Fission products and nuclear fuel behaviour under severe accident conditions part 1: Main lessons learnt from the first VERDON test

Journal of Nuclear Materials 495 (2017) 363e384

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Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat

Fission products and nuclear fuel behaviour under severe accident conditions part 1: Main lessons learnt from the first VERDON test Y. Pontillon, E. Geiger 1, C. Le Gall*, S. Bernard, A. Gallais-During, P.P. Malgouyres, E. Hanus, G. Ducros CEA, DEN, CAD, DEC, F-13108 Saint-Paul-lez-Durance, France

h i g h l i g h t s
The general classification of fission products in relation to their released fractions and specific behaviour is obtained.
Fission gases and volatile fission products which have almost been totally released (iodine, caesium, tellurium, antimony).
Semi-volatile fission products which are highly sensitive to oxygen potential conditions (molybdenum and barium).
Low or non-volatile fission products (ruthenium, europium, niobium, cerium, zirconium, neodymium).

a r t i c l e i n f o

a b s t r a c t

Article history: Received 2 March 2017 Received in revised form 11 August 2017 Accepted 11 August 2017 Available online 24 August 2017

This paper describes the first VERDON test performed at the end of September 2011 with special emphasis on the behaviour of fission products (FP) and actinides during the accidental sequence itself. Two other papers discuss in detail the post-test examination results (SEM, EPMA and SIMS) of the VERDON-1 sample. The first VERDON test was devoted to studying UO2 fuel behaviour and fission product releases under reducing conditions at very high temperature (~2883 K), which was able to confirm the very good performance of the VERDON loop. The fuel sample did not lose its integrity during this test. According to the FP behaviour measured by the online gamma station (fuel sight), the general classification of the FP in relation to their released fraction is very accurate, and the burn-up effect on the release rate is clearly highlighted. © 2017 Elsevier B.V. All rights reserved.

Keywords: LWR PWR VERDON tests HEVA-VERCORS tests Severe accidents Fission product behaviour Fission product releases

1. Introduction It is well-known in the field that one of the most important aspects of research on severe accidents in pressurised water reactors (PWR) is determining the source term, i.e. quantifying the nature of FP, their release rate, and global released fraction of these FP and any other radioactive materials. This is in great part due to the consequences of the accidents at Three Mile Island (1979), Chernobyl (1986) and more recently Fukushima. In this type of

* Corresponding author. E-mail address: [email protected] (C. Le Gall). 1 Current address: Department of Chemistry and Chemical Engineering, Royal Military College of Canada, 11 General Crerar Crescent, Kingston, ON, K7K 7B4, Canada. http://dx.doi.org/10.1016/j.jnucmat.2017.08.021 0022-3115/© 2017 Elsevier B.V. All rights reserved.

scenario, the chain of events can result in primary coolant boiling and draining, meaning that the core is no longer being cooled. A direct result is core melting, which can lead to the release of FP and structural and/or activated control rod material, e.g. activation products (AP), into the containment building. If there is a failure in the various protective barriers, these FP and AP can leak out of the containment building and into the environment. A large number of research programmes have thus focused on this subject in various countries. In line with this approach, IRSN (France) has been the driving force behind such studies. It has carried out specific programmes to determine the source term, with focus on understanding the mechanisms that lead to the release of FP. This is because only very exhaustive knowledge of the phenomena governing the behaviour of FP/AP under such constraints will make it possible to define the actions required to

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minimise emissions and optimise the protection of both people and the environment. The HEVA2 [1] and VERCORS [2e5] programmes were thus initiated by the CEA. VERCORS has considerably broadened the field of application by exploring higher temperatures and by testing a wider range of fuels (UO2, MOX, debris bed configurations, high burn-ups) in a more complex experimental facility with better instrumentation. This programme was composed of 17 tests which were conducted over 14 years with three different experimental phases. A first series of six tests (VERCORS 1 to VERCORS 6, Table 1) was conducted between 1989 and 1994 on UO2 fuel in a higher temperature range (close to fuel relocation) than that of the HEVA programme [5]. This series made it possible to integrate certain FP with low volatility into the HEVA database. Two series of tests eVERCORS High Temperature (HT)3 and Release of Transuranics (RT)4 (Table 2) e were conducted alternately throughout 1996e2002, which made it possible to extend the database to include the less volatile FP. These analytic experiments simulating severe PWR accidents aimed at i) quantifying the released fraction and release rates of FP from irradiated nuclear ceramics (UO2 or MOX, typically three PWR pellets in their original cladding), ii) determining the type of the gases and aerosols emitted (particle size analysis and speciation), and iii) understanding the fuel degradation mechanism. These experimental sequences were carried out in a hot cell and were commonly considered to be complementary to the PHEBUS FP [6] integral tests. They are also and comparable with certain tests carried out abroad, i.e. HI/VI [7] in the United States, VEGA [8] in Japan, and the programme conducted in Canada [9]. The experimental results of this programme are used to (a) define the envelope values for released fraction within the scope of assessing reference source terms for all French PWR, and (b) validate the semi-empirical or mechanistic models on FP releases and transport while qualifying the simulation codes by integrating these models [10e12]. However, major uncertainties still remain with respect to the assessment of risks for populations and the environment [13]. As a consequence, it was decided to build a co-operative research programme between teams involved in severe accident phenomenology all over the world (US-NRC, IRSN, CEA, EDF, PSI, European Commission, EACL, KAERI, etc.) based on separate-effect experiments and called the “International Source Term Programme (ISTP)”. The results of these separate-effect experiments would make it possible to improve models used for source term evaluation studies. Four main R&D research areas have been included in this programme: (1) iodine study, (2) study of the boron carbide effect, (3) study of the air effect on fuel behaviour and (4) study of the fission product releases from the fuel. A total of four VERDON tests were considered for source term quantification. They focused on FP releases from high burn-up UO2 fuel, MOX fuels and air ingress scenarios. They were performed in

