Spent fuel conditioning

Currently, there are several different spent fuel conditioning technologies, at different levels of development. In this report we will consider only those that are considered potentially 

Can-in-canister. A critically safe quantity of non-processed spent fuel is placed in a can. This can is placed in a canister into which HLW glass is poured to form a solidified unit. 

Press and dilute/poison. To minimize volume, the spent fuel is mechanically compressed and either diluted with depleted uranium or mixed with a neutron poison. 

Melt and dilute. The spent fuel is melted and diluted with depleted uranium. This process was developed at the Savannah River Technology Center (SRTC), USA, and involves melting of the entire spent fuel assembly, diluting the uranium alloy with depleted uranium for isotopic dilution and aluminium for eutectic formation. 

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Emergency public or on-site personnel. This includes A major decrease in the level of protection provided to the core or large amounts of spent fuel Conditions in which any additional failures could result in damage to the core or spent fuel and High doses on- or off-site approaching the urgent protective actions interventions levels. 

Spent fuel conditioning Spent fuel Facilities in which spent fuel is conditioned for longer... 

This TECDOC is based on the results of TC Regional Project RLA/4/018. This project was successful in identifying and assessing a number of viable alternatives for RRSF management in the Latin American region. Options for operational and interim storage, spent fuel conditioning and final disposal have been carefully considered. 

The option for spent fuel management activity dealt with the identification of technically viable possibilities for research reactor spent fuel management, particularly for operational and interim storage (wet and dry), spent fuel transport (development of a dual purpose cask) and spent fuel conditioning. Moreover, it aimed to be the main source of information for the design and implementation of related facilities. 

Spent fuel conditioning. A special processing operation to prepare spent fuel for disposal. 

Spentfuel processing. Either spent fuel conditioning or spent fuel reprocessing. 

Spent fuel conditioning extra-regional context... 

Spent fuel conditioning for storage or for final disposal... 

In general, most techniques to be applied for spent fuel conditioning are based on processes already used in the nuclear fuel cycle industry or conventional industry. Up to now, however, spent fuel conditioning technology has not yet been demonstrated at an industrial stage and remains in the design phase. 

Figure 13.1 SFR pyroprocess development plan. PRIDE, pyroprocessing integrated inactive demonstration facility ACRE, advanced spent fuel conditioning process facility DFDF, dupic fuel development facility 3S, safety, security, safeguards.

While public understanding of nuclear issues may lack sophistication and is often based on inadequate or even misleading information, the public s assessments are not irrational. Having been told over many years that spent fuel is nuclear waste, it is only natural that the public should insist on its disposal. If and when effectively informed ofthe fact that spent fuel is not a waste but an energy resource, there is every reason to believe that the public will reject its deliberate burial and favor its storage under secure conditions, just as it now favors consuming, rather than immobilizing, surplus weapons plutonium. 

In order to assess the integrity of the system, we should know what kind of reactions would take place when the groundwater invades and the overpack is corroded. Consequently, the solidified waste or spent fuel itself will be in contact with groundwater. Since the waste would still be seriously activated, radiolysis of groundwater will take place and change the chemical condition, which might affect the dissolution of the solidified waste or UO2 of the spent fuel. 

Spent fuels vary in microstructure, and phase and elemental distribution depending on the in-core reactor operating conditions and reactor history. The chemical stability of spent U oxide fuel is described by local pH and Eh conditions, redox being the most important parameter. However, the redox system will also evolve with time as various radionuclides decay and the proportion of oxidants and reductants generated at the fuel/water interface changes with the altering a-, (J-, y-radiation field and with the generation of other corrosion products that can act as... 

Most repository sites under consideration for commercial spent fuel disposal are in reducing environments, such as in Boom clay formations of the Mol site, Belgium, where U02 is thermodynamically stable. In oxygen-free conditions, Spahiu et al. (2002) have shown that fuel in an... 

Detailed analysis of corroded spent fuel reacted under moist-air conditions revealed a precipitate... 

Finn, P. A. Finch, R., Buck, E. Bates, J. 1998. CoiTosion mechanism of spent fuel under oxidizing conditions. Materials Research Society Symposium Proceedings, 506, 123-131. 

Spahiu, K., Eklund, U.-B., Cui, D. Lundstrom, M. 2002. The influence of near field redox conditions on spent fuel leaching. Materials Research Society Symposium Proceedings, 713, 633-638. 

Two master variables, pH and pe, can be used to define the limits of stability of the UO2 spent fuel matrix under repository conditions. The Pourbaix diagram depicted in Fig. 2 clearly indicates the stability space of UO2 and consequently the desired chemical conditions that ensure the correct performance of the waste matrix. 

The inherent radioactive characteristics of the spent nuclear fuel condition determine many of the key processes to be studied. Owing to its energy content, spent fuel relaxes by transferring alpha, beta, and gamma radiation to water when contacting it. This originates what is known as radiolysis reactions. The key processes occurring at the spent fuel water interface are depicted in Fig. 8. 

Depending on the water composition other radical species are formed, such as carbonate and chloride radicals. This imposes net oxidizing conditions at the water—fuel interface because the generated oxidants, molecular oxygen and hydrogen peroxide, predominate under a radiation, and other radical species like OH- or CQf- are more active than the generated reductants, mainly molecular hydrogen. This is why we propose that the spent fuel-water interface is a dynamic redox system, independently of the conditions imposed on the near field (Merino et al. 2001). 

This is a process that is at present being integrated in the current models of spent fuel stability in repository conditions, but some work is still necessary in order to ascertain the reactions and mechanisms that control the reductive passivation of the U02 surface and the inhibition of the radiolytic production of oxidants. 

Once this common thermodynamic framework is established for the solubility of U02 under nominally reducing conditions, we have to ascertain the most probable pathway for the oxidative alteration of U02 spent fuel in geological repository conditions. There is a large body of evidence on the processes involved in the oxidative alteration of natural uraninites and unirradiated U02. Long-term unsaturated tests performed by Wronckiewicz et al. (1992) on groundwater from Yucca Mountain (the so-called J-13 groundwater), indicated that the formation of schoepite, as described by process (20) and (21), occurs, but is a transient event and that the alteration proceeds towards the precipitation of... 

In this chapter we have attempted to give an overview of current developments concerning waste/water interactions with special emphasis on U02 spent fuel disposal in saturated ground-water conditions and with a specific look at the... 

Merino, J., Cera, E., Bruno, J., Erikssen, T., Quinones, J. Martinez-Esparza, A. 2001. Long term modelling of spent fuel oxidation/ dissolution under repository conditions. ICEM 01. Session 23, V. The 8th International Conference on Radioactive Waste Management and Environmental Remediation. 30 September-4 October 2001, Bruges (Brugge), Belgium. 

Fig. 3, Evolution of Am(HI), Eu(III) and U concentrations with time in spent fuel pellet leaching experiments (leachate 5 mol/kg NaCl solution anaerobic conditions) radionuclides found in ultrafiltered samples (uf filter pore size 1.8 nm) arc considered as truly dissolved radionuclide concentrations found in filtered samples (f filter pore size 450 nm) are attributed to truly dissolved + colloidal species the grey shaded area marks the fraction of colloidal radioelement species in solution the black arrow indicates the pH increase in solution during the leaching experiment (Geckeis et al. 1998).

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