1 g. ducros international vercors seminar, october 15-16th, 2007 – gréoux les bains, france fp...

G. DucrosInternational VERCORS Seminar,October 15-16th, 2007 – Gréoux les Bains, France 1

FP release from VERCORS tests:semi-volatile, low-volatile FP and actinides

Classification by volatility degree Semi-volatile FP: Mo, Ba, Rh, Pd, Tc Low-volatile FP: Ru, Nb, Sr, Y, La, Ce, Eu Non volatile FP: Zr, Nd, Pr Actinides: U, Np, Pu, Am, Cm

Analysis of the main parameters affecting their release Temperature Oxidising/Reducing conditions Material interactions Fuel burn-up Fuel nature

UO2/MOX steady state fuel/debris bed/molten pool


Semi-volatile: Molybdenum, main characteristics

Only one significant radioactive isotope: 99Mo … 2,7 days of half life acting in the short term Low radiological effects, as well as low impact on residual power

… but several stable isotopes High mass inventory and ability to be chemically associated with

other FP (Cs to form Cs2MoO4) and to modify their volatility

Essentially under metallic precipitate within the fuel Associated with Ru, Rh, Pd


Mo release: impact of oxidising conditions

Test number

Fuel and conditions

Test atmospher

e

Mo release

Ba releas

e

Rh releas

e

Vercors 4 UO2 3 cycles

2570 K

Reducing 47% 80% 45%


2570 K

Oxidising(pure

steam)

92% 55% 20%


2620 K (collapse)

Oxidising(H2O-H2)

79% 28% 4%

HT3 UO2 4 cycles

2680 K (collapse)

Reducing 33% 85%

HT2 UO2 4 cycles

2420 K (collapse)

Oxidising(pure

steam)

100% 38%

RT7 MOX 3 cycles2890 K

(collapse)

Reducing 7% Mox effect ?

64%


(collapse)

Oxidising(H2O-H2)

~ 50%

RT6 UO2 6 cycles

2470 K (collapse)

Oxidising(H2O-H2)

BU effect ?


0.0E+00

2.0E+14

4.0E+14

6.0E+14

8.0E+14

1.0E+15

1.2E+15

1.4E+15

0 20 40 60 80 100 120 140 160 180 200 220

Cote relative (mm)

No

mb

re d

'ato

me

s Z

r95

0.0E+00

2.0E+16

4.0E+16

6.0E+16

8.0E+16

1.0E+17

1.2E+17

1.4E+17

1.6E+17

No

mb

re d

'ato

me

s M

o9

9

Zr 95

Mo 99

Cs 137

Mo release: interaction with Cs (from VERCORS 6)

Same location of deposit on the upper part of the sleeve Same location in the fuel-corium zone

And different from 95Zr distribution, representative of the fuel-corium location

Upper sample(extracted from the crucible)

Lower sample(remained within the crucible)

Downstream depositalong the sleeve


Mo retention in the fuel: large metallic precipitates associated with Ru (from RT3)

0

5E +14

1E +15

1,5E +15

2E +15

2,5E +15

3E +15

-18-821222324252

C ote relative (mm)

Nom

bre

d'at

omes

0,00E +00

1,00E +15

2,00E +15

3,00E +15

4,00E +15

5,00E +15

6,00E +15

7,00E +15

8,00E +15N

ombr

e d'

atom

es

Zr95

Mo99

Mo99 AVS

Ru103

0

5E +14

1E +15

1,5E +15

2E +15

2,5E +15

3E +15

-18-821222324252

C ote relative (mm)

Nom

bre

d'at

omes

0,00E +00

1,00E +15

2,00E +15

3,00E +15

4,00E +15

5,00E +15

6,00E +15

7,00E +15

8,00E +15N

ombr

e d'

atom

es

Zr95

Mo99

Mo99 AVS

Ru103


Semi-volatile: Barium, main characteristics

Main radioactive isotope: 140Ba, parent of 140La 12,7 days of half life acting in the middle term

Important radiological effect Released fraction

High impact on residual power "Corium" fraction, but also deposit within the upper part of the RPV 140Ba/140La holds 15-20% of the total residual power between 1 day and 1

month


Ba release: impact of reducing conditions

Test number

Fuel and conditions

Test atmospher

e

Mo releas

e

Ba release Rh releas

e


2570 K

Reducing 47% 80% 45%


2570 K

Oxidising(pure

steam)

92% 55% 20%


2620 K (collapse)

Oxidising(H2O-H2)

79% 28%Lower

release after melting

4%

HT3 UO2 4 cycles

2680 K (collapse)

Reducing 33% 85%

HT2 UO2 4 cycles

2420 K (collapse)

Oxidising(pure

steam)

100% 38%


(collapse)

Reducing 7% 64%


(collapse)

Oxidising(H2O-H2)

~ 50%

RT6 UO2 6 cycles

2470 K (collapse)

Oxidising(H2O-H2)

BU effect ?