2

These tests were performed on three pellets from a standard PWR fuel rod in its original cladding, heated in a high-frequency furnace up to 2300 K in a steam and hydrogen environment. The volatile FP release rates were measured by gamma spectrometry. Post-test examinations supplied further information on FP behaviour, i.e. aerosol particle sizes and the chemical speciation of the deposits [1]. 3 The HT configuration was used to study FP releases and transport, as well as the potential interactions of FP with elements from the degradation of PWR neutron absorbers. To do so, new equipment was installed, such as a thermal gradient tube (TGT) and a May-Pack (selective trap for the various chemical forms of iodine) [2]. 4 The RT configuration was designed to collect all releases as close as possible to their emission point. The objective was to improve the measurement precision of the FP released fractions and to extend the “release” database to include both FP that do not emit gamma rays but are important for safety (e.g., 90Sr), and heavy nuclei (U, Np, Pu, Am and Cm) by means of post-test analysis of the loop components [2].

the VERDON laboratory at the CEA Cadarache centre. This paper deals with the VERDON-1 test itself. The main issue addressed by this first test concerns high burn-up UO2 fuel behaviour and corresponding fission product releases under reducing conditions at very high temperature (up to 2883 K). Moreover, the first part of the test (i.e. up to the end of the oxidation plateau at 1773 K) is performed under the same atmosphere conditions compared with the VERCORS RT6 test, which was conducted with a very similar UO2 high burn-up fuel, in order to check continuity between the VERCORS and future VERDON databases [2e4]. The second and third parts of this article [14,15], deal in detail with the post-test examination results (SEM, EPMA and SIMS) of the VERDON-1 sample. The second part focuses on the fuel behaviour during the VERDON-1 test, and the third part describes a promising methodology to assess non g-emitter FP releases thanks to post-test characterisations. Section 2 describes the experimental set-up, section 3 presents both the fuel sample and the progress of the accident sequence, while section 4 discusses the results of FP releases and fuel behaviour. The main results are discussed in the last part of the paper with a special focus on: (1) loss of fuel integrity, (2) continuity between VERCORS and VERDON and (3) FP volatility and release kinetics. Two other articles will focus on (1) the fuel behaviour in terms of microstructure and chemical variations and (2) the FP speciation during the VERDON-1 test. This series of articles will describe the fuel and FP behaviour in reducing conditions corresponding to a severe accident scenario involving significant H2 production.

2. VERDON experimental set-up This section details the VERDON experimental circuit and the apparatus used to measure FP releases. The experimental loop has been extensively described in Refs. [2,16e19] so only the main characteristics are recalled in this section.

2.1. Experimental circuit The VERDON laboratory at the LECA-STAR facility comprises 2 hot cells (called C4 and C5) and a glove box, as illustrated in Fig. 1. The C4 cell is used for receiving the sample, for performing pre-and post-test gamma scanning and loop component storage. The C5 cell contains the experimental circuit itself (i.e. VERDON loop). It is specifically used to perform the accident sequence and online measurements. The main glove box functions are to analyse and store the fission and carrier gases. The VERDON loop in its release configuration (as used for VERDON-1 test) is illustrated in Fig. 2. This experimental loop comprises (along the path of gas flow): (1) The fluid injection system (2) The furnace (see below) (3) An aerosol filter located directly on the top of the furnace. Its filtering device is made of poral® stainless steel which is designed to stop all FP in aerosol form. The aerosol filter is heated at 423 K ± 10% (4) A May-Pack filter. Half of this filter is filled with zeolite (impregnated with silver) to stop potential molecular iodine, while the other part is empty and is used for gas gamma spectrometry sighting, even though the filter design and detector are not well suited; the May-Pack is heated at 423 K ± 10% to avoid condensation (5) A condenser whose function is to condense steam from the experimental gas and to recover the water for analysis

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Table 1 VERCORS 1 to 6 test matrix parameters. Test

VERCORS 1

VERCORS 2

VERCORS 3

VERCORS 4

VERCORS 5

VERCORS 6

Date of test

11e1989

06e1990

04e1992

06e1993

11e1993

06e1994

Fessenheim 42,9 Siloe

Bugey 38,3 Siloe

Bugey 38,3 Siloe

Bugey 38,3 Siloe

Bugey 38,3 Siloe

Gravelines 60 Siloe

2130 Mixed H2O þ H2 17 0,15 0,003

2150 Mixed H2O þ H2 13 1,5 0027

2570 Mixed H2O þ H2 15 1,5 0,03

2570 Hydrogen 30 1,5e0 0,012

2570 Steam 30 1,5 0

2620 Mixed H2O þ H2 30 1,5 0,03

Fuel PWR irradiation Fuel burn-up (GWd/tU) Re-irradiation Test conditions Max fuel temperature (K) Atmosphere (end of test) Last plateau duration (min) Steam flow rate (g/min) Hydrogen flowrate (g/min)

