The VERDON-3 and -4 tests were part of the VERDON-ISTP programme that aimed at studying the fuel and fission products (FP) behaviour in severe accident conditions. The main objective of these two complementary tests was the study of MOX fuel behaviour and FP release under oxidising (VERDON-3) and reducing (VERDON-4) conditions at very high temperature (>2300 °C). Complementary to the on-line gamma spectrometry measurements performed during the two tests, post-test characterisations were carried out in order to tackle these tasks. The two samples recovered after the VERDON-3 and -4 tests were compared to a third one extracted from the same father rod and left as irradiated. This comparison enabled to highlight the effect of temperature and atmosphere on the fuel behaviour. These three samples were characterised by several techniques available at the LECA-STAR facility of the CEA Cadarache. Experimental observations showed that an interaction between the fuel and the cladding occurred in both types of conditions by interdiffusion mainly between U and Zr. This phenomenon led to the formation of a U_yZr_1-yO_2±x cubic phase at the periphery of the fuel pellet which melted in the VERDON-4 test conditions, penetrating through the cracks of the sample and dissolving the fuel matrix. No liquid was formed during the VERDON-3 test despite the formation of a large fuel-cladding interaction zone.

VERDON-3和-4测试是VERDON-ISTP计划的一部分，该计划旨在研究严重事故情况下的燃料和裂变产物（FP）行为。这两个互补测试的主要目的是研究在高温（> 2300°C）下在氧化（VERDON-3）和还原（VERDON-4）条件下的MOX燃料行为和FP释放。与两次测试期间进行的在线伽马能谱测量法相辅相成，进行了测试后表征以解决这些任务。将在VERDON-3和-4测试后回收的两个样品与从同一父棒中提取的第三个样品进行比较，并保持照射状态。这种比较能够突出温度和气氛对燃料性能的影响。这三个样品通过CEA Cadarache的LECA-STAR工厂可用的几种技术进行了表征。实验观察表明，在两种类型的条件下，燃料和覆层之间的相互作用都是通过U和Zr之间的相互扩散而发生的。这种现象导致了U的形成_y Zr _1-y O _2±x立方相，位于燃料颗粒的外围，在VERDON-4测试条件下熔化，穿透样品的裂缝，溶解了燃料基质。尽管形成了大的燃料-包层相互作用区，但在VERDON-3试验期间没有形成液体。