interpretation of fission product transport and chemistry in vercors ht and phebus tests

Interpretation of fission product transport Interpretation of fission product transport and chemistry in Vercors HT and Phebus tests and chemistry in Vercors HT and Phebus tests

N. Girault, C. Fiche

Institut de Radioprotection et de Sûreté Nucléaire

Direction de la Prévention des Accidents Majeurs

- - 1 VERCORS SEMINAR, Gréoux, October 15-16th, 2007

CONTENTSCONTENTS

1. Objectives 2. Approach - Modelling3. Main Experimental findings4. Calculation results 5. Discussion (sensitivity

analyses)6. Summary - Conclusions

Fission Product Transport

Isotope treatment & Activity

Containmentout-leakage

Lower head failure, Collapse of corium,Core/concrete int.

Aerosol nucleation/transport in primarycircuit, break and release in containment

Aerosol behavior In

Containment

FP release, conveyed by gas emission from degraded core

Source Term

Nuclear power plant context : stakes ? What importance ?

FP transport/retention in primary circuit determines the source term

significant FP deposition for containment by-pass sequences, FP

retention in primary circuit is the only possibility to reduce radioactive releases in the environment

possible delayed FP releases (re-vaporisation is a source term factor in late phase)

at the break, aerosol/vapour-gas split for some FP (especially for Ru and I which poses main short-term radiological risk for human populations)

Obtain a realistic assessment of possible releases in environment to optimise management of accident’s consequences

Approach Calc. results ConclusionsObjectives DiscussionExp. Findings

- - 2 VERCORS SEMINAR, Gréoux, October 15-16th, 2007Fission Product Transport

Provide predictability of FP retention in primary circuit (I, Te, Cs, Ru) Provide predictability of volatile iodine (and Ru) speciation exiting the RCS in

900/1300 PWR undergoing a severe accident through different scenarios Phebus containment chemistry analyses can not explain the early gaseous iodine fraction

Analyses of PHEBUS FP (integral) and VERCORS HT (analytical) tests with SOPHAEROS (equilibrium chemistry in gas) to investigate FP retention and speciation within different oxido-reducing conditions and SIC/B release kinetics

Phebus FPT2 for FP speciation in TGT/TL under H2 and H2O with SIC/B

Vercors HT1/3 for FP speciation in TGT mostly under H2 with SIC/B

Earlier and on-going work SOPHAEROS analyses continuously progressed with regards to thermo

dynamic code MTDATA/SGTE (check of thermodynamic data of elements)

besides analysis of potential chemical kinetics limitations (because of low residence time, strong thermal gradient in some parts of RCS (CHIP)

Objectives and means


VERCORS SEMINAR, Gréoux, October 15-16th, 2007Fission Product Transport - - 3 VERCORS SEMINAR, Gréoux, October 15-16th, 2007Fission Product Transport

What needs to be explained in Phebus and Vercors HT RCS ?

423K

steam generator, ~8m

973 K

cold line, ~5m

hot line,

~13m

20-rod bundle

driver core

POINT C POINT G

containement

~10m3

KEY POINT = VAPOUR PHASE CHEMISTRY (TGT analyses with SOPHAEROS including ext. chemical database)



Phenomena Where ? Remarks

Phebus RCS Vercors loop

vertical line f urnace tube chemisorption + condensation deposition

SG hot leg TGT condensation + thermophoresis

hot leg (700°C)

TGT I and Cd vapours only (Phebus) Cs, I , Te, Cd vapours (Vercors)

vapour/aerosol split

cold leg (150°C)

downstream loop

aerosols (% I gas in Phebus)

in the non heated part of the oven where non stationnary thermal conditions prevailed and in upstream zone of TGT significant deposits can occur


VERCORS HT


PHEBUS FPΔtsampling ~150s

200°C

700°C

fluid

200°C

700°C

fluid

thermal gradient tube

Δtsampling ~2-3 h

transition zone

TGT

Zone de T variable

heating power 600 W

linear thermal

Gradient

non heated transition zone

800°C

150°C

Transport phenomenology• Simultaneous occurrence of:

chemical interactions (vapour-vapour, vapour-surfaces) vapour supersaturation condensation on structures, aerosol formation aerosol agglomeration & deposition (phoretic effects, diffusion, etc.)

