APPROACHES TO THE MANAGEMENT OF SEVERE ACCIDENTS IN SFP AT ARMENIAN NPP Eduard YEGHOYAN Head of Laboratory, “ Armatom ” Institute Artashes HOVHANNISYAN I Cat. Engineer, “ Armatom ” Institute IAEA Technical Meeting on the Phenomenology, Simulation and Modelling of Accidents in Spent Fuel Pools 2-5 September 2019, Vienna, Austria 1
The nuclear energy sector of Armenia includes one nuclear power plant - Armenian NPP, which is located in the western part of Ararat valley, 30 km west of the capital Yerevan. 2
Introduction Armenian NPPconsists of two units with Soviet design WWER-440- ➢ 270 model reactor that is a version of the WWER-440-230 serial model with special seismic considerations in the design. Unit 1 started its commercial operation in 1976 and the Unit 2 in ➢ 1980; Both units were shut down shortly after the earthquake of December 7th, 1988; Following the completion of repair and safety upgrading activities ➢ Unit 2, after 6.5 years of shutdown, restarted operation in 1995 and it has been operational since then. Unit 1 remains in long-term shutdown. The Spent Fuel Pools (SFPs) of both units are currently in ➢ operation - the fuel discharged from the operating unit 2 reactor core is put first in the SFP of that unit, then, after several years of storage, is transferred to the SFP of the unit 1. 3
Plant layout SFPs at Armenian NPP are of “ at-reactor ” type. The SFPs are located close to the reactor, but outside of the containment hermetic boundary. The pools are constructed of reinforced concrete with a two-layer steel liner – stainless steel and carbon steel with 4mm of thickness each one and 4mm gap between them. The so called “ Central hall ” of the reactor building, to which the SFPs are connected, is a relatively big area – width 39m, length 126m, height 28.3 m (the Central hall is common for two units of the plant; it is an airtight room in the upper part of the reactor building, not designed for overpressure). When the fuel pool is not in refueling mode, it is covered by panels (panels don ’ t ensure tightness of the pool and the pool is considered to be connected to the Central hall). 4
Plant layout SFP1 SFP2 FIG 1. Simplified layout of the Reactor building 5
Plant layout SFP Refueling Pool FIG 2. Section of the Reactor building 6
SFP ACCIDENT STUDY SPENT FUEL POOL AND COOLING SYSTEM DESIGN The SFP has rectangular form, and has 2 separate parts – main pool and container compartment. The racks in the SFP can have 2 levels. The lower level is used for permanent storage of the fuel assemblies, and the upper level of racks is used temporarily - installed and used in short term in case of full off-load of reactor core. The number of fuel assemblies ’ cells in the SFP lower racks is 372 and in the upper level - 351. The number of fuel assemblies in reactor core – 347. The pools have also several cells for installing special containers in the SFP for the storage of damaged or leaking FAs. Depending on operation mode, different coolant levels are maintained in the pool. Heat removal from SFPs is ensured by forced circulation of the coolant through the heat exchangers of the dedicated cooling system. The lowest level penetration of the pool is the connection of circulation pumps suction line (pool outlet line). 7
SFP ACCIDENT STUDY SPENT FUEL POOL AND COOLING SYSTEM DESIGN +11.8 Container Passage connecting compartment with refuelling pool +6.0 +5.67 +5.47 Upper level racks +5 44 . +4.5 +2 24 . Spent Fuel Assemblies +1.4 + 1. 715 +1.03 Lower level racks -1.48 -1. 8 6 -1.8 SFPHE-1 SFPHE-2 SFPP-2 SFFP-1 Cooling water to heat exchangers FIG 3. SFP heat removal system simplified principal scheme 8
SFP ACCIDENT STUDY ACCIDENT SCENARIOS The main scope of studies of accident scenarios in SFPs is performed within the development of plant EOPs for SFPs and corresponding analytical justification documentation. Main two categories of accident scenarios considered in the studies are: “ Loss of SFP coolant ” 1) “ Loss of cooling of SFP ” . 2) “ Loss of SFP coolant ” scenario can take place in case of break of cooling system tube. For the case of coolant inlet pipe break, in order the siphon effect is “ broken ” there is special small size line connected to the containment atmosphere to insure air inlet to the cooling system in case the highest point is under vacuum. The elevation of tube connection to the pool prevents uncovering of fuel when only lower level racks contain spent fuel assemblies. 9
