1

Osaka University

Akira Yamaguchi

Uncertainty of Source Terms Evaluation in

Severe Accident and Level 2 PRA

Contents

2

• Defense in depth – Source term evaluation is one of the most important for nuclear safety

• Key Phenomena in Source term evaluation

• Fukushima #1 Accident

• Uncertainty in Source Term Evaluation and Ambiguity of Scenario

Defense in Depth is the Basis of Nuclear Safety

3

• Defense in Depth

• Prevention

• Mitigation

• Emergency Preparedness / Response

• The first and the most important step is to define object

• Safety purpose is to protect public health and safety as well as environment

• Safety object is to to avoid significant radioactive release to offsite

• Management object is to contain radioactive materials

• Design object is to prevent severe accident

Defense in Depth for Safety Object

(Significant Radioactive Release to Offsite)

4

• Prevention (safety design)

• To prevent severe accident to occur

• Mitigation (severe accident management)

• To mitigate the consequence to prevent significant radioactive release

• Emergency preparedness / response (offsite evacuation)

• To prepare for significant radioactive release to offsite

• Phenomenology in severe accident

• Quantification of source term

• Level 2 Probabilistic Risk Assessment

Key Phenomena in Severe Accident

5

Combustion and explosion

of flammable gas Coolant boundary failure

FP transport in HTS

Load on penetration

AESG-SC-P009:2008, A standard for procedures of probabilistic safety assessment

of nuclear power plant during power operation (Level 2 PSA): 2008

FP behavior in CV Core melt and relocation

Debris-coolant interaction

Debris-concrete interaction

Key Phenomena in FP Transport and Removal

6

Flushing

Gaseous FP removal Aerosol growth

Aerosol deposition

AESG-SC-P009:2008, A standard for procedures of probabilistic safety assessment

of nuclear power plant during power operation (Level 2 PSA): 2008

Pool scrubbing

Filtering

Containment spray

FP origin

Gaseous FP

Aerosol FP

Gas

Water

Fukushima #1 Accident

Radioactive Release – from NSC, NISA, TEPCO

7

Organizati

ons

Published

Date Evaluation Period

Radioactive release (PBq)

Noble

gas I-131 Cs-134 Cs-137 INES

JAEA

NSC

Apr. 12, 2011

May 12, 2011 Mar. 11, 2011- Apr. 5, 2011 - 150 - 13 670

JAEA

NSC Aug. 22, 2011 Mar. 12, 2011- Apr. 5, 2011 130 11 570

JAEA Mar. 6, 2012 Mar. 11, 2011- Apr. 10, 2011 120 9 480

NISA Apr.12, 2011 - - 130 - 6.1 370

NISA Jun 6, 2011 - - 160 18 15 770

NISA Feb. 16, 2012 - - 150 - 8.2 480

TEPCO Mar 12, 2012 Mar. 12, 2011- Mar. 31, 2011

(<1% for Apr. 1 and after) 500 500 10 10 900

Major Difference in Evaluation Method

8

• JAEA Estimate

• Monitoring data

• SPEEDI analysis

• JNES

• MERCOR analysis

• TEPCO

• Monitoring data

• Weather conditions

• Air dose rate - estimate the ratio: noble gas / I / Cs

Radioactive Release Path

9

Source Radioactive Release (PBq)

Noble gas I-131 Cs-134 Cs-137

CV Venting ~5 ~1 ~0.02 ~0.01

RB

Explosions

~10 ~3 ~0.07 ~0.05

RB ~500 ~500 ~10 ~10

Total ~500 ~500 ~10 ~10

TEPCO (May, 2012)

Radioactive Release during RB Explosions

10

Unit Date/time Radioactive Release (PBq)

Noble gas I-131 Cs-134 Cs-137

1 Mar.12 15:36 ~10 ~3 ~0.05 ~0.04

3 Mar.14 11:01 ~1 ~0.7 ~0.01 ~0.009

4 Mar 15 06:12 - - - -

Total ~10 ~3 ~0.07 ~0.05

TEPCO (May, 2012)

Monitoring Car Measurement of Dose Rate （Report of Japanese Government, June 2011）

11

Monitoring post 4

Main gate

Gymnasium

Monitoring post 5

Administration Bldg.

