Tailoring the radiation tolerance of vanadate–phosphate fluorapatites by chemical composition control

Abstract

The apatite-type structure of A I 4 A II 6 (BO 4 ) 6 (OH, F, Cl) 2 (A I , A II = Ca, Na, rare earths, fission product elements such as I and Tc, and/or actinides; B = Si, P, V, or Cr) offers unique structural advantages as an advanced nuclear waste form because a wide variety of actinides and fission products can be incorporated into the structure through coupled cation and anion substitutions. However, apatite undergoes a radiationinduced crystalline-to-amorphous transition, and previously, the effect of composition on the radiationinduced transformation has not been well understood. In this study, we demonstrate that vanadatephosphate fluorapatite's radiation tolerance can be controlled by varying the composition. Enhanced radiation tolerance is achieved by replacing vanadium with phosphorus at the B-site or by replacing Pb with Ca at the A-site. Correlations among chemical composition, radiation performance and electronic to nuclear stopping power ratio were demonstrated and suggest that the ionization process resulting from electronic energy loss may enhance annealing of defects upon radiation damage.

Radiation Stability of Spark‐Plasma‐Sintered Lead Vanadate Iodoapatite

“…Temperature dependence curves of D c for SPS ‐700C, BM ‐300C and the previously reported Ca 10 (VO 4 ) 6 F 2 showing a decreasing radiation tolerance from left to right. The open diamond on the far right side indicates that Ca 10 ( VO 4 ) 6 F 2 is still crystalline at this point.…”

“…The lead vanadate iodoapatite Pb 9.85 (VO 4 ) 6 I 1.7 BM‐300C exhibits significantly higher radiation tolerance than the previously reported isostructural calcium and lead vanadate fluorapatite (Pb x Ca 1− x ) 10 (VO 4 ) 6 F 2 . As a comparison, the temperature dependence curve of D c of calcium vanadate fluorapatite Ca 10 (VO 4 ) 6 F 2 is also plotted in Fig.…”

“…A TEM heating stage was used for irradiations at elevated temperatures, to obtain the temperature dependence of the radiation‐induced amorphization. The Kr 2+ fluences were converted into the universal displacive radiation damage unit of displacements per atom (dpa) by the SRIM 2008 program using a displacement energy ( E d ) of 50 eV for all the atoms, as previously suggested by other studies based on theoretical calculations …”

“…Therefore, the radiation tolerance of iodoapatite must be carefully evaluated. The radiation stability of various apatite compositions has been investigated previously, including natural F‐rich apatite, oxyapatite, hydroxyapatite, and fluorapatite . However, the radiation stability of the I‐129‐bearing iodoapatite waste form has not been investigated.…”

“…The radiation stability of various apatite compositions has been investigated previously, including natural F-rich apatite, 21 oxyapatite, [22][23][24] hydroxyapatite, 25 and fluorapatite. [25][26][27] However, the radiation stability of the I-129-bearing iodoapatite waste form has not been investigated. In this work, systematic irradiation experiments on the HEBM-synthesized and SPS-densified iodoapatite samples were performed to simulate the radiation effects caused by alpha, beta, and gamma decays, and the phase stability of the samples under the different radiation conditions was studied for evaluating their long-term performance as nuclear waste forms.…”