Volatile FP release from VERCORS tests

International VERCORS Seminar,October 15-16th, 2007 – Gréoux les Bains, France 1

Volatile FP release from VERCORS tests

Preamble :

What have we learnt from VERCORS tests ?

Volatile FP behaviour

VERCORS HT Loop

Parameters affectingtheir release


What have we learnt ?

VERCORS program

VERCORS- 6 tests(from 1989 and 1994)

VERCORS HT/RT-11 tests(from 1996 to 2002)

Fuel collapsetemperature

FP classificationby volatility degree


What have we learnt : Fuel collapse temperature

1000

1200

1400

1600

1800

2000

2200

2400

2600

2800

3000

3200

Te

mp

era

ture

(°K

) .

UO2 RT1, 47 GWd/t HT1, 47 GWd/t HT2, 50GWd/t HT3, 49 GWd/t V_6, 60 GWd/t RT6, 70GWd/t

Fuel Collapse Temperature

70 GWd/T47 - 50 GWd/T 60 GWd/T

Without irradiation

Since the beginning of the RT/HT grid : Systematic fuel collapse for T between 2400/2600 K without

significant difference for high burn up fuel in the range of 45-70 GWd/t:

Relocation at T < UO2 melting point


What have we learnt : Fuel collapse temperature

HT1

(~47 GWd/t)

HT2

(~47 GWd/t)

HT3

(~47 GWd/t)

T (K) ~2500 ~2300 ~2500

Atm reducing oxidizing reducing

Same fuel rod

Similar temperature evolution histories

Atmosphere effect

Beginning of fuel collapse

Beginning of fuel collapse


What have we learnt : FP classification

From VERCORS program

Volatile :gases, I, Cs, Te, Sb, Ag, Rb, Cd

Semi-Volatile : Mo, Ba, Rh, Pd,

Tc

Low-Volatile: Ru, Nb, Sr, Y,

La, Ce, Eu

Non-Volatile: Zr, Nd,

Pr

+ actinides : U, Np, Pu, Am, Cm



FPs Volatility for irradiated nuclear fuel Volatile FP :

Present lecture

Semi-volatile FP :

Release can be as high as for volatile FP, but :

High sensitivity to oxidizing/reducing conditions

Mo very volatile in oxidizing conditions (MoO3)

Ba more volatile in reducing than in oxidizing conditions

Significant retention close to the fuel

Low volatile FP:

Release from few % to 10% BUT potentially higher release (~30-40%) at high burn-up and/or very oxidizing conditions

Deposit very close to the fuel

Non volatile FP:

No significant release (<1%)



-20%

0%

20%

40%

60%

80%

100%

16:00 17:00 18:00 19:00 20:00 21:00 22:00

0

400

800

1200

1600

2000

2400

Cs 137

Ba 140

Ru103

Zr97

Te mpé ra ture fond du cre us e t (°C)

Time


Volatile FP Behaviour


Volatile FP behaviour

For each case :

Kinetics (release from the fuel)

Global release

(Transport : G. Ducros, Tuesday, 16)

{gases} {Cs and I} {Te, Sb and Ag}1 2 3


Fission gas release: Generalities

Fission gases (Kr and Xe) are composed of isotopes whose half-lives have a very different radiological impact over time under severe PWR accident conditions:

Long half-life for krypton (10.71 years for 85Kr); active over the mid and long term. The other tracer isotopes of the element have sufficiently short half-lives for having no significant impact in the hours following reactor shutdown, with the exception of 85mKr (half-life of 4.48h) whose effects are felt for a little longer.

Short half-lives for the main isotopes of xenon (2.19 days, 5.24 days and 9 h respectively for 133mXe, 133Xe and 135Xe); active in the short term.


Fission gas release: Kinetics

0,0E+00

2,0E-03

4,0E-03

6,0E-03

8,0E-03

1,0E-02

1,2E-02

1,4E-02

1,6E-02

1,8E-02

10:48 12:00 13:12 14:24 15:36 16:48 18:00 19:12 20:24

Elapsed time

Inst

anta

neo

us

rele

ase

.