Portable MP (main gate)

Portable MP (west gate)

Unit 1 vent

Unit 1 RB explosion

Unit 3 RB explosion

Unit 3 vent

Unit 3 vent

Unit 3 vent

Unit 3 vent

Radioactive Release Path to Environment

12 TEPCO, May 2012

Analytical condition consistent

with measured data

I and Cs to

atmosphere

I and Cs to S/P

SRV open

Deposition of I and Cs

on RV structure

Deposition of I and Cs

on CV structure

Emission of I and

Cs from fuel

Distribution of CsI in RPV and CV and Release

Based on MAAP Analysis of Unit 3

13

Sensitivity Analysis of Radioactive FP Release

Fraction with MERCOR

14

• Unit 1

• Case 2 Two ICs operated

• Case 3 Water injection depends on RPV pressure

• Unit 2

• Case 2 Water injection depends on RPV pressure, larger D/W and S/C failure

• Case 3 PCV no dry well failure

• Case 4 PCV larger dry well failure

• Case 5 PCV larger S/C failure

• Unit 3

• Case 2 Smaller water injection

Uncertainty in CsI and Cs Release Fraction

– Evaluated Based on MERCOR Analysis

15

Report of Japanese Government to the IAEA Ministerial Conference on Nuclear Safety

- The Accident at TEPCO's Fukushima Nuclear Power Stations -, June 2011

0%

1%

2%

3%

4%

5%

6%

7%

8%

CsI Cs

Rle

ase

Fra

ctio

n

U1-Case1

U1-Case2

U1-Case3

U2-Case1

U2-Case2

U2-Case3

U2-Case5

U2-Case5

U3-Case1

U3-Case2

Unit 1 Unit 2 Unit 3 Unit 1 Unit 2 Unit 3

Event Tree for Plant Damage State (BWR)

16

Initiating Event

LOSP

Emergency

power supply

Reactivity

control

CRD Boron

Pressure

control

High pressure

injection and LL

RCIC HPCS

Depress

urization

Low press.

Injection

LPCS/CI

Decay heat

removal

RHRS

AESJ Level 2 PSA Standard, AESJ, 2008

Plant Damage State (BWR)

17

AESJ Level 2 PSA Standard, AESJ, 2008

Core damage

sequence

Containment vessel

failure timing Reactor pressure

vessel pressure Core damage

timing Debris and CV

cooling Plant damage state

High

Low

Late

Early

Late

Early

Before CD

After CD

Core Damage Scenario and Classification

18

Core damage

RPV failure

RB Explosion Core damage RB Explosion

Core damage

SBO

PCV Failure Time Sequences

19

Uncertainty in NUREG-1150

20

12

ソースタームの不確実さの評価例

NUREG-1150

• Reduction of Source Term depends on

integrity of containment and time

sequence

• Longer the grace period, less the

source term release by facto of 10

• Source term uncertainty comes from

scenario ambiguity as well as model

uncertainty

Source Term Analysis for BWR – Release Fraction

21

Summary

22

• Ambiguity and Uncertainty

• Severe accident code uncertainty is less than accident scenario ambiguity

• Defense-in-depth for uncertainty and ambiguity

• Key Issues

• Grace Period Evaluation

• Low power low flow cooling (debris cooling, natural circulation)

• Containment Cooling

• Containment Spray, submergence from outside

• FP Release Suppression

• Pool Scrubbing, Containment Spray

• Hydrogen Risk Control – Severe Accident Management

• Loss of Monitoring – Robust I&C and Monitoring, Simulation Tool

• Multi-Phase Flow Research and Level 2 PRA - Research Needs Based on Risk Consideration