Ba release: interaction with the cladding (trapping by Zr) – From VERCORS 5

-7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7

-7

-6

-5

-4

-3

-2

-1

0

1

2

3

4

5

6

7

int clad

ext clad

15% of initialInventory insidethe cladding


Ba release: high upstream retention (from VERCORS 5)

0,0E+00

2,0E+14

4,0E+14

6,0E+14

8,0E+14

1,0E+15

1,2E+15

1,4E+15

-50 -30 -10 10 30 50 70 90 110 130 150

Distance from the bottom of the fuel before the test (mm)

140

Ba

ax

ial

dis

trib

uti

on

(a

tom

s b

y m

m)

Fuel before the test

Fuel after the test

Zirconia sleeve surrounding the fuel


Ba release without cladding – Debris bed configuration from RT3

-20,00%

0,00%

20,00%

40,00%

60,00%

80,00%

100,00%

120,00%

14:00 15:00 16:00 17:00 18:00 19:00 20:00 21:00Heure

Fra

ctio

n r

elâc

hée

ho

rs d

u c

om

bu

stib

le

0

500

1000

1500

2000

2500

3000

3500

Tem

per

atu

re (

°C)

137Cs

Début de délocalisation

Délocalisation/Fusion complète

140Ba

99Mo

95Zr

T (°C)

Higher kinetics than Mo (the only time) Nearly total release at the end of the test (60% at the beginning

of debris melting)


Semi-volatile FP: Rh, Pd, Tc - main characteristics

Very low radiological impact

Rh: only 105Rh 1,5 days acting in the short term

Pd, Tc: no radioactive isotopes

Essentially under metallic precipitate within the fuel Associated with Ru and Mo


Rh release: similar behaviour than Ba

Test number

Fuel and conditions

Test atmospher

e

Mo releas

e

Ba releas

e

Rh release


2570 K

Reducing 47% 80% 45%Higher release

in reducing cond.


2570 K

Oxidising(pure

steam)

92% 55% 20%


2620 K (collapse)

Oxidising(H2O-H2)

79% 28% 4%Lower release after melting

HT3 UO2 4 cycles

2680 K (collapse)

Reducing 33% 85%

HT2 UO2 4 cycles

2420 K (collapse)

Oxidising(pure

steam)

100% 38%


(collapse)

Reducing 7% 64%


(collapse)

Oxidising(H2O-H2)

~ 50%

RT6 UO2 6 cycles

2470 K (collapse)

Oxidising(H2O-H2)

BU effect ?


Pd, Tc: similar behaviour than Mo ?

Data obtained by chemical analysis at TUI

Test numbe

r

Fuel and condition

s

Test atmosphe

re

Mo releas

e

Pd releas

e

Tc releas

e

RT1 UO2

3 cycles2440 K

Oxidising(H2O-H2)

70% 34% 21%

RT2 MOX cycles2440 K

Oxidising(H2O-H2)

53% 42% 11%

RT3 UO2

Debris bed

Reducing(H2O-H2)

26% 16% 0% ?

