Yehuda Eyal1

1, Technion - Israel Institute of Technology, Haifa, , Israel

A previous study of lattice dilatation attributed the structural stability of the α-active 238,239,240PuO2, 241AmO2, and 244CmO2 to radiation annealing [1]. We present a similar study on fuel-like (U,Pu)O2, (U,241Am)O2, (Pu,241Am)O2 and (Pu,244Cm)O2. As commonly supposed, an α-decay event creates ~1,500 Ac (actinide) and O Fps (Frenkel pairs), which cause lattice expansion. But despite continuous self-irradiation, accumulation of Fps is limited to a sufficiently low damage level, which prevents metamictization. In our model [1], ingrowth of Fps involves event-by-event production of local defect zones, and prompt annihilation of randomly-occurring closely-spaced interstitial-vacancy pairs. Guided defect mobility is generated by electronic and mechanical distortions, so activation energy is not required. At ambient temperature, mostly O Fps are expected to reside [1], so only similar Fps have been considered in our analyses. Any fresh interstitial or vacancy remains stable if created outside crystal volumes that engulf each a vacancy or interstitial surrounded by empty lattice sites of unstable complementary interstitials or vacancies, respectively. These critical annihilation volumes will expand if a few defect jumps will be generated by local transient thermal spikes. The model has been validated by molecular dynamics simulations on UO2 at <5K [2]; interstitial and vacancy at closest separation and at some more distant separations combine promptly in both the U and O sublattices.

The current fourteen analyses and the ten previous analyses [1] have resulted in similar parameter values. The mean volume of damage that is created by a single α-decay event is 490±80 nm3, and thus covers ~3,070 unit cells. Damage is saturated even within non-overlapping damage sites. The mean fractional lattice dilatation at damage saturation is 0.0030±0.0003. A steady state of damage and recovery is reached under ~0.2 dpa (displacements per atom). Assuming that the volume increment per Fp is the mean volume of an atom in an undamaged crystal cell, only one surviving Fp is contained in ~9.9 unit cells and ~310 Fps reside in a single damage zone. These features, together with the observation that residual damage is independent of the nature, decay rates, and chemical properties of the Ac cations, manifest the great rigidity of the AcO2 lattice.

Understanding of mechanisms that prevent radiation-induced crystalline-to-metamict transformations in materials such as nuclear fuels is of practical and academic interest.

1. Yehuda Eyal, A radiation annealing model for maintenance of crystallinity in self-damaged actinide dioxides, Proc. 6th Intern. Conf. on Radioac.Waste Manag. and Environ. Remed., Singapore, Oct 12-16, 1997, pp. 303-307.
2. Yehuda Eyal, Spontaneous annihilation of radiation-induced point defects in uranium dioxide: A molecular dynamics simulation, Proc. 7th Intern. Conf. on Radioac. Waste Manag. and Environ. Remed., Nagoya, Japan, Sept 26-30, 1999, pp. 761-766.