Community Research and Development Information Service - CORDIS

Final Report Summary - ALISA (Access to Large Infrastructures for Severe Accidents)

Executive Summary:
Transnational access to large-scale research facilities in Europe and in China is offered in the ALISA project to allow the optimal use of the resources in the complex field of severe accident analysis for the existing power plants. This research involves very substantial human and financial resources and, in general, the research field is too wide to allow investigation of all phenomena by any national program. To optimize the use of the resources, the collaboration between nuclear utilities, industry groups, research centers and safety authorities in Europe and in China is very important. This is the main objective of the ALISA project, which aims to provide these resources and to facilitate this collaboration by providing large-scale experimental platforms in Europe and in China for transnational access. Activities within the ALISA project focus on the large-scale experiments under prototypical conditions addressing the remaining R&D issues on severe accident management in light water reactors: coolability of a degraded core, corium coolability in the reactor pressure vessel, possible melt dispersion to the reactor cavity, and hydrogen mixing and combustion in the containment. The major aspect is to understand how these events affect the safety of existing reactors and how to deduce soundly-based accident management procedures, such as e.g. in-vessel melt retention or hydrogen control.

Eight experimental facilities from six Chinese research organizations provide access to European users and six facilities from KIT and CEA provide access to the Chinese partners. The project started on June 1st, 2014 and has four years duration. Two calls for proposals have been undertaken during the project followed by the evaluation and selection of proposals by the User Selection Panel. All the facilities offered for access in Europe and six facilities in China have received proposals. The European facilities, QUENCH, LIVE, DISCO, HYKA at KIT, and KROTOS, VITI at CEA, have received approved proposals from Chinese partners; and the Chinese facilities, namely COPRA from Xi´an Jiaotong University (XJTU), HYMIT and WAFT from Shanghai Jiaotong University (SJTU), and IVR2D, IVR3D from CNPRI and MCTHBF from Nuclear Power Institute of China (NPIC), have received proposals from European users. The nature of most Chinese proposals reflects the high demand to check the safety design of their own reactor types. Since some EU and Chinese proposals investigate similar phenomena but in different scale and geometry, such as LIVE and COPRA, HYKA, HYMIT and MCTHBF, the comparison of the test results will provide a broader range of applicability. Other proposals investigate different aspects of a same severe accident strategy, such as LIVE and IVR2D/IVR3D. The combined knowledge from the experiments can provide comprehensive understanding of the phenomena of in-vessel melt retention with external cooling.

The major findings of ALISA experiments were widely disseminated in a variety of sources, such as yearly QUENCH Workshop, Annual ERMSAR meetings, peer-reviewed papers in scientific journals, presentations in multi-lateral partner´s Meeting and at international conferences. Special session on ALISA results is organized at the NUTHOS-12 International Conference in fall 2018. This strengthens the collaboration between different partners and will maximize the benefits of the research results.
Initiated from the high interest of the bilateral access to large infrastructures on severe accident research, endure a long way of the project preparation, with a successful start and an excellent performance during the project lifetime, ALISA demonstrated, how the expertise of European and Chinese researchers can be efficiently used to maximize the human and financial resources, to learn from each other and to disseminate the outcomes to the open society as a whole.

Project Context and Objectives:
Enhanced transnational access to large research infrastructures in Europe and in China is offered to allow the optimal use of the resources in the extremely complex field of severe accident analysis for the existing power plants. This research involves very substantial human and financial resources and, in general, the research field is too wide to allow investigation of all phenomena by any national program. To optimize the use of the resources, the collaboration between nuclear utilities, industry groups, research centers and safety authorities, at both European and Chinese levels is very important. This is precisely the main objective of the ALISA project, which aims to provide these resources and to facilitate this collaboration by providing large-scale experimental platforms in Europe and in China for transnational access.

Large-scale facilities of the ALISA project are designed to resolve the most important remaining severe accident safety issues, ranked with high or medium priority by the SARP group for SARNET NoE. These issues are:
- Core coolability during reflood and debris cooling;
- Ex-vessel melt pool configuration during Molten Corium Concrete Interaction (MCCI), ex-vessel corium coolability by top flooding;
- Melt relocation into water, ex-vessel Fuel Coolant Interaction (FCI);
- Hydrogen mixing and combustion in containment;
- Oxidizing impact (Ruthenium oxidizing conditions/air ingress for High Burn-up and Mixed Oxide fuel elements) on source term;
- Iodine chemistry in Reactor Coolant System (RCS) and in containment.

The major aspect is to understand how these events affect the safety of existing reactors and how to deduce soundly-based accident management procedures. The aim is not only to understand the physical background of severe accidents but to provide the essential knowledge that can help to reduce the severity of the consequences. It is crucially important to understand the whole core melt sequences and identify opportunities to lower the risk, which can be done by e.g.:

- altering the timing or magnitude of reflooding the degraded core,
- in-vessel melt retention in the lower plenum of the reactor pressure vessel,
- ensuring the upper bound of system pressure at vessel failure by dedicated depressurisation valves,
- installation of devices or implementing accident management procedures to mitigate melt dispersion into the containment,
- delaying basement penetration or even by regaining melt cooling before containment failure or basement penetration occur,
- implementing hydrogen mitigation measures in the containment (ignitors, recombiners etc.).

Activities within the ALISA project are focused on the large scale experiments under prototypical conditions addressing the remaining R&D issues on severe accident management in light water reactors. These will help the understanding of core degradation, melt formation and relocation as well as core coolability in real reactors in two ways – firstly by scaling-up and secondly by providing data for the improvement and validation of computer codes applied for safety assessment and planning of accident mitigation concepts, such as ASTEC. ALISA offers a unique opportunity for all parties to get involved in the networks and activities supporting safety of existing and advanced reactors and it allows European and Chinese researches to work together as equal partners so as to arrive at a wider common safety culture.

