 Not part of the BSAF OECD Benchmark Project

(1)

FUKUSHIMA DAIICHI UNIT 1 (MELCOR 1.8.6)

BRAUN Matthias, DTIPP7-G

Zagreb, 04/26/2018

(2)

Restricted - Framatome

 Not part of the BSAF OECD Benchmark Project

 Did not receive undisclosed information from TEPCO / JAEA

 Relying only on publically available input data

 No legal restrains for usage & publication (project and export control)

 Why

 Test of simulation capabilities (code and user) and as MELCOR 1.8.6 to 2.x conversion test base

 Support for training courses (accident awareness trainings for crisis team members and sometimes operators )

 Improvement of Severe Accident Management Guidelines (SAMG)

 Optimization of usage of severe accident hardware

(Passive autocatalytic recombiners, Filtered containment venting systems)

 Contribution to the international MELCOR community

 Model description report containing all input data (FGF_D02-ARV-01-111-828)

 revision B(hardcopy) on EMUG2018

 revision C(electronically) and the MELCOR input model at CSARP/MCAP 2018. (planned)

p.2

Introduction

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 RPV & Core

 Typical GE BWR RPV geometries (well standardized)

 Generic 7x7 fuel assemblies

 Experimental (non-recommended) features

 RPV leakage before failure via TIP dry tubes

 More CVH volumes in lower plenum

 Separating lower plenum in channel & bypass, representing the control rod guide tubes

 Fuel assembly skeleton as supporting structure

 Maximizing the COR detail depth to

7 rings and 35 levels (more leads to crash)

 Additional core collapse criteria

(collapse of fully oxidized assemblies without lateral support)

 Maximum void 0.4 -► 0.7 (RPV water inventory)

 Time-dependent oxidized rod collapse model (Critical thickness of unoxidised Zr-►0.0)

Overview of the MELCOR model for 1F1

(4)

 Nuclear systems

 Operational systems like main steam, feedwater (steady state initiation)

 Safety systems like Isolation Condenser,

core spray, External water injection (fire engines)

 RPV liquid level measurement system (failure of the measurement)

 Noticeable from core spray system

 Spray package can malfunction at high pressure

 Noticeable from IC modelling:

 MELCOR overestimates departure from nucleate boiling at low pressures

 IC operation did not work until secondary side pressure was increased from 1 bar-abs to 2 bar-abs

p.4

Overview of the MELCOR model for 1F1

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 Containment

 Wet-well / suppression pool sub-divided to allow for thermal stratification

(-►spool scrubbing issue)

 Dry-well sub-divided to address over-temperature failure of hatches

(-►too fast convection in lumped parameter codes)

Overview of the MELCOR model for 1F1

CV750 CV745

CV721

CV723

CV780

CV790

HS721.01 HS750.01 HS723.01

751.11

HS761.11HS741.21

771.12 HS771.13

HS770.14 HS763.11HS743.21

773.12 HS773.13

HS783.11

CV760

FL750 FL755

HS721.11 HS721.12 HS731.11 HS731.12 HS731.13

HS731.15 HS731.16

FL753

CV741 CV743

HS790.15

HS741.11 HS741.12 HS741.13 HS791.11

HS781.11 HS783.12

HS781.12

HS723.12 HS733.11

HS733.12 HS733.13

HS731.14 HS733.14

HS733.16 HS733.17 HS791.12

HS741.14

HS743.11 HS743.12 HS743.13 HS793.11

HS743.14 HS793.12 HS799.14

FL721 FL723

FL733 FL731

FL743 FL741

FL780 FL780

FL909

FL763

FL760 FL765

CV770 FL781 FL786

FL783 FL788

Sector 1 (315° - 45°) Sector 3 (135° - 225°)

HS711/8.16 HS711/8.14

HS711/8.13 CV7HS711/811/8.15 CV661/8

CV621/8 CV600

FL701/8

FL611/8 HS711/8.11 HS711/8.12

CV641/8 FL631/8

HS600.01 HS600.02 HS621/8.01 HS641/8.01 HS641/8.02 HS641/8.03 HS661/8.01 HS661/8.02 HS661/8.03 HS661/8.04 HS661/8.05 HS661/8.06 HS661/8.07

HS661/8.08 HS661/8.09

HS661/8.10

HS723.11 Access door (1.9m x 0.77m)

