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First safety approach of the DHR system of XT-ADS. B. Arien. General purpose. Main objective: identification of the possible failure modes of the DHR system and its weaknesses, its limits Methodological approach: master logic diagram (MLD) method Accidents into consideration:

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First safety approach of the DHR system of XT-ADS

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First safety approach of the dhr system of xt ads l.jpg

First safety approach of the DHR system of XT-ADS

B. Arien


General purpose l.jpg

General purpose

  • Main objective: identification of the possible failure modes of the DHR system and its weaknesses, its limits

  • Methodological approach: master logic diagram (MLD) method

  • Accidents into consideration:

    • Loss of heat sink (LOHS)

    • Loss of flow (LOF)

    • Combination of LOF and LOHS

    • Protected and unprotected cases


Main design assumptions l.jpg

Main design assumptions

  • Primary system:

    • 2 groups pump-HX (2 pumps, 4 HXs)

    • Emergency electrical supply to pumps

    • Free convection if total loss of pumps

  • Secondary system:

    • 2 independent loops

    • Emergency electrical supply to pumps

    • Possibility of natural circulation to be considered

  • Tertiary system: no design information, supposed to work in natural circulation and is treated as a whole

  • Vault system (RVACS): no design information, supposed to work in natural circulation mode and treated as a whole


Slide4 l.jpg

XT-ADS

Sketch of the Secondary System and DHR System (Proposal)


Slide5 l.jpg

SCKCEN’s proposal


Mld procedure l.jpg

MLD procedure

For each accident type:

  • Step 1: identification of the failure modes that initiate the accident

  • Step 2: development of a MLD for the protected case

  • Step 3: development of a MLD for the unprotected case


Mld procedure7 l.jpg

Qi

OK

unsuccess

question

?

no

yes

MLD procedure

Symbols:

  • DHR system fulfills its function (= .false.)

  • may contribute to DHR system failure (=.true.)

  • question related to any unresolved problem

Accident initiating event

Failure in DHR system


Slide8 l.jpg

LOHS accident

HX blockage (primary side)

Secondary pump failure

Pipe break in SCS

Depressurization in SCS

HX blockage (secondary side)

Tertiary cooling system failure

Accompanied by LOF

Partial blockage

Blockage by debris

LOF&LOHS

LBE freezing in HX

Blockage by debris

LOHS: step 1


Slide9 l.jpg

Failure of core cooling in LOHS conditions

Unprotected accident

Protected accident

Failure of core cooling under protected LOHS conditions

Failure of core cooling under unprotected LOHS conditions

Accelerator shutdown failure

A

B


Slide10 l.jpg

OK

OK

OK

OK

OK

DHR possible at atm. p in SCS

?

yes

no

DHR possible by free convection in SCS

A

Q1

Q2

?

unsuccess

no

yes

LOHS: step 2

Failure of core cooling under protected LOHS conditions

If total secondary pump failure

If single secondary pump failure

If pipe break in 1 SCS loop

If pipe breaks in whole SCS

If depressur. in 1 SCS loop

If depressur. in whole SCS

If partial HX blockage (water side)

If tertiary cooling system unavailable

Failure of tertiary cooling system

Vault System failure

SCS pipe breaks caused by external accident

Vault System failure

Over- pressure in SCS

Safety valve failures

Vault System failure

Failure of emergency electrical supply to secondary pumps

Vault System failure

Failure of electrical supply to secondary pumps

Failure of SCS pressurization

Vault System failure

Free convection fails to take place in the secondary system


Slide11 l.jpg

Failure of core cooling under unprotected LOHS conditions

If total secondary pump failure

If single secondary pump failure

If pipe break in 1 SCS loop

If pipe breaks in whole SCS

If depressur. in 1 SCS loop

If depressur. in whole SCS

If partial HX blockage (water side)

If tertiary cooling system unavailable

Debris formation in SCS

Failure of pressurization in 1 SCS loop

Failure of SCS pressurization

Failure of tertiary cooling system

Single secondary pump failure

Over- pressure in 1 SCS loop

Safety valve failure

Single pipe break in SCS

Over- pressure in SCS

Safety valve failures

SCS pipe breaks caused by external accident

B

Q3

Q3

Nominal power can be removed by free convection in SCS

unsuccess

Failure of electrical supply to secondary pumps

Failure of emergency electrical supply to secondary pumps

yes

?

Free convection fails to take place in the secondary system

no

LOHS: step 3


Slide12 l.jpg

LOF accident

HX blockage (primary side)

Accidental core bypass

Primary pump failure

Accompanied by LOF

LOF&LOHS

LBE freezing in HX

Blockage by debris

LOF: step 1


Slide13 l.jpg

Failure of core cooling under LOF conditions

Unprotected accident

Protected accident

Failure of core cooling under protected LOF conditions

Failure of core cooling under unprotected LOF conditions

Accelerator shutdown failure

C

D


Slide14 l.jpg

Failure of core cooling under protected LOF conditions

If single primary pump failure

If total primary pump failure

If accidental core bypass

OK

OK

Core bypass formation

DHR possible in free convection mode and with core bypass

?

unsuccess

no

yes

Free convection fails to take place

C

Primary pumps fail to stop

Q4

LOF: step 2


Slide15 l.jpg

Failure of core cooling under unprotected LOF conditions

If accidental core bypass

If single primary pump failure

If total primary pump failure

Nominal power can be evacuated when 1 group is operating

Nominal power can be evacuated in free convection mode

OK

OK

Failure of electrical supply to primary pumps

Failure of emergency electrical supply to primary pumps

?