The importance of the ALISA project for the European and Chinese severe accident research is reflected in three aspects:
1) The access to large scale experimental facilities is proposed to investigate all important processes from the early core degradation to late in-vessel phase pool formation in the lower head, continuation to ex-vessel melt situations and to the hydrogen behaviour in the containment. Therefore four high priority issues identified by the SARP group will be addressed in the project.
2) The results of the project will be applicable both to the European and Chinese reactor fleet taking into account the main light water reactors.
3) The project offers a unique opportunity for Chinese experts to get an access to large scale facilities in Western research organization and vice versa, to improve understanding of material properties and core behaviour under severe accident conditions, and to become familiar with the high level safety concepts in nuclear power plants.
Project Results:
1. ALISA EXPERIMENTS IN EUROPEAN FACILITIES

1.1 QUENCH
QUENCH-ALISA (QUENCH-18) was the first large scale bundle test including a prototypical experiment phase in air + steam mixture [6]. The bundle contained 20 heated and 2 unheated rods with M5® cladding as well as 2 Ag-In-Cd absorber rods. The test was performed at KIT on September 27, 2017 in the framework of the ALISA project. It was proposed by XJTU Xi’an (China) and supported by PSI (Switzerland) and GRS (Germany). Three typical features of QUENCH-18 were: moderate pre-oxidation to ≈ 80 µm of oxide layer (less than in QUENCH-16), a long period of oxygen starvation during the air and steam ingress phase (1770 s instead 800 s for the QUENCH-16 test performed without steam injection during air ingress), and reflood initiation at the melting point of the cladding (≈ 2000 K instead of 1700 K for QUENCH-16).

The claddings of unheated and pressurized (6.0 MPa) rods burst at 1045 K at a heat-up rate of 0.3 K/s. These burst temperature is lower in comparison to burst temperatures observed during the bundle test QUENCH-L2 (Tpct = 1138 ± 34 K) due to lower heat-up rate and thinner cladding wall.

The temperature escalation during the air ingress between elevations 150 and 850 mm was significantly stronger than for QUENCH-16 mainly due to additional exothermal cladding oxidation in steam (corresponding additional chemical energy of ≈4 kW was even slightly higher than electrical power). The metallographic investigations of the Zry corner rod, withdrawn at the end of escalation, showed formation of ZrN inside α-(ZrO) layer formed above the oxide layer during oxygen and steam starvation.
Releases of aerosols and helium were registered at the beginning of temperature escalation (failure of absorber rods). Simultaneously, the readings of cladding surface thermocouples below elevation of 550 mm indicated the relocation of absorber melt. Aerosol measurements were performed with two systems: 1) on-line device ELPI (electrical low pressure impactor) and 2) two particle collection devices BLPI (Berner low pressure impactor). Additionally, three polycarbonate filters were installed in parallel to BLPI and withdrawn successively during the air ingress phase. Effective diameter of sampled particles (ELPI and BLPI) was measured to be between 0.4 and 10 µm (main part of released particles had diameter of about 1 µm). Based on the EDX analysis, a rough estimation of absorber material releases during the whole test was performed: 12% Cd, 0.7% Ag, and 0.4% In were released as aerosols.

During the starvation period about 100 and 450 g oxygen and steam were consumed. During the steam consumption period about 45 g hydrogen were released. In the same time a partial consumption of nitrogen (about 120 g) was registered.
Initiation of reflood with 50 g/s water caused strong temperature escalation to about 2450 K at elevations between 750 and 1150 mm resulting in about 238 g hydrogen release (128 g for QUENCH-16). During re-oxidation of zirconium nitrides more than 54 g nitrogen were released. Final quench was achieved after about 800 s.

First metallographic investigations of the bundle at elevations between 1090 and 1500 mm show strong cladding oxidation and Zr melt formation below the elevation of 1430 mm. The melt relocated below 1350 mm was completely oxidized. No remaining nitrides or nitrides re-oxidized during reflood were indicated at these upper elevations.

1.2 LIVE
In-vessel melt retention by flooding the reactor vessel externally is regarded as an effective severe accident management (SAM) strategy for existing and advanced light water reactors (LWR). One key question of this strategy is the influence of different upper cooling conditions on the heat flux through the vessel wall. High uncertainty also exists on the up/down heat transfer splitting rate of a melt pool under different top boundary conditions. In the frame of the ALISA project, LIVE-ALISA experiment was proposed by CNPRI to quantify the influence of top cooling condition on the general heat transfer rate and the heat flux distribution of a melt pool with otherwise same features, e.g. decay power, pool height and external cooling [7].

1.2.1 LIVE-3D Facility
The LIVE 3D test facility consists of a hemispherical test vessel with external und upper cooling system, a volumetric heating system and a heating furnace for the preparation of liquid simulant melt. The inner diameter of the vessel is 1m. The vessel is made of stainless steel, and the wall thickness is ~25 mm. It is enclosed with a cooling vessel to enable the external cooling. The cooling water flows in at the bottom and flows out through one outlet at the top of the cooling vessel. The test vessel is covered with either an insulation lid or a cooling lid to realize various modus of external cooling and top cooling. The cooling lid has for water inlets at the periphery and one outlet at the central. The flow rate can be individually adjusted to guarantee a uniform distribution of water on the whole lid.

The decay power of the melt is simulated by 8 horizontally-oriented electrical heating coils, and their power inputs can be controlled individually for the homogenous power generation in the melt pool.
The maximum homogenous heat generation is 29 kW. The homogeneity of the heating system is evaluated by CONV CFD calculations [7], and proves that there is little local effect in bulk pool; however over heating at vessel bottom can happen due to the massive formation of crust. The liquid simulant melt is prepared in the heating furnace, which can tilt and pour the liquid melt into the test vessel either centrally or near the vessel wall via a pouring spout. Different relocation modus during transient state after melt relocation can be therefore captured. At the end of one test, liquid melt can be extracted back to the heating furnace and uncovering the crust in the vessel.