HS714/5.14

HS71 4/5.13 HS714/5.15

CV714/5 HS714/5.16 FL704/5

CV624/5 CV644/5

HS600.01 HS600.02

HS624/5.01 HS644/5.01

HS644/5.02 HS644/5.03 HS664/5.01 HS664/5.02 HS664/5.03 HS664/5.04 HS664/5.05 HS664/5.06 HS664/5.07 HS664/5.08 HS664/5.09 HS664/5.10

CV664/5

CV600 FL614/5

HS714/5.11 HS714/5.12

FL634/5 CV731

CV733

HS733.91 Inner HS 10t steel 5mm thick HS731.91

Inner HS 10t steel 5mm thick HS741.91 Inner HS 5t steel 5mm thick

HS743.91 Inner HS 5t steel 5mm thick

HS760.91 Floor grating

753.11 780.01

HS790.14HS790.13

+2.055 - Torus 05/16 height^th +2.560 - Torus 06/16 heightth +3.065 - Torus 07/16 heightth +4.075 - Torus 09/16 heightth +4.650 - Header center +4.580 - Torus 10/16 heightth +3.570 - Torus center elevation +7.609 - Torus inner top +7.624 - Torus outer top

+1.046 - Torus 03/16 heightth +1.551 - Torus 04/16 height^th +1.461 - Downcomer end

+0.541 - Torus 02/16 heightth +0.036 - Torus 01/16 heightth +0.000 - RB zero

-4.000 - Soil -1.230 - Torus room floor -0.484 - Torus outer bottom -0.469 - Torus inner bottom +7.104 - Torus 15/16 heightth +6.599 - Torus 14/16 heightth +12.200 - Sphere equator

+15.100 - RPV outer bottom +27.800 - Shield wall top

+19.572 - Sphere - Cylinder +25.000 - 2nd floor ceiling +25.900 - 3rd floor

+18.700 - 2nd floor +11.700 - 1st floor ceiling +30.400 - 3rd floor ceiling +30.700 - Reactor well floor +35.536 - PCV head outer top +35.500 - PCV inner top +38.900 - Service (5th) floor

+31.000 - 4th floor +33.000 - PCV cylinder top

+9.550 - Torus room ceiling +10.200 - 1st floor

+38.500 - 4th floor ceiling

+27.100 - SFP Floor +31.300 - Storage pit floor 2.565 - RPV outer

2.870 - Shielding wall inner 3.370 - Shielding wall outer

5.500 - Reactor well inner 7.100 - Reactor well outer 3.111 - RPV stud ring outer

+8.173 - Vent pipe top

+6.527 - Vent bottom +6.180 - D/W floor +6.777 - Vent bottom + 25cm +9.708 - Floor grating top +13.340 - CRD hydraulic openings +14.660 - Lower pedestal top

+33.412 - Stud ring top

+30.888 - Stud ring bottom +31.650 - RPV seal surface +34.290 - RPV outer top

2.500 - Lower pedestal inner 3.500 - Lower pedestal outer 1.877 - Skirt inner radius

4.877 - PVC cylinder inner

14.785 - Torus major radius

8.858 - PCV shpere outer 8.850 - PCV shpere inner

10.100 - PVC basemat outer 10.600 - PVC walls (ground floor)

7.000 - PCV walls (1st & 2nd floor) 1.800 - RPV support ring inner

1.917 - Skirt outer radius

CV721

HS72 HS721.11 HS721.12 HS731.11 HS731.12

FL721

HS711/8.16 HS711/8.14

HS711/8.13 CV7HS711/811/8.15 CV661/8

CV621/8 CV600

FL701/8

FL611/8 HS711/8.11 HS711/8.12

CV641/8 FL631/8

HS600.01 HS600.02 HS621/8.01 HS641/8.01 HS641/8.02 HS641/8.03 HS661/8.01 HS661/8.02 HS661/8.03 HS661/8.04 HS661/8.05

HS661/8.06 HS661/8.07

HS661/8.08 HS661/8.09

HS661/8.10 +8.173 - Vent pipe top

+6.527 - Vent bottom +6.180 - D/W floor +6.777 - Vent bottom + 25cm +9.708 - Floor grating top

(6)

 Surroundings

 Reactor Building (-► CF-based add-on to calculate dose rate in building)

 HVAC, SGTS & stack (deposition and release of radionuclides)

 Filtered Containment Venting System (fictional, for “what-if” studies)

