Keywords: Thermalhydraulics, VVER-440, safety, strainer, clogging, downstream effects, fuel element, sump, risk.

Transcription

1 SAFETY IMPACT OF THE INSULATION FIBERS PENETRATING SUMP STRAINERS AND ACCUMULATING IN LOVIISA VVER-440 FUEL BUNDLES Seppo Tarkiainen, Olli Hongisto, Timo Hyrsky, Heikki Kantee, Ilkka Paavola Fortum Power, Helsinki, Finland The emergency core cooling systems of Loviisa NPPs have been equipped with dedicated filters to prevent insulation debris and other impurities of the sump water from entering to the process during the accidents. However, during some phases of ECCS operation it is inevitable that, small amounts of fibers penetrate the filters. The phenomenon has been studied both experimentally (Loviisa-specific SUPA experiments) and with process simulations (APROS analyses). The conditions and parameters of the core cooling modelling during recirculation phase depend on the size and location of the break. The parameter selection for the test model was based on the process analyses with the APROS simulation tool. The test set-up consisted of a heated water tank for mixing the boric acid to the water, a tank for mixing insulation debris material and a sedimentation tank for the strainer with the filter element. Additional devices were a fine filter element for catching the fibers passing the strainer filter, recirculation water pumps and a full-scale VVER-440 fuel bundle model. The experiments were carried out in two phases. In the first phase the main objective was to define the amount of the fibers passing through the strainer filter element and the pressure loss over the fiber bed attached on the outer surface of the filter. In the second phase of the tests the amount of fibers carried to the fuel bundle and resulting pressure loss in the test arrangement were studied. The flow restrictions in the fuel bundle caused by accumulating fibers were considered when the acceptability for the amount of penetration was assessed. The test results were included into the simulation model of a large-break loss-of-coolant-accident and analyzed as an envelope case for core cooling. Finally, the safety impact of flow restrictions on the fuel cooling was evaluated with the aid of the Probabilistic Risk Analysis for Loviisa NPP. It turned out necessary to decrease the amount of the penetrating fibers. The solution was to install an enhanced filter surface structure on the sump strainers. Thus, the resulting risk level has been kept within acceptable limits. Keywords: Thermalhydraulics, VVER-440, safety, strainer, clogging, downstream effects, fuel element, sump, risk.

2 SAFETY IMPACT OF THE INSULATION FIBERS PENETRATING SUMP STRAINERS AND ACCUMULATING IN LOVIISA VVER-440 FUEL BUNDLES Seppo Tarkiainen, Olli Hongisto, Timo Hyrsky, Heikki Kantee, Ilkka Paavola Fortum Power, Helsinki, Finland Abbrevations: ECCS -Emergency Core Cooling System LWR - Light Water Reactor LOCA - Loss of Coolant Accident NPP - Nuclear Power Plant HPI - High Pressure Injection LPI - Low Pressure Injection IRWS - Interim Reactor Water Storage APROS -Advanced Process Simulator Tsed - Sedimentation Time NPSH - Net Positive Suction Head dp - Pressure Difference PRA - Probabilistic Risk Assessment 1 INTRODUCTION During the operation of the Emergency Core Cooling system (ECCS) in accident conditions, most of the LWR designs rely on recirculating water from the containment to the reactor vessel after initial Loss-of-Coolant-Accident (LOCA). This ensures long term cooling of the fuel and limits the contamination of the plant inside the containment circulation system. To ensure the coolant flow and heat transfer conditions, a sump system is designed to capture and filter the debris from the circulating water. Depending on the nature and amount of the debris, considerable damage to the circulating system equipment or deterioration of the heat transfer can occur if large amount of debris can enter the downstream flow through the sump. This paper presents a design case and a study to examine the behaviour of the sump system and debris penetration effects. 2 GENERAL DESCRIPTION OF THE ECCS SYSTEM IN LOVIISA NPP Loviisa NPP is of VVER-440-design, equipped with 2-redundant HPI and LPI and containment spray systems. In the first phase of the accident operations, all these systems take suction from the common IRWS-tank not connected to the containment. After the water in the tank is finished, automatics change all pumps suction to the containment water sumps. Safety automatics also initiate containment isolation to maintain the water inventory, and pumped water together with other water sources (accumulator tanks, ice condenser, primary circuit) result in sufficient water level for suction strainers on the containment bottom floor (+10.00). Both trains have dedicated suction filter systems common to HPI and LPI. These systems discharge water to the primary vessel. Heat transfer to the intermediate cooling system is via the containment spray with separate suction filters.