Table 2 VERCORS HT-RT test matrix parameters. VERCORS tests

HT 1

HT 3

HT 2

RT 1

RT 2

RT 7

RT 6

Date of test

June 1996

June 2001

April 2002

March 1998

April December June 1999 November 1998 1998 1999

RT 5

RT 4

RT 3

April 2000

September November 2002 2002

RT 8

Fuel

UO2

UO2

UO2

UO2

MOX UO2

MOX

UO2

Burnup (GWd/tU) Re-irradiation

47 SILOE

~47 OSIRIS

~47 OSIRIS

47 No

41 No

2900/2500 Max fuel temperature (K)/ Fuel collapse

2750/2500

2600/2300

2570

H2 (mg/s) H2O (mg/s) Air (mg/s)

0,2 0 0

0,2 0 0

0 25 0

0,45 25 0

Main objective

H2 atm., high temperature, HT reference test

MOX High Boric acid and Boric acid and RT SIC injection SIC injection reference fuel Burnup test

UO2/ZrO2 debris bed 3 cycles No

UO2 debris bed 3 cycles OSIRIS

2440 Fuel collapse

Fuel collapse

Fuel melting Fuel Fuel melting melting

Fuel melting

0,45 25 0

0,4 14,6 0

1,25 1,25 0

0,45 25 0

0 0 0,8

Phebus FPT4 support

MOX Fuel volatilization fuel

High burn up fuel

High burn up fuel/air injection

60 OSIRIS

0,45 25 0

Fig. 1. Schematic view of the VERDON laboratory.

3 cycles 6 cycles OSIRIS OSIRIS

0,2 0 0

UO2 6 cycles OSIRIS

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Fig. 2. The release experimental loop.

(6) A final safety filter constituted of poral® stainless steel to stop any residual traces of fission products (other than gaseous Xe, Kr) (7) Upstream from the condenser, the circuit is constituted of stainless steel tubes heated at 423 K ± 10% (8) Downstream, the condenser is linked to the final safety filter thanks to a flexible stainless tube at ambient temperature (9) Outside the cell, a “linking line” connects the C5 cell to the glove box. This glove box is equipped with a flow-meter measuring the total flow-rate of the loop, a pressure sensor and a safety filter similar to the final safety filter of the C5 cell (at the end of the release experimental loop). It is mainly dedicated to gas analysis and storage. As was the case with the previous VERCORS furnace, this VERDON furnace is based on induction technology [2]. Schematically speaking (Fig. 3), the furnace comprises a coil surrounding a tungsten susceptor tube, which is the heating component of the furnace. A high-frequency power supply generates a current in the coil. A current is generated by electromagnetic coupling into the susceptor tube and the corresponding electric energy is converted into thermal energy by Joule effect to heat the susceptor tube. The fuel sample located inside the susceptor tube is then heated, mostly by thermal radiation. The external part of the furnace includes a quartz tube sealed between 2 stainless steel bases via to 2 joints.5 The internal part can

5 These joints need to be adequately cooled to guarantee the leaktightness of the furnace.

be schematically divided into two areas delimited by a ceramic column. The latter is composed of a double stack of concentric dense ceramics; hafnium dioxide is used in the central area where the furnace temperature is the highest, while zirconium dioxide is used at the ends where the furnace temperature is the lowest. The fuel sample is inserted into the crucible before the sequence. This crucible is located inside this ceramic column, together with several cups which are designed to support the crucible and to allow the flow of gases. The susceptor and a double stack of concentric porous zirconia and hafnia components are positioned outside the ceramic column for the purpose furnace insulation. A “susceptor gas” (He) flows between the quartz tube and the “ceramic column” through the porous insulators and along both sides of the susceptor tube. This gas is slightly over-pressurised compared with the experimental gas to protect the susceptor tube (in tungsten) against oxidation from the experimental fluid. A pyrometer, a line of sighting under the crucible and three thermocouples (TC) inside the furnace insulators are used to assess the fuel sample (centre) temperature. Two of these TC are positioned in front of the fuel sample while the third is placed underneath the sample (and used as a safety TC, in case the first two become unusable). These TC are used to monitor the power supply of the furnace at low temperature up to 1273 K. At higher temperature, the TC give indicative values which can be used to check the consistency of the pyrometer measurements since the pyrometer is used to monitor the power supply of the furnace at high temperature, from 1273 K to 2883 K.

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Fig. 3. The VERDON furnace.