0

500

1000

1500

2000

2500

Tem

per

atu

re (

°C)

133Xe

85Kr

Fuel relocation

Below 1000°C

1000°C < T <1200°C

T >> 1200°C

RT6, UO2,~70GWd/t


Consistent with previously reported results (T<1200°C) :

FIRST PEAK (600-800°C) Grain boundary cracking

MAIN PEAK (T > 1000°C)Bubbles interconnection and release

- Diffusion of intra-granular gas atoms < 2%

0,0E+00

2,0E+12

4,0E+12

6,0E+12

8,0E+12

1,0E+13

1,2E+13

1,4E+13

1,6E+13

1,8E+13

0 40 80 120 160 200 240 280 320

Elapsed time (mn)

85K

r re

leas

e ra

te

(at

/s/g

)

0

200

400

600

800

1000

1200

1400

Tem

per

atu

re (

°C)

Measurement

Calculation

Température

85Kr releaseMETEOR

Y. Pontillon et al., Proceedings of the 2004 International Meeting on LWR Fuel Performance, Orlando, USA, September 2004

UO2, ~70 GWd/t

Fission gas release: Kinetics


Fission gas release: Global release

Since VERCORS 6: Total release (100% of the initial inventory)

From VERCORS 1 to 5: Released fraction is a function of :

Temperature 1860-1880°C

Test VERCORS 2 VERCORS 1

Release 23% 33%

Temperature 2300°C


Release 86% 87%

Final temperature Duration of high T° plateau



Duration 13 minutes 17 minutes

Release 23% 33%

Temperature 2300°C



Release 77% 86%


Cs and I release: Generalities

FP of great importance with regard to the radiological consequences following a severe accident in a PWR core. They are composed of isotopes with very different half-lives:

Short half-life for iodine (from 1 hour for 134I to 8 days for 131I); the short-term radiological effects are very high in the first few days following an accident, but are negligible after 1 month. Iodine carries 15% of the core's decay heat 1 day after the emergency shutdown;

Long half-life for caesium (30 years for 137Cs); the radiological effects, which are more or less negligible in the short term (there are nevertheless 138Cs and 136Cs with respective half-lives of 30 min and 13 days) stretch into long term over several decades.


Cs and I release: Kinetics

From VERCORS program

Parameters affecting their release rate:

Burn-up,

Oxidizing or reducing conditions,

Fuel nature:

MOX versus UO2

Initial morphology


Cs and I release: Kinetics - BU effect

Comparison between RT1 (reference test) and RT6 (High BU test):

Significant increase in release rates for RT6 compared to RT1

00:00 01:12 02:24 03:36 04:48 06:00 07:12 08:24

Elapsed Time

Temperature (RT1)

Temperature (RT6)

Cs137 (RT1)

Cs137 (RT6)

VERCORS RT1:UO2, 47 GWd/TMixed H20/H2



Cs and I release: Kinetics - Atm effect

Comparison between HT2 and HT3 (same fuel used):

Significant increase in release rates for HT2 compared to HT3

VERCORS HT2 et HT3 : iode, césium, lanthane

0%

20%

40%

60%

80%

100%

17:00 17:30 18:00 18:30 19:00 19:30 20:00 20:30 21:00

Durée (h:mn)

0

500

1000

1500

2000

2500

Iode 132 HT2

Température °C

Cs 137 HT2

La 140 HT2

Température HT2Température HT3

Iode 132 HT3

Cs 137 HT3

La 140 HT3

VERCORS HT2:UO2, 50 GWd/T

steam

VERCORS HT3:UO2, 50 GWd/T

hydrogen


Cs and I release: Kinetics – Fuel nature (MOX versus UO2)

Significant increase in release rates for RT2 compared to RT1

0:00:00 1:00:00 2:00:00 3:00:00 4:00:00 5:00:00

Time (h:min:s)

MOXtemperature

UO2 (RT1)Cs release

MOX (RT2)Cs release

UO2temperature

Oxidizing plateauT=1770K

Comparison between RT1 (reference test) and RT2 (MOX test):


VERCORS RT2:MOX, 46 GWd/T

Mixed H20/H2


Cs and I release: Kinetics – Fuel nature (Initial morphology)