cooler

Gas phasechemical reactions

Inlet flow Supersaturated vapours

Condensation Deposition

ReleaseAgglomeration

Sorbtion

Nucléation

Aerosol

WALL



CARRIER GAS H2O, H2, O2, N2, He,Xe, Kr, Ar

Important assumption : time constant of gaseous reaction is sufficiently smaller than that of vapour condensation (gas species essentially in local chemical equilibrium but may be in non equilibrium with respect to the condensed species)

Chemical equilibrium and mass balance equations

oThe chemical equilibrium in the gas phase is computed using constant partition for each species i and element k

krk

ri

kkkii

i ki

PP

PP

RT

TGTGTK

)()()(


- - 7 Fission Product Transport VERCORS SEMINAR, Gréoux, October 15-16th, 2007

o 5 physical states vapour aerosols vapour condensed on wall

deposited aerosols sorbed vapour

Case of state :

mswsatcw

fsatcpf

upupf Jmmmmmmms

dt

dm 111111

1 )()()(

Heterogeneous nucleation or evaporation

Circuit inlet Sorption

Condensation Homogeneous nucleation Carrier fluid transfer

(including fall back/down)

Approach Calc. results ConclusionsObjectives Discussion

Phebus test Overview

Main characteristics FP SM releases very variable

and depends on fuel degradation events (50g in FPT2 -150g in FPT1)

H2/H2O ratio : main H2 peak lasts ~ 2 (FPT1) to 20 min (FPT2/3) : never complete steam starvation

SM releases : most of SIC release in oxidising conditions, B constant in FPT2 (0.5 µg/s)

volatile FP releases (Mo excepted) initiated during Zr oxidation phase, Mo release starts after this phase

Significant H2 releasemain Zr oxidation phasemax (FPT2) : 95%

Exp. Findings

FPT1FPT2FPT3

131I at point G(cold leg of RCS)

FPT1FPT2FPT3


HT 3Vercors test Overview

Main characteristics FP releases depend on Tfuel and

H2O/H2 access to fuel (≠ HT1/HT3)

H2/H2O ratio : Zr oxid. lasts ~ 40 min but low amount of H2 (15% max)

SM releases : Ag, In vaporisation starts during oxidation plateau at 1800 K, Cd at 1250 K, B2O3 only in H2 rich phase (~ 10 µg/s) (constant mass flow rates in tests)

volatile FP releases (Mo excepted) starts before Zr oxid. in steam, when H2 phase starts ~50% of volatiles already released; Mo release in HT3 starts during steam phase (~ 30%)

Iodine

- - 9 VERCORS SEMINAR, Gréoux, October 15-16th, 2007

Fission Product Transport



Results:Main differences between HT1/3 and FPT2 tests

Ag/I In/I Cd/IB/Cs

Mo/CsTe/Cs

VERCORS H2O

VERCORS H2

PHEBUS H2OPHEBUS H2

0

10

20

30

40

50

60

70H2/H2O ratio: in FPT2 no complete steam

starvation while in HT1/3 pure H2 phase volatile FP releases: in HT3 (Cs, I, Mo)

release starts during steam rich phase while in FPT2 Cs and I release initiated under H2 (with no Mo); lower Te/Cs in FPT2/3 TGT due to high its retention in hot zones

SM releases: Ag, In and B mainly released under H2 in HT3 (with low Mo release) while in FPT2 ~ constant (in excess/ I & Cs);

constant Cd release in HT3 while in Phebus tests release «puffs» are suspected Others : fluid velocity ~2 times higher in FPT2/3 (≥1m/s) but conc. 50 times

higher : impact on gas phase chemistry kinetics ? (no gas. iodine measured but upstream high capacity filter could trapped gaseous iodine if any (res 3 s))

Res. times upstream TGT very low in Vercors: impact on aerosol/vapour split ?


Cs/I, Mo/Cs similar≠ B/Cs, Re/Cs, SIC/I

iodine species not only depends on oxido reducing conditions BUT on FP release kinetics (molar ratios : I/Cs, Cs/Mo-B-Re)

in H2O/H2 mixtures when Cs <<< I : volatile I not associated with Cs when Mo release low (during H2 phase) : CsI

after H2 release : large increase of Mo (CsI with others volatile I species)

evidence of volatile iodine not associated to Cs in FPT2 whatever H2O/H2 and releases

in Vercors I always associated to Cs even in HT2-3 with no clear impact of SIC (B)

Te condensation in TGT suggests Cs-tellurides in both HT1-3 tests

Main experimental findings1) FP vapour speciation (FPT2/HT1-3) )



0,00E+00

2,50E-05

5,00E-05

7,50E-05

1,00E-04

1,25E-04

1,50E-04

1,75E-04

2,00E-04

-50 50 150 250 350 450 550 650 750 850 950

TGT Level (mm) [level -50 mm corresponds to TGT inlet]

Depo

sitio

n pr

ofile

(m

mol

/mm

)

0

100

200

300

400

500

600

700

800

Temperature (

°C)

Caesium deposition profile

Iodine deposition profile

Temperature profile

I Cs TGT 700

Cs : 2 10-6 mol

I : 6 10-7 mol

HT-1

Main experimental findings2) Volatile iodine formation Total Volatile

iodine (% i.b.i.)

