LEADER Project: Task 5.4 Analysis of Representative DBC Events of the ETDR with RELAP5 G. Bandini - ENEA/Bologna LEADER 5 th WP5 Meeting JRC-IET, Petten, 26 February 2013

2 Outline  RELAP5 modelling  Steady-state at EOC  Analysed DBC transients  Transient results  Conclusions

3 ALFRED modelling

4 Steady-state at EOC ParameterUnitRELAP5 Reactor thermal powerMW300 Total primary flowratekg/s25250 Active core flowratekg/s24970 Average FA flowratekg/s145.8 Hottest FA flowratekg/s174.3 Pressure loss through the primary circuitbar1.5 Pressure loss through the corebar1.0 Core inlet lead temperature°C400 Average FA outlet lead temperature°C480 Hottest FA outlet lead temperature°C483 Upper plenum lead temperature°C480 Average pin max clad temperature°C500 Hottest pin max clad temperature°C508 Average pin max fuel temperature°C1594 Hottest pin max fuel temperature°C1991 SG inlet lead temperature°C480 SG outlet lead temperature°C400 Total SG feedwater flowrate (8 SGs)kg/s192.8 SG feedwater temperature°C335 SG steam outlet temperature°C450 SG inlet pressurebar188 SG outlet pressurebar182 Steam line outlet pressurebar180

5 Main events and reactor scram threshold Analysed DBC transients

6 TD-1: Spurious reactor trip (1/2) Total reactivity and feedbacks ASSUMPTIONS:  Reactor scram at t = 0 s  Reactivity insertion of at least 8000 pcm in 1 s  Secondary circuits are available  constant feedwater flowrate Core and MHX powers  Core power reduced down to decay level at t = 0 s  Power removal by secondary circuits reduces with decreasing primary temperatures

7 Core temperatures  Initial temperature gradient on the fuel rod clad is about -8 °C/s  No risk for lead freezing since the feedwater temperature (335 °C) remains above the solidification point of lead (327 °C) TD-1: Spurious reactor trip (2/2) Primary lead temperatures

8 TD-3: Loss of AC power (1/2) Active core flowrate ASSUMPTIONS:  At t = 0 s  Reactor scram, primary pump coastdown, feedwater and turbine trip  At t = 1 s  DHR-1 system activation (4 IC loops  risk of lead freezing) Core temperatures  No initial core flowrate undershoot (lead free levels equalization)  No significant clad temperature peak in the initial phase of the transient

9 TD-3: Loss of AC power (2/2) Core decay, MHX and IC powers Primary lead temperatures  After the initial transient the natural circulation in the primary circuit stabilizes around 2% of nominal value  DHR power (7 MW) exceeds the decay power after about 15 minutes  Risk of freezing at MHX outlet is predicted by RELAP5 after about 2 hours (no mixing in the cold pool around MHXs) Active core flowrate

10 TD-7: Loss of primary pumps (1/2) ASSUMPTIONS:  At t = 0 s  All primary pumps coastdown  Reactor scram at t = 3 s on second scram threshold (Hot FA ΔT > 1.2 nominal value)  At t = 4 s  DHR-1 system activation (3 IC loops  maximum temperatures) Active core flowrateCore temperatures  No initial core flowrate undershoot (lead free levels stabilization)  More significant clad temperature peak than in case of LOOP transient due to delayed reactor scram

TD-7: Loss of primary pumps (2/2) Active core flowrate Core decay, MHX and IC powers Primary lead temperatures  After the initial transient the natural circulation in the primary circuit stabilizes around 1.5% of nominal value  DHR power (5 MW) exceeds the decay power after about 45 minutes  Risk of freezing at MHX outlet is predicted by RELAP5 after more than 3 hours (no mixing in the cold pool around MHXs)

12 TO-1: FW temperature reduction (1/2) ASSUMPTIONS:  Loss of one preheater (FW temperature from 335 °C down to 300 °C in 1 s)  reactor scram at t = 2 s on low FW temperature  At t = 3 s  DHR-1 system activation (4 IC loops) Primary lead temperatures  DT through the core and the MHX reduces quickly down to few degrees  After some fluctuations the primary lead temperatures stabilizes around 410 °C

13 Core decay, MHX and IC powers Primary lead temperatures TO-1: FW temperature reduction (2/2)  No risk of lead freezing in the initial phase of the transient due to prompt reactor scram  After about 15 minutes the DHR power (7 MW) exceeds the decay power  The risk of lead freezing in the primary system is predicted after about 3 hours (no mixing in the cold pool around MHXs)

14 TO-1: FW flowrate +20% Core and MHX powers Primary lead temperatures ASSUMPTIONS:  Feedwater flowrate +20% in 25 s  No significant perturbations on both primary and secondary sides  The system reaches a new steady-state condition in about 10 minutes without exceeding reactor scram set-points  Slight increase in core power (+6%) leads to max fuel temperature increase of 70 °C

15 Maximum core temperatures Transient DescriptionCode SystemMax Temperatures [°C] FuelCladdingCoolant NominalSteady state, peak pin - ENEARELAP TD-1Spurious reactor tripRELAP TD-3Loss of AC powerRELAP TD-7Loss of all primary pumps (PLOF)RELAP TO-1Reduction of FW temperatureRELAP TO-4Increase of FW flowrate by 20 %RELAP

16 Conclusions In all analysed DBC accidental transients the protection system by reactor scram and prompt start-up of the DHR-1 system for core decay heat removal is able to bring the plant in safe conditions in the short and long term. The core temperatures (clad and fuel) always remain well below the safety limits and no significant vessel wall temperature increase is predicted. The time to reach lead freezing at the MHX outlet after start-up of DHR-1 system strongly depends on the assumptions taken on the lead mixing in the cold pool surrounding the MHX that involves the largest part of the primary lead mass inventory. In the RELAP5 calculations the cold lead flowing out of the MHX does not mix with hotter lead of the cold pool surrounding the MHX, before to move downward into the lower plenum towards the core inlet. Therefore, in the calculations of TD-1, TD-7 and TO-1 transients, the decrease of lead temperature in the primary system is significantly accelerated by the lack of coolant mixing in the cold pool, that decreases noticeably the effective thermal inertia of the primary system. The absence of cold pool mixing effect (observed with the analysis with the CATHARE code by CEA) mainly explains the large difference between RELAP5 and CATHARE results, regarding the time needed to approach the risk of lead freezing following DHR-1 start-up.