?

yes

yes

no

no

Single primary pump failure

Core bypass formation

D

Nominal power can be evacuated in free convection mode and with core bypass

Q5

Q6

Q7

unsuccess

unsuccess

?

unsuccess

no

yes

Free convection fails to take place

Primary pumps fail to stop

LOF: step 3


Slide16 l.jpg

LOF&LOHS accident

Common cause failure generating LOF and LOHS

Independent combinations of LOF and LOHS

Dependent combinations of LOF and LOHS

HX blockage (primary side)

Partial blockage

Freezing induced by LOF

Blockage by debris

LBE freezing in HX

LOF&LOHS: step 1


Slide17 l.jpg

Failure of core cooling under LOF&LOHS conditions

Unprotected accident

Protected accident

Failure of core cooling under protected LOF&LOHS conditions

Failure of core cooling under unprotected LOF&LOHS conditions

Accelerator shutdown failure

E

F


Slide18 l.jpg

Failure of core cooling under protected LOF&LOHS conditions

Common cause failure for LOF and LOHS

LOHS induced by LOF: HX blockage (primary side)

Independent combinations of LOF and LOHS

OK

Total HX blockage by LBE freezing

Partial HX blockage by debris

Station black-out

Failure of emergency electrical supply

DHR possible by total free convection in the primary, secondary and tertiary systems

Vault System failure

?

DHR possible via VS in ‘degraded’ free convection mode

Total primary pump failure

Overcooling

no

yes

E

Free convection fails to take place in the secondary system

?

unsuccess

Q9

Q8

no

unsuccess

yes

Failure of electrical supply to primary pumps

Failure of emergency electrical supply to primary pumps

Vault System failure

LOF&LOHS: step 2


Slide19 l.jpg

Failure of core cooling under unprotected LOF&LOHS conditions

Common cause for LOF and LOHS

Independent combinations of LOF and LOHS

LOHS induced by LOF: HX blockage (primary side)

Station black-out

Failure of emergency electrical supply

Nominal power can be evacuated by total free convection in the primary, secondary and tertiary systems

Total HX blockage by LBE freezing

Partial HX blockage by debris

Q10

Debris formation in primary system

?

Total primary pump failure

Overcooling

unsuccess

no

yes

F

Free convection fails to take place in the secondary system

Failure of electrical supply to primary pumps

Failure of emergency electrical supply to primary pumps

LOF&LOHS: step 3


Problems to be solved l.jpg

Problems to be solved

  • Q1: is the DHR possible with the SCS working in free convection mode?

  • Q2: is the DHR possible when the SCS is at atmospheric pressure?

  • Q3: can the nominal power be evacuated with the SCS working in free convection mode?

  • Q4: is the DHR possible with the primary system working in free convection mode and with the presence of a core bypass?

  • Q5: can the nominal power be evacuated when only one pump-HX group is operating in the primary system?

  • Q6: can the nominal power be evacuated with the primary system working in free convection mode ?

  • Q7: can the nominal power be evacuated with the primary system working in free convection mode and with the presence of a core bypass?

  • Q8: is the DHR possible with the primary, secondary and tertiary circuits working in free convection mode?

  • Q9: is the DHR possible via the VS with the primary system working in free convection mode and with a total blockage of the PHXs?

  • Q10: can the nominal power be evacuated with the primary, secondary and tertiary circuits working in free convection mode?


Cut sets for protected lof l.jpg

Core bypass formation

Primary pumps fail to stop

and

if Q4 true

Core bypass formation

if Q4 false

DHR possible in free convection mode and with core bypass

Q4

?

Cut sets for protected LOF

Failure of core cooling under protected LOF conditions

:


Cut sets for unprotected lof l.jpg

Single primary pump failure

Accelerator shutdown failure

if Q5 false

and

Failure of electrical supply to primary pumps

Failure of emergency electrical supply to primary pumps

Accelerator shutdown failure

if Q6 false

and

and

Failure of core cooling under protected LOF conditions

:

Core bypass formation

Primary pumps fail to stop

Accelerator shutdown failure

if Q7 true

and

and

Core bypass formation

Accelerator shutdown failure

if Q7 false

and

Nominal power can be evacuated in free convection mode

Q5

Q7

Q6

Nominal power can be evacuated in free convection mode and with core bypass

Nominal power can be evacuated when 1 group is operating

?

?

?

