Results of the QUENCH-18 Bundle Experiment on Air Ingress and AgInCd absorber behavior

Abstract

The experiment QUENCH-18 on air ingress and aerosol release in an electrical heated test bundle with 24 rods and a length of about 2 m was successfully conducted at KIT on 27 September 2017. This test was performed in the frame of the EC supported ALISA program. 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 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-stage bundle degradation (companion test to the QUENCH-13 experiment), and of two pressurized unheated rod simulators (60 bar, He). The low pressurized heater rods (2.3 bar, similar to the system pressure) were Kr-filled. In a first transient, the bundle was heated from the peak cladding 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 pressurized rods burst at a temperature of 1045 K. The attainment of Tpct ≈ 1400 K marked the start of the pre-oxidation stage to achieve a maximum cladding oxide layer thickness of about 80 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 stage. 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 0.21 g/s. The change in flow conditions had the immediate effect of reducing the heat transfer so that the temperatures began to rise again. 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 air and steam caused release of higher chemical energy (power about 8 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 occurred and was followed by almost complete steam consumption and partial consumption of the nitrogen, indicating the possibility of formation of zirconium nitrides. Following this the temperatures continued to increase and stabilized at melting temperature of Zr bearing materials until water injection. The total consumption of oxygen, steam and nitrogen was 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 with cladding melt release, relocation and oxidation. 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. Zirconium nitrides and re-oxidized nitrides were found in the middle part of the bundle. In this bundle region, the claddings and cladding melt were strongly oxidized, the melt was collected mostly inside the grid spacer. Partially oxidized Zr-bearing melt was found down to elevation 160 mm; this elevation was the lowest with evidence of relocated pellet material. At the bundle bottom, only frozen metallic melt containing Zr, Ag, In and Cd was observed between several rods. The data of the experiment are used for validation of severe accident code systems.