. The anticipated earthquake is much, much more fierce than 1200 gal. What we have learned from the 2011 Tohoku earthquake is that the only way to survive from a tsunami is just running away from it. In Taro in Iwate, people had constructed a gigantic breakwater of 10 meters high and 2 kilometers long to defend their town, but the actual tsunami came far beyond the breakwater and destroyed it like pudding. Therefore the best and only solution is to put the spent nuclear fuel in the dry casks and move them to safe places beyond the reach of the tsunami.
2. 12 active nuclear reactors
The first and the utmost important step is to stop 12 active nuclear reactors in the south part of Japan. The government anticipates that 320,000 people are going to be killed by tsunami in the Nankai Trough earthquake. Why are they still running the nuclear power plants in that area? Currently we have 19,000 tons of the spent nuclear fuel stored in the cooling pool without any protection. If we lost the control of one nuclear reactor, our world would change completely. The radioactive contamination can not be undone and we are going to lose safe water and safe food forever. We have to stop the 12 active nuclear reactors to save our lives.
The nuclear power plants are the facilities to produce the billion-times intense radioactive substances after the one year operation and we have to take care of them for 100,000 years. The Japanese government advocates the nuclear fuel cycle. (Agency for Natural Resources and Energy [5]) However, Japan is located on the top of the boundary of four tectonic plates and it is impossible to safely carry out the geological disposal of radioactive waste. After 100,000 years, Japan in map must be in a quite different shape. Therefore the cycle is not closed. In the situation of facing the Nankai Trough earthquoke, running the 12 nuclear reactors for the “nuclear fuel cycle” is nothing but a suicide attack on Japanese people and the whole human beings.
Here we assume that we were successful in stopping the 12 reactors, we continue our argument. The spent nuclear fuel retrieved from the nuclear reactor is too hot for the dry cask storage. We have to wait for at least one year for the normal uranium fuel and at least three years for the MOX fuel. The MOX fuel is currently used in four reactors; Genkai(unit 3), Ikata(unit 3), Takahama(unit 3, unit 4). The first option is just to wait. However, it would be painful to wait for at least three years for the MOX fuel. The second option is to transport one spent fuel rod at a time, which has been cooled in the pool for several months, using the normal cask for the transportion of spent fuel rods. If it is not safe, we can develop the bigger cask which is filled with a prenty of water and transport one fuel rod kept in the middle of the cask. We are going to construct another cooling pool in the safe place beyond the reach of the tsunami and store the spent fuel there until the dry cask storage.
3. High-level radioactive waste in Tokai and plutonium in Monju
JAEA (Japan Atomic Energy Agency) keeps 6,500 high-level radioactive solid waste (in the 200-liter vessel) and about 400 cubic meters of high-level radioactive liquid waste in Tokai which is located 100 km north-east of Tokyo. The former (the solid waste) consists of the end-piece and the cladding of the fuel rods and the latter (the liquid waste) was generated by the failure of the vitrification process. The liquid waste is extremely dangerous and needs to be immobilised as soon as possible. JAEA had planned to vitrify the high-level radioactive liquid waste until 2028, but after the series of accidents they postponed the schedule until 2038. Since the opening of the reprocessing plant in the 1970s, they could not immobilise the nuclear waste for the last 50 years. The main problem of their failure is platinoids in the radioactive waste, which will be explained below. (Kazuyoshi Uraga et al. [16], Energy Frontline [17]) Currently Japan possesses 8.6 tons of plutonium in Monju and in other places. This extremely dangerous substance needs to be kept safely in the event of earthquakes. Our solution for both problems is to use Synroc and the professional services by ANSTO (Australian Nuclear Science and Technology Organisation). “Synroc” is the assemblage of multiple ceramics and has been established by Ted Ringwood and ANSTO. (World Nuclear Association [7], ANSTO [6]) ANSTO provides professional services for the immobilisation of radioactive wastes and has experience of similar projects such as Idaho National Laboratory. (Eric Vance et al. [15]) Therefore using Synroc and the professional services by ANSTO we can obtain the satisfactory results immediately. In the following sections we are going to explain why Synroc is the best solution.