Experiment

FPT-0 2

FPT-1 < 1

FPT-2 < 0.1

FPT-3 30

In Phebus tests : no direct evidence of gaseous I in primary circuit (FPT3 excepted) BUT early

detection of gaseous iodine in containment,higher fraction in FPT0 (higher steam flow rate/lower conc.) and without SIC (FPT3)


FPT1

H2 peak

scram

In Vercors HT tests (to be compared to FPT2 for fluid velocity): no direct evidence of gaseous I in the gaseous bulb nor in maypack downstream

TGT (<0.05 % detection limit)

HT-1

HT-2

HT-3

< 0.05


iodine mainly deposited in SG in Phebus and in TGT in Vercors ( 25-40% )

in Vercors, I retention in TGT more significant in HT3 (under H2 + SIC)

low I retention in Phebus UP : 5-10%, no I retention in Vercors hot zones

Other FP retention (Cs, Te, Ru) high Te retention upstream SG in all Phebus tests (20-40%); high Cs (Mo) retention in FPT1 UP (40%)

in Vercors, Cs, Te retention favoured in hot zones in oxidising conditions (HT2 : 18% compared to 4% in HT3

Ru retention favoured in hot zones but in oxid. conditions 5% deposited in TGT

Main experimental findings3) Retention in Phebus/Vercors loops



case of iodine

Results:Revaporisation phenomena

decrease in Cs deposited activity

HT3

In TGTIn H2O(600°C)

In HLIn H2O(700°C)

FPT2

Evidence of partial Cs revaporisation at high T (600-700°C) from SS (HT-3) and inconel surfaces (FPT-2 after core shutdown) in steam rich conditions

no significant decrease of I, Te, Mo deposited activity in HT-3/FPT2

no significant revaporisation of Cs in HT1



0

500

1000

1500

2000

2500

13:12 14:24 15:36 16:48 18:00 19:12 20:24 21:36 22:48

time (hh:mn)

Fuel

tem

péra

ture

(°C)

0,00%

0,20%

0,40%

0,60%

0,80%

1,00%

1,20%

1,40%

1,60%

1,80%

Cs de

posit

ed fr

actio

n on T

GT

(gam

ma s

crut

ation

at 60

0°C)

.

Star

t of h

eat u

p ph

ase

Core

shu

t dow

n

Star

t of a

eros

ol ph

ase

8000 10000 12000 14000 16000 18000 20000 22000 24000 26000

Time [s]

Activ

ity [

MBq

/mm

]

137Cs Station 3 (661.66 keV, 30 a)

Experimental phases

137Cs Station 2

Synthetic signal

Core Power [a.u.]

H2 production [a.u.]

Results:1.1) Calculated integrated I speciation in Phebus and Vercors loops

In FPT0 large impact of SIC on I speciation : I (CdI2) [because of Cs (CsReO4)] In FPT2/3 small/no AIC impact : I (CsI) (CsOH not fully consumed by Mo and B) More volatile iodine species (HI) in FPT3 because no Cd (Cd + HI CdI2) and in FPT0 because less Cs/CsOH to react with (CsOH + HI CsI) In Vercors main predicted I species is CsI in HT1/3 (in agreement with FPT2/3) calculations and with exp. results ; no HI is predicted



1) CdI2

2) CsI

3) Volatile I

Phebus

relative fraction

BaI2 (BaI)CsI (Cs2I2)

HT1

HT3

0

20

40

60

80

100 Vercors relative

fractionCs/I10



1.1) Calculated integrated Cs speciation in Phebus and Vercors loops

Results:

In FPT0 large impact of Re : Cs (CsReO4) main species Impact of B (Mo) in FPT2/3 but CsI not prevented and (Cs2MoO4 = BCsO2) In Vercors HT1 : large fraction of Cs remained unreacted (Cs/CsOH ~ 70 %) In Vercors HT3, though Cs2MoO4 becomes significant Cs2Te and CsI formed in similar proportions/HT1 (decreased unreacted fraction of Cs) Compared to FPT2, in HT3 Cs2MoO4 and above all BCsO2 are formed in lower amounts (BCsO2 < Cs2Te)

