ITER and Fusion Safety Aspects

H.-W. Bartels, ITERPrague, 13.November 2006

Fusion and nuclear safety

D

T n

He-4

+ 17.6 MeV

==> nuclear safety related issues:

1) radioactivity of tritium (~5 kg in reactor)2) activation from 14 MeV neutrons (~1/2 of activity of PWR)

Tritium

• Half-life: 12.3 a

• ß—radiator: <Ee-> = 5.7 keV range organic matter: 6 µm Horny layer skin: 70 µm

Incorporation: Inhalation, skin absorption, Ingestion

Very Mobile: HTO, HT

Biological half-life:~10 days

Hazard/Bq: tritium <1000*Cs137

Tritium in human body:i) fast component t1/2 ~10 daysii) slow component t1/2 ~ 30 – 300 d (organically bound tritium)

0.01

0.10

1.00

10.00

100.00

1000.00

0 50 100 150 200 250 300 350 400 450

days

mic

ro-C

i/l

Activation Products

Problematic isotopes:steel:

60Co: t1/2 ~ 5 years1.2 MeV -radiation

tungsten:

185W: t1/2 ~ 75 days 0.4 MeV -radiation

Favorable elements: - Vanadium (V)- Chromium (Cr)- Titanium (Ti)

1.E-09

1.E-07

1.E-05

1.E-03

1.E-01

1.E+01

1.E+03

1.E+05

1.E+07

0 1 100 10000 1000000

time after plasma shutdown [years]

dose

rate

[Sv/

h]

Ag

Mo

W

FeV

recycling limit

hands-on-limit

natural dose 2 mSv/a

Neutron activation in some elements(5 MW/m2 in 2.5 years)

Schematic Fusion Reactor Build-up

cryostat

roomfor systems:1) diagnostics2) fueling3) heating4) maintenance

Magnets

plasma

vacuum vessel

plasma-facingcomponents

Normal operational effluents

• ALARA principle– lower project release limits

• Tritium 1 g/a• Activated dust 10 g/a• Corrosion products 50 g/a

• Dose limits:– vary by country: 0.1-5 mSv/a– average natural dose: 2 mSv/a

• Doses < 1% of natural dose

• Technical precedence used:– tritium plants– chemical plants (beryllium)– fission reactors

(esp. CANDU’s)– tokamak (D_T: JET, TFTR)

• Analysis for last year ITER shielding blanket operation

• Initial releases small:– limited use of tritium– low level of activation– time delay permeation of

tritium into coolant

Accidents:Design guideline atmospheric release

Ultimate safety margin:Evacuation threshold for ground level releases:Tritium: < 100 gTungsten dust: < 4 kg

Event

Category Incidents AccidentsHTO [g/event] 0.1 5Dust [g/event] 1 50

Accidents: Energy Sources

0.1 1 10 100 1000

plasma

fusion power-10s

magnetics

decay heat-1day

decay heat-1week

chemical

thermal

Energy Inventory [GJ]

Schematics of computer model for integrated accident analysis

Cryostat

Upper HTS Vault

Lower HTS Vault

ConnectingDuct

SuppressionPool

HTO &CPs

HTO &CPs

LeakFilteredDried

LeakFilteredDried

LeakFilteredDried

GenericBypassRoom

Leak

Cold magnetstructure,crowns,thermalshields

HTO &CPs

VV H-3

Dust

FW/ShieldandDivertor

EquatorialPit

PenetrationBypass Line

Accidents: Pressurization and decay heat

In-vessel decay heat driventemperature transient

VV cooled by natural circulation

• Accident scenario:

– multiple FW failure – all FW cooling pipes in

two toroidal rings damaged

– fast pressurization plasma chamber

– pressure limited by suppression system

– Maximum pressure ITER: 2 bar

– no in-vessel cooling

0

50

100

150

200

250

300

350

400

0.01 0.10 1.00 10.00 100.00

time [d]

T [C

]

FWshield-frontshield-backVV

1.E-12

1.E-10

1.E-08

1.E-06

1.E-04

1.E-02

200 400 600 800 1000 1200Temperature [C]

[kg

-H2/

m2

s] Be

W

C

Accidents: Hydrogen in ITER

Combustion wave propagates ~2000 m/s

Pressures from 15 to 20 bar

H2 formation in fusion:chemical reactions with hotplasma facing components, e.g.:

