RELAP/SCDAP Sensitivity Study on the Efficiency in Severe Core Degradation Prevention of Depressurization and Water Injection into Steam Generators following SBO at a CANDU-6 NPP National Commission for Nuclear Activities Control, Romania (CNCAN) “Politehnica” University of Bucharest, Romania (UPB) Elena Dinca – CNCAN Daniel Dupleac - UPB Ilie Prisecaru – UPB IAEA International Conference on Topical Issues in Nuclear Installation Safety: Safety Demonstration of Advanced Water Cooled Nuclear Power Plants, Vienna, Austria, 6–9 June 2017

Outline Introduction Analysis Conclusions References Analysis Methodology and Models Analysed cases, for SBO initiation event Results of analysis Conclusions References 2

Introduction The paper has the objective: to demonstrate by calculations the efficiency of the Steam Generators as a heat sink in case of a Station Black-Out (SBO) accident at a CANDU-6 NPP. Analysis is performed using: RELAP/SCDAPSIM/MOD3.6. It is a sensitivity analysis, on timing of implementation considered preventive actions Preventive actions consist in: Steam Generators depressurization, and 100 s later water addition into SGs , at a flow of 40 l/s for all SGs The paper is in line with lessons learned from Fukushima Daiichi accident, to increase prevention and mitigation at operating NPPs, as well as to verify by analyses the considered actions efficiency 3

CANDU-6 reactor distinctive characteristics: Introduction – Background CANDU-6 reactor distinctive characteristics: 380 horizontal fuel channels, with fuel natural uranium, heavy water as coolant (100 bars, 312°C in ROH), ~120 t heavy water as moderator (~ 230 t, 70 °C, 1,1 MPa), cooling capacity of about 5% FP heat sink in case of LOCA+LOECC 4

CANDU-6 NPP schematic presentation Introduction – Background (1) CANDU-6 NPP schematic presentation

CANDU-6 NPP behaviour during a SBO event, with management actions (1) All sources of AC electrical power are considered lost Batteries are still available Reactor shutdown and containment isolation happen The main concern: reactor fuel cooling Without any cooling, CANDU fuel channels will break and discharge hot water into moderator then into containment The fuel channels rupture can be precluded if preventive actions are taken, and fuel cooling is ensured: Steam Generators are depressurized AND Water is added into SGs, gravitationally from the Dousing Tank

Analysis of SBO at a CANDU-6 NPP Analysis methodology A realistic approach has been used for this analysis The input data, and analysis assumptions and failure criteria are the same in IAEA TECDOC 1727, ref. 1), Selected computer code: RELAP/SCDAPSIM/MOD3.6(a), developed by ISS (Innovative Software System), [2], [3] Extended of analysis: only PHTS, SGs, and Reactor Core Analysis models: CANDU-6 models for RELAP/SCDAP code Prepared in many years to UPB, [4], [5], [6] Similar with models used in IAEA TECDOC 1727, [1] (small differences to reactor core, SGs model and PHTS auxiliaries), see [10]

Nodalization scheme, models The primary circuit nodalization scheme for CANDU-6 [1] Schematic representation and fuel channel model for CANDU-6 [1]

Schematic representation and model of calandria vessel for Nodalization scheme for CANDU-6 reactor core Schematic representation and model of calandria vessel for CANDU-6, [1]

Unmitigated SBO - results from [10] SBO without credit for any cooling for the nuclear fuel (unmitagated accident) (Ref. 8, 9, 10) showed that fuel channel breaks at ~ 13000 s PHTS pressure [10] PHTS inventory [10]

Evolution of the unmitigated SBO accident – reference case [10] Unmitigated SBO, - results from [10] Evolution of the unmitigated SBO accident – reference case [10] Event Time (h) Time (s) Loss of all power sources (Class IV, Class III, Emergency Power Supply unavailable) – except batteries 0.0 Reactor trip Secondary side of SGs is dry (at all SGs) 2.086 7510 First time LRVs open 2.408 8970 DGC-RVs open first time and discharge D2O into containment 2.561 9220 First fuel channel breaks 3.6 12960 CV opening and moderator expulsion due to CV rupture disks breaking Start of CANDU core disassembly (TCT > 1473°K) 3.971 14301 Core collapse on the calandria vessel bottom 5.861 21100

SBO analysis - with preventive actions Calculations were done to verify the efficiency, if preventive actions are performed at different moments since SBO initiation SGs are depressurized (and 100 s later water is added into SGs from Dousing Tank, with a flowrate of 40l/s, total for all 4 SGs) as EOP for SBO, according to [7]. At: ~ 2200 s, as the specific SBO EOP requires A calculation was done for a water flow rate of 30 l/s.

