Experiment QUENCH-18 on air ingress and aerosol release was successfully conducted at KIT on 27 September 2017. This test was performed in the frame of the EC supported ALISA programme. It was proposed by XJTU Xi’an (China) and supported by PSI (Switzerland) and GRS (Germany). The primary aims were to examine the oxidation of M5® claddings (OD=9.5 mm, wall thickness 570 µm) in air/steam mixture following a limited pre-oxidation in steam, and to achieve a long period of oxygen and steam starvations to promote interaction with the nitrogen. QUENCH-18 was thus a companion test to the earlier air ingress experiments, QUENCH-10 and -16 (in contrast to QUENCH-18, these two bundle tests were performed without steam flow during the air ingress stage). Additionally, the QUENCH 18 experiment investigated the effects of the presence of two Ag/In/Cd control rods on early-phase bundle degradation (companion test to the QUENCH-13 experiment), and two pressured unheated rod simulators (60 bar, He). The low pressurised heater rods (2.3 bar, similar to the system pressure) were Kr-filled.
In a first transient, the bundle was heated from the peak cl ... mehradding temperature Tpct ≈ 900 K in an atmosphere of flowing argon (3 g/s) and superheated steam (3.3 g/s) by electrical power increase to the peak cladding temperature of Tpct ≈ 1400 K. During this heat-up (with the heat-up rate 0.3 K/s), claddings of the two pressurised rods were burst at temperature of 1045 K. The attainment of Tpct ≈ 1400 K marked the start of the pre-oxidation phase to achieve a maximum cladding oxide layer thickness of up to 120 µm. Then the power was reduced from 9 to 3.8 kW (simulation of decay heat) which effected a cooling of the bundle to Tpct ≈ 1080 K, as a preparation for the air ingress phase.
In the subsequent air ingress stage, the steam flow was reduced to 0.3 g/s, the argon flow was reduced to 1 g/s, and air was injected with the flow rate of 0.2 g/s. The change in flow conditions had the immediate effect of reducing the heat transfer so that the temperatures began to rise again. After some time measurements demonstrated a gradual increasing consumption of oxygen. The first Ag/In/Cd aerosol release was registered at Tpct = 1350 K and was dominated by Cd bearing aerosols. Later in the transient, a significant release of Ag was observed along with continued Cd release, as well as a small amount of In. In contrast to the QUENCH-16 test (performed with the air ingress stage without steam flow), oxidation of bundle parts in steam caused release of additional chemical energy (power about 4 kW) and consequently acceleration of bundle heat-up. A strong temperature escalation started in the middle of the air ingress stage. Later a period of oxygen starvation was occurred and was followed by almost complete steam consumption and partial consumption of the nitrogen, indicating the possibility of bundle. Following this the temperatures continued to increase and stabilised at melting temperature of Zr bearing materials until water injection. The total uptakes of oxygen, steam and nitrogen were 100±3, 450±10 and 120±3 g, respectively. During the starvation period a noticeable production (about 25 mg/s, totally 45±1 g) of hydrogen was measured. Almost immediately after the start of reflood there was a temperature excursion in the mid to upper regions of the bundle, leading to maximum measured temperatures of about 2450 K. Reflood progressed rather slowly and final quench was achieved after about 800 s. A significant quantity of hydrogen was generated during the reflood (238±2 g). Nitrogen release (>54 g) due to re-oxidation of nitrides was also registered.