Evolution of amorphous structure under irradiation: zircon case study

A Diver,K Trachenko,O Dicks,A M Elena,I T Todorov

doi:10.1088/1361-648x/ab9f51

Abstract

The nature of the amorphous state has been notably difficult to ascertain at the microscopic level. In addition to the fundamental importance of understanding the amorphous state, potential changes to amorphous structures as a result of radiation damage have direct implications for the pressing problem of nuclear waste encapsulation. Here, we develop new methods to identify and quantify the damage produced by high-energy collision cascades that are applicable to amorphous structures and perform large-scale molecular dynamics simulations of high-energy collision cascades in a model zircon system. We find that, whereas the averaged probes of order such as pair distribution function do not indicate structural changes, local coordination analysis shows that the amorphous structure substantially evolves due to radiation damage. Our analysis shows a correlation between the local structural changes and enthalpy. Important implications for the long-term storage of nuclear waste follow from our detection of significant local density inhomogeneities. Although we do not reach the point of convergence where the changes of the amorphous structure saturate, our results imply that the nature of this new converged amorphous state will be of substantial interest in future experimental and modeling work.

Highlights

Research into materials suitable for the encapsulation of nuclear waste is both a societal and scientific challenge
We do not reach the point of convergence where the changes of the amorphous structure saturate, our results open up the way to study this process and imply that ascertaining the nature of this new converged amorphous state will be of substantial interest in future experimental and modelling work
An important conclusion from our analysis is that the local structures in the amorphous system substantially a) vergence eventually takes places due to the need to fulfill chemical and connectivity constraints at the local level[23]. The nature of this converged amorphous state will be of substantial interest in future experimental and modelling effort, in view of continuing changes of coordination and density at the nanoscale

Summary

Introduction

Research into materials suitable for the encapsulation of nuclear waste is both a societal and scientific challenge. Part of this challenge is how to deal with existing stockpiles of plutonium. Many materials have been proposed as potential immobilization matrices (waste forms), but no matter how resistant any material is to radiation damage after a few thousand years[2] any initial crystalline waste form with a typical waste loading will undergo a transformation, becoming completely amorphized long before it is deemed safe. It is typically considered that waste forms need to be an effective barrier for long-lived nuclear waste encapsulation for up to a 1,000,000 years. For a model waste form, zircon, this transition to a fully amorphous state has been measured experimentally[2] and occurs at doses between 1018 and 1019 alpha decay events/g, which corresponds to a waste storage time in the order of 1,700 years

Methods

Results

Conclusion