2.2. Fission product measurements In the case of the VERDON-1 configuration, the FP release kinetics were measured by means of three complementary online gamma spectrometry stations and one micro gas chromatography device:
One gamma station was aimed directly at the fuel sample and used during the entire test. This gamma station made it possible

6 Releases of at least 10% must be recorded by this station to guarantee a significant value, particularly as the changes in the object's geometry measured during heating (swelling, fracturing, fuel collapse, etc.) significantly complicate the quantitative use of the measurement, as does the axial migration of the FP.

to measure the FP remaining in the fuel as a function of temperature (presented in section 4.1), which explains why the release kinetics could not be quantified more precisely.6 The two advantages of this station lie in its ability to (i) perform direct measurements at the source (all the FP were measured, unlike at the other stations where deposits upstream could occur), and (ii) indicate the precise moment when the fuel relocates by detecting the disappearance (or significant decrease) in the signal from non-volatile FP. This last point was well illustrated in the case of the VERCORS series [4,20]. This gamma station is also used to perform pre- and post-qualitative gamma scanning of the sample before and after the accident sequence respectively (see section 4.2).
Another gamma station was aimed at the large-capacity aerosol filter. It provided a very precise measurement of the FP

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deposited at this point, where most of the volatile FP were found (see section 4.1). It is highly complementary with the previous station (fuel line-of-sight).
The last gamma station was aimed at the May-Pack. The MayPack was designed to make it possible to measure potential iodine deposits (after the test).
Gas analysis was also performed online by a micro gas chromatograph (mGC) (or sequentially by 4 sampling aliquots) located inside the glove box. The m-GC extended the analysis of active gases to all gases. Within the context of the VERDON programmes, the m-GC was able to analyse H2 (at concentrations less than 1%), Kr and Xe. In addition to the online gamma spectrometry measurements which gave us access to the FP release kinetics, the overall released fractions were obtained by quantitative gamma spectrometry of all the loop components. The measurements were carried out on a gamma scanning bench located in the C4 hot cell at the VERDON laboratory. The initial FP inventory (see section 3.1 for more details) was first assessed by scanning the fuel sample before the experimental sequence7 and was completed by calculations for the nongamma-emitting elements. Calculating the FP production also made it possible (i) validate the coherence of the FP measurements and (ii) recalculate the irradiation conditions in a PWR (fuel burnup) and in the Material Testing Reactor (MTR, re-irradiation power level). 3. Fuel sample characteristics and progress of the accident sequence 3.1. VERDON-1 fuel sample characteristics The sample was taken from UO2 fuel irradiated up to around 72 GWdd.t1 HM (i.e. six irradiation cycles in the Gravelines-5 PWR) in a PWR operated by EDF. The power history of the fuel rod is given in Table 3. A standard industrial process was used to manufacture these UO2 fuel pellets. The 235U initial enrichment was 4.5%. The sample was taken from a fuel rod section located at span 4 of the initial rod (Fig. 4). Its characteristics are summarised in Table 4. The sample was composed of two irradiated pellets in their original cladding (M5 alloy). Two half-pellets of depleted (and unirradiated) uranium oxide were placed at each end of the sample and held there by crimping the cladding (Fig. 4). The cladding was therefore not fully sealed. At this stage, it is important to point out that the sample used in this test was very similar to that used in the VERCORS RT6 test (i.e. same fuel assembly, same power history and very similar burn-up, see Table 2). Before the experimental sequence, the sample was re-irradiated at low linear power in the OSIRIS MTR for ten days to recreate the short half-life FP without any in-pile releases. As a consequence, these FP (i.e. 99Mo, 132Te, 133I, 131I, 140Ba, etc.) which are known for their radiobiological effects, were measurable by using online gamma spectrometry during the experiment. The initial FP inventory was used as reference to calculate the released fraction and was determined in two different methods depending on the evaluated FP. The first involved measuring the contents of the VERDON-1 sample (before and after re-irradiation in MTR) by quantitative gamma spectrometry. The second required calculating the FP content of the sample by using a computer code like CESAR [21]. This was done not only because some

7 This initial inventory prior to the sequence is often determined in two stages: before re-irradiation in the experimental reactor to precisely measure the FP with long half-lives, then just after re-irradiation to measure the FP with short half-lives.

Table 3 Power history of the VERDON-1 father rod. Cycle n

1

2

3

4

5

6

Mean linear power (kW.m1)

20

5

20

15

15

20

fission products cannot be measured inside the pellet (e.g. 85Kr and all stable FP and/or pure g emitters), but also because the power history during both the base irradiation in a PWR and MTR reirradiation needed to be rescaled. The initial FP inventories of the VERDON sample (PWR UO2)8 have been compared to the calculations performed with the Cesar 5.1 code. Generally speaking, the consistency between the calculated and experimental results is very good (Table 5), except for 125Sb and 154Eu which are well known to be incorrectly calculated by the CESAR code. The corresponding rescaled burn-up is 72 GWdd.t1 HM for the PWR irradiation and the rescaled power history from the MTR irradiation is 10 W cm1. These rescaled values correspond to the average gamma spectrometry measurements over the calculated values from the CESAR code. 3.2. VERDON experimental sequence The VERDON thermal-hydraulic sequence is illustrated in Fig. 5. Three main phases can be distinguished according to the temperature and atmosphere (Table 6). Phases 1 and 2 are similar to VERCORS RT6 and correspond to the initial phases of a severe accident induced by a loss-of-coolant accident (LOCA). The third phase of the test was performed under reducing conditions (oxidising for RT6) corresponding to a severe accident scenario involving important H2 production due to the oxidation of both the fuel cladding beginning around 1473 K and the metal-rich melt mixture during core reflooding [22e24]. There was also a 1.5 h troubleshooting resolution phase between phase 1 and phase 2. During this whole period, it proved impossible to inject steam as initially defined due to a technical problem. Consequently, it was impossible to produce steam. As the trouble occurred at relatively low temperature (less than 1273 K), however, most of the FP had not yet been released and the impact of both the temperature stabilisation and the slight decrease in the FP releases was minor. As explained above, some helium was injected into the susceptor circuit (HeSus) during the sequence so that the over-pressure between the experimental circuit and the susceptor circuit could be maintained and the W susceptor tube could be protected against oxidation. The criterion used to determine the end of the sequence was either when the loss of fuel sample integrity was detected, or when the maximal temperature (Tmax ¼ 2883 K) was reached. As illustrated in Fig. 6a, the gamma station did not record any significant decrease in the 140La signal at the end of the sequence (1973K < T < 2883 K). The end of the accident sequence (HF power supply switched off) was performed when the maximal temperature of 2883 K was reached. 4. FISSION products releases and fuel behaviour 4.1. Fission product releases This section describes the release kinetics recorded by the ‘fuel’ gamma station according to the well-established FP classification