3.1. Immobilisation
One year operation of a nuclear reactor generates about 100 tons of spent fuel which contains the 95% mass of uranium (93.8% 238U, 0.8% 235U, 0.4% 236U), 1% of plutonium (0.8% 239Pu, 0.2% 240Pu), 4% of fission products (FP) and 0.1% of minor actinides (MA). In reprocessing plants, after retrieving uranium and plutonium the remaining substance is called high-level radioactive waste (HLW) which contains FP, MA and the additional substances (P, Na, Fe, Ni, Cr,..) coming from the retrieving process and the corrosion of the tanks. Table 2 and Table 3 show the half life and the weight of main radionuclides. (Caurant, Daniel et al. [8])
HLW must be isolated from the biosphere until their radiotoxicity level drops back to that of the natural uranium ore. One way to carry out the isolation is to immobilise HLW in a highly durable matrix. Currently we have three major waste forms: glass, glass-ceramic and ceramic.
3.2. Glass
Currently glasses are the main waste forms for the immobilisation of non separated HLW. Borosilicate glasses which mainly consist of SiO2, B2O3 and Al2O3 are preferably chosen because of the good chemical durability and the flexibility to incorporate a variety of chemical elements in HLW. The generation process of the glass waste form is simply melting the wastes with the glass frit in high temperature (1100°C) and cooling them in the canisters. Joule heated ceramic melter (JHCM) which is used in USA, Russia, China and Japan directly heats the melt by Joule effect with the submerged electrodes. The AVM process used in France and UK firstly evaporates and calcines the HLW solution in a rotating kiln (500°C) and then puts the calcined wastes with glass frit into an induction-heated metallic melter (1150°C). (Caurant, Daniel and Majérus, Odile [9])
In the room temperature glass is an amorphous solid which does not posess a long-range order of atomic organization. The structure of the simplest pure silica glass is a continuous random network of SiO4 tetrahedra. Each tetrahedron is linked to four tetrahedra and each oxygen atom is shared between two Si atoms which form strong directional bonds. Now let’s add other substances to the pure silica. The Addition of Na2O to the silicate network decreases the liquidus temperature and the viscosity. The reason is that it transforms two strong Si-O bonds into two weak Na-O ionic bonds. (Figure 4) The further addition of B2O3 results in the reinforcement of the silicate network. B2O3 participates in the network and generally reduces the weak ionic bonds in the network. Na2O is called a network-modifying oxide and B2O3 is a network-forming oxide. To predict those two different behaviors the field strength F of a cation M in an oxide is defined as F = Z/d2 where Z is the cation charge of M and d is the mean distance between M and oxygen in Å (10-10m) unit. Dietzel Theorem: If F >= 1.5, it is the network-forming oxide and if F <= 0.4, it is the network-modifying oxide. (Caurant, Daniel et al. [8]) If the oxide has high field strength (F >= 1.5) and cannot participate in the network, it tends to create its own network (phase). In the following sentences the glasses mean the borosilicate glasses.
1. alkalis and alkaline-earths
The alkalis and alkalis-earth cations such as Cs+, Rb+, Ba2+, and Sr2+ have low field strength and play the role of network modifier or charge compensator. They are easily dissolved in the glass structure. However, they are also dissolved well in water. When there are poorly soluble anionic entities such as orthophosphate (PO43-) and molybdate (MoO42-), they form the separate phases and deteriorate the overall durability of the glass wasteforms. (Caurant, Daniel and Majérus, Odile [9])
2. Transition metals
2.1. Zirconium
Zirconium Zr (20% of which is 93Zr with half-life of 1.5 106 years) has the high field strength and can participate in the glass network as ZrO6 octahedra. Zirconium can form separate phases such as ZrO2 and ZrSiO4 which are well known for their good chemical durability. (Caurant, Daniel and Majérus, Odile [9])
2.2. Molybdate
Molybdate Mo6+ is a stable fission product and has high field strength. Molybdate is poorly soluble and forms the separate network as MoO42- tetrahedra, called “yellow phase”, which deteriorates the durability of the wasteform. (Caurant, Daniel and Majérus, Odile [9])
3. Platinoids
Platinoids are almost insoluble in the melt and exist as crystalline phase RuO2 and polymetallic (Pd, Rh, Te) alloy. (Caurant, Daniel and Majérus, Odile [9]) They increase the viscosity of the melt and the electrical conductivity. Because of the high density (dpd = 12, dRu02 = 7 compared to dglass = 2.5), they tend to segregate at the bottom of the furnaces. The increased viscosity may block the flow of the melt to the canisters. In JHCM, the increased local electrical conductivity deprives the other part of the Joule heat which is necessary for the vitrification process. It may also cause the risk of short circuit. (Caurant, Daniel et al. [8])
4. Rare earths and actinides
4.1. Lanthanides and actinoides (Am, Cm)
(La, Ce, Pr, Nd) represent more than 90 wt% of lanthanides. Lanthanides (La, Ce, Pr, Nd) and actinoides (Am, Cm) possess similar chemical properties. Let’s use the notation Ln to represent those elements. Ln3+ has high field strength and is generally well dissolved in the glass network. They form strong Ln-O-Si bonds. (Caurant, Daniel and Majérus, Odile [9])
4.2. Actinoids (U, Pu, Np)
Lighter actinoids (U, Pu, Np) behave differently from the heavier actinoids (Am, Cm). Pu4+ exists in the normal and the oxidizing conditions. Pu3+ occurs only in the reducing conditions. Pu3+ is more soluble in the glass network than Pu4+.