The LIVE-3D test vessel is extensively instrumented. 3D melt temperature distribution and 3D heat flux distribution can be determined with 80 thermocouples (MT) in the bulk melt, 26 pairs of thermocouples on inner and outer surface on the vessel wall and 7 arrays of thermocouples (CT: crust temperature) mounted perpendicularly to the wall in the wall boundary area. The boundary-area temperatures are important parameters for the determination of the boundary position at the melt/crust interface, and the crust growth rate during the transient. Besides, two video cameras are installed for the observation of melt pouring process and one IR camera records the turbulent pattern on the melt surface. More detailed descriptions of LIVE test facility are given in [9][10][11].

1.2.2 Experimental procedure
For the LIVE-ALISA experiment, three top boundaries were defined: top water cooling, top air cooling and top insulation. There are two power levels for each condition. The cooling lid is placed on the pool height of 409mm, and allows slight vertical adjustment.
Since the melt volume and thus the surface position varied slightly with power levels, the melt surface was frequently controlled to ensure the contact between melt and cooling lid. The vessel wall was continuously cooled with subcooled water. As simulant material, the eutectic mixture of 50 mol.% - 50 mol.% KNO3-NaNO3 with melting temperature is 220°C. Six test phases were performed with following general test conditions:
• External cooling: cooling vessel was filled with water and water flow was started 4 minutes before melt pour. Flow rate was varied to maintain the water inlet and outlet temperature difference at about 10°C
• Top water cooling: variable flow rate to maintain the water inlet and outlet temperature difference at about 10°C
• Top air cooling: compressed air with flow rate of 16m³/h
• Pool height: 409 mm in upper insulation test; and 409 mm as virtual height in top cooling phases if there were no cooling lid;
• Cooling lid position: 403 mm is the lowest positon of the cooling lid
• Heating power: 28.5 kW and 24 kW each upper cooling test
• Melt temperature during melt pour: 300°C

1.2.3 Experimental results
The selection of parameters for the calculation of Nu and Ra are: property values at film temperature Tfilm; the characteristic length both for Rai and Nudn and Nuup is pool height. The internal Raleigh number Rai is in the range of 1013 for the LIVE experiment. Concerning the heat transfer through vessel wall, although the Nudn under the top water cooling condition was higher than the cases under top insulation and top air cooling, the temperature difference in pool, Tmax-Tint, is also significantly lower than the upper insulation case. As consequence, the average heat flux under top cooling is 45% of the one under surface insulation. More notably, the maximum heat flux is even lower, which is only 27% of that under top insulation. The heat flux up/down ratio, qup /qdn, is 3.5 under top cooling. The high upward heat flux under top cooling contributes effectively to maintain the low level of melt temperature and heat flux through the wall, especially at the very critical position near the melt surface.

1.3 DISCO
The DISCO-AP1000 experiment was designed to investigate the consequences of a failure of the In-Vessel-Retention method in an AP1000 plant during a postulated core meltdown accident. When the lower head of the reactor pressure vessel fails under a residual pressure higher than 0.2 MPa, molten corium will be ejected under pressure into the reactor pit which is flooded with water. The molten corium will fragment into small droplets and interact with the water leading to an intense heat transfer process. The experimental results can be useful to improve the modelling of fuel coolant interaction (FCI), specifically for the geometry of the AP1000 reactor cavity, which is characterized by a narrow gap between reactor pressure vessel and cavity wall and small flow paths out of the reactor pit. Since the DISCO facility simulates also the containment, the experiment can also lead to better understanding of effects of FCI in respect to the containment pressure and its integrity. The experiment was proposed by China Nuclear Power Technology Research Institute, Shenzhen, P.R. China, and was carried out on 26.6.2018 in the DISCO-facility at Karlsruhe Institute of Technology.

The DISCO facility models the containment, the pit and the pressure vessel together with the reactor cooling system. The containment is modelled by a cylindrical pressure vessel with an empty volume of 14 m³. In respect of the diameter of the RPV the linear scale of the model is 1:13.4. Initially, the RPV is filled with 16 kg of an iron-oxide aluminum thermite powder, which results in 8.5 kg iron and 7.5 kg aluminum oxide after reaction at a temperature of approximately 2200 K. The break at the bottom of the melt generator is modelled by a graphite annulus with a diameter of 30 mm (flow area 7 cm²). It is closed with a brass plate, which melts at contact with the thermite melt. The cavity and containment is filled with water (55°C) up to a level of 0.965 m. The atmosphere in the containment and cavity is air at room temperature and 0.1 MPa pressure. The RCS-RPV vessel is filled with nitrogen at 0.476 MPa.

There are two flow paths out of the cavity into the containment. The first flow path is through the annular gap between the RPV and the cavity leading to the vessel support with the minimum cross section of 31 cm² and leading further to the cold and hot legs. Near the bottom of the cavity the connection to the RCDT is a small flow path of 10 cm². Data registered consist of temperatures and pressures in all compartments and debris collected post-test, which is analysed by size and location.

The experiment is started by ignition of the thermite and after 5 seconds the reaction front has reached the bottom of the RPV. The brass plate is melted and opens the hole and the blowdown starts. The pressure in the RPV at start of blowdown was 1.88 MPa. It took 2 seconds to reach pressure equilibrium with the containment at 0.2 MPa. The maximum pressure in the cavity reached 0.6 MPa after 20 ms close to the bottom, no steam explosion occurred. In the containment the pressure reached 0.21 MPa after 4 seconds and decreased to 0.14 MPa at 28 seconds.