 Spent fuel pool (heat-up, boil-off, RN deposition)

p.6

Overview of the MELCOR model for 1F1

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 14:46:23 - Tohoku earthquake about 150 km from Fukushima Daiichi

 Based on seismic readings of the KNET Station FKS005⁽¹⁾ relative distance to the Fukushima site and wave propagating speed ~4 km/s, seismic trip threshold reached in Fukushima Daiichi at 14:47:32±1s

 1F1 alarm recorder⁽²⁾ states first seismic scram signals 14:46:46

 Correction of the alarm recorder clock by 46 s (consistent with TEPCO timing)

 Correction of the 1F1 transient recorder⁽³⁾ by additional 33 s

 Timing of the paper strip recorders manually corrected

First hour of the Incident in 1F1

Epicenter

F. Daiichi FKS005

(1) http://www.strongmotioncenter.org/cgi-bin/CESMD/iqrStationMap.pl?ID=Japan_11Mar2011_usc0001xgp (2) http://www.tepco.co.jp/en/nu/fukushima-np/index10-e.html#anchor02

(3) http://www.tepco.co.jp/en/nu/fukushima-np/index10-e.html#anchor05

(8)

 14:47:32 - First seismic trip signals

 T + 12 s - 2 out of 2 signals causes SCRAM, terminating nuclear fission

p.8

First hour of the Incident in 1F1

(9)

 After SCRAM steam generation stalls, but turbine still draws steam

 T + 15 s - RPV pressure drops

First hour of the Incident in 1F1

(10)

 Turbine control valves automatically close to stabilize PRV pressure

 T + 30 s - Steam flow to turbine stops and RPV pressure stabilizes

p.10

First hour of the Incident in 1F1

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 T + 30 s - Void in the RPV core collapses, seemingly decreasing the RPV liquid level

 Measurement malfunctioning between loss of offsite power and start diesel generators

First hour of the Incident in 1F1

(12)

 T + 30 s - Lower RPV level causes automatic ramp-up of the feedwater injection

p.12

First hour of the Incident in 1F1

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 T + 40 s - Increased feedwater injection causes RPV pressure transient

First hour of the Incident in 1F1

(14)

 T + 20 s – RPV recirculation system runs down to 30% after SCRAM

 T + 63 s – Recirculation pumps stops at LOOP

p.14

First hour of the Incident in 1F1

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 14:53:05 (T +333 s) – RPV pressure exceeds 72.2 bar,

automatic activation of the Isolation Condenser (IC) train A & B

First hour of the Incident in 1F1

(16)

 Plant situation prior the arrival of the Tsunami

 Plant is in a stable hot-shutdown state, decay heat controlled by operating the IC

 RPV is filled with water (significantly more than during power operation)

 Small water injection into RCS by pump seal / CRD purge water (supplied by diesel generators)

 Slight PCV heat-up / pressure rise by stop of dry-well cooling system (not supplied by diesel)

 Analytical aspects of initial transient

 Transient timing and bottom lying physics is well understood

 The MELCOR thermohydraulics corresponds well to the time-corrected plant data

 Based on deviations between simulation and recorded measurements

a timing error of ±15 min can be expected for predictions of the start of core damage

 Fit parameters within the MELCOR model

 Closure time of the turbine control valves (direct fit to measurements)

 Run-down of the primary loop recirculation pumps (direct fit to measurements)

 Feedwater flow dependence on RPV liquid level (simplified modelling)

 Flow/Void within and outside the RPV steam separator tubes (no detailed design knowledge)

p.16

First hour of the Incident in 1F1

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Escalation into an Accident

 15:37 to 15:38 Tsunami flooding of the plant site

 Loss of instrumentation and control

 Loss of all core cooling functions

 IC currently inactive, and if it would have been active, fail-safe tube rupture signal would have shut it down

 Further accident progression

 Decay heat ~10 MW vaporizes coolant

 PRV pressure rises until a SRV opens

 Coolant is discharged into the wet-well

 RPV liquid level drops

(18)

Escalation into an Accident

 17:45 (T+2.5h) Top of active fuel (TAF) is reached

Quake Tsunami

TAF

BAF 18:18 - IC on18:25 - IC off

21:30 - IC on

0 2 4 6 8 10 12 14

12:00 14:00 16:00 18:00 20:00 22:00 0:00 2:00 4:00 6:00

Reactor water level [mRPV0]