0

20

40

60

80

100

CsI

(Cs2I2)

Cs CsOH

(CsOH)2

Cs-Te Cs2MoO4 BCsO2

HT1

HT3

rela

tive

fract

ion Vercors

1) CsReO4 Phebus relative fraction

2) Cs2MoO4

BCsO2

Results:1.2) Predicted I & Cs speciation in FPT2 HL samplings

SOPHAEROS mainly predicts CsI and CdI2 (minor amount of Cs2I2 and RbI) : in accordance with similar deposition profiles in 2/3 TGT for I and Cs in contradiction with no/small Cs detection in some TL (trigerred during

early release phase)


CsICs2I2

210-450°C

RbI

TL(H2)10050s

Explained as following : when low Mo release (under H2), large amount of CsOH and HI to form CsI when high Mo release (late H2O phase) CsOH consumes by H2MoO4 to form Cs2MoO4, leaving large fraction of HI to react with Cd at lower T limited impact of B (especially under H2 conditions)

CsI475°C

275°CCdI2

TGT (H2O)15000s

Mo/Cs < 1early release phase

Mo/Cs ~ 1late release phase



Results:1.2) Predicted I & Cs speciation in HT1 TGT (under H2) without B and SIC

SOPHAEROS mainly predicts Cs2Te and CsI too a less extent in agreement with exp. condensation profiles of Cs, I and Te in TGT (small CsOH chemisorption at inlet tube)

As in reducing phases of Phebus, Cs2MoO4 is not favoured leaving large amount of CsOH to react with HI to form CsI (in HT1 Mo starts to release only during H2 release phase)

Main difference with Phebus tests is Cs-Te species condensation in TGT; in FPT2 these species were not evidenced in TGT (may be deposited upstream)


BaI2

CsI450°C

Cs2Te

CsI

650°CCsOH

Results:1.2) Predicted Cs speciation in HT3: Cs revaporisation



SOPHAEROS predicts partial vaporisation (~50%) of Cs2MoO4 deposited during the steam injection phase : explains by a decrease of Mo release kinetics during the subsequent H2 phase in HT3 due to early release of Mo (under H2O : > 20 % i.i. is released) Mo has a large impact on Cs speciation in calculations :

totally preventing CsI formation during H2O phase partly inhibiting its condensation (Cs2Te) in TGT during H2 phase

CsI is the only iodine species formed and predicted : no interaction of I with Cd

Cs2MoO4

Mo/Cs 1.5

at end of steam injection

Cs2MoO4

Mo/Cs 0.7at test end

Results:1.2) Predicted FP speciation in HT3 TGT

Cs-Te species formed under H2 release rapidly become dominant species (in calc. Cs-Te condensation in TGT disturbed by aerosol particles, mainly metallic B)

no predicted interaction of Cd with I because all iodine has reacted with Cs in TGT

similar calculated behaviour for B, Ag and In that nucleates as metallic particles (limiting their interaction with FP, while metallic Cd vapours predicted to condense at TGT outlet

This explains : small impact of B on Cs speciation : boron mainly deposited in furnace tube and in TGT by thermophoresis

only small interaction of In with Te (In2Te) because In mainly transported as aerosols



at test end

Cs2Te CsI650°C ~450°C

Results:2) Volatile iodine at low T

According to base-case analysis, HI is the main candidate in FPT0/1/2 HI predicted amount doesn’t significantly depend on H2O/H2 as measured

Some other volatile iodine species are also predicted but only if no CdTotal volatile I in circuit (% of I. release)

FPT3

Psat (Pa) at 400 K

Experiment

HI (gas) SnI2 SnI4 I2MoO2

Total volatile I in containmt (% i.b.i.)

~4.3 3.7

11.8

6.2

18

gas

108 8

30


In HT1(~FPT3 but without B) no HI is predicted in agreement with exp.data (only CsI due to limited Mo (metallic) release under H2)


measured in containment

Results:3) FP retention



Basic Analysis

Experiment

I RCS (%)FPT-0FPT-1FPT-2 FPT-3

~ 53 ~ 42 ~ 28 ~ 19

> 30 ~ 25 > 18 > 15

in Phebus total iodine retention factor overestimated by 1.8 mainly due to overestimation in SG

volatiles (I,Cs,Te) : low retention of in hot zones well reproduced while retention in TGT well calculated (40- 60%)

low volatiles (Ba, Ru) : underestimation of their retention in hot zones

metallic particles were found to disturb FP condensation in TGT in calculations

aerosol /vapour split need to be investigated

in Vercors :

0

20

40

60

80

100

I Cs Te I Cs Te I Cs Te

Meas.