The thermocouples in the cavity were destroyed by the melt after 20 ms at the bottom and after 260 ms at the top. In the containment the peak temperature of 490°C was reached after 2 seconds above the water level. The average temperature in the containment atmosphere was between 250°C and 300°C lasting for about 10 seconds Due to the small gap between RPV and cavity and the small path at the vessel support the melt welded together both parts. A separation is only possible by destroying the cavity and the RPV. Only little of the debris was carried out through the openings of the cold and hot legs and the connection to the RCDT. The analysis of the debris will be conducted when it will be dried, which will take still some time.

1.4 HYKA-A2
The largest safety vessel A2 of the KIT HYKA test site with main dimensions of 6 m id and 9 m height provides an empty test volume of about 220 m3. It is designed for fire and explosion tests with an operating over-pressure from -1 to 10 bar. Depending on the purpose, large samples or structures can be tested inside them, or the whole vessel can be used as a test volume. The vessel can be evacuated or filled with inert atmosphere of nitrogen or steam and be heated up to 150 °C. The vessel is equipped with many vents and ports for experiment and measurement set-ups as well as with windows for visual observations. It has 3 vents of 2000 mm id, 4 vents of 700 mm id, 5 vents of 400 mm id and about 40 vents of smaller inner diameters (50-250 mm). The measuring system consists of thermocouples array (gas temperature, flame arrival time); piezoelectric and piezoresistive gauges (initial pressure, explosion pressure); gas analyzer and mass spectrometer (to control mixture composition); sonic hydrogen sensors, photodiodes and ion probes (flame arrival time, flame speed), strain gauges (deformations). The data acquisition system is based on multi-channel (64) ADC with a sampling rate of 1 MHz. The vessel was successfully tested within LACOMECO Project using 2 large scale combustion experiments of hydrogen-steam-air mixture (10:25:75 = H2:H2O:air) at 1.5 bar of initial pressure and 90 °C temperature [16].

During a hypothetical severe accident in a reactor of NPP and reactor core degradation hydrogen can be produced and then accumulated as a stratified layer of hydrogen-air mixture at the top of reactor building [17][18]. Different flame propagation regimes of such a mixture may occur. Water spay as a combustion suppression system can be used.

Shanghai Jiao Tong University has proposed to perform a series of experiments on flame propagation in a stratified hydrogen-air mixture in a large scale facility HYKA-A2 (220 m3). Three different vertical linear hydrogen concentration gradients of 14→0, 12→2 and 10→4%H2 with the same amount of hydrogen equal to 7% of the average concentration should be investigated. Four experiments with central ignition point with uniform and non-uniform hydrogen concentration to be performed. A mitigation test with water spray on flame suppression will also be conducted.

Dynamics of the combustion process to be registered by measuring of the temperature, pressure, acoustic effects and optical observation using Background Oriented Schlieren Method (BOS).

1.4.1Test matrix and experimental details
The measurement system consists of 7 pressure sensors, 24 thermoelements, 7 H2-sampling probes, 3 Stemmer high speed cameras (70 fps), 2 Canon cameras (30 fps), 2 finger cameras (25 fps), 2 microphones.pressure. The facility is also equipped with a gas filling system, sampling probes and concentration measurements and ignition device. Two ventilators and a system of pneumatic valves also belong to the gas filling system. Safety alarm sensors were installed inside the A2-vessel to control a flammable hydrogen concentration and minimum oxygen concentration for personal in between the experiments to be able to work for test preparation inside the vessel A2.

Three different vertical linear hydrogen concentration gradients of 14→0, 12→2 and 10→4%H2 with the same amount of hydrogen equal to 7% of the average hydrogen concentration have been created using a gas filling system. Required amount of hydrogen equal to 7%H2 in average was injected with or without mixing by fans. Mixing option was only used for uniform compositions. To create a gradient of concentration, hydrogen was injected at different altitude and then equilibrated due to a turbulent diffusion. Local hydrogen concentration was measure by 7 sampling probes and analyzed by Rosemount gas analyzer as time dependence. 5 measuring points were located at the centerline and two at the side wall. The level of 7%H2 was always kept at the ignition point in the center. Required hydrogen concentrations at the top and bottom have also been established.

A hot wire provided an ignition of the test mixtures in 2-3 minutes after the mixing procedure to suppress a turbulence generated by mixing fans. A center ignition (CI) position at the centerline H = 3.15 m from floor level was used in the tests where the concentration kept constant 7%H2. The only one test with upper ignition position (H = 6.95 m) at highest hydrogen concentration was used. Pressure, temperature records simultaneously with video observations of combustion process were performed in the tests. Total record time was about 10.5-21.0 seconds for fast controllers and about 1400 seconds for slow controllers. All the pressure and temperature records and video cameras were synchronized with an ignition moment with a pre-record time of about 0.5 s.
A water dispersion system has built on top of the A2 vessel at H = 8.14m. It was based on the WhirlJet Spray Nozzles type 1CX-SS15, full cone spray, with a capacity 100 liter/min. The water spray provides a 120o of opening angle.

It includes a series of experiments with stratified compositions of 3 different gradients, two tests with uniform mixture of 6.5 and 7%H2 one test with upper ignition position and two experiments with water spray. Characteristic combustion velocity can roughly be evaluated as a ratio Uf = R/ t1/2.

1.4.2 Experimental results and discussion
Strong influence of hydrogen stratification and ignition position was found in the tests. The maximum combustion pressure increases 10 times for stratified hydrogen mixture (1.7 bar) as compared with the same amount of hydrogen equal to 7% H2 (0.16 bar). The time for maximum pressure roughly corresponds to complete combustion time equal to ~2·t1/2, which is proportional to the average flame speed. Then we may tell that the average combustion velocity for a stratified mixture (~1m/s) is about 2 times higher than that (0.6m/s) for uniform mixture of the same amount of hydrogen (7%H2). Maximum combustion temperature behaves almost the same way as pressure. Namely, the maximum combustion temperature 1300-1600C for stratified combustion is much higher than for uniform combustion (260C for 6.5% H2 and 370C for 7% H2). Figure 10 shows that for stratified compositions the combustion rate governs by highest hydrogen concentration at the sealing rather than an average hydrogen concentration of the mixture. The changing of highest hydrogen concentration from 10 to 14%H2 leads to maximum combustion pressure increase from 1.4 bar to 1.7 bar and combustion temperature increase from 800C to 1300C. An additional confirmation of the importance of maximum hydrogen concentration on combustion process has been done by upper ignition position. The ignition at highest hydrogen concentration 14%H2 leads to maximum combustion pressure increase to 1.7 bar and two times higher average combustion velocity compared to center ignition at 7%H2 for the same stratified mixture 14→7→0%H2.