Local time [3/11/2011 hh:mm]

W/R A (transient recorder 1min) W/R B (transient recorder 1min) MELCOR (CF49504)

Liquid level visible in MCR MELCOR (Core swell level)

(19)

Escalation into an Accident

 Core oxidation releases large amounts of hydrogen

18:18 - IC on 18:25 - IC off

21:30 - IC on Quake Tsunami

0 100 200 300 400 500 600 700 800

0 500 1000 1500 2000 2500

12:00 14:00 16:00 18:00 20:00 22:00 0:00

Hydrogen in RPV [kg]

Cladding max. temperatures [ K ]

max. Cladding (CF200728) hydrogen release (COR- DMH2-TOT)

(20)

Escalation into an Accident

 Hydrogen causes strong pressure buildup in BWR containments

Quake Tsunami

0 1 2 3 4 5 6 7 8 9 10

12:00 14:00 16:00 18:00 20:00 22:00 0:00 2:00 4:00 6:00 8:00 10:00 12:00

Containment pressure [bar]

D/W (paper strip record) W/W (paper strip record) MELCOR (CVH-P.795) MELCOR (CVH-P.661)

Dry-well presssure recorded data Wet-well presssure recorded data

(21)

Escalation into an Accident

 Observed vs. calculated dose rates in RB due to containment design leakage

(22)

Escalation into an Accident

 Failure of water level measurement only achievable by early RCS leakage, not by bare heat-up of containment through intact RCS walls

Quake Tsunami

TAF

BAF

0 2 4 6 8 10 12 14

12:00 14:00 16:00 18:00 20:00 22:00 0:00 2:00 4:00 6:00 8:00 10:00 12:00 14:00 16:00 18:00

Reactor water level [m]

Fuel range A (CF480.04) Fuel range B (CF480.04) Wide range (CF495.04) F/R A (parameter data) F/R B (parameter data)

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2. SCIENTIFIC CONDUCT

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 Example BSAF – RPV pressure Unit 3

 What do we see here:

Fit of the simulations to measured pressure values

 What is NOT-necessarily seen:

Numerical simulated accident progression in U3 with a high degree of accuracy

 Every model (good or bad) can be forced onto plant data

 Such pictures can suggest a higher accuracy than one really have

 Over-confidence in simulations can result in “negative training”

p.24

Scientific Conduct

(25)

 Fitting is not worthless!

 With fitting one can access unknown information

 Fitting allows to assume a certain situation, andthen one can evaluate derivative quantities

 Example BSAF - PCV pressure of U3 as result of the fitted RPV pressure

 Good practices when Fitting

 Disclose the fit

 Do not state a fit / boundary condition as simulation result

 Evaluate the invasion strength on the simulation

 Choose a reasonable fit parameter

(e.g. not an opening/closing containment leakage area to fit PCV pressure)

Scientific Conduct

(26)

Fukushima Daiichi Unit 1– BRAUN Matthias, DTIPP7-G – Zagreb, 04/26/2018 p.26

3. EXAMINATION OF THE NUCLEAR FALLOUT

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 Database for Radioactive Substance Monitoring Data

 by of the Japan Atomic Energy Agency http://emdb.jaea.go.jp/emdb/en/

 Deduction of the core release fraction from the fallout isotope composition

 Insights into the core degradation

 ~ 2400 K to release silver

 << 2800 K to not release americium

 Sub-stoichiometric melts, and probably no large-scale liquefaction of oxides

 No ruthenium release due to lack of oxygen (RuO2 boils at 1200°C, but Ru

can not be oxidized by steam)

 Low niobium but high molybdenum release, but in MELCOR both are the same class -►Cs2MoO4 class definition

Examination of the Nuclear Fallout

Element Release fraction

from Core Iodine, Cesium, Tellurium,

Technetium, Molybdenum up to 100 %

Silver up to 100 %

Antimony ~ 5 %

Barium ~ 1 %

Niobium, Barium, Strontium ~ 0.1 %

Ruthenium < 0.1 %

Americium < 5E-4

Uranium, Plutonium Not measurable

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 Plutonium release of high public interest

 Numerous papers claim to have observed reactor plutonium

 Plutonium background from surface nuclear weapon test

 Separation of reactor plutonium from weapon plutonium by different isotope composition

 Signal vanishes with increasing signal strength

-► measurement errors?

p.28

Examination of the Nuclear Fallout

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