3 The main debris sources are the fibrous insulation material installed in primary piping and steam generators. The LOCA accident will release and destroy large amounts of fibrous insulation, and these fibers, due to their small size, can transport and accumulate to the filtering surfaces. Other sources can be common dust, remains of insulation materials inside the containment, but compared to the released insulation, the main bulk of the blocking material is LOCA-originated fibers. Considering the core cooling, the LPI- and HPI-systems contribute by injecting water into the primary system. Depending on the LOCA-type (hot leg, cold leg), some of the pumped water can bypass the core thorough the leak, but the ECCS configuration ensures cooling water to the core via upper or lower vessel injection. The flow patterns will be discussed more in following chapters. The cooling water is also the route of the fiber/particle penetration inside the core on special conditions. At the beginning of the recirculation, the filtering surfaces are submerged and clean. At the start of the circulation, clean filtering surfaces allow small amount of fine fibers to penetrate the filtering surface, perforated plate ( 2 mm) or woven mesh (# 0,7 mm, the new design). After a short period (depending on the flow rate), enough fibrous debris has accumulated on the filtering surfaces to form a uniform layer which now prevents further fiber penetration into the interns of the filter structure. The internal losses in the sump configuration also regulate the influx so, that the most distant surfaces of the sump can remain open until the pressure boundary is uniform over the whole filter system. At this point of time, the penetration can be considered finished. On special cases the penetrating phase can start again, when the covering layer on the strainers is disrupted. Reasons can be stopping of the suction flow, or in some cases, initiated cleaning of the filter surface with backflush due to the excess pressure build up over the strainer. In these cases, restart of the circulation can initiate a similar phase of penetration as in the beginning of the recirculation, introducing a new pulse of fibers into the reactor internals. 3 TEST FACILITY DESIGN REQUIREMENTS The purpose of the tests was to examine the effect of the penetrating fibers to the cooling of the fuel during accident conditions. Test facility was designed to give realistic flow and other geometrical conditions to study the penetration and the effect on the heat transfer inside the fuel element. The test programs took into account the relevant parameters and conditions for realistic results. Studies also covered the filter upgrade possibilities. Overall design of the test facility considered: Actual flowrates for filter surface and fuel dummy 1:1 horizontal geometry models for filters and core barrel Sampling system to determine the quality and amount of fiber penetration. Test program development considered: Sufficient coverage of the relevant parameters The flow conditions for the test must be determined with simulations for the most damaging case. Provide data for simulations and calculations Necessary flexibility for expert judgement 3.1 Analysis of the flow regimes inside the core The flow regimes inside the core are strongly influenced by the location of the LOCA break. The ECCS configuration allows water injection to the downcomer and upper dome, whereas the break location determines the water flow path out of the vessel and therefore the direction of the coolant flow. Other determining factor is the natural circulation occurring inside the core, depending on the decay