A weak influence or even promoting effect of water spray on combustion process have been found. The spray was initiated 60 ms after ignition of the mixture when the flame develops quite well (about 1m radius). Higher combustion pressure (1.6-1.7 bar) and faster combustion time (t1/2 = 2.72s) were registered due to an additional turbulence in presence of water spray. Combustion temperature has also increased in 100-200 C compared to dry mixture.

1.4.3 Summary
(1) Hydrogen distribution experiments in HYKA-A2 vessel were performed in order to create a relatively stable vertical hydrogen concentration gradients.
(2) Flame propagation experiments with uniform hydrogen concentration of 6.5 and 7%H2 for center ignition (CI) point have been carried out as reference tests.
(3) Flame propagation experiments with center ignition point for three different vertical hydrogen concentration gradients of 14→0, 12→2 and 10→4%H2 with the same amount of hydrogen equal to 7% of average concentration have been performed. Strong influence of hydrogen stratification was found. The maximum pressure increases 10 times (1.7 bar) for stratified mixture compared to uniform one of the same amount of hydrogen equal to 7% H2 (0.16 bar). The temperature is also increased from 370C (7%) to 1300-1700C (14→7→0%H2)
(4) The governing role of highest hydrogen concentration on top on combustion process for stratified mixture was experimentally shown. There is no effect of average hydrogen concentration.
(5) One test with upper ignition (UI) point and vertical hydrogen concentration gradient of 14→0%H2 was performed. It leads to the highest combustion over-pressure (1.7 bar) due to two times higher combustion velocity as compared with center ignition (CI) point. It also demonstrated dominating role of highest hydrogen concentration.
(5) An effect of water spray on flame propagation was studied in two tests with center ignition and vertical hydrogen concentration gradient of 14→0%H2. No suppression effect of water spray (100 l/min) was found on combustion. Maximum combustion temperature increases from 1020C to 1400C due to an additional turbulence in presence of water spray.

1.5 KROTOS
The KROTOS facility consists of three main parts [13]: the furnace (1), the transfer channel (3), the test section with its X Ray radioscopy system. The furnace is a water cooled stainless steel container designed to withstand 4 MPa pressure and is equipped with a three-phase cylindrical heating resistor made of tungsten. In order to avoid heat losses, the heating element is surrounded by a series of concentric reflectors and closed by circular lids made of molybdenum. The tungsten crucible is hanged inside the heating element; its net volume is 1 liter, which enables the melting of up to 6 kg of corium. The furnace has been designed to operate in inert atmosphere or vacuum at temperatures up to 2800 °C.

The transfer channel is a vertical tube, connecting the furnace and the test section. It is used to transfer the crucible containing the melt to the test section. At the top of the transfer channel a fast hydraulic ball valve (2) is positioned; at the bottom are placed the puncher and melt release cone. A fusible tin membrane guarantees zero initial velocity and gravitational release conditions. The water-filled test section is installed within a cylindrical containment. Both vessels are made of 7075 aluminium alloy (Fortal) in order to reduce X-ray attenuation enabling X-ray imaging of the interaction [14]. A 9 MeV Linatron linear accelerator emits the X-ray. A Ta-Gd scintillator placed at the opposite side of the test section is captured by a 200 fps video camera.

Hong Kong City University has proposed to perform a test similar to past KROTOS test KFC [14] and KS 4 [15] but with shallower water pool depth (645 mm vs. 1245 mm in previous KROTOS tests at CEA Cadarache). It must be noted that while KROTOS was in JRC Ispra, the dozen of tests with UO2-ZrO2 have also been performed in water pools between 0.9 m and 1.14 m depth [19][20][21].

1.5.1 Test conditions
This experiment which has been carried out on November 17th, 2016 [22]. In order to have similar jet velocity at the water pool surface, the test section bottom plate has been translated upwards (in order to keep a 492-mm free fall distance in gas). Thermocouples have been installed on the test section central line at 100 (ZT3), 200 (ZT35), 300 (ZT4), 500 (ZT5) and 700 (ZT6) mm above the bottom plate where a pressurized gas capsule, serving as steam explosion trigger is installed. Other thermocouples have been positioned on the vessel wall, as well as static and rapid pressure sensors. A capacitive water level sensor completes this instrumentation layout. Asides from the X-ray imaging, there is a video imaging of the jet in the gas phase.

1.5.2 Outline of the experiment
5150 g of (80 wt% UO2- 20 wt% ZrO2) mixture are first molten in a tungsten crucible under helium atmosphere in KROTOS furnace after a two-hour heating phase. A Keller bichromatic pyrometer measures a plateau at 2831°C on the crucible lateral surface. As the liquidus temperature of this mixture is estimated at 2650°C, this corresponds to a superheat of 180 ±25°C.

The crucible has been released, transferred to the puncher and a cylindrical jet has been observed by the video camera in the air space between injection nozzle and water. A smooth, continuous cylindrical shape is observed showing the improvement in initial jet conditioning compared to earlier KFC experiment [14]. The crucible is then released, transferred to the puncher and the jet is observed by the video camera in the gas space between injection nozzle and water. A continuous and coherent shape is observed showing the improvement in initial jet conditioning compared to the earlier KFC experiment [14].

