Nuclear Spent Fuel Disposal: Long Term Performances Challenges

Kazi Mahmuda Tasneem Mimmi

Email: moushumy@yahoo.com

Received 30 January 2012; received in revised form 29 June 2012; online published 16 July 2012

 

Of the many issues that instigate the nuclear debate, none appears more unsettling and of so much concern to the public as the problem of nuclear waste management.Albeit, around 6% of the world’s energy and 13–14% of the world’s electricity are provided by nuclear power plant [1]; nuclear power industries, in all over the world, are encountering challenges towards minimization of potential impact of radioactive waste disposal into the environment.

While energy is the critical issue of the 21st century, it maybe anticipated that one of the most significant challenges in this century will be ensuring that the energy supplies required to raise the global standard of living will confront the threat of global climate change and decreasing availability of “clean” fossil fuels. Although not a panacea, fission-based nuclear power is playing a key role in meeting this challenge. Nuclear power on the fissioning of Uranium and Plutonium produces virtually no greenhouse gas emissions and sufficient nuclear fuel is available to support humankind’s energy needs for decades at present consumption levels. However, for fission-based nuclear power to contribute significantly to future energy supplies, it is pressing to ensure safe waste management. One such possibility for the management of spent nuclear fuel (SNF) is its permanent disposal in a deep geological repository. Nowadays, long term performance of geological storage, expected by considering immobilization of the waste product, is of great concern.

The problem associated with radioactive waste disposal essentially starts with the discussion of radioactivity: what it is, how it is produced, why it is hazardous to health, and how dangerous it is. Radioactive waste is hazardous primarily due to its energetic nature. Unlike the chemical energy associating with the burning of coal or oil, nuclear power exploits the binding energy of atomic nucleus. In the heart of the nuclear reactor, splitting of atoms causes unstable energy balances among the fragments of split atoms. Radioactivity is a process by which energetic stability is restored. Excess energy is released in terms of radioactive emission or radiation, ejecting from spitted atomic nuclei. Penetrating characteristic of such emission makes it biologically hazardous. Unlike many chemical toxins that can be neutralized, the threats of radioactivity only disappear through natural decay, which may take hundreds, thousands, or even millions of years.

The main nuclear reactor type in use in USA and throughout the world is the light water reactor (LWR). It is named so as it uses ordinary water formed from hydrogen (not deuterium, as in the heavy water reactor). The water is used as moderator, the substance composed of light elements with which neutrons collide and slow down. The water is also used as coolant that removes the fission heat. Modern reactors use higher percentage of 235U (3%) than that found in nature (0.7%). The fuel comes in small pellet of Uranium di-oxide (UO2) [2].

Spent nuclear fuel refers to uranium-bearing fuel elements that have been used at commercial nuclear reactors and that are no longer producing enough energy to sustain a nuclear reaction. It results from nuclear energy production that mainly includes UO2 in the waste product. It also contains radionuclides which include fission product and transuranium element, which are called epsilon particles[3].

Fission: 235U + 1no = fission fragment + 2-3 neutrons
Neutron capture and beta decay: 235U + 1no = 239U = 239Np = 239Pu

Once the spent fuel is removed from the reactor the fission process stops, but the spent fuel assemblies still generate significant amounts of radiation and heat. High-level nuclear waste mainly consists of this spent nuclear fuel. A typical spent nuclear fuel pellet consists of a UO2 matrix, in which fission products (e.g., Ru, Sr, Zr and Tc), actinides- chemical elements having atomic number from 89 to 103 (e.g., Np, Pu and Am)and neutron activation products (e.g., Mo, Zr and Sr), are present in various amounts [4].

The disposal of radioactive wastes deep within the Earth’s crust is considered the most promising amongst the various proposed disposal techniques. It is also rather pragmatic. Geologic disposal is attractive because, in principle, it appears that wastes canbe safely isolated from the biosphere for thousands of years or longer. However, geologic isolation of radioactive waste will not be easy to implement, and ensuring long-term isolation of radioactive wastes will not be possible without extensive research and development program. Long term performance of the disposal of waste is needed to be addressed since it becomes more complicated due to the complex interaction between fission product of radioactive waste and the surrounding ground water over time.
In spent nuclear fuel, most of the radionuclides remain in the matrix, embedded within dense grains. They can only be released into the grain boundaries via diffusion or if the matrix itself dissolutes. So, fuel oxidation/corrosion and dissolution arethe main processes for the release of the majority of radionuclides.