4 heat from individual fuel elements and core bypass routes from fuel dummies and other open bypass channels. During the hot leg break flow direction in the core is mainly upwards as in the normal operations. Cold cooling water entering the downcomer passes through the core and is discharged from the hot leg break to the containment. Instead, cold leg break results in more complicated flow pattern, combining natural circulation inside the core with injected cold water on top of the core. Following figure (Fig 1) indicates averaged flowrates and directions used for design of the test facility and developing the test programs. HLLOCA, liquid flow rates CLLOCA, liquid flow rates YD10 YD20 YD30 YD80 YD40 YD70 YD10 YD20 YD30 YD80 YD40 YD70-0,24 1,23-0,19 1,08-0,16 0,93-0,17 0,95 0,19-0,23 0,51-2,11-0,96 1,7-0,98 2,1-0,96 2,1-0,98 2,1 0,82-5,3-1,0 3,4 YD60 YD60 1,07 (51,3) 0,95 (45,4) 0,82 (9,8) 0,86 (107,8) 0,19 0,11-0,16 (-6,8) -3,61 0,81 (39,2) 1,1 (53,8) 1,2 (14,3) 1,1 (150,2) -5,8 0,49-4,47 (-187,8) -26,0 0,29 (10,5) -1,6-5,34 (-192.4) 2,4 3.2 Facility design Fig 1. Averaged flow rates and directions inside the core The sump configuration consists of small strainer modules connected together to form a filtering surface. For testing purposes, a single module can represent the behaviour of the sump in this case. The fuel bundles are housed inside a hexagonal tube, which restricts the cross flows from element to element, allowing the mixing only inside one hexagonal tube. Single dummy VVER-440-fuel element was available to model the core clogging inside the hexagonal box. Since the testing objective was to provide information of the amount and conditions for maximum penetration and their impact for the reactor internals, it was necessary to divide the test program into two phases. 3.3 Test process development Test process development was conducted keeping in mind the complex process conditions. The first phase of the test concentrated on the fiber penetrating phenomenon. A test matrix was created to separate and combine some parameters for Phase 1 testing purposes. To evaluate the most damaging setup for the penetration, some parameters were varied in the first phase of the test. Such parameters were fiber settling time, heat treatment time, debris injection method, concentration of the fibers in water, flow rate through the strainer. The testing methods and parameters of Phase 2 were then selected according to these results. The second phase of the test program concentrated on modeling the flow conditions inside the core in different LOCA-accidents. The main difficulty was to determine representative flow rates for cold leg LOCA, since during this type of accident the flow directions inside the core are determined by natural circulation, and are subject to the fluctuations and special conditions arising from the decay heat production rates in the fuel elements. This question was approached by studying the flow patterns

5 inside the core by simulations, and utilizing an "envelope" for test flow velocity, since maximum flow was determined to be the controlling factor in clogging and pressure loss development for the fuel element. The simulator code APROS was used to model the behaviour of the flow inside the core. 3.4 Acceptability criteria Effects of fiber accumulation to the heat transfer in the fuel element were determined with simulator study. Since fibers are inorganic material, only flow restrictions were considered, and the facility and test programs were designed to give input data for such study. The overall risk effect was determined by risk analysis methods. The study provides necessary information to conduct a limited risk analysis, to verify the overall effect to the plant risk. 4 TEST FACILITY The test facility was built and the tests were carried out by the Lappeenranta University of Technology. Facility consisted of three pumping circuits, strainer element, fiber trap and fuel element model. Facility was operated by computer control, and necessary measurements were processed and stored to the computer. The model of the facility for both phases is presented in Figure 2. T1 L1 T2 T5 T3 F1 T4 F3 T7 DP1 Fig 2. The test facility for phases 1 and 2

