*3.3. Chemical Durability Measurement by PCT Leaching Test*

The leaching performance of nuclear waste forms is a significant indicator for estimating the chemical stability [37]. The 1–42 days normalized elemental leaching rates of Si, Na, and Ce of the Cu-20 sample are depicted in Figure 7. With the extension of soaking time, the normalized leaching rates of Na and Si show a downward trend from 1 to 14 days. Then, *NR*Na and *NR*Si exhibit slight ascension within 14–28 days, and finally both of them reach the lowest values in 42 days. The day 1 and day 42 *NR*Si values are 2.04 × <sup>10</sup>−<sup>2</sup> and 1.86 × <sup>10</sup>−<sup>3</sup> <sup>g</sup>·m−2·d<sup>−</sup>1, which represent excellent stability. The lowest value of *NR*Na is 1.63 × <sup>10</sup>−<sup>2</sup> <sup>g</sup>·m−2·d−<sup>1</sup> after 42 days of leaching. This rule is not applicable for Ce, whose leaching rate has no definite regulation. The *NR*Ce varies from 7.20 × <sup>10</sup>−<sup>5</sup> <sup>g</sup>·m−2·d−<sup>1</sup> of 42 days to 2.20 × <sup>10</sup>−<sup>6</sup> <sup>g</sup>·m−2·d−<sup>1</sup> of 7 days, which is about five orders of magnitude lower than the *NR*Na values. The *NR*ce values vary irregularly but remain at a low level (10−5–10−<sup>6</sup> <sup>g</sup>·m−2·d−1). Compared with our previous research results [38], the leaching rate of Si is slightly lower in this study. The *NR*Na of the Cu-20 sample is similar to that of borosilicate glass-ceramics [39]. Nevertheless, the leaching performance of simulated nuclide in Cu-20 sample is significantly lower than that of typical vitreous products [40].

**Figure 7.** Normalized leaching rates of Si, Na, and Ce from day 1–42.