Cut sets for unprotected LOF


Cut sets for protected lohs l.jpg

Free convection fails to take place in the secondary system

Failure of emergency electrical supply to secondary pumps

Vault System failure

Failure of electrical supply to secondary pumps

if Q1 true

and

and

and

Failure of emergency electrical supply to secondary pumps

Vault System failure

Failure of electrical supply to secondary pumps

if Q1 false

and

and

Failure of core cooling under protected LOHS conditions

:

Overpressure in SCS

Safety valve failures

Vault System failure

SCS pipe breaks caused by external accident

Vault System failure

DHR possible by free convection in SCS

and

and

and

?

Q1

Q2

Failure of tertiary cooling system

Vault System failure

and

DHR possible at atm. p

?

Failure of SCS pressurization

Vault System failure

if Q2 false

and

Cut sets for protected LOHS


Cut sets for unprotected lohs a l.jpg

Accelerator shutdown failure

Failure of emergency electrical supply to secondary pumps

Free convection fails to take place in the secondary system

Failure of electrical supply to secondary pumps

and

and

and

Single secondary pump failure

Accelerator shutdown failure

Free convection fails to take place in the secondary system

and

and

Single pipe break in SCS

Accelerator shutdown failure

and

SCS pipe breaks caused by external accident

Accelerator shutdown failure

and

Overpressure in 1 SCS loop

Safety valve failure

Accelerator shutdown failure

and

and

Failure of core cooling under unprotected LOHS conditions

Nominal power can be removed by free convection in SCS

:

Overpressure in SCS

Safety valve failures

Accelerator shutdown failure

and

?

and

Failure of SCS pressurization

Accelerator shutdown failure

and

if Q3 true

Q3

Depressur. of 1 SCS loop

Accelerator shutdown failure

and

Debris formation in SCS

Accelerator shutdown failure

and

Failure of tertiary cooling system

Accelerator shutdown failure

and

Cut sets for unprotected LOHS (a)


Cut sets for unprotected lohs b l.jpg

Accelerator shutdown failure

Failure of emergency electrical supply to secondary pumps

Failure of electrical supply to secondary pumps

and

and

Single secondary pump failure

Accelerator shutdown failure

and

SCS pipe breaks caused by external accident

Accelerator shutdown failure

and

Overpressure in 1 SCS loop

Safety valve failure

Accelerator shutdown failure

and

and

Failure of core cooling under unprotected LOHS conditions

Nominal power can be removed by free convection in SCS

Single pipe break in SCS

Accelerator shutdown failure

:

and

Overpressure in SCS

Safety valve failures

Accelerator shutdown failure

and

?

and

Failure of SCS pressurization

Accelerator shutdown failure

and

if Q3 false

Q3

Depressur. of 1 SCS loop

Accelerator shutdown failure

and

Debris formation in SCS

Accelerator shutdown failure

and

Failure of tertiary cooling system

Accelerator shutdown failure

and

Cut sets for unprotected LOHS (b)


Cut sets for protected lof lohs l.jpg

if Q9 true

if Q9 true

if Q8 true

Station black-out

Vault System failure

Failure of emergency electrical supply

Free convection fails to take place in the secondary system

and

and

and

Station black-out

Vault System failure

Failure of emergency electrical supply

if Q8 false

and

and

Failure of electrical supply to primary pumps

Failure of emergency electrical supply to primary pumps

Vault System failure

and

and

and

Overcooling

?

DHR possible via VS in ‘degraded’ free convection mode

Failure of electrical supply to primary pumps

Failure of emergency electrical supply to primary pumps

and

and

Overcooling

?

Q9

Q8

Independent combinations of LOF and LOHS

DHR possible by total free convection in the primary, secondary and tertiary systems

Cut sets for protected LOF&LOHS

Failure of core cooling under protected LOF&LOHS conditions

:


Cut sets for unprotected lof lohs l.jpg

Station black-out

Accelerator shutdown failure

Failure of emergency electrical supply

Free convection fails to take place in the secondary system

if Q10 true

and

and

and

Station black-out

Accelerator shutdown failure

Failure of emergency electrical supply

if Q10 false

and

and

Q10

Failure of electrical supply to primary pumps

Failure of emergency electrical supply to primary pumps

Accelerator shutdown failure

?

and

and

and

Overcooling

Nominal power can be evacuated by total free convection in the primary, secondary and tertiary systems

Independent combinations of LOF and LOHS

Accelerator shutdown failure

and

Accelerator shutdown failure

Debris formation in primary system

and

Cut sets for unprotected LOF&LOHS

Failure of core cooling under unprotected LOF&LOHS conditions

:


Conclusions and future work l.jpg

Conclusions and future work

  • A qualitative analysis was performed:

    • to provide first indications on the DHR performance

    • to guide the future work

  • Some unresolved questions require a quantitative analysis

  • Design needs to be completed

    • Choice of the SCS (Ansaldo or SCKCEN)

  • RELAP (or TRAC) model has to be developed for the simulation of the whole system in most of the transients

  • CFD model of the primary system has to be developed

    • Free convection simulation

    • Calibration of the RELAP model

  • Reassessment of the DHR system behaviour in accidental situations


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