Be + H2O BeO + H2

H2 + air explosion

Accident: Loss of coolant w/o shutdown

Vault

Crane hall

Divertor

In-vessel LOCA

Pressuriser

Heat exchanger

VVPSS

Cryostat

Ex-vesselLOCA

Vacuum Vessel

10%/day

Environment No confinement credit

10%/day

Accident: Loss of coolant w/o shutdown

• In case of on-going plasma burn temperature increase in affected components.– failure of in-vessel

components at elevated temperatures

– ingress of steam into VV– Be/steam chemical

reactions– hazard of hydrogen

formation– bypass 1. confinement

barrier

• ==> plasma burn will be terminated by fusion shutdown system

Ex-vessel LOCA w/o plasma shutdown

0

400

800

1200

Tem

pera

ture

(°C

)

FW

0

0.050.1

0.150.2

0.25

Pre

ssu

re (

MPa)

PHTS Vault

VV

0

1

23

4

5

0.01 1 100 10000 1000000

Time (s)

Hyd

rog

en

(kg

)

in VV

20

40

kg-H

2

Accident analysis margins

• Large margins maintained because:

– limitation of radioactive inventories;

– inherent plasma termination processes;

– long time for component heat-up;

– gross structural melting impossible;

– multiple layers and lines of defense to implement radioactive confinement;

– design tolerant to safety system failures

• Maximum doses < 2 mSv (annual natural dose)

• ITER accident analysis has confirmed safety potential of Fusion Energy.

0.1 1.0 10.0 100.0

Loss of confinement in hot cell

Cryostat water/helium ingress

Cryostat air ingress

Arc near confinement barrier

Toroidal filed coil short

Failure of fueling line

Isotope separation system failure

Transport hydride bed

Stuck DV cassette

Air ingress during maintenance

DV ex-vessel coolant pipe break

VV coolant pipe break

Heat exchanger tube rupture

Pump seizure in divertor HTS

Loss of vacuum

Multiple FW pipe break

Blackout for one hour

Loss of off-site power (32 hours)

Loss of plasma control

% of release limit

totalACPdusttritium

Accident: Wet bypass

• Hypothetical event:– loss of coolant plus 2 failures

in one heating or diagnostic line (“wet bypass”)

– Analysis results:• plasma chamber

pressurizes• opening bypass /

suppression tank• transport radioactivity into

room• capture tritium, dust in

tank, settling of dust by gravity, condensation of HTO in room

• cleaning of room after 8 hours:

~15 g tritium released

Building

Divertor

In-vessel LOCA

VVPSS

Cryostat

Tube breakVacuum Vessel

Environment

Generic bypass room

100%/day

Heating or diagnostic system

Window break

Accidents: Loss of cooling Fusion Reactor

-300

0

300

600

900

1200

0.01 0.10 1.00 10.00 100.00days

tem

pe

ratu

re [

C]

First Wall

Vacuum Vessel

TF magnet

cryostat

Is it true ?

• V&V: verification & validation–verification

• correct coding• comparison between different codes and performers

–validation• comparison codes / data

Verification thermo-hydraulic codes

• Two codes used in ITER:– MELCOR (US)– INTRA (EU)

• benchmark: large loss of cooling accident in ITER vacuum vessel– initially some differences,

but both below design pressure 5 bar

– differences could be explained by different treatment of mixed flow of steam and water

• ==> feedback to design: lower pressures for separation of phases, e.g. pressure suppression system on top of vacuum vessel

0

0.5

1

1.5

2

2.5

3

ab

solu

te p

ress

ure

[atm

]

3.5

4

0 5 10 15 20 25

time [s]

30 35 40

INTRA

MELCOR-NSSR-2

MELCOR-seperate-phase-flow

Validation experiments steam vacuum

Validation thermo-hydraulic codes

• Two codes used for ITER:– MELCOR (US)– INTRA (EU)

• Comparison of code results with experimental data of water injection into vacuum vessel

• Problem is scaling: length 1/10 of ITER

• ==> larger surface/volume

0

100

200

300

400

500

0 20 40 60 80 100

seconds

pre

ssu

re [

kP

a]

ExperimentMELCORINTRA

14 MeV n-Source Experiment

SS316

Be

H2O

SS316

D-T Source #1 Beam Duct (160x80)

R

Z

50.8660.4609.8203.2

101.6

SS316 or CopperSupport

W alloy

600

400

382.