A continuous jet is clearly visible. It is brighter in the central part where the X-ray intensity is larger. When the jet reached the prescribed depth, the trigger has been actuated leading to a steam explosion. At this time, only about half of the corium melt has been injected in the test section. A significant volume of water has been splashed out of the test vessel. Similar behavior has for instance been observed in the DISCO-LACOMECO test where only 4% of water remained in the reactor pit mockup.

Corium pouring has been halted by the pressurization induced by the explosion, then resumed. It appears that the pool height has been reduced by more than 50 %. With such a small pool height and the deflection of its surface close to the jet impact, it seems that part of the corium reached the test section bottom without having been totally fragmented.

1.5.3 Post-test examinations
After test completion, the test section has been dismounted. Due to the presence of very fine particles, the dried top surface looks like dried soil. The larger blocks are assumed to be part of unfragmented jet that have been poured after explosion in the very shallow remaining water pool. A large quantity of debris has also been found in the lower part of volume between the test section and the facility containment. A similar transfer of debris to the annular space between the test section and its containment vessel has been observed during FARO L-33 test with triggered steam explosion. Several sizes of particles are visible in the debris from the test section bottom which have been spread over a plate for photography: fine debris (sub-millimetric), medium-sized debris (between mm and cm) and strongly bonded coarse aggregate (above 10 mm). Debris that has been found in the space between the two cylinders. They are assumed to have experienced the steam explosion and to have been projected out of the test section. There are fine (sub-millimetric) black debris and larger (millimetric) white-greenish debris.

1.6 VITI
CGN has proposed to test in VITI facility at CEA Cadarache one oxidic mixture characteristic of in-vessel retention scenario, as well as two oxidic mixtures characteristic of ex-vessel retention scenarios but with different compositions. Sample one is actually a sample from the previously performed ALISA-KROTOS test.

1.6.1 VITI MBP Experimental set-up
Corium powders are placed into a tungsten crucible. The crucible is positioned onto a dedicated support system. The whole system is heated by so-called “direct” induction method. The water-cooled inductor is connected to the power generator at a given working frequency f (up to 400 kHz) and electric power P (up to 25 kW). This inductor is electromagnetically coupled with the electroconductive crucible. The latter heats the sample up to the target temperature by mainly conductive heat transfer (up to 3000°C). The thermal shield is placed around the susceptor, in order to decrease thermal losses (e.g. those linked to radiative heat transfer) and thermal gradients in VITI High-Temperature Chamber (HTC).

Once liquid state is reached, a tungsten-made capillary tube is immersed into liquid corium, the immersion depth being accurately determined by means of the micrometer translation stage. A very low gas-flow rate is imposed by the flowmeter. Thus, a continuous bubbling is imposed inside liquid corium due to the gas (typically argon) flowing through the capillary. The related pressure oscillations are measured by the differential pressure sensor, designed for high-accuracy pressure measurements (range: 0-100 mbar, manufacturer accuracy: 0,04%). Finally, the experiments are carried out within the water-cooled confinement vessel, in an inert atmosphere of argon with absolute pressure 1.3-1.5 bar.

The bi-chromatic video pyrometers are focused on the crucible through adequate windows and holes provided in the HTC design. Finally, the whole acquisition data is collected by the data bus and sent to the data acquisition computer, while the imaging computer allows for crucible monitoring, both computers being synchronized in time. The final stage consists of the full post-treatment analysis in an integrated tool.

1.6.2 Main experimental results
The compositions corresponding to the LOCA and LOOP ex-vessel retention scenarios have been measured on VITI. The tests performed on the VITI-MBP facility give access to the following values for the surface tension and density measurements of LOCA liquid corium. For the KROTOS-ALISA in-vessel sample, the MBP technique has provided a value of 0.57 N.m-1 for the surface tension. It must be noted that the surface tension of the corium-concrete mixture is significantly lower than that of the (UO2-ZrO2) melt.

2. ALISA EXPERIMENTS IN CHINESE FACILITIES

2.1 IVR2D-2D IN-VESSEL MELT RETENTION
The two-dimensional IVR facility, REVECT-II at CNPRI is designed to investigate the External Reactor Vessel Cooling (ERVC) of the lower plenum of RPV in order to achieve in-vessel melt retention. The objectives of the experiment are the identification of Critical Heat Flux (CHF) along the outer wall of RPV, optimization and verification of the flow channel, the characteristics of two-phase flow in different flow conditions, the effect of coolant (additives on CHF, etc. Test data are of great significance for the mechanical research and engineering application of the reactor pit flooding system. The test facility is a 2D quarter slice with 1:1 scale in vessel radius and a 7.6 m high loop, where the water level in the upper tank is 6.8m. The whole heating block is composed of 23 copper heating sections, the heating power of each heating section can be controlled individually. With totally 631 heating rods in the heating sections, a maximum heating power of 400 kW, corresponding 2.4 MW/m² can be realized. The stainless steel flow channel is constrained with 3 parts of baffle, simulating the gap between RPV lower head and its insulation. The width of the flow channel is 15 cm. Besides, a lower water tank is located under test section and it is in connection with the stainless steel flow channel, aimed to simulate prototype reactor pit space under the RPV insulation. An upper water tank is located at the top of the facility and simulates the prototype’s annular region around the RPV support rings. The height of water level at the upper tank is 6.8 m, the height of the steam outlet at the upper tank is 7.6 m. Both nature circulation and forced circulation can be applied.

In each heating zone, there are 3 pairs of thermocouples (Ths,01~Ths,06) installed within the outer region of the heating section, with the distance of 18.5mm and 5.5mm to the outer surface. Since the heat flux gradient is in radial direction, the heat flux can be calculated according to the measured temperature gradient. To get the local wall fluid temperature, 19 thermocouples (one for each zone except zone 4, 16, 17, 19 as space restriction) are mounted in the flow channel at 4mm distance to the heating surface.