8 A similar approach was performed on the un-irradiated half pellet, with the corresponding measured amount then included in the total FP initial inventory.

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Fig. 4. Location of the fuel rod section used for the VERDON-1 sample.

Table 4 VERDON-1 father rod characteristics. Cladding material

Fuel

Pellets mean height (mm)

Outer diameter of clad (mm)

Pellets mean diameter (mm)

M5

UO2

14

9.5

8

Table 5 FP's initial inventory of the VERDON 1 sample, comparison between gamma spectrometry measurements and Cesar 5.1 calculations. FP

Half life

M/C

Zr95 Nb95 Mo99 Ru103 Ru106 Sb125 Sb127 Te132 I131 I133 Cs134 Cs137 Ba140 La140 Ce141 Ce143 Nd147 Eu154 Eu156 Np238

63.98 d 65 d 2.75 d 39.3 d 1.017 y 2.76 y 3.85 d 78.20 h 8.02 d 20.8 h 2.07 y 30.17 y 12.8 d 1.68 d 32.5 d 33.0 h 11.0 d 8.8 y 15.19 d 2.12 d

0.98 1.06 1.02 0.96 1.01 0.37 0.83 1.01 1.01 1.02 0.99 1.00 1.01 1.06 1.05 1.03 0.87 0.46 1.03 1.05

(see discussion), i.e. non-volatile, low-volatile, volatile (including fission gases), and semi-volatile fission products. The release kinetics shown in Fig. 6a and b show that no significant releases of Zr, La, Eu, Np and Ru were measured at this stage. Furthermore, no global loss of the fuel sample's integrity was recorded by this gamma station, i.e. no global loss of the signal was recorded up to the end of the test at 2883 K. The total release of the iodine and caesium species was measured at the end of the test, with equivalent release rate behaviours between these two elements (Fig. 7). The released fraction obtained at the end of the oxidation plateau proved to be very high (around 60% of the initial inventory) with releases starting at ~1473 K. Additionally, right before the beginning of release (1273 K < T < 1473 K), a slight decrease in the “released fraction” of 137 Cs was detected (Fig. 7). This is due to the axial and longitudinal migration of caesium inside the fuel pellet.9

9 This axial and/or longitudinal migration of FP inside the fuel pellet is seen as an over-concentration by the gamma station, resulting in an apparent decrease in the “released fraction” as has been previously observed during the VERCORS programme.

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Fig. 5. The VERDON experimental sequence.

Table 6 VERDON-1 qualitative thermohydraulic sequence. Atmosphere

Phase 1 Troubleshooting resolution phase Phase 2 Phase 3 Phase 4

Neutral Reducing Oxidising Reducing Neutral

H2O

H2

He

Hesus

None None Yes Yes reduced None

None Yes until power supply shut down Yes unchanged Yes reduced None

Yes Yes None Yes Yes

Yes Yes Yes Yes Yes

The fission gas release kinetics (Fig. 8) are characterised by a series of burst releases. In each case, the amplitude tends to be greater for Xe than for Kr, in line with the expected Xe/Kr ratio. The first burst release starts at around 973e1073 K (about 4% of Kr and 6% of Xe). There is a second small burst release around 1273 K (~2% both). The following burst release starts at about 1373 K (16% of total Kr and 14% of total Xe). The main burst release occurs during the oxidation plateau at 1773 K (43% of total Kr and 47% of total Xe). A series of low burst releases occurs from 2073K to the end of heating (20% of total Kr and 21% of total Xe). The last puff (15% of total Kr and 10% of total Xe) occurs at the beginning of the temperature decrease. The fission gas release kinetics show a time lag between Kr and Xe at every burst end. This behaviour may be due to the zeolite contained in the May-Pack which tends to separate these two gases10. The global Kr releases were slightly underestimated because of the detection limit (about 1 ppm), and its relative low concentration (compared with Xe). The loss of the Kr signal (calculated between first and last burst release) did not exceed 4% of total Kr measured. The loss of signal of the last puff,

due to the end of measurement, can be evaluated up to 7% of the total Xe and Kr measured (symmetrical burst end). The last puff probably indicates the complete drainage of the residual intragranular inventory of fission gases. Fig. 9b1 and b2 shows the Ba and Mo release kinetics up to 1773 K so a direct comparison can be made with the VERCORS RT6 test (see section 5). We monitored the start of Mo releases during the beginning of the oxidation plateau at 1773 K, with very strong release kinetics up to approximately 40% of the initial inventory two thirds of the way through the plateau. More or less like molybdenum, the release of barium did not start during the 1773 K plateau. Only a slight decrease in the “released fraction” of Ba is visible in Fig. 9b1 and b2. This is probably due to axial and/or longitudinal migration of barium inside the fuel pellet, as was observed in many VERCORS test (see section 2, [3]). In order to illustrate the general behaviour of the FP belonging to the four volatility categories, Fig. 10 compares the release kinetics of Zr, Cs, I, Ba and Mo.