4

Fusion Neutron Source (FNS)

ITER Decay Heat R&D

.

0

-0.1

C-E

/E

time, days

0 1 2 3 4 5 6 7

AC T- 4/ FE NDL /A-2

ANI TA- 4M / FEN DL/A- 2

DKR -PUL SAR 2.0 / FEND L/A- 2

FISPAC T / FEND L/A- 2

R EAC - 3/ FE NDL/A -2

-0.05 .

.

.

.

.

.

0 1 2 3 4 5 6 7

A CT-4/ F EN DL/A -2

A NITA/ F EN DL/A -2

D KR-PULSA R2.0/ FEND L/ A-2

F ISP ACT/ FEN DL/A -2

REA C-3/ F EN DL/A -2

0.1

0

-0.1

C-E

/E

time, days

- 14 MeV n-irradiation at FNS at JAERI- Decay heat measurement: sum of ß, radiation

SS-316, 7 hours irradiation

-International code validation effort:- Uncertainties < 15%

Cu, 7 hours irradiation

JA dust mobilization experiments

Russian hydrogen detonation experiments

What if it is not true ?

”No-evacuation” limit and cliff-edge effects

• Release assumptions: ground level, duration 1 h, worst case weather

• No-evacuation limit (early dose):IAEA, ITER: 50 mSv

ITER “no-evacuation” limit met for tritium release < 90 g, in HTO form

No cliff-edge effect for tritium (For a hypothetical tritium release of 1 kg no-evacuation limit exceeded for < 1 km2)

Are

a [

km

2]

Long term contamination

Tritium concentration in soil after contamination

0.01

0.10

1.00

10.00

100.00

0.1 1.0 10.0 100.0 1000.0

days

[Bq

/g]

background level

Waste volumes fusion reactor

• Fusion optimized materials:

– V-alloys

– steel without Ni, Co

– impurities need careful attention (Nb, Ag, Co, U)

• Significant part (~30%) of activated material can be cleared

• Volume ~1-2 x larger than fission waste (not counting U-mining~1.5 Mm3)

• large fractions could be recycled

0

20

40

60

80

100

120

140

160

PM-1 (V-alloy/He)

PM-2(LAM/H20)

PM-3(LAM/He)

Plant Model

Acti

vate

d m

ate

rial (t

on

nes x

1000)

Non Active Waste (clearable)

Simple Recycle Material (<2 mSv/h)

Complex Recycle Material (2-20 mSv/h)

Permanent Disposal Waste (>20 mSv/h)

No burden to future generations

1.E-07

1.E-06

1.E-05

1.E-04

1.E-03

1.E-02

1.E-01

1.E+00

0 100 200 300 400 500

Time after final shutdown [years] .

PWR

Fusion-Vanadium

Fusion-steel-He/Be

Fusion-steel-H2O/LiPb

Coal

Radiotoxicity index for power plant waste

Conclusion

• Normal operation– dose < 1% of natural background

• Accidents– source term ~ 1000 times smaller compared to fission– if properly designed: no destruction due to internal

accidents– large reactions times– tritium and dust largest hazard >> allowable releases

• Waste– volumes comparable (~2 * larger) compared to fission– toxicity 1000 times smaller compared to fission– recycling might be feasible

• Safety and environmental features dependent on design

Other fusion reactions

(1a) D + D 3He + n + 3.3 MeV

(1b) D+D T + p + 4.0 MeV

(2a) D + 3He 4He + p + 18.4 MeV

(2b) D + T 4He + n + 17.6 MeV

(3) p + 11B 3 * 4He + 8.7 MeV

(4) p + 6Li 3He + 4He + 3.9 MeV

Equations (1a) – (2b) can be summarized as

3D 4He + p + n + 21.6 MeV

D in water ~3.3*e-5 energy content 1 liter water ~ 350 l gasoline

Advantages of fusion

• No radioactive raw material

• No chain reactions (small amount of fuel ~1 g in plasma)

• Moderate decay heat (large surfaces)

• Low biological toxicity and half-life time of activation products

• Generates no greenhouse gases (no SO2, NOx)