SFP ACCIDENT STUDY ACCIDENT SCENARIOS The scenario with most serious consequences - both level of racks full of spent fuel assemblies (reactor core off-load mode) and break of cooling system tube (pump suction line). In such a scenario coolant level in SFP can decrease very fast till reaching the elevation of cooling system suction tube (see figure 4 ). Thus, the higher part of fuel assemblies in the upper level racks can be uncovered very soon after beginning of the transient . According to the calculations ’ results, in the beginning phase of the transient, decrease of the coolant level in SFP is very fast. The fuel top level is uncovered in less than 5 min. From this moment increase of fuel temperature starts at the uncovered part and takes place significantly fast. 10
SFP ACCIDENT STUDY ACCIDENT SCENARIOS Elevation, m Coolant level Fuel top level Outlet tube level Time, hours FIG 4. Change of coolant level in the SFP in the scenario of cooling system tube break (without SFP make-up) 11
SFP ACCIDENT STUDY ACCIDENT SCENARIOS Quick loss of coolant ceases when the coolant level reaches the elevation of the tube connected to the pool. Heat removal from the pool is lost and the coolant temperature increases continuously. Starting from this period, during about 50 minutes the coolant level is unchanged – the loss of coolant (mainly through evaporation) is compensated by its thermal expansion. About 1 hour after beginning of the transient coolant temperature reaches the boiling temperature and decrease in coolant level starts again due to more intensive evaporation process (with no more effect of thermal expansion of the coolant). In parallel to the warming up of the coolant in the lower part of the pool (below upper level racks), the rate of steam generated in the upper level assemblies increases, thus, improving the heat removal by steam from the uncovered part of the fuel, and starting from about 45-46 min. of the transient the fuel temperature starts to decrease (see figure 5). 12
SFP ACCIDENT STUDY ACCIDENT SCENARIOS Fuel hottest point o temperature, C 1200 1000 800 600 400 200 Time, hours 0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0 FIG 5. Change of fuel maximum temperature in the scenario of cooling system tube break (without SFP make-up) 13
SFP ACCIDENT STUDY ACCIDENT SCENARIOS Continuous decrease in coolant level results in less steam generation and bigger part of fuel uncovered, i.e. bigger decay power to be removed by steam. After 25 minutes of fuel temperature decrease, it starts to increase again. Overheating of the fuel cladding till 1200 o C takes place about 4.23 hours after starting of the transient. Another accident scenario with the same initiating event and early SFP make-up was studied. Due to significant loss of coolant through the break the coolant level is almost the same as in the first scenario. Some kind of “ feed-and-bleed ” takes place in the pool, and the mean temperature of the coolant does not practically increase. However, heat removal conditions for upper level rack assemblies are deteriorated – low temperature of coolant feeding the upper level assemblies results in low rate of steam generation and, thus, in continuous increase in fuel temperature in the upper uncovered part of fuel. In this 2nd scenario the conditions of fuel damage are reached significantly earlier (less than 1.5 hours after transient beginning). 14
SFP ACCIDENT STUDY ACCIDENT SCENARIOS In the scenario with loss of cooling of the SFP, even in case of reactor core full off-load, the fuel top part will be uncovered in a time longer than 34 hours (coolant boiling starts about 2 hours and 40 minutes after the beginning of the transient). For the scenarios with only lower level racks containing spent fuel assemblies the cooling system line break will not create quick change of heat removal conditions – at least 2.5 m layer coolant inventory will be available in the beginning phase of the transient. Severe accidents in SFPs of ANPP are not studied yet. Draft versions of SFP SAMGs were developed based on known general phenome- nology of the accident progression in SFPs. Currently the model is under development (MELCOR 1.8.6 version is used) to support analytical justification of the strategies considered in the SAMGs. 15
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