4.2. General behaviour of fission products 10

This zeolite “pre-column” effect was previously seen on calibration gas Kr and Xe during commissioning tests before VERDON-1 but it was not quantified.

Just before loop dismantling and in order to collect preliminary

Y. Pontillon et al. / Journal of Nuclear Materials 495 (2017) 363e384

Fig. 6. Low- or non-volatile FP release kinetics, (a)

information on both FP releases and the final FP distribution inside the sample, the fuel gamma station was used as a longitudinal bench. The sample then underwent gamma scans inside the furnace. Comparison between the results obtained before and after the test was then performed using a qualitative method (i.e. the efficiency was considered equal to 1 in both cases). Comparisons of the FP distributions in the sample before (blue curve) and after

154

Eu,

95

Zr and

140

La, (b)

103

Ru,

371

97

Zr and

238

Np.

(pink curve) the VERDON-1 test are shown in Figs. 11, 12 and 13 for non or low-volatile (95Zr, 154Eu, 103Ru), semi-volatile (99Mo and 140Ba) and volatile FPs (137Cs and 131I) respectively. They were obtained by gamma scanning of the sample inside the furnace. From a general point of view, the main information deduced from the online measurements (see section 4.1) is confirmed by the FP distribution along the sample:

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Fig. 7. Volatile FP (I, Cs) release kinetics.

Fig. 8. Fission gas release kinetics.

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Fig. 9. Comparison of release kinetics between VERCORS RT6 and VERDON-1: (a1 and 2) volatile FPs, (b1 and 2) semi-volatile FPs.

373

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Fig. 9. (continued).

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375

Fig. 10. Release kinetics of Zr, I, Cs, Ba, Mo during the whole VERDON-1 test.


No significant releases were measured, at this stage (using this approach), for non- or low volatile FPs, except for Ru where some small deposits are observed just above the crucible.
No general degradation in the sample was highlighted by these post-test gamma signals (for the non-volatile FP), since the final shape of the sample (after the test) illustrated by the FP distribution is more or less the same as that before the sequence.
Great and quasi-total releases were measured for semi-volatile and volatile FPs respectively, as well as the greater release of Ba compared with Mo. After these measurements, the furnace column was extracted from the furnace and measured on the C4-gamma bench. Fig. 14 gives the total gamma count as a function of the displacement along the furnace column. The corresponding FP location is indicated in this figure. These results are very consistent with the previous results and the well-known class of FP volatility, with:
no or very low deposits for non- (Zr, Nd) and low-volatile (Nb, Np, Ru) FP respectively. In the latter case, the deposits are in the high temperature section of the furnace column (i.e. just above the crucible).
Semi-volatile FP contain a great deal of deposits all along the furnace column, with significant retention in the sample.
Volatile FP (I, Cs, Te) show no or very little retention in the sample with low deposits in the extreme upper part of the furnace column just before the filter entrance. As previously measured, the non-volatile FP distributions do not evidence any significant fuel collapse. In fact, the general shape of the VERDON sample is still visible: two half un-irradiated pellets and the two PWR fuel pellets. The major change is due to the global

swelling in the sample, increasing its length by about 20%. Finally, as observed during the VERCORS programme for volatile FP:
There was limited caesium retention inside the PWR fuel
Iodine was practically only detected inside the un-irradiated half pellet. Finally, as explained in above, all the loop components were gamma-scanned in order to ascertain the global FP balance. From a general viewpoint, the different methods produced results that show very good consistency in terms of the released fraction (Fig. 15): (1) differences between the measurement of the sample before and after the test (RF APS/AVS), and (2) sum of the deposited fractions. This is probably one of the most important facts inferred from this first VERDON test as it confirms the very good behaviour of the loop during this type of accident sequence. Moreover, the released fractions measured by the online gamma fuel sighting device are also very consistent with the previous values (Table 7). 5. Discussion The VERDON-1 results given in the first section of this paper with respect to FP and fuel behaviour may be examined from three different perspectives: (1) fuel behaviour with special focus on the fuel melting temperature, (2) comparison with the VERCORS RT6 test, and (3) general volatility of FP. 5.1. Loss of fuel integrity The experimental facility was totally refurbished for the “VERCORS HT and RT” series, with two complementary test loop

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Y. Pontillon et al. / Journal of Nuclear Materials 495 (2017) 363e384 4,50E+08

Zr95

4,00E+08

ZR95 APS

Total count rate (c/s)

3,50E+08

ZR95 AVS

3,00E+08

2,50E+08

2,00E+08

1,50E+08

bottom

top

1,00E+08

5,00E+07

0,00E+00 6,00E+01

7,00E+01

8,00E+01

9,00E+01

1,00E+02

1,10E+02

1,20E+02

1,30E+02

1,40E+02

Arbitrary distance (mm)

