LIKELIHOOD THAT SHORT TERM STATION …psa.ans.org/wp-content/pdf/22515_PSA17-Vanover... 8 RELEVANT CONSIDERATIONS FOR LERF NRC SOARCA study investigated likelihood of SLR (only assumed

LIKELIHOOD THAT SHORT TERM STATION BLACKOUT SCENARIOS LEAD TO A

LARGE AND EARLY RELEASE Donald E. Vanover

September 27, 2017

www.jensenhughes.com 2

PRESENTATION TOPICS

Background Modeling approach Relevant considerations for a Large and

Early Release (LER) Integrated LER likelihood estimate results Conclusions


LERF IN STSBO BACKGROUND

Large Early Release Frequency (LERF) risk metric used in risk-informed applications Mark I BWRs generally have assumed a high

conditional likelihood of LER in Short Term Station Blackout (STSBO) scenarios • Contribution from liner melt-through of

containment following vessel failure Considerable uncertainty associated with

exact timing and magnitude of release • Potential exists that release is not large

and/or not early


LERF IN STSBO MODELING APPROACH

Identify key inputs to estimate LER likelihood Relevant considerations

• General Emergency (GE) declaration and Evacuation Time Estimates (ETEs)

• Core melt progression characteristics (e.g., stuck open relief valve, steam line rupture, timing of vessel failure, fission product location)

• Timing of “large” threshold release Provide uncertainty distribution for each key

input • Integrate using Monte Carlo uncertainty

evaluations


RELEVANT CONSIDERATIONS FOR LERF

General Emergency (GE) declaration and Evacuation Time Estimate (ETEs) • The STSBO scenario will lead to an early

declaration of a GE • ETEs are available from recent emergency

planning study for a variety of conditions Conditions Evacuation

Advisory (Hours)

ETE for 100%

Population out to EPZ

(Hours)

Evacuation Time from Sequence Initiation (Hours)

Good to Poor 1.0 to 1.5 4.0 to 5.0 5.0 to 6.5



Assumed cumulative probability distribution for time to 100% evacuation

5.000 6.5005.0% 0.0%95.0%

0.0

0.2

0.4

0.6

0.8

1.0

4.8

5.0

5.2

5.4

5.6

5.8

6.0

6.2

6.4

6.6

Prob

abili

ty

Hours

Cumulative Distribution Mean for 100% Evacuation



Core melt progression characteristics Two key phenomena have first order impact on

fission product location at time of containment failure • Likelihood of a Stuck Open Relief Valve (SORV) • Likelihood of Steam Line Rupture (SLR)

NRC SOARCA study indicated very high likelihood of SORV (i.e., assumed SORV in base case analysis) • Large number of SRV cycles • Thermal degradation mechanisms

In our analysis, assume 95% likelihood of a SORV during core melt progression



NRC SOARCA study investigated likelihood of SLR (only assumed in sensitivity cases) • Extreme variations to the SORV failure criteria

were needed to obtain conditions that would allow SLR to occur − Prolonged SRV cycling, or − Partial opening of SORV such that additional SRV

cycles are precluded but RPV pressure stays high

In our analysis, assume 10% likelihood of a SLR during core melt progression • This is assumed to be a subset of the SORV

likelihood



Core melt progression characteristics – base case likelihood assumptions

Designator Description Likelihood

SORV Stuck Open Relief Valve Occurs

0.85

SLR Steam Line Rupture Occurs

0.10

HP RPV Remains at High Pressure

0.05



Timing of “large” threshold release SOARCA results from MELCOR for STSBO with

SORV and no injection Results indicate virtually no chance of a large

and early release



However, not all case results are in agreement Event MAAP4.06 MELCOR MAAP5.03 MAAP5.04

H2 Production Begins

36 Min. 55 Min. 50 Min. 42 Min.

SRV Sticks Open 1.8 Hrs. 1.8 Hrs. 1.8 Hrs. 1.1 Hrs.

Vessel Failure 3.8 Hrs. 8.2 Hrs. 8.0 Hrs. 3.6 Hrs.

Drywell Liner Mel-Through

3.9 Hrs. 8.5 Hrs. 8.2 Hrs. 3.8 Hrs.

Iodine or CsI release exceeds 1%

4.0 Hrs. (~4% shortly after that and slowly climbs)

9.7 Hrs. > 24 Hrs. 4.0 Hrs . (~2% shortly after that and slowly climbs

later on) Iodine or CsI release exceeds 10%

> 24 Hrs. 27 Hrs. N/A (at 48 Hrs.)

N/A (at 48 Hrs.)



Timing of “large” threshold release Assign probability distribution based on

uncertainty involved for SORV case

6.45 20.265.0% 5.0%90.0%

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0 5 10 15 20 25

Per H

our F

requ

ency

Hours

Time of Large Release



Timing of “large” threshold release Assign probability distribution based on

uncertainty involved for SLR and HP cases • Much more likely to have ‘large’ release occur

earlier in time

3.457 5.7175.0% 5.0%90.0%

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

2.5

3.0

3.5

4.0

4.5

5.0

5.5

6.0

6.5

Per H

our F

requ

ency

Hours

Time of Large Release


INTEGRATED LER LIKELIHOOD RESULTS

0.00 14.92

1.7% 5.0%93.3%

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.10

-5 0 5 10 15 20

Per H

our F

requ

ency

Hours

SORV Case: (LR - EV)0.000 0.618

72.5% 5.0%22.5%

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

-4.0

-3.5

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5 0.0

0.5

1.0

1.5

Per H

our F

requ

ency

Hours

SLR Case: (LR - EV)

SORV AND SLR MONTE-CARLO CASE RESULTS

Only 1.7% chance of LER is SORV scenario 73% chance of LER is SLR scenario Similarly, 59% of LER in HP scenario


INTEGRATED LER LIKELIHOOD RESULTS

OVERALL MONTE-CARLO CASE RESULTS

Considering assigned uncertainty distributions, less than 12% LER likelihood estimated for STSBO scenarios

0.00 14.5811.6% 5.0%83.4%

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

-5 0 5 10 15 20

Per H

our F

requ

ency

Hours

Combined Case: (LR - EV)


CONCLUSIONS

NRC SOARCA study indicates very high probability of SORV in STSBO scenario and much lower probability of SLR MELCOR and MAAP results both indicate time to

“large” threshold may be significantly delayed or not occur at all in SORV scenarios Time to “large” threshold will occur much sooner

in SLR and HP scenarios Integrating various assumptions leads to <12%

LER likelihood using Monte-Carlo analysis Recommend bounding value of 0.2 be used for

risk-informed applications


Contact Donald E. Vanover +1 610-431-8260

[email protected]

For More Information Visit www.jensenhughes.com

QUESTIONS?

http://www.jensenhughes.com/

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LIKELIHOOD THAT SHORT TERM STATION …psa.ans.org/wp-content/pdf/22515_PSA17-Vanover... 8 RELEVANT CONSIDERATIONS FOR LERF NRC SOARCA study investigated likelihood of SLR (only assumed