Despite its radioactivity for at least 100,000 years, UO2 matrix, having low solubility, can act as protective barrier under deep geologic condition and can prevent the unwanted release of radionuclides. The entire fission and neutron activation products are radioactive.Therefore, when this radioactive spent fuel will come in contact with water, it will undergo redox reaction due to the radiolysis of water, and UO2 matrix will be oxidized to form UO22+, which is soluble inwater [5].
Spent fuel matrix (UO2) is the first barrier for the stability of the long term disposal of the waste in deep geologic storage. It is encapsulated in a steel canister to avoid contact with water. But, spent fuel will come in contact with water if the canister barrier function fails. Continuous radiation will ionize the water and will change the redox condition of the system that will increase the oxidative dissolution of spent fuel [5].

Alpha- radiolysis of water

Radiolysis of water will occur as shown in Figure 1.


Figure 1: Radiolysis of water [5].

From the alpha radiolysis of water, oxidizing and reducing species and radical (HO•, O2•-, HO2•, e-aq, H•) and molecule like H2O2 are produced. These reactive species create oxidizing condition at the interface of UO2-matrix and water and thus enhance dissolution under repository condition [6, 7].

Figure 2: Groundwater and UO2 matrix interaction [4].

Figure 2 shows how UO2 matrix interacts with groundwater.Not only does the radiolytic production of oxidants by the alpha, beta, gamma radiolysis of water control the oxidative dissolution, but it is also coupled with the cathodic corrosion processes to anodic fuel dissolution (Shoesmith, 2007).

Oxidative dissolution of UO2

Under reducing condition, UO2 has a very low solubility. But, the solubility increases while UIV oxidizes to UVI form. Simultaneous oxidation and dissolution of spent fuel is shown in Figure 3. When spent fuel interacts with water, active oxidants (HO•, O2•-, HO2•, H2O2) produced due to radiolysis of water and thus dissolution of fuel is increased and fission product and actinides are produced. The reductants (H2,e-aq, H•) are also produced, but they have little effect. e-aq, H• are present in minimum level and H2 is produced by radiolysis as well as by the corrosion of steel in the canister that was used for encapsulation.

Figure 3: Oxidation and dissolution of spent nuclear fuel [8].

The mechanism involved in oxidative dissolution of UO2 comprises of two steps [9]:

UO2 + OX → UO22+
UO22+ (s) → UO22+ (aq)

Spent nuclear fuel emits alpha, beta and gamma rays, of which alpha – radiation will be the dominant one after the passage of 1000 years. So, oxidative dissolution due to alpha-radiolysis was examined and the rate of oxidation was found to be governed by the H2O2[5]. Redox condition at the fuel surface is the key factor for the fuel corrosion and it is influenced by the presence of H2 which is the corrosion product of waste container steel due to the reaction between H2O2 and Fe3+[4, 5]. While gamma/beta radiations are dominant (<1000 year), there is an oxidizing environment and eventually with time while alpha radiation is prevalent, mild oxidizing condition is achieved.

Along with the major factor of alpha radiolysis of water, dissolution of UO2 will be enhanced by the presence of carbonate ion ([NaHCO3] = 10-3mol.L-1) in groundwater (Tribet et al, 2009). It indicates that groundwater condition is another important factor of concern while considering long-term performance of nuclear waste disposal in deep geologic repository.

Possible scenario under oxidizing geologic repository condition

The main features of the oxidative dissolution of fuel with water consist of two corrosion fronts, one being the fuel surface and the other, the waste container steel liner. Radiolysis induced corrosion reactions occur on the fuel surface, where the main oxidant is H2O2 and the secondary oxidant is O2 with slow reaction rate. On the steel surface, water reacts with Fe2+ and H2. Thus steel corrosion product underdoes homogeneous redox reactions along with radiolytic reactions [10].
Figure 4 shows the possible scenario of spent nuclear fuel corrosion in deep geological condition.

Figure 4: Possible key reaction paths inside groundwater in contact with nuclear fuel containing waste[4].

Groundwater geologic condition is another important factor. Bicarbonate forms complex with fuel surface that hinders the formation of corrosion layer and increases the solubility. Thus, fuel corrosion is enhanced by the formation of bicarbonate-uranyl complex. Again, bicarbonate/carbonate couple buffers the pH near the surface that also controls pH to suppress local corrosion. So, fuel dissolution is enhanced due to the presence of bicarbonate in the water [4].