(a) 4,00E+08

Ru103 3,50E+08 RU103 APS RU103 AVS

Total count rate (c/s)

3,00E+08

2,50E+08

2,00E+08

1,50E+08

bottom

top

1,00E+08

5,00E+07

0,00E+00 6,00E+01

7,00E+01

8,00E+01

9,00E+01

1,00E+02

1,10E+02

1,20E+02

1,30E+02

1,40E+02

Figuredistance 17 Arbitrary (mm)

(b) 1,80E+10

Eu154

1,60E+10

EU154 APS

Total count rate (c/s)

1,40E+10

EU154 AVS

1,20E+10

1,00E+10

8,00E+09

6,00E+09

bottom

top

4,00E+09

2,00E+09

0,00E+00 6,00E+01

7,00E+01

8,00E+01

9,00E+01

1,00E+02

1,10E+02

1,20E+02

1,30E+02

1,40E+02

Arbitrary distance (mm)

(c) Fig. 11. Gamma scanning of the sample inside the VERDON furnace ((a) 95Zr, (b) 103Ru and (c) 154Eu) before (in blue) and after (in pink) the test. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Y. Pontillon et al. / Journal of Nuclear Materials 495 (2017) 363e384

377

2,50E+08

Significant release confirmed

Mo99 MO99 APS

2,00E+08

Total count rate (c/s)

MO99 AVS

1,50E+08

1,00E+08

bottom

top

5,00E+07

0,00E+00 6,00E+01

7,00E+01

8,00E+01

9,00E+01

1,00E+02

1,10E+02

1,20E+02

1,30E+02

1,40E+02

Arbitrary distance (mm) (a) 3,00E+08

Significant release confirmed

Ba140

2,50E+08

BA140 APS

Total count rate (c/s)

BA140 AVS

2,00E+08

1,50E+08 c

1,00E+08

bottom

top

5,00E+07

0,00E+00 6,00E+01

7,00E+01

8,00E+01

9,00E+01

1,00E+02

1,10E+02

1,20E+02

1,30E+02

1,40E+02

Arbitrary distance (mm) (b) Fig. 12. Gamma scanning of the sample inside the VERDON furnace (99Mo and 140Ba) before (in blue) and after (in pink) the test. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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1,40E+12

Quasi-total release confirmed

Cs137

1,20E+12 CS137 APS CS137 AVS

Total count rate (c/s)

1,00E+12

8,00E+11

6,00E+11

4,00E+11

bottom

top

2,00E+11

0,00E+00 6,00E+01

7,00E+01

8,00E+01

9,00E+01

1,00E+02

1,10E+02

1,20E+02

1,30E+02

1,40E+02

Arbitrary distance (mm) (a) 7,00E+07

Quasi-total release confirmed

I131

6,00E+07 I131 APS I131 AVS

Total count rate (c/s)

5,00E+07

4,00E+07

3,00E+07

2,00E+07

bottom

top

1,00E+07

0,00E+00 6,00E+01

7,00E+01

8,00E+01

9,00E+01

1,00E+02

1,10E+02

1,20E+02

1,30E+02

1,40E+02

Arbitrary distance (mm) (b) Fig. 13. Gamma scanning of the sample inside the VERDON furnace (137Cs and 131I) before (in blue) and after (in pink) the test. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Y. Pontillon et al. / Journal of Nuclear Materials 495 (2017) 363e384

379

7000

Volatiles FPs 6000

5000

Top of the column 4000

Top of the sample 3000

2000

Semi-Volatiles FPs Low-volatiles FPs

1000

Bottom

0 0

50

100

150

200

250

300

350

400

Arbitrary distance (mm) cote X du banc (mm)

Fig. 14. Total gamma counting rate along the VERDON-1 furnace column.

120,00% RF (from the sample) Sum on the loop elements 100,00%

80,00%

60,00%

40,00%

20,00%

N p2 39

Eu 15 4 N p2 38

N d1 47

C e1 43

7 Ba 14 0 La 14 0

s1 3 C

XE 13 3 C s1 34

I1 33

I1 31

Te 13 2

Sb 12 5 Sb 12 7

R u1 03

M o9 9

Zr 97

b9 5 N

Zr 95

0,00% Kr 85

Released fraction inventory) Released Fraction (% (% of initial initial inventory)

Total count rate Taux de comptage global(c/s) (c/s)

Sample

-20,00% Fission Product Fission Products Fig. 15. Comparison between the released fractions deduced from the sample analysis (RF APS/AVS) and by the sum on the loop elements.

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Table 7 Comparison between released fractions obtained by the differential approach (before versus after test) and by on-line gamma fuel sight.