The objective of the experiment in the REVECT-II facility focuses on the influence of different heat flux profiles either in a homogenous oxide pool or in a two-layer stratified pool in the RPV lower plenum on the natural convection and CHF. The test was performed in 2017 using two different heat flux distribution profiles. The LIVE-HF profile simulates the situation of a homogenous pool, whereas the ULPU –HF profile corresponds to a two-layer pool with very high heat flux at the upper layer. The total heating power was increased proportionally according to the heat flux profiles until CHF occurs at a local heating section. The adjudgement of the CHF is temperature escalation from 140°C to 200°C at a very short time.

During the test of LIVE-HF curves, the CHF was reached at a total power of 1.36MW/m² at the heating section 20, corresponding to polar angle 78°. During the UPLU-HF test, the flow instability occurred before CHF could reached. The flow instability was featured with large fluctuation of flow rate at a set point of upper tank water level and the total steam generation. The total heating power at the set point of flow instability was 337 kW.

2.2 IVR3D-3D VESSEL EXTERNAL COOLING
IVR3D test facility at CNPRI will study three-dimensional vessel external cooling under integrated reactor component mode. The aim is to compare CHF under different cooling channel geometries and two-phase flow patterns of the coolant. The research program includes a) investigation of two-phase coolant circulation process of natural convection, venting and condensation and b) determination of the CHF in 3D geometry. This new facility was constructed and put into operation in 2017. The 1:5 scaled RPV lower head has a wall outer diameter of 0.92 m and a height of 1.68 m. The total height of the facility is 2.8 m. The simulation of RPV wall heat flux is realized with copper heating block bordered with stainless steel outer surface. The maximum heat flux of 1.8MW/m² can be realized.

The instrumentation includes high-speed camera, temperature measurement at different level and the high-frequency pressure sensors. The IVR-3D test in the ALISA project will study the effects of the influence of non-symmetrical top conditions due to venting and non-symmetrical gap around the RPV on the CHF. The experiment is planned in middle 2018.

2.3 COPRA IN-VESSEL MELT POOL
The COPRA facility at XJTU is designed to study the natural convection heat transfer in corium pools with high Rayleigh numbers up to 10^16 [23]. The test vessel is a two-dimensional ¼ circular slice. With an inner radius of 2.2 m, it simulates full scale for CNNC’s ACP-1000. The inner width of the slice is 20 cm. All the vertical walls of the vessel are kept thermally insulated. The curved vessel wall thickness is 30 mm and is enclosed with a regulated external cooling path. The top surface of the vessel is either in adiabatic boundary condition with an insulated upper lid, or be cooled by a specially designed cooling device. The main objective of the proposed experiment is to gain knowledge about the transient evolution of a layer evolving towards steady-state convection. The background is the lacking knowledge of the melt pool transient heat transfer. In the tests two steady-state boundaries are supposed: a) adiabatic boundary condition on the top of the pool and adiabatic boundary condition on the side at the beginning of the experiment, and b) after a defined time, the curved vessel wall boundary is changed quickly to isothermal condition by flooding. The lateral heat flux, related to the lateral Nusselt number, the evolution of pool temperature, and the time required to reach fully established convection are the main interests of the investigation. The test results can be compared with BALI experiments due to the similar geometry and with LIVE tests due to the same simulant material. The tests were performed in 2017.

2.4 HYMIT - HYDROGEN BEHAVIOR IN CONTAINMENT
The HYMIT facility at SJTU is a medium-scale hydrogen mitigation test facility designated for investigations of hydrogen recombination and combustion behavior. The facility is designed to test the characteristics of current available ignitor and passive autocatalytic recombiners (PAR) as well as new hydrogen mitigation technologies. It can operate with hydrogen concentrations between 0 and 30 vol.%, as well as steam concentrations between 0 and 50 vol %. The main part of the facility is a steel cylinder tank which is 4 meters high and 2 meters in diameter. The tank volume is about 12 m3. The content of gas mixture, gas temperature and pressure and speed of flame can be measured with gas analyzer, thermocouple arrays, piezoelectric pressure sensors and photodiode array respectively.

The focus of the ALISA experiment is hydrogen deflagration with upward flame propagation in a homogeneous air-steam-hydrogen atmosphere. The purpose of the experiment was to perform a test with conditions similar to the ones in previous experiments in other vessels, with a similar shape but a larger volume: THAI vessel, with a volume of 60 m3, and HYKA A2 vessel with a volume of 220 m3. This will enable the investigation of the influence of scaling from small to large volumes on the characteristics of hydrogen deflagration (eventually leading to adequate modelling of hydrogen deflagration in containments of real nuclear power plants), as well as to further develop theoretical models of flame propagation. The experiment was conducted successfully in October 2016. A homogeneous mixture of air, hydrogen and steam was ignited at the vessel centre line, near the vessel bottom. Hydrogen burning did occur. The combustion was almost complete, as the measured final hydrogen concentration at three different points was less than 0.1 vol.%.

2.5 MCTHBF - CONTAINMENT THERMAL HYDRAULICS
MCTHBF is a medium-size hydrogen mitigation test facility at NPIC. The main part is a steel cylinder tank which is 5 meters high and 2.8 meters in diameter. The tank volume is about 21 m3. It is designated to test the hydrogen recombination and combustion behavior. The mixed gas content, gas temperature and pressure and speed of flame can be measured with gas analyzer, thermocouple array, piezoelectric pressure sensors and photodiode array. It can deal with hydrogen concentrations between 0 and 35 vol.%, as well as steam concentrations between 0 and 40 vol %. The proposal on MCTHBF investigates stratification of atmosphere containing hydrogen in the upper part of the vessel and a low-momentum vertical air jet. The topic of studies includes scaling from small to large volumes by comparing results of other previous experiments and validation of lumped-parameter and CFD codes [24].

