Geological conditions of safe long-term storage and disposal of depleted uranium hexafluoride

Abstract

The production of enriched uranium used in nuclear weapons and fuel for atomic power plants is accompanied by the formation of depleted uranium (DU), the amount of which annually increases by 35-40 kt. To date, more than 1.6 Mt DU has accumulated in the world. The main DU mass is stored as environ mentally hazardous uranium hexafluoride (UF 6 ), which is highly volatile and soluble in water with the for mation of hydrofluoric acid. To ensure safe UF 6 storage, it is necessary to convert this compound in chemi cally stable phases. The industrial reprocessing of UF 6 into U 3 O 8 and HF implemented in France is highly expensive. We substantiate the expediency of long term storage of depleted uranium hexafluoride in under ground repositories localized in limestone. On the basis of geochemical data and thermodynamic calcula tions, we show that interaction in the steel container-UF 6 -limestone-groundwater system gives rise to the development of a slightly alkaline reductive medium favorable for chemical reaction with formation of ura ninite (UO 2 ) and fluorite (CaF 2 ). The proposed engineering solution not only ensures safe DU storage but also makes it possible to produce uraninite, which can be utilized, if necessary, in fast neutron reactors. In the course of further investigations aimed at safe maintenance of DU, it is necessary to study the kinetics of conversion of UF 6 into stable phases, involving laboratory and field experiments.

Synthesis and multi-scale properties of PuO2 nanoparticles: recent advances and open questions

“…66,67 This overall environmental topic also includes the safety questions related to the potential intrusion and leaching behavior of water in the case of contact with the spent nuclear fuel under surface or deep underground storage. [68][69][70] This analysis explains the current, ongoing and exciting research dealing with the synthesis and characterization of PuO 2 nanoparticles. Understanding and predicting the behaviour of Pu nanoparticles in the environmental context or under storage conditions require identifying their formation mechanism, deciphering their structure and evaluating their reactivity.…”

“…66,67 This overall environmental topic also includes the safety questions related to the potential intrusion and leaching behavior of water in the case of contact with the spent nuclear fuel under surface or deep underground storage. 68–70…”

Spent fuel canister for geological repository: Inner material requirements and candidates evaluation

“…Plutonium, also present in the spent fuel UO 2 matrix, is incorporated into the UO 2 crystal structure, preventing its possible release until the matrix itself is destroyed. Additionally, plutonium is less soluble than uranium under reducing repository conditions [19,80] and so it is unlikely that it would get separated, specially because 239 Pu (the main fissile plutonium isotope) decays to 235 U with a half-life of 24 110 years, which somewhat limits the extent of hypothetical 239 Pu lixiviation prior to conversion to 235 U. The other plutonium fissile isotope likely to be present in significant amounts in spent fuel is 241 Pu, which decays with a half-life of only 14.4 years to 241 Am, an important neutron absorber (around 700 barns for thermal neutrons and [n, c] reaction [81]).…”

Crystal Chemistry of Titanates and Zirconates of Rare Earths—Possible Matrices for Actinide Isolation

“…The existence in nature of environments in which there is no migration of radionuclides underlies the method alternative to transmutation for handling long-lived isotopes, including actinides. It consists in introducing them in stable matrices and placing them at depths of several hundred meters in a special repository of highly active materials [13,[18][19][20][21][22]. The reports of the International Atomic Energy Agency and the Nuclear Energy Agency argue for the safe disposal of SNF and solidifi ed HLW [23][24][25].…”

“…Some of the techniques have been tested on real active solutions and have a technology readiness level of 6 or higher [35]. The presence of stable phases of actinides and the existence in nature of media in which there is no their migration [13,[18][19][20], underlie handling the REE-MA fraction by including them in matrices and burials. The localization of radionuclides will be ensured by the engineering barriers of the repository and the protective properties of the host rocks.…”