LEADER Project: Task 5.4

Analysis of Representative DBC Events of the ETDR with RELAP5

G. Bandini - ENEA/Bologna

LEADER 5th WP5 MeetingJRC-IET, Petten, 26 February 2013

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Outline

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

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ALFRED modelling

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Steady-state at EOCParameter Unit RELAP5

Reactor thermal power MW 300Total primary flowrate kg/s 25250Active core flowrate kg/s 24970Average FA flowrate kg/s 145.8Hottest FA flowrate kg/s 174.3Pressure loss through the primary circuit bar 1.5Pressure loss through the core bar 1.0Core inlet lead temperature °C 400Average FA outlet lead temperature °C 480Hottest FA outlet lead temperature °C 483Upper plenum lead temperature °C 480Average pin max clad temperature °C 500Hottest pin max clad temperature °C 508Average pin max fuel temperature °C 1594Hottest pin max fuel temperature °C 1991SG inlet lead temperature °C 480SG outlet lead temperature °C 400Total SG feedwater flowrate (8 SGs) kg/s 192.8SG feedwater temperature °C 335SG steam outlet temperature °C 450SG inlet pressure bar 188SG outlet pressure bar 182Steam line outlet pressure bar 180

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TRANSIENT Initiating Event (t = 0 s) Reactor scram and threshold

Primary pump trip

MHX FW trip

MSIV closure DHR startup

TD-1: Spurious reactor trip Reactor scram 0 s, Spurious trip No No No No

TD-3: Loss of AC power Station blackout 0 s, CR magnet

de-energization 0 s 0 s 0 s DHR-1 at 1 s (4 IC loops)

TD-7: Loss of all primary pumps All primary pump coastdown 3 s, ΔT hot FA =

120% nominal 0 s 3 s 3 s DHR-1 at 4 s (3 IC loops)

TO-1: Reduction of FW temperature

FW temperature from 335 °C down to 300 °C in 1 s

2 s, Low FW temperature No 2 s 2 s DHR-1 at 3 s

(4 IC loops)

TO-4: Increase of FW flowrate

20% increase in FW flowrate in 25 s

No, No scram threshold reached No No No No

Main events and reactor scram threshold

Analysed DBC transients

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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

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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

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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

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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

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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 flowrate Core 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)

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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

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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)

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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

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Maximum core temperatures

Transient Description Code System Max Temperatures [°C] Fuel Cladding Coolant

Nominal Steady state, peak pin - ENEA RELAP5 1991 508 483

TD-1 Spurious reactor trip RELAP5 1991 508 483

TD-3 Loss of AC power RELAP5 1991 556 534

TD-7 Loss of all primary pumps (PLOF) RELAP5 1991 639 592

TO-1 Reduction of FW temperature RELAP5 1991 508 483

TO-4 Increase of FW flowrate by 20 % RELAP5 2060 508 483

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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.