Experiment QUENCH-20 with BWR geometry simulation bundle was successfully conducted at KIT on 9th October 2019. This test was performed in the framework of international access SAFEST infrastructure with the users from Swedish Radiation Safety Authority (SSM) in cooperation with Westinghouse Sweden, GRS and KTH. The test objective was the investigation of a BWR fuel assembly degradation including a B4C control blade. The test bundle mock-up represents one quarter of a BWR fuel assembly. The 24 electrically heated fuel rod simulators were filled separately with krypton (overpressure of 4 bar). According to the pre-test calculations performed with ATHLET-CD, the bundle was heated to a temperature of 1230 K at the cladding of the central rod at the hottest elevation of 950 mm. This pre-oxidation phase in steam lasted 4 hours. Towards the end of this phase, the reference rod was extracted from the test bundle for determination of the oxide thickness axial distribution. During the transient stage, the bundle was heated to a maximal temperature of 2000 K. The cladding failures were observed at temperature about 1700 K and lasted about 200 s. Massive absorber melt relocation was observed 50 s before the end of transition stage. The test was terminated with the quench water injected with a flow rate of 50 g/s from the bundle bottom. Fast temperature escalation from 2000 to 2300 K during 20 s was observed. The mass spectrometer measured release of COx and few CH4 during the reflood as products of absorber oxidation; corresponding production of B2O3 should be about 97 g. Hydrogen production during the reflood amounted to 32 g (57.4 g during the whole test) including 10 g from B4C oxidation.

The QUENCH-19 bundle experiment with FeCrAl(Y) claddings and 4 FeCrAl(Y) spacer grids as well as 8 KANTHAL APM corner rods and KANTHAL APM shroud was conducted at KIT on 29th August 2018. This was performed in cooperation with the Oakridge National Laboratory (ORNL). The test objective was the comparison of FeCrAl(Y) and ZIRLO™ claddings under similar electrical power and gas flow conditions. In common with the previous QUENCH-15 experiment, the bundle was heated by a series of stepwise increases of electrical power from room temperature to a maximum of ≈600 °C in an atmosphere of flowing argon (3.45 g/s) and superheated steam (3.6 g/s). The bundle was stabilised at this temperature, the electrical power being ≈4 kW. During this time the operation of the various systems was checked. In a first transient, the electrical power was controlled with the same electrical power history as the QUENCH-15 test. As a result, the bundle was heated to peak cladding temperature of about 1000 °C reached at about 4000 s. It showed a slowed bundle heating than for the QUENCH-15 bundle (1200 °C reached at about 3000 s). In this test phase about 0.3 g of hydrogen were produced (QUENCH-15: 23.3 g). In the following phase, the power was increased continuously to 18.12 kW (corresponds to maximal power of the QUENCH-15 test). After reaching of this value the power was kept constant during about 2000 s. At the end of this phase the maximal peak cladding temperature of Tpct≈1500 °C was reached. Much lower heating rate in comparison to QUENCH-15 was measured. Exceeding Tpct≈1400 °C sharp increase of hydrogen release rate was observed. Then reflood was initiated at ≈9100 s, connected with switching the argon injection to the top of the bundle, first rapidly filling the lower plenum of the test section with 4 kg of water, and continuing by injecting ≈48 g/s of water. The electrical power was reduced to 4.1 kW during the reflood. A temperature excursion was not observed. The temperatures at all elevations decrease immediately after water injection. The total hydrogen release during the whole test was 9.2 g compared to 47.6 g in the QUENCH-15 test with much shorter high electrical power phase. The videoscope observation of the bundle at the positions of the withdrawn corner rods showed the damage of several claddings at the bundle elevations between 850 and 1000 mm. The claddings were failed either due to interaction with melted thermocouples (mostly) or by spalling of small annular cladding parts.

The QUENCH-LOCA-5 test with pre-hydrogenated optimised ZIRLO™ claddings (≈300 wppm H) was performed according to a temperature/time-scenario typical for a LBLOCA in a German PWR with the similar parameters as the QUENCH-LOCA-3 test with fresh optimised ZIRLO™ claddings: maximal heat-up rate 8 K/s, cooling phase lasted 120 s and terminated with 3.3 g/s/rod water flooding. Significant difference was the lower temperature before the start of the transient (to decrease the dissolution of hydrides during the conditioning stage of experiment). Accordingly to this, the lower maximum temperature of 1250 K was reached on the end of the heat-up phase at elevation 950 mm (1350 K for QUENCH-LOCA-3). Tangential temperature gradient across a rod was up to 30 K on the burst onset. Due to higher ballooning rate, the maximum blockage ratio of cooling channel (25% at 940 mm) was higher in comparison to QUENCH-L3 (21% at 923 mm). Due to moderate blockage good bundle coolability was kept for both bundles. Some rods have up to three ballooning regions for both tests. The reason is successive onset of ductile temperature threshold at different elevations. The cladding burst occurred at temperatures between 1027 and 1151 K (QUENCH-L3: 1064 and 1188 K). The inner rod pressure relief to the system pressure during less 30 s (similar to QUENCH-L2). The average burst temperatures of 1081 K (808°C) for QUENCH-L5 is lower than for QUENCH-L3 (1117 K or 844°C) due to lower α-Zr→β-Zr phase transition temperature. No secondary hydrogenation was indicated for the QUENCH-L5 claddings due to very short high temperature period. During quenching, following the high-temperature phase, no fragmentation of claddings was observed (residual strengths or ductility is sufficient). Tensile tests at room temperature showed cladding fracture mostly due to stress concentration in the region of burst opening.

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 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 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.