A better method to immobilise plutonium is to use the ceramic wasteform, which will be explained in the ceramic section.
3.3. Glass-ceramic
Glass-ceramics are composite materials in which crystalline ceramics are distributed homogeneously in the host glass. Separate phases in the glass network would weaken the chemical durability of the wasteform. However as in the case of zirconium shown above some phases have good chemical durability. One strong motivation for the research of glass-ceramics is the idea of the double containment barrier. In this model ceramics contain long-lived radionuclides as the first barrier and they are dispersed in the glass matrix which serves as the second barrier. (Caurant, Daniel et al. [8])
1. Control crystallization
Ceramics in glass can be generated in two steps: nucleation and crystal growth. After mixing the ingredients of glass and ceramics, first we keep the temperature suitable for the nucleation and then we set the temperature for the crystal growth and wait for the development.
In (Caurant, Daniel et al. [8]) an experiment was conducted to form the glass-ceramic for zirconite. At first the glass frit (SiO2, Al2O3, CaO, Na2O) with the zirconite ingredients (TiO2, ZrO2) and the MA surrogate (Nd2O3) are mixed together at 1550°C to form homogeneous glass matrix. Then the mixture was kept at 810°C for 2 hours for the nucleation and at 950 – 1350°C for 2 hours for the crystal growth. After 2 hours of annealing, the glass was converted to glass-ceramic with only zirconite ceramic. In this experiment and in others in the literature, however, the MA surrogate always remains in the residual glass. Therefore the idea of the double containment barrier has not yet been realised with this approach. (Caurant, Daniel and Majérus, Odile [9])
2. Encapsulation
A simpler approach to realise the double containment barrier is to encapsulate the ceramics which already contain the waste into a glass matrix. In (S. Pace et al. [13]) particles of pyrochlore ceramic were encapsulated in a soda borosilicate glass matrix by hot-pressing. Using the relatively low temperature (620°C), the structure of pyrochlore remained unaltered.
3.4. Ceramic
Ceramics are multiple phase or single phase crystalline materials which exist in geological nature in the earth and other planets such as the moon and mars. Currently ceramics are the de facto standard for the immobilisation of separated long-lived FP and actinoids such as 135Cs, Pu and MA due to the following reasons:
1. Ceramics are generally more durable by several orders of magnitude than borosilicate glasses because of the crystalline structure. The existence of very old ceramics in nature confirms the superior durability.
2. Ceramics are thermally more stable than glasses (Solid glasses have the transition temperature Tg. When the temperature is greater than Tg, the viscosity of the glasses rapidly decreases.)
3. Ceramics can accommodate higher concentration of the waste such as Pu and MA which are generally soluble limitedly in borosilicate glasses.
Ceramics can be produced using several standard methods such as cold pressing and sintering, hot pressing, hot isostatic pressing and spark plasma sintering. (Albina I. Orlova and Michael I. Ojovan [12])
Many types of ceramics have been intensively investigated. We show some examples of the ceramics used for the immobilisation of FP and actinoids.