RT4 UO2-ZrO2

Débris bed

Oxidising(H2O-H2)

100% 45% 42%

Slightly lower release than Mo Slightly lower release in reducing conditions


Semi-volatile FP release: conclusion

Significant released fraction Can be as high as for volatile FP, but …

High sensitivity to oxidizing/reducing conditions Mo more volatile in oxidizing conditions

And probably also Pd, Tc

Ba, Rh more volatile in reducing conditions

Materials interactions, affecting their release Ba trapped by Zr of the cladding Potential chemical interaction of Mo with Cs, limiting Cs release

Additional Burn-Up effect Seems to increase Ba release in oxidising conditions


Low volatile: ruthenium, main characteristics

2 important radioactive isotopes 103Ru: 39 days acting in the middle term 106Ru: 1 year acting in the long term High inventory in MOX fuel Metallic precipitate within the fuel

Potentially very high radiological impact

Volatile oxide forms can be produced in very oxidising environment, particularly under air ingress Among them, RuO4 may stay gaseous at low temperature within the

containment


Ru release: impact of oxidising conditions

VERCORS 4 and VERCORS 5 comparison (UO2 38 GWj/t) Low Ru release in both tests: 6%

Deposit mainly located at high temperature, but … For Vercors 5 (pure steam conditions), 10% of the released fraction

reaches the impactor heated at 870K

Vercors HT3 and HT2 comparison (UO2 50 GWj/t) Same low release (6%) for HT3 as for VERCORS 4, both performed

in reducing conditions, with total deposit at high temperature HT2 (pure steam conditions) exhibits a large release (65%), with 20%

of the released fraction recovered at low temperature (400 K)

Significant release may also occur in steam (even mixed steam and hydrogen) conditions

Significant impact of burn-up is also evidenced


Ru release in HT2 test

0

5E+14

1E+15

2E+15

2E+15

3E+15

3E+15

210 260 310 360 410 460

Cote (mm)

FP

dis

trib

uti

on

(at

om

/mm

)

Ru103 before HT2 test

Ru103 after HT2 test

Zr95 after HT2 test

53% of Ru depositedin a hot zone

(12% downstream ina colder zone)


Ru release kinetics for HT2 and RT6 tests

-5%

0%

5%

10%

15%

20%

25%

30%

15:00 16:00 17:00 18:00 19:00 20:00

Time (hh:mm)

Rel

ease

d f

ract

ion

500

1000

1500

2000

2500

Fu

el t

emp

erat

ure

Ru103_HT2

Ru103_RT6

HT2 fuel temperature

RT6 fuel temperature

Same kinetics up to fuel delocation:• Burn-up effect for RT6• More oxidising conditions for HT2


Low volatile: Niobium, main characteristics

Refractory element, with 2 radioactive isotopes, both daughter of a radioactive Zr isotope 97Zr/97Nb (16,9/1,2 hours) acting in the short term 95Zr/95Nb (64/35 days) acting in the middle term

No stable FP isotope very low mass inventory

Under oxide forms within the fuel (dissolved and precipitates) Nb is less refractory than Zr, with several oxides, one of them (Nb2O5)

having a relatively low melting point (~ 1500°C)


Nb release: only detected in HT/RT grid at very high temperature

Test number

Fuel and conditions

Test atmosphere

Nb release

Comment

HT1 UO2 4 cycles

2900 K (collapse)

Reducing 9% Lower release in oxidising condition

?

But earlier fuel collapse

HT3 UO2 4 cycles

2680 K (collapse)

Reducing 18%

HT2 UO2 4 cycles

2420 K (collapse)

Oxidising(pure steam)

10%

RT3 UO2

Debris bed

Reducing(H2O-H2)

40% Large release in debris bed

configuration

RT6 UO2 6 cycles

2470 K (collapse)

Oxidising(H2O-H2)

High release

Significant BU effect


Large Nb release in debris bed configuration (RT3)

-20,00%

0,00%

20,00%

40,00%

60,00%

80,00%

100,00%

120,00%

14:00 15:00 16:00 17:00 18:00 19:00 20:00 21:00Heure

Fra

ctio

n r

elâc

hée

ho

rs c

om

bu

stib

le

0

500

1000

1500

2000

2500

3000

3500

Tem

pér

atu

re (

°C)

95Nb

Début de délocalisation

Délocalisation/Fusion complète

95Zr

T (°C)


Nb release: main parameters

Firstly high temperature

Burn-up effect No release measured below 50 GWj/t Significant increase of the release at 70 GWj/t (RT6)

Debris bed configuration Seems to be an important factor for Nb (RT3 test, highest release of all

the Vercors grid)

Impact of oxidising conditions ? Seems to be less important than expected The comparison between HT2/HT3 highlights a potential compensation

effect of a long duration time at high temperature before fuel collapse


Low volatile FP: Sr, Y, La, Ce, Eu – main characteristics (1/2)