SBO analysis - with preventive actions (1) SGs are depressurized (and 100 s later water is added into SGs from Dousing Tank, with a flow of 30 l/s, or 40l/s, total for all 4 SGs) At: ~ 7200 s, when SGs are almost empty At: ~ 9000 s, when PHTS pressures increases over Liquid Relief Valves opening setpoint At: ~ 10800 s, when a good part of primary coolant inventory was discharged into Degasser Condenser by LRVs and then to the containment by Relief Valves At: ~ 12550 s, when PHTS inventory is less than half

SBO analysis - with preventive actions (2) SBO sensitivity study results: Parameters of interest for this analysis: PHTS pressure, in ROH (Pa); The SGs water level (from the tube-sheet) (m); Maximum fuel surface temperature (MFST almost equal to cladding temperature) (K degrees) PHTS coolant inventory, in the two loops (kg) The results of analysis presented in graphs, for selected cases

SBO analysis - with preventive actions (3) 1) SGs depressurization at 2200 s and water addition from the Dousing tank at 2300 s, at a total flow, for 4 SGs of 40 l/s 1) Steam Generators water level 1) PHTS pressure 1) Maximum fuel suurface temperature (K) 1) Fuel channels flow (kg/s)

SBO analysis - with preventive actions (4) 3) SGs depressurization at 9000 s and water addition from the Dousing tank at 9100 s 1) PHTS pressure 1) Fuel channels flow (kg/s) 1) Steam Generators water level 1) Maximum fuel suurface temperature (K)

SBO analysis - with preventive actions (5) 4) SGs depressurization at 10800 s and water addition from the Dousing tank at 10900 s 1) Maximum fuel suurface temperature (K) PHTS liquid inventory (kg) 2) SGs depressurization at 7200 s 5) SGs depressurization at 12550 s 1) Maximum fuel suurface temperature (K) 1) Maximum fuel suurface temperature (K)

SBO Sensitivity Study conclusions The study results indicated that: SGs can become and remain an efficient heat sink following a SBO event at a CANDU-6 NPP, when SGs depressurization is performed before 3 h following the SBO Water is added in around 100 s, with a minimum flow rate of 40 l/s (for case of SGs depressurization at 2200s). The best results are obtained when: Primary inventory is still intact (till LRWs discharge liquid from PHTS to Degasser Condenser) –till 9000 s The time window is large enough to put into service a Mobile Diesel Generator (at least 2.5 h, according to [7])

SBO Sensitivity Study conclusions (1) The study is important for: Validation through analytical calculations of the measures considered in EOP for SBO Situations with batteries unavailable after a SBO (determined by a common cause failure) and the preventive measures are implemented later (by operator manual actions or using a mobile Diesel generator for valves actuation) In fact, multiple possibilities have been implemented after Fukushima to ensure SGs as an efficient heat sink for nuclear fuel cooling, to avoid fuel channel break.

References International Atomic Energy Agency, IAEA TECDOC 1727, “Benchmarking Severe Accident Computer Codes for Heavy Water Reactor”, Vienna, 2013 SCDAP/RELAP5 Development Team, SCDAP/RELAP5/MOD3.2 Code Manual, Vol. 1-5, NUREG/CR- 6150, INEL-96/0422, 1998 L. J. Siefken, C. M. Allison, J. K. Hohorst, RELAP/SCDAPSIM/MOD3.5 - Improvements Resulting from QUENCH and PARAMETER Bundle Heating and Quenching Experiments, 2010, ISS, Idaho Falls, USA Ghe. Negut, Al. Catana, I. Prisecaru, D. Dupleac, Thermal Hydraulics of the CANDU Degraded Cores, 2007 M. Mladin, D. Dupleac, I. Prisecaru, SCDAP/RELAP5 application to CANDU6 fuel channel analysis under postulated LLOCA/LOECC conditions, 2008 M. Mladin, D. Dupleac, I. Prisecaru, Evaluation of the RELAP5/SCDAP Accident Analysis Code Applicability to CANDU Nuclear Reactors, U.P.B. Sci. Bull., Series C, Vol. 71, Iss. 4, ISSN 1454-234x, Bucharest, 2009 CNCAN, „National Report on the Implementation of the Stress Tests‖, Bucharest, Romania, December 2011 E. Dinca, D. Dupleac, I. Prisecaru “Verification by analytical means of the efficiency of some accident management measures for SBO at CANDU6 NPP, International Nuclear Safety Journal, vol. 4, issue 3, pages 9 – 22, ISSN 2285 – 8717, 2015, E. Dinca, D. Dupleac, I. Prisecaru “, Analysis of the CANDU-6 Plant Behavior in Case of Very Late Steam Generators Depressurization and Water Injection Following a Station Black-Out Accident, U.P.B. Scientific Bulletin, Series C, Vol. 78, Issue 4, ISSN 2286-3540, Bucharest, 2016 E. Dinca, D. Dupleac, I. Prisecaru “RELAP/SCDAP Simulation Results for CANDU 6 Accident Management Measure: Primary Heat Transport System Voluntary Depressurization following a Station Blackout, U.P.B. Scientific Bulletin, Series C, Vol. 78, Issue 3, ISSN 2286-3540, Bucharest, 2016

Thank you for your attention! Questions? This year CNCAN has approved the scope and programme of the PSR for Cernavoda NPP Unit1, together with the Quality Assurance plan. Preparatory work is currently done by the utility as part of the 1st Phase of the PSR.