6 Since the amount of strainer modules in single sump did not correspond to the amount of fuel elements, the flow area in the strainer was scaled so that the flow velocity at the filtering surface and inside the fuel element was the same as in plant during simulated accident conditions with nominal ECCS flow. The debris load of the filtering surface was scaled accordingly. Facility was designed to allow necessary measurements and sampling. The test facility of Phase 1 was then extended for the tests in Phase 2, to ensure the uniform filtering conditions for both tests. The facility was equipped with openings for photographs and video. Since no credit of sedimentation during containment transportation for fibrous debris was taken in the design of the strainers, the volume of the filter pool could be kept at minimum, also allowing control of the fiber concentration in the suction pool. Some effort was taken to allow the capturing the penetrating fibers during the separate phases with multi-screen fine fiber trap, which could be used multiple times during the test period. The fuel element was integrated to the facility in the Phase 2, hosting the fuel bundles and spacers and directing flow from the bottom connection to the top outlet. The element was equipped with pressure difference measurements to indicate the clogging effect along the flow paths thorough the element. A small container was included to the bottom of the fuel element to represent a mixing volume for the fibers inside the reactor pressure vessel and to allow possible fiber sedimentation. Fig 3. The fuel element model and the section pressure difference measurements dp2 5 and overall measurement dp6

7 The debris material used in the tests was mineral wool similar to that of primary circuit insulation. Debris was generated with cold water jet, bar working pressure, by forcing the insulation material through wire screen (#5 mm) to assure that all the insulation material was crushed into homogenous debris, slurry. For every experiment, necessary amount of new heat treated insulation material was crushed. The base debris material diameter was about 5 µm, and the mean length from a sample from the slurry was 0.7 mm. 4.1 Phase 1 test program The test program developed to the Phase 1 included tests with varied parameters. The main objective was to determine the amount of fiber penetration and conditions for the phase 2 test with fuel element. In addition the results of the tests in Phase 1 provided necessary parameters for the process simulation and risk analysis studies. Phase 1 tests also served as a licensing test for the new filter surface configuration. In most of the tests strainer cleaning was initiated at the end of the test by air backflush, and the amount of fibers by new penetration process with restarted water circulation was studied. Test Filter (hole/area) [ 2 / #0.7, 0.08 / 0.44 m 2 ] Time to Sedimentation [min] The test matrix of Phase 1 tests Initial Fiber concentration [kg/m 3 vettä] Mass Flow [kg/s] Temperature [ C] SED1 2, SED2 #0.7, SED3 2, SED4 #0.7, SED7 #0.7, FILL1 2, VER1 2, MIN1 2, LIS1 2, LIS2 #0.7, Table Phase 2 test program The objective of the phase 2 studies was to study effects of selected parameters to the amount of fiber penetration and resulting fuel element clogging behaviour. Among the chosen parameters were sedimentation time, fiber concentration, nominal mass flow. The Phase 2 tests were carried out with 25 min sedimentation time (T sed ), 1.1 kg/m 3 and 1.4 kg/m 3 fiber concentrations and nominal flux of 7.6 kg/s/m 2 through the strainer. The flow rates through the fuel element were determined from the results of selected process simulations. Since the first test did show no or very little penetration with scaled filter surface, the following adjustments were made in order to test the most demanding flow and fiber penetration conditions. Thus the filter surface was increased to allow more open surface for initial penetration phase, and at the same 1 Starting time for the recirculation

8 time, more fiber load and increased flow rate through the fuel element. These changes are variations of the possible flow patterns for the filter in the containment and also for the fuel element positions inside the core assembly. Test Filter [ 2 / #0.7, 0.08 / 0.44 m 2 ] Time to sedimentation [min] Phase 2 test parameters. Concentrati on [kg/m 3 vettä] Flow thorough fuel element [kg/s] Flow in Filter circuit [kg/s] Temperatur [ C] NIPPU1 #0.7, NIPPU2 #0.7, NIPPU2-2 #0.7, Table 2. 5 TEST RESULTS AND EVALUATION 5.1 Test result Test results of Phase 1 (e.g. pressure losses over the strainer surface) were comparable to the results carried out previously. The amount of fibers passing through the filter of new design (#0.7 mm wire mesh covering the old 2 mm perforated plate) was significantly smaller (up to 80 % less penetration) The mass of fibers penetrating both surface types was measured with fine fiber trap (Figure 4). Total measured fiber mass for LIS1 test was about 30 g, which was decreased to ~ 10 g with the new surface in test LIS2. Fig 4. The fine filter trap screens after the intial flow start (left) and after the cleangin and restart of the flow (right) in the LIS1 test