The most refractory elements of this group of low volatile FP Dissolved oxides within the fuel High melting temperature of the oxides (> 2300°C), but low melting

temperature of the metallic form (< 1000°C, except Y) Like for Ba, we can expect an higher release in reducing conditions

(but at higher temperature)

Sr: high radiological impact of 90Sr (30 years) But also 91Sr (10 hours), acting in the short term

Y: low radiological impact 93Y (10 hours), acting in the short term All other isotopes have a behaviour imposed by their parent (91Sr/91Y,

92Sr/92Y), all acting in the short term


Low volatile FP: Sr, Y, La, Ce, Eu – main characteristics (2/2)

La: 140La (1,7 day), daughter of 140Ba (12,7 days), which imposes the inventory evolution Less volatile than Ba Difficult to quantify precisely High impact on residual power for 140Ba/140La

Ce: very high impact on residual power (and activity) 144Ce (285 days) acting in the long term (20% of the core residual power

after one month, 45% after one year) 141Ce (32 days) and 143Ce (1,4 day) acting in the middle and short term

Eu: low inventory, low radiological impact Activity (Pres) always < 1% of total core activity (Pres) 154Eu (8,8 years), 155Eu (5 years) acting in the long term 156Eu (15 days) acting in the middle term


Sr, Y, La, Ce, Eu release: main results from VERCORS testsTest

numberFuel samp

le

Test atmosphe

re

Sr Y La Ce Eu Comment

Vercors 3

UO2 3 cycles

Oxidising(H2O + H2)

17%

Vercors 4

UO2 3 cycles

Reducing 3% Reducing effect

Vercors 5

UO2 3 cycles


ND

HT1 UO2 4 cycles

Reducing 8% 5% 9% Reducing effect

HT3 UO2 4 cycles

Reducing 13% 0,8% 11%

HT2 UO2 4 cycles


5% 1% 1%

RT6 UO2 6 cycles

Oxidising(H2O + H2)

++ %

++ %

BU effect(La, Ce)

RT7 MOX 3 cycles

Reducing 14% Additional MOX effect (Ce) ?

RT1-2-3-4

Various Various ~ 1%

~ 1-2%

Chemical analysis


Sr, Y, La, Ce, Eu … Zr, Nd, Pr: main conclusion Sr: very low volatility, nearly non volatile

Y: could be measured only one time (VERCORS 3), because of very short half life of 93Y Seems to confirm its low-volatile behaviour

La: Higher release in reducing conditions Higher release for high burn-up fuel

Ce: Same tendency than La, but with lower amplitude of the release Seems to have an additional effect of higher release in MOX fuel

Eu: higher release in reducing conditions

Non volatile elements: Zr, Nd, Pr


Actinide release: U, Np, Pu, Am, Cm

Main characteristics Generally alpha emitters and very long half life isotopes …

High U mass inventory impact on aerosol transport, even with low release

… Except Np, which has 2 short half life isotopes 239Np and 238Np (2 days) acting in the short term 239Np loads 20% of the core residual power after 1 day

Np release Very similar to Ce (La) release

Total release up to 10% Favoured in reducing conditions Favoured at high burn-up

U, Pu release: measured by chemical analyses (RT1 to 4) U release can reached 10% in oxidising conditions (RT1 – RT4) Pu release seems to be lesser than U release by a factor 10


General conclusion: FP release classification Volatile FP: (Kr, Xe), Cs, I, but also Te, Sb, Rb, Ag, Cd

Nearly complete release Kinetics sensitive to oxidising/reducing conditions Release delay for Te et Sb (trapping in the clad when not fully oxidized)

Semi-volatile FP: Mo, Ba, Rh, Pd, Tc Release can be as high as for volatile FP, but : High sensitivity to oxygen potential Sensitivity to material interaction and burn-up (Ba) Significant retention close to the fuel

Low volatile FP: Sr, Y, Nb, Ru, La, Ce, Eu Release level from some % to 10%, BUT: Potentially higher release (~30-40%) at high burn-up for some of them Sensitivity to oxygen potential Deposit very close to the fuel

Non volatile FP: Zr, Nd, Pr No significant release measured up to now (<1%)

Actinides: U, Np similar to low volatile – Pu to non volatile

1 g. ducros international vercors seminar, october 15-16th, 2007 – gréoux les bains, france fp...

Documents