2.6 WAFT - WATER FILM TEST FACILITY
The WAFT facility is dedicated to study Passive Containment Cooling System (PCCS) of GEN-III plants. The facility consists of a stainless plate (5 m long, 1.2 m wide) mounted on a metallic frame allowing for plate inclination. The surface of the plate undergoes a preparation by painting with organic zinc to maintain designated wettability. The plate surface is heated by heating oil, and the power can reach 100 kW/m². A parallel plate 30 cm apart from the test plate with visualization windows constitutes a part of a rectangular channel which enables the simulation of the air gap between the containment steel envelope and the metallic baffles in the reference passive reactor design. A blower at the bottom of the channel simulates the natural circulation air flow occurring in the reference plant conditions. Variation of the reference air velocity from 0 to 12 m/s is allowed. The water film distribution box is mounted on the top of the facility to ensure uniform water film distribution of the inlet. The water film and air temperatures are measured with thermocouples, and water film thickness is measured with optical sensor, air flow velocity is measured with hot-wire anemometer, the surface wave velocity is measured with high-speed camera, and heat flux is measured with heat flux sensors.

The WAFT experiment in the ALISA project investigates the formation of rivulets (change of wetted surface fraction). The results will be used for the validation of the applied minimum energy principle used inside existing rivulet model of the COCOSYS code developed by GRS. Additional investigation of specific boundary conditions will include film boiling to validate heat transfer model under these conditions and “closed chimney” (no atmospheric flow) to validate heat transfer model under steam rich bulk conditions.

3. CONCLUSIONS

Enhanced transnational access to ALISA large research infrastructure allow the optimal use of the R&D resources in Europe and in China in the complex field of severe accident analysis for existing and future power plants. This research involves substantial human and financial resources and, in general, the research field is too wide to allow investigation of all phenomena by any national program. To optimize the use of the resources, the collaboration between nuclear utilities, industry groups, research centers and safety authorities, at both European and Chinese levels is very important. This is precisely the main objective of the ALISA project, which aims to provide these resources and to facilitate this collaboration by providing state-of-the-art large-scale experimental platforms in Europe and in China for transnational access. Large-scale facilities of the ALISA project are designed to resolve the most important remaining severe accident safety issues, ranked with high or medium priority by the SARP group for SARNET Network of Excellence. These issues are coolability of a degraded core, corium coolability in the RPV, possible melt dispersion to the reactor cavity, molten corium concrete interaction and hydrogen mixing and combustion in the containment. The major aspect is to understand how these events affect the safety of existing reactors and how to deduce soundly-based accident management procedures. The aim is not only to understand the physical background of severe accidents but to provide the underpinning knowledge that can help to reduce the severity of the consequences. It is crucially important to understand the whole core melt sequences and identify opportunities to lower the risk.

A wide range of European and Chinese organizations have participated in preparation of the experimental proposals and in the preparation and analysis of the experiments. Due to strong links to other European projects, ALISA offers a unique opportunity for all partners to get involved in the networks and activities supporting safety of existing and advanced reactors and to get access to largescale experimental facilities in Europe and in China to enhance understanding reactor core behavior under severe accident conditions. A special session on ALISA has been set up at the NUTHOS-2018 (Topical Meeting on Nuclear Thermal Hydraulics, Operation and Safety) at Qindao, P.R. China in October 2018 to disseminate the results of this project.

A continuation of this project is being under preparation for the September 2018 HORIZON2020 EURATOM call for proposal.

REFERENCES

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Potential Impact:
At present, knowledge of various core melt sequences and the consequences of possible operator actions are not yet sufficient as they are too dependent on specific characteristics of the power plant under consideration. ALISA project provided the resources for a better understanding of possible scenarios of core quenching, different core melt sequences and hydrogen behaviour for different reactor designs by offering state-of-the-art experimental facilities for the transnational access.

The experiments performed in the ALISA project in both European and Chinese facilities addressed important issues of early in-vessel core degradation for various accident scenarios, the detailed study of the in-vessel melt pool behaviour, the cooling strategy of the ex-vessel cooling, thermochemical interactions of the corium and its influence on the fuel coolant reaction and the study of the hydrogen behaviour in the containment. The topics addressed not only the safety issues ranked with high and medium priority by the SARP group for SARNET NoE, but also reflected a high interest in the safety management in the current and future nuclear power plants in both China and in Europe.

The knowledge obtained in the project will help improving severe accident management measures, which are essential for reactor safety and in addition offer competitive advantages for the nuclear industry. The experimental results will be used for the development of models and their implementation in the severe accident codes such as ASTEC, MELCOR, SCDAP. This helps to capitalise the knowledge obtained in the field of severe accident research in the severe accident codes and scientific databases, thus preserving and diffusing this knowledge to a large number of current and future end-users both in Europe and in China.

Due to strong links to other European activities like NUGENIA association and SAFEST and IVMR projects, ALISA has provided a unique opportunity for all parties to get involved in the networks and activities supporting safety of existing and advanced reactors and to get access to large-scale experimental facilities in Europe and in China to enhance understanding of material properties as well as reactor core behaviour under severe accident conditions. In the ALISA project both European and Chinese researchers worked together as equal partners so as to arrive at a wider common safety culture. By doing this, the project also helped diffusing the European culture of research and innovation as a support to regulatory and industrial decision making both in Europe and in China.

EU and China organizations profited in their own research field by using the facilities both in the Chinese and in European research institutes. ALISA project provided a platform for the exchange of experience in the experimental techniques and the philosophies between the EU and Chinese partners. The expertise of EU facility operators could be better understood and appreciated in the course of rapid development of Chinese nuclear safety research.
List of Websites:
http://alisa.xjtu.edu.cn

Contact details:
Prof. Yapei Zhang
Xi’an Jiaotong University
Email: zhangyapei@xjtu.edu.cn

Related information

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Reported by

Karlsruher Institut fuer Technologie
Germany

Subjects

Nuclear Fission
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