Comparison between RT1 (reference test), RT3 and RT4 :

0

500

1000

1500

2000

2500

3000

0:00 1:00 2:00 3:00 4:00 5:00 6:00 7:00 8:00 9:00

heures relatives

Tem

pér

atu

re (

°C)

0%

20%

40%

60%

80%

100%

120%

Fra

ctio

n r

elâc

hée

tempéaturer_RT1

température_RT4

température_RT3

Cs137_RT1

Cs137_RT4

Cs137_RT3


VERCORS RT3:UO2, debris bed

reducing

VERCORS RT4:UO2, debris bed

oxidising

RT1

RT3

RT4

RT1

RT3

RT4

Release rate


Cs and I release: Global release

Since VERCORS 6: release almost complete whatever the nature of the test

From VERCORS 1 to 5: Released fraction is a function of :



Release 30-40%

Temperature 2300°C


Release 87 - 93%

Final temperature Duration of high T° plateau

Temperature 2300°C

Test V_ 3 V_4 and V_5


Release 70% 87-93%


Te, Sb and Ag release: Generalities

Te: Main isotopes 132Te (3.26 d) and 131mTe (1.25 d). The short-term radiological effects

are very high in the first few days following an accident. Parent of the corresponding Iodine.

Sb, main isotopes composed of isotope with very different half-lives :

125Sb (2.76 y), acting in the long term 127Sb (3.85 d), acting in the short term

Ag: Main isotope 110mAg (250 d), acting in the middle/long term


Te, Sb and Ag release: Kinetics

Results obtained are relatively restricted because of:

Problems with detecting antimony and silver in all the VERCORS tests

This made it impossible to monitor their release from the fuel over time

The loss of detectability of 132Te (best tracer isotope for Te) with the use of thoria in the furnace component after VERCORS 6

Data available up to VERCORS 5


Te, Sb and Ag release: Kinetics

Hour

0 ,0 0 %

2 0 ,0 0 %

4 0 ,0 0 %

6 0 ,0 0 %

8 0 ,0 0 %

1 0 0 ,0 0 %

1 2 0 ,0 0 %

1 3 :5 5 1 4 :2 4 1 4 :5 2 1 5 :2 1 1 5 :5 0 1 6 :1 9 1 6 :4 8 1 7 :1 6 1 7 :4 5 1 8 :1 4

0

5 0 0

1 0 0 0

1 5 0 0

2 0 0 0

2 5 0 0

Mo9 9

Te1 3 2

Cs1 3 7

TUO2

Fuel temperature (° C)Released fraction

Tellurium retention in the cladding until the latter was completely oxidised

VERCORS 4:UO2, 38 GWd/T

hydrogen


Te, Sb and Ag release: Global release

For tellurium and silver :

Global release was comparable and almost total for all of the most severe VERCORS tests, i.e. from VERCORS 6 onwards

The main difference between these two FP was in terms of the quantities deposited in the hot zones of the experimental loop (transport effect)

For antimony:

Release delay by trapping into the clad For the entire RT grid, the release rates were generally lower than those obtained for

VERCORS 4, 5 and 6 (typically around 80-95% and 97-100% respectively for the RT grid and VERCORS 4 to 6)

Partial retention in the solidified corium


Sb release: Global release

This retention sometimes (tests VERCORS RT1, RT2 and RT7) involved the dissociation of this element from the solidified corium:

0

1E+13

2E+13

3E+13

4E+13

5E+13

6E+13

7E+13

320 330 340 350 360 370 380 390 400

nu

mb

er o

f at

om

s

0

2E+14

4E+14

6E+14

8E+14

1E+15

1,2E+15

1,4E+15

1,6E+15

1,8E+15

nu

mb

er o

f at

om

s

Sb127 Sb127 AVS Zr95

Sb before the test

Sb after the test

Zr after the test = corium position


Conclusion

Volatile FP:

Nearly complete release since VERCORS 6, whatever the nature of the test

Up to VERCORS 5: the release is a function of the final Temperature and duration at high temperature plateau

Sensitive to :

Burn up Atmosphere of the test Fuel nature

Global release

Kinetics

Volatile FP release from VERCORS tests

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