9 In addition the pressure loss over the fiber bed of the new design was also smaller in the licensing experiments. During the experiment, flow rate was varied to examine the behaviour of the pressure low over debris bed with different flows (Figure 5) LIS1 LIS2 LIS1, LIS Paine-ero [Pa] Virtaus [kg/s] Fig 5. Pressure loss over filter element against flowrate (licensing experiments) Fig 6. The filter before (left) and after the stopping the flow and cleaning in LIS2. In the first test of the Phase 2 only a small amount of fibers accumulated inside the fuel element model with nominal flow for the filter-fuel element - system. Increasing the filter area to a whole filter element for a fuel element also increased the accumulated mass of fibers in the fuel element. Pressure difference measurements indicated that fibers accumulated on all spacer grids inside the element. The pressure difference measurement locations are indicated in previous fuel element drawing (Fig 3). The measured fiber mass inside the fuel element was ~ 3 g, with pressure difference over fuel element around 9 kpa in NIPPU2-2 tests. The amount of fibers accumulating in the fuel element varied; the

10 measured masses in Phase 2 were approximately 3-5 g of debris and resulting pressure loss over the fuel element 9-13 kpa, respectively (Figure 7). Paine-ero [Pa] DP2 DP3 DP4 DP5 DP6 NIPPU Virtaus [kg/s] Fig 7. dp measurements over the fuel element in NIPPU2-2 Fig 8. The dismantled fuel element and collected debris in spacer lower part after the NIPPU2-2 experiment.

11 5.2 Evaluation of the results The APROS simulator model for Loviisa NPP consists of core model and system models for primary circuit and connected systems for secondary systems (feedwater, steam), and safety systems for primary circuit. In the core model, the fuel elements are grouped according to their thermal effect and properties and modelled as interconnected groups to simulate the behaviour of the whole core. The pressure loss across the fuel element was defined as a function of accumulated fiber in the fuel element. The measured data was utilized in the APROS simulator analyses which were carried out to study the amount of fiber in the fuel element the core can tolerate without overheating. A Large Loss- Of-Coolant-Accident was chosen for the case to be analyzed since in that case sump circulation starts in the early phase of the accident. This is conservative from the overheating point of view because of high decay heat generation level. 5.3 Risk modelling The resulting risk to the plant was evaluated with Loviisa Probabilistic Risk model. In order to take into account the different behaviour and requirements for the mitigating systems arising from different LOCA-configurations, an application was developed to evaluate the effect of break size and break location. The model consists of the exclusion of initiating events not relevant for the current case on technical basis and selection of the relevant LOCA-cases and their potential impact on filter penetration phenomenon. Selection of the size of the break has impact on the success criteria for the mitigating systems, the generation of the debris and debris transportation to the sump filters. The model evaluate the frequencies for the break and estimates probabilities for debris destruction, debris transportation to the sump filter and fiber penetration. Also some special features of the filtering system are taken into account, like filter cleaning with backflush. The risk analysis compares the risk levels of old strainers with the new, improved structure. The risk decrease with the new strainer structure can be twofold in the analyzed PRA-sequences. 6 CONCLUSIONS The results of the fuel element tests for the Loviisa NPP show that the fibrous debris can accumulate to the fuel spacers and develop additional pressure loss. The phenomenon can be reduced with better filtering surfaces without causing unacceptable strain for the pump NPSH. Even for the conservative cases, the pressure loss of 9 kpa to 13 kpa is well below the threshold where the cooling of the fuel element can be compromized. The coolability of the fuel element was determined with APROS simulation model where the additional pressure loss caused by the fiber load was included in the core model according to the test results. Risk model indicates significant improvement with the new filtering structure with all considered LOCA cases.