Nathália N. Araújo, Maritza R. Gual & Marcos C. Maturana

Brazil

Technical Meeting on the Phenomenology, Simulation and Modelling of Accidents in Spent Fuel Pools

2-5 September 2019, Vienna, Austria

Summary

• PROBABILISTIC SAFETY ASSESSMENT (PSA)

• DESCRIPTION OF THE SFP COOLING SYSTEMS

(SFPCS)

• EVENT SEQUENCES

• SPF MODELING WITH MELCOR

• RESULTS OF THE SIMULATIONS

• CONCLUSIONS AND FUTURE WORK

Probabilistic Safety Assessment (PSA)

• Selection of the most frequent accident sequences

through PSA level 1

• Preparation of an SFP model in MELCOR

• Support the calculation of frequency and release of

radioactive material – PSA level 2 – with the

accident simulation results

PSA LevelsAt each level the risk associated

with the indicated event

is analyzed.

SFP COOLING SYSTEMS (SFPCS)

FUEL BUILDING (FB)

CONTROL ROOM (CR)

Compartment

FSP-I

Compartment

FSP-II

Compartment

FSP-III

B1 B2

B3 B4

T2

T3

P1

P2

S1

FE

Cleaning and

Purification

System (CPS)

T1

FE

Safety Water

Cooling System

(SWCS)

Drainage

System (DS)S3

V34

Tra

nsf

er

Ch

ann

el(T

C)

V47

V36

V62

V65

V72

V22

V23

V27

V25

V35

V28V31

V57

V71

V70

V52

V56

V4

V5

V7

V9

V11

V64V10V14

V17

V63

V44

V38

V76

V45 V75

V40

V73

V46

V74

V53

Sampling

System (SS)

V51

V37

S2

SFPCS OPERATIONAL MODES

Operational

Mode

Cooled

CompartmentsDescription

B

FSP-II

Normal operation. Circulation by pump B3.

Cooling by the heat exchanger T2.

B.1Degraded operation. Circulation by pump B3.

Cooling by the heat exchanger T3.

B.2Degraded operation. Circulation by pump B4.

Cooling by the heat exchanger T2.

B.3Degraded operation. Circulation by pump B4.

Cooling by the heat exchanger T3.

EVENT SEQUENCES TO BE

CONSIDERED IN PSA LEVEL 2

• PSA Level 1 results:

– Failure to open valves for

redundant circuit: first three

cases;

– CCF of B3 and B4 to start:

fourth case (3.43E-6 / yr);

– One pump unavailable due

to testing and maintenance

and standby pump failure to

start: fifth and sixth cases

(5.00E-7 and 4.70E-7 / yr);

• The set of abnormal conditions

identified in the PSA Level 1 will

be considered for the spent fuel

pool accidents in PSA Level 2.

SHUTDOWN PSA LEVEL 1 TOP 3 SFP FUEL DAMAGE FREQUENCY CUTSETS

Cutset

NumberEvent Description

Event

Probability

Frequency

(yr)

1

Plant is shutdown based on

annual availability8.22E-2

8.56E-6

Shutdown Phase II, III, IV

duration7.33E-1

Loss of SFPCS 1.00

Flag: Spent fuel elements in

FSP-II.1.00

Manual Valve _036_ Failure to

Open1.42E-4

2

Plant is shutdown based on

annual availability8.22E-2

8.56E-6

Shutdown Phase II, III, IV

duration7.33E-1

Loss of SFPCS 1.00

Flag: Spent fuel elements in

FSP-II.1.00

Manual Valve _034_ Failure to

Open1.42E-4

3

Plant is shutdown based on

annual availability8.22E-2

3.66E-6

Shutdown Phase II, III, IV

duration7.33E-1

Loss of SFPCS 1.00

Flag: Spent fuel elements in

FSP-II.1.00

Check Valve _032_ Failure to

Open6.07E-5

MELCOR SFP MODELING

• MELCOR 2.2.9541

– SFP – PWR

– 3 rings × 12 axial levels (6 for active fuel)

– 21 fuel assemblies with UO2 ~5% fuel rods

– 48 MW (NPP) ~ time decay heat of 5 days

MELCOR SFP MODELING

MELCOR RESULTS

MELCOR RESULTS

MELCOR RESULTS

CONCLUSIONS & FUTURE WORK

• The Shutdown PSA Level 1 of a reference NPPwas analysed and a set of accident sequenceswas selected;

• A MELCOR model for SFP was obtained;

• A better understand of the LOFA physicalphenomena was reached;

• The SFP accidents evaluation will be based on theresults of these simulations;

• Improvement of MELCOR model of the SFP;

• The resulting MELCOR model will be useful for thecalculation Large Early Release Frequency (LERF)of the reference NPP.

Thank You!