Analysis of Representative DEC Events of the ETDR with RELAP5

LEADER Project: Task 5.5

G. Bandini - ENEA/Bologna

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

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Outline

Analysed DEC transients at EOC Transient results Conclusions

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Analyzed DEC transients at EOC

Main events and reactor scram thresholdTRANSIENT Initiating Event

Reactor scram

Primary pump trip

MHX FW trip

MSIV closure

DHR startup

TR-4: UTOP Insertion of 250 pcm in 10 s

No No No No No

TDEC-1: ULOF All primary pumps coastdown

No 0 s No No No

TDEC-3: ULOHS All MHX feedwater trip No No 0 s 1 s DHR-1 at 2 s (3 IC loops)

T-DEC4: ULOHS+ULOF All primary pumps and MHXs feedwater trip

No 0 s 0 s 1 s DHR-1 at 2 s (3 IC loops)

T-DEC5: Partial blockage in the hottest FA

10% to 97.5% blockage at the hottest FA inlet

No No No No No

TO-3: All prim. pumps stop + reduction of FW temperature

T-fw: 335330°C in 1s + all p. pumps stop

2 s, low pump speed

0 s 2 s 2 s DHR-1 at 3 s (4 IC loops)

TO-6: All prim. pumps stop + increase of FW flow rate

FW-flow +20% in 25 s + all p. pumps stop

2 s, low pump speed

0 s 2 s 2 s DHR-1 at 3 s (4 IC loops)

T-DEC6: SCS failure Depressurization of all secondary circuits

2 s, low sec. pressure

No 2 s No No

UN

PRO

TECT

EDPR

OTE

CTED

TR-4: Reactivity insertion (UTOP) (1/2)

Core and MHX powers

ASSUMPTIONS: Insertion of 250 pcm in 10 s without reactor scram No feedwater control on secondary side Fuel-clad linked effect fuel expansion according to clad temperature (closed gap)

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Total reactivity and feedbacks

The reactivity insertion is mainly counterbalanced by Doppler effect initial core power rise up to 680 MW

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TR-4: Reactivity insertion (UTOP) (2/2)

Core temperatures

MAIN RESULTS: Maximum clad temperature remains below 650 °C Maximum fuel temperature of 2930 °C at t = 57 s (hottest FA, middle core plane,

fuel pellet centre) exceeds the MOX melting point (~2670 °C) only local fuel melting no extended core melting

Core temperatures

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T-DEC1: ULOF transient (1/2)

ASSUMPTIONS: All primary pumps coastdown without reactor scram No feedwater control on secondary side Fuel-clad not-linked effect fuel expansion according to fuel temperature (open gap)

Active core flowrate Core and MHX powers

Natural circulation in the primary circuit stabilizes at 23% of nominal value Core power reduces down to about 200 MW due to negative reactivity feedbacks

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T-DEC1: ULOF transient (2/2)

Core temperatures

Core temperatures

MAIN RESULTS: Initial clad peak temperature of 764 °C Max clad temperature stabilizes below

650 °C No clad failure is expected in the short

and long term No vessel wall temperature increase

Total reactivity and feedbacks

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T-DEC3: ULOHS transient (1/2)

ASSUMPTIONS: Loss of feedwater to all MHXs without reactor scram Startup of DHR-1 (3 out of 4 IC loops of in service) No heat losses for the external vessel wall surface

Core power progressively reduces down towards decay level Maximum clad and vessel temperatures rise up to 700 °C after about one hour

Core and MHX powers Core and vessel temperatures

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Total reactivity and feedbacksCore temperatures

T-DEC3: ULOHS (2/2)

MAIN RESULTS: No fuel rod clad rupture is expected in the medium term No vessel failure is expected in the medium term (to be verified) Enough grace time is left to the operator to take the opportune corrective actions

and bring the plant in safe conditions in the medium and long term

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T-DEC4: ULOHS+ULOF transient (1/2)

