Radiation stability of natural britholites
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Radiation stability of natural britholites Tatiana S. Yudintseva Institute of Geology of Ore Deposits RAS, Staromonetny 35, Moscow, 119017, Russia
ABSTRACT Britholite, Ca-REE silicate with apatite structure, is an actinide host phase occurred in vitreous borosilicate waste forms. Such glass-ceramics are considered as potential host phases for immobilization of actinide-containing high-level waste. Crystalline phases have to be radiation resistant for this application. Radiation stability of the britholites was mainly studied by either heavy ions irradiation or incorporation of Cm-244 or Pu-238 and 240. A wide range of critical doses (0.15 - 0.6 dpa at 250C) and temperatures have been obtained depending on the compositions of the samples. Natural analogue study of the waste forms allows to predict the behavior of actinide host phases for long periods after disposal. Britholites with age from 320 to 2600 millions years, ThO2+UO2 content from 1.0 to 12 wt.%, and cumulative doses from 0.6×1019 to 7.7×1019 α-decays/g have been studied. The britholite becomes amorphous at a dose of 1 dpa (0.9×1019 α-decays/g) and higher. Critical doses for natural minerals are higher than those for synthetic samples, most likely due to re-crystallization during annealing.
INTRODUCTION Britholite, Ca-REE-An silicate with an apatite structure, is a major actinide host phase in devitrified borosilicate high level waste (HLW) glasses [1]. Due to its radiation resistance and chemical durability, britholite can be used to retain actinides during the long-term storage of vitrified HLW. Glass-ceramics with the britholite crystalline phase are also considered to be potential matrices for an actinide fraction of HLW [2]. For such application, the crystalline phases have to be radiation resistant. Radiation resistance can be estimated by ion-irradiation experiments of synthetic compounds or by their natural analogues using (U,Th)-containing minerals [3]. The latter approach also let us to predict a behavior of the actinide host phases under long-term storage in underground repository conditions. Radiation stability of the britholite-type phases was previously investigated by the studying of synthetic samples [4-6]. A very limited number of data on radiation stability of the natural britholite also exist [7]. It is probably because the minerals of this group contain high concentrations of radioactive elements (up to 20 wt.% Th and U in sum) and therefore they are normally amorphous in X-ray analysis. Temperatures of the thermal recovering of the structure of natural (U,Th)-containing britholites are unknown. The goal of this work is to evaluate the radiation resistance of britholites with various ages and ThO2+UO2 contents and compare the obtained data with the results of investigation of the synthetic compounds.
STRUCTURE AND CHEMICAL COMPOSITION OF BRITHOLITES The minerals of the britholite group (P63/m) belong to the apatite structure type. Their composition corresponds to a general formula A10B6O24X2. Ions which can fill “A”, “B” and “X”-site
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