1. Hollandite
Hollandite Ba(Ti,Al)2Ti6O16 can be used for the immobilisation of Cs. Cs can be accomodated in the tunnels in the framework of (Ti,Al)O6 octahedra. (Figure 5)
2. Zirconolite
Zirconolite CaZrTi2O7 is very durable and has a good capacity to incorporate MA(Np, Am, Cm) and Pu. The structure of zirconolite consists of the Ca/Zr layer (CaO8, ZrO7) and Ti layer (TiO6, TiO5). MA and Pu can be accomodated in the Ca/Zr layer. In the case of Pu we also need to add Gd together to suppress the risk of criticality. (Figure 6)
Synroc (Synthetic Rock) is the assemblage of multiple ceramics. The research started by Professor Ted Ringwood and co workers in 1978 at Australian National University and further developed in collaboration with ANSTO (Wikipedia (Synroc) [14]). Synroc C is a basic formulation and consists of three titanate minerals: hollandite, zirconolite, perovskite, plus minor titanium oxides. It has the capacity to incorporate almost all elements in HLW. (Jostons, A and Kesson, SE [11])
Synroc is mixed with the HLW solution and the mixture is dried and calcined at 750°C to produce a powder. The powder is compressed in the process known as HIP (hot isostatic pressing) at temperature 1150 – 1200°C, which results in a cylinder of dense synthetic rock. (Wikipedia (Synroc) [14])
3.5. Solution
Our solution for the following two problems:
1. high-level radioactive waste (solid, liquid) in Tokai
2. plutonium in Monju
is to use Synroc and the professional service by ANSTO.(ANSTO [6])
The reprocessing plants in Tokai and in Rokkasho use JHCM (Joule heated ceramic melter). Due to the problem caused by the platinoids (mentioned above), the operation of the plants stopped.
Glasses are still the main wasteform for the immobilisation of non separated HLW. However, because of the amorphous structure we need to fully understand not only the property of each element but also the relationships among the elements in HLW in order to prevent the undesirable phases in the glass matrix. Ceramics have the crystal structure. They are more thermally stable, more chemically durable, and capable of incorporating more waste than glass matrix. The structure of ceramics does not change before and after the intake of wastes. Therefore ceramics have the better properties and are much easier to handle than glasses.
Sysroc provides a kind of versatile solution. We can adjust the combination of ceramics in Synroc according to the components of the target waste. The high-level radioactive liquid waste in Tokai would contain SiO2 coming from the glass frit and the high-level solid waste would contain metallic substances. We need to adjust the ceramics in Synroc or remove some elements from the waste as the impurities before applying the Synroc process.
4. Middle and long term solution
We proposed the dry cask for the storage of the spent nuclear fuel in the cooling pool and Synroc for immobilisation of the high-level radioactive waste in Tokai and the plutonium in Monjyu. These are the solutions to survive the Nankai Trough earthquake.
With regards to the long term solution for the nuclear waste, it is impossible to carry out the geological disposal in Japan. Therefore we have no clear endpoint for the handling of the nuclear waste. We have to wait and keep the waste safely until we get better solutions.
With respect to the medium term solution, we still have a lot of things to do. In the first several centuries, FP such as 137Cs and 90Sr are the main source of the radiotoxicity and the radioactivity of HLW. After that period the contribution of MA is 1000 times larger than that of FP. Currently we conduct separation of the spent nuclear fuel to produce the MOX fuel, but we should do it in order to optimise the waste management. It is important to separate the long-lived FP and actinoids from the rest of waste. If possible, we should try to transform the long-lived radioactive substances to the stable substances. (National Research Council [10]) We should also improve the dry cask storage by the immobilisation of the separated waste and the improvement of the dry cask itself. The skill of separation and transmutation is also useful for handling the melted waste in Fukushima.
We didn’t mention the cost of the proposed solutions. One reason is that we have to solve the problems to survive the Nankai Trough earthquake. Another reason is that there are actually a lot of funds for the nuclear projects in Japan. The reprocessing plant in Rokkasho has used 15 trillion yen (150 billion dollars) for the last 30 years and could not immobilise the HLW. The breeder reactor Monju has used 1 trillion yen (10 billion dollars) and has never run. JAEA submitted a 30 years plan for the dicomission of the unused plant and it was approved. We don’t have time to argue about political things, but we should recognise that the nuclear policy in Japan is led by this rotten system.
References
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