On the other hand, presence of calcium and silicate will form protective layer of their hydrated product, which leads to the formation of UVI insoluble phase. It will decrease the corrosion of fuel surface [4].

H2 can play the role of an inhibitor; it is a product of container steel corrosion process. This reaction between H2 and H2O2 to lower the dissolution rate is very slow without a catalyst. UO2 itself cannot catalyze the reaction [5]. Pb is a well-known catalyst. It is found that, a 100 year old fuel can be inhibited completely if the Pb catalytic effect exists in the UO2 fuel matrix. It would reduce UVI back to insoluble UIV, and thus would inhibit the dissolution of spent nuclear fuel, preventing the release of radioactive fission products and actinides for the long term [5].

Besides, there are some studies that proposed noble material incorporation into UO2 matrix [11]. Due to oxidative dissolution of UO2, fission product radionuclides and transuranium elements are released. The altered UO2 fuel matrix can incorporate many of these highly reactive nuclides,therefore, immobilizing them for the long term isolation. For example, experiments are conducted for the incorporation of one important radionuclide 237Np; its half-life is 2.14 million years [11]. More research should be conducted in this area forthe better understanding of the chemistry and stability of such compound formation so that highly reactive fission product can be inhibited.Further researchshould also be conducted to consider the perspective of long term performance of deep geological safe disposal of spent nuclear fuel.

Conclusion

In the coming decades, the combined effect of increasing industrialization around the world, the threat of global climate change, and decreasing availability of clean fossil fuels will make the development of alternative energy supplies more important and pragmatic. Nuclear energy offers a clean energy alternative that frees us from the shackles of fossil fuel dependence. But, large quantities of nuclear waste have already been generated.As it is evident that the spent fuel breaks down over a long period of time, and the probability of accidental chain reactions developing must be assessed. These questions indicate that spent nuclear fuel disposal will not be a simple matter. While the prime objective of nuclear waste disposal is to isolate the radioactive waste from the environment for as long as necessary so that there is no threat to public health and safety, the nuclear power plant stands on the border between humanity’s greatest hopes and its deepest uncertainties associated with the future safeguard of human life and environment.

References

[1] World Nuclear News (2010) Another drop in nuclear generation. World Nuclear Association, 05 May. Available at:http://www.world-nuclear-news.org/newsarticle.aspx?id=27665&terms=another+drop+ [Accessed on 07 December 2011]

[2] Murray, R.L., (1989) Understanding of radioactive waste. 3rd ed. Batelle Press, Columbus, Ohio.

[3] Bruno, J. and Ewing,R.C., (2006) Spent Nuclear Fuel. Element 2, 343-349

[4] Shoesmith,D.W., (2007) Used Fuel and Uranium Dioxide Dissolution Studies – A Review. Nuclear Waste Management Organization. Report No.: NWMO TR-2007-03

[5] Nilson, S., (2008) Influence of fission products and irradiation on the rate of spent nuclear fuel –matrix dissolution. Licentiate thesis KTH Stockholm

[6] Muzeau, B., J´egou, C., Delaunay, F., Broudic, V., Brevet, A., Catalette, H., Simoni, E. and Corbel, C., (2009) Radiolytic oxidation of UO2 pellets doped with alpha-emitters (238/239Pu). Journal of Alloys and Compounds. 467, 578-589

[7] Tribet, M., J´egou, C., Broudic, V., Marques, C., Rigaux, P. and Gavazzi, A., (2009) Leaching of UO2 pellets doped with alpha-emitters (238/239Pu) in synthetic deep Callovian-Oxfordian groundwater. IOP Conf. Series 9, doi: 10.1088/1757-899X/9/1/012009

[8] Ekeroth, E., (2003) Effect of radiolysis on the dynamics of UO2 dissolution. Licentiate thesis KTH Stockholm

[9] Chopping,G.R., Lijenzin,J.O. and Rydberg, J., (1995) Radiochemistry and Nuclear Chemistry, 2nd ed. Oxford: Butterworth-Heinemann

[10] Shoesmith,D.W., Kolar, M. and King, F., (2003) A mixed potential model to predict fuel (uranium dioxide) corrosion within a failed nuclear waste container. Corrosion. 59, 802-816.

[11] Burns, P.C. and Klingensmith,A.L., (2006) Uranium Mineralogy and Neptunium Mobility. Element 2, 351-356.

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