Active core flowrate Core and MHX powers

ASSUMPTIONS: Loss of feedwater to all MHXs and all primary pumps without reactor scram Startup of DHR-1 (3 out of 4 IC loops of in service) No heat losses from the external vessel wall surface

Natural circulation in primary circuit reduces down to very low value (around 1%) Core power progressively reduces down towards decay level

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T-DEC4: ULOHS+ULOF transient (2/2)

MAIN RESULTS: Max T-clad rises up to 800 °C after about 15 minutes and stabilizes around 825 °C

no fuel rod clad rupture is expected in the short and medium term (to be verified) No vessel failure is expected in the medium and long term Enough grace time is left to the operator to take the opportune corrective actions

and bring the plant in safe conditions in the medium and long term

Core temperatures Total reactivity and feedbacks

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TO-3: FW temp. reduction (1/2)

Active core flowrate (short term)

Active core flowrate (long term)

ASSUMPTIONS: Loss of one preheater (FW temperature

from 335 °C down to 300 °C in 1 s) + all primary pumps coastdown

Reactor scram at t = 2 s on low primary pump speed signal

Startup of DHR-1 (4 IC loops in service)

Core temperatures

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TO-3: FW temp. reduction (2/2)

Core decay and MHX powers Primary lead temperatures

MAIN RESULTS: Power removal by 4 IC loops of DHR-1 system is about 7 MW No risk of lead freezing after DHR-1 startup Lead freezing at MHX outlet is reached after about 2 hours (cold lead at the MHX

outlet flows to the core inlet without mixing with the hotter lead of the cold pool surrounding the MHXs)

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TO-6: FW flowrate + 20% (1/2)

Core temperatures

Active core flowrate (short term)

Active core flowrate (long term)

ASSUMPTIONS: FW flowrate increase of 20% + all primary

pumps coastdown Reactor scram at t = 2 s on low primary

pump speed signal Startup of DHR-1 (4 IC loops in service)

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TO-6: FW flowrate + 20% (2/2)

MAIN RESULTS: Power removal by 4 IC loops of DHR-1 system is about 7 MW No risk of lead freezing after DHR-1 startup Lead freezing at MHX outlet is reached after about 2 hours (cold lead at the MHX

outlet flows to the core inlet without mixing with the hotter lead of the cold pool surrounding the MHXs)

Core decay and MHX powers Primary lead temperatures

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T-DEC6: SCS failure (1/2)

Secondary pressure

Core and MHX powers

Primary lead temperatures

ASSUMPTIONS: Depressurization of all secondary circuits

at t = 0 s (no availability of the DHR) Reactor scram at t = 2 s on low

secondary pressure

Initial MHX power increase up to 850 MW no risk for lead freezing

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T-DEC6: SCS failure (2/2)

Core decay and MHX powers Core and vessel temperatures

MAIN RESULTS: No risk for lead freezing in the initial transient phase Slow primary temperature increase due to large thermal inertia of the primary

system (effective mixing in the cold pool surrounding the MHXs) large grace time for the operator to take opportune corrective actions

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TDEC-5: Partial FA blockage

ASSUMPTIONS: Total ΔP over the FA = 1.0 bar ΔP at FA inlet = 0.22 bar Partial flow area blockage

at FA inlet No heat exchange with

surrounding FAs

MAIN RESULTS: 75% FA flow area blockage 50% FA

flowrate reduction 85% blockage T-max clad = 700 °C No clad melting if area blockage < 95% Fuel melting if area blockage > 97.5% 50% inlet flow area blockage can be

detected by TCs at FA outlet

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Conclusions

The analysis of DEC transients with RELAP5 code has highlighted the very good intrinsic safety features of ALFRED design thanks to:

Good natural circulation characteristics, Large thermal inertia, and Prevalent negative reactivity feedbacks

In all analyzed transients there is no risk for significant core damage or risk for lead freezing large grace time is left to the operator to take the opportune corrective actions and bring the plant in safe conditions in the medium and long term

The RELAP5 results for unprotected transients (UTOP, ULOF, ULOHS and ULOHS+ULOF) are confirmed by the results of the analyses performed with the CATHARE code