Calc.HT3

furnace tube TGT downstream

Discussion:

Impact of Cd and Mo release kinetics

assuming a continuous Cd release leads to overprediction of I retention in SG AND low amount of volatile HI (cf FPT0/1)

if limited, a better agreement with I retention factor in SG BUT overprediction by 10 of HI formation (cf FPT3)

in HT3, complete I consumption by Cs prevents any reaction of Cd vapour with I

Total volatile I Basic Analysis

Exp.

(% i.b.i.)

FPT-2 non limited Cd FPT-2limited Cd

~ 0.03

~9< 0.1



Saturation pressure MDB 1.3

-15

-10

-5

0

5

0 500 1000 1500 2000 2500 3000

T [K]

log

(p

sa

t/p

atm

)

H2MoO4

CsI

Cs2MoO4

CsOH

MoO3

CdI2

HMoO2

at 700°C, high volatility of H2MoO4 that leads to significant formation of Cs2MoO4

(CsI prevented and large fraction of HI)

if Psat (H2MoO4) is decreased ( MoO3) CsI (RbI) is formed (Cd do not compete with CsOH to form CdI2)

in HT3 Cs2MoO4 predominant during H2O phase, CsI-Cs2Te then formed during H2

CdI2

CsOHCsIH2MoO4

700°C

Overview of “FP” Chemistry in RCS gas phase

Steam GeneratorCold leg

COLD Temperature700 <T< 150°C

condensation condensation

CORE

VERY HIGH T up to 2800°C

CORE FUSION

Upper PlenumHot leg

HIGH Temperature2800 <T< 700°C

FP/SM RELEASES FP/SM RETENTION

I HI, I, (CsI)

Cs CsOH, Cs, (Cs2MoO4)

Mo MoO3, H2MoO4

Re ReO, CsReO4,(ReO2 Re2O7)

B BO2,H3B3O6 (BO HBO2 H3BO3)

BH3, BH2, B (in H2)

Cd CdTe H2Te, SnTe ,CsTe, AgTe (in H2)

Cd + HI CdI2 ↓CsOH +

Cd

HI H2MoO4 ReO H3B3O6

CsI ↓Cs2MoO4↓CsReO4 ↓BCsO2 ↓Cs2Te ↓

Potential kinetics limitations due locally to low residence time and high T


cooling

---- gas ---- vapour ---- condensed state



Summary and Conclusions

Volatile iodine species at low T

SOPHAEROS implies strong sensitivity to Cd, main species = HI (small amounts of SnI2, SnI4 and I2MoO2 in FPT3 with no Cd) in Phebus (FPT3 excepted) when volatile iodine correctly predicted, its condensation in SG is under estimated

FPT3 calc. results show a very high volatile I fraction (18% /i.b.i) when Cd is completely missing in accordance with exp. data (~30%) not observed in Vercors (HT1) because of Cs not completely consumed by Mo

no clear impact of SIC materials in Vercors HT tests under H2 (Ag, In mainly under metallic aerosol forms, HI completely react with Cs) analysis of VERCORS HT2 test : impact of SIC under H20

in Phebus more volatile iodine measured in FPT0 with high flow rate and low FP concentrations role of kinetics limitations still need to be investigated (CHIP) - - 25 VERCORS SEMINAR, Gréoux, October 15-16th, 2007Fission Product Transport

Summary and Conclusions

Iodine & Caesium vapour speciation

good agreement between iodine exp. data and SOPHAEROS only when measured I is associated with Cs

Cs2MoO4 favoured to detriment of CsI with high Mo release (under H2O), CsI/Cs2Te with low Mo release under H2

limited impact of B on Cs speciation (too much ?)

volatile iodine in Phebus only with low Cs or high Mo release not well

predicted (only CdI2 ?) : impact of others SM (Fe, Cr, Si…) VERCORS HT2 and FPT3

Sensitivity calculations

depending on Mo chemistry more or less CsI is calculated

CHIP (investigation of simplified systems under equilibrium) Continuous check/development of MDB (polymolybdates ?)



interpretation of fission product transport and chemistry in vercors ht and phebus tests

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