FP Kr85 Zr95 Nb95 Zr97 Mo99 Ru103 Sb125 Sb127 Te132 I131 I133 XE133 Cs134 Cs137 Ba140 La140 Ce143 Nd147 Eu154 Np238 Np239

RF (APS/AVS) crucible

RF fuel sight Kinetics

100,0% 0,3% 0,0% 0,9% 63,7% 1,3%

ND 0,0% <5-10% 0,0% 60,0% 0,0% ND 61% à 2120°C 67% à 1670°C 100,0% 100,0% ND 100,0% 100,0% ~75% 0,0% 0,0% 0,0% 0,0% 0,0% 0,0%

95,0% 93,7% ND 100,0% 99,9% 99,4% 76,4% 0,0% 0,2% 0,0% -1,1% 2,8% 2,1%

configurations: 1) complex HT configuration [25] (three tests), integrating the additional aim of studying FP transport in the primary system of a PWR and their potential interaction with the elements composing PWR neutron absorbers (Ag, In, Cd and B, [26]). 2) compact RT configuration (eight tests), which more specifically focused on the release of low-volatile FP and transuranium elements. All these tests were conducted by bringing the fuel sample up to the loss-of-fuel-integrity point to quantify the entire volatility range of FP [2e4] Fig. 16 shows fuel collapse temperatures as a function of different VERCORS tests. Systematic fuel collapse was detected within a temperature range of 2373e2623 K for burn-ups from 1 47 GWdd.t1 HM to 70 GWdd.tHM. It can therefore be said that high burn-up does not have a great impact. Furthermore, whatever the test atmosphere conditions, the temperature at which the fuel loses its integrity is systematically inferior to both the melting point of un-irradiated UO2 and the solidus temperature of the ZrO2-UO2 eutectic [27]. The fuel collapse temperature also seems to decrease in oxidising conditions. This point is well highlighted by the HT1, HT2 and HT3 tests which were performed on the same fuel section in reducing conditions for HT1 and HT3 and in oxidising conditions for HT2. The corresponding fuel collapse temperatures were approximately ~2573 K for HT1 and HT3 and ~2273 K for HT2. This general behaviour has been already discussed in detail elsewhere [20,28]. New thermodynamic modelling of the UeO phase diagram has been defined on the basis of this data analysis in the UO2þx composition domain. An important consequence of this new optimisation is that a liquid phase may appear in the OeUO2eZrO2 composition domain of the UeOeZr phase diagram at 2603 K at atmospheric pressure (this temperature decreases as the pressure increases, about 2503 K at 2 atm.). These temperatures can be associated with the temperature at which the fuel assembly could lose its integrity in oxidising conditions and also with what was observed in some of the VERCORS tests (quite different from the

reducing test conditions) and in the PHEBUS tests. The VERDON-1 test does not result in a global and/or clear loss of fuel integrity at the end of the test (i.e. up to a temperature of 2883 K). This behaviour may be connected, by way of comparison with the previous explanation, to the final atmosphere of the test (i.e. reducing conditions). However, the temperature reached by the VERDON-1 sample is certainly one of the main points of the test. At this stage, the exact origin of this behaviour needs to be clearly demonstrated and investigated more thoroughly [14]. 5.2. Comparison with RT6 As explained in the introduction, the first part of the VERDON-1 test (i.e. up to the end of the oxidation plateau at 1773 K) was performed under the same atmosphere conditions as the VERCORS RT6 test. It was also performed on a very similar UO2 fuel with a high burn-up in order to check continuity between VERCORS and the future VERDON databases. In order to perform these analyses, Fig. 9a1, a2 and 9b1, b2 compare the release kinetics of the semivolatile and volatile fission products during VERCORS RT6 and VERDON-1 tests respectively. The general FP behaviour (release rate and kinetics) in both cases seems to be very similar when examining the middle of the oxidation plateau. The releases of Ba and Mo reached approximately 0% and 40e45% respectively in the two experiments. For volatile species, around 60% was obtained in the two experiments in question. These points are very important since they underline the perfect continuity between the two facilities in terms of FP releases [29]. 5.3. FP volatility In general terms, analysis of the released fractions obtained during the VERCORS programme [2] made it possible to classify the FP into four categories of decreasing volatility: 1) volatile FP (including fission gases, iodine, caesium, antimony, tellurium, cadmium, rubidium and silver), 2) semi-volatile FP such as molybdenum, rhodium, barium, palladium and technetium, 3) low-

Y. Pontillon et al. / Journal of Nuclear Materials 495 (2017) 363e384

381

VERDON-1 No relocation

Fuel collapse temperarture

Un-irradiated UO2 UO2 un-irraodated 3200 3000

47 - 50 GWd/T

Temperature (K) Temperature (°K) .

2800

60 GWd/T

72 GWd/T

2600 2400 2200 2000 1800 1600 1400 1200 1000 UO2

RT1, 47 GWd/t

HT1, 47 GWd/t

HT2, 50GWd/t HT3, 49 GWd/t

V_6, 60 GWd/t

RT6, 72GWd/t

VERDON 1

Fig. 16. Fuel collapse temperature for several VERCORS tests and VERDON-1 compared with the melting point of non-irradiated UO2 (i.e. 2869  C from Ref. [27]).

volatile FP such as ruthenium, niobium, strontium, yttrium, lanthanum, cerium and europium, and finally 4) non-volatile FP such as zirconium, neodymium and praseodymium.11 The released fraction obtained thanks to the data analysis presented in this paper are in good agreement with the VERCORS results and with what we can expect regarding FP behaviour in VERDON-1 thermal-hydraulic conditions (i.e. reducing/neutral at high temperature). The following information may be highlighted thanks to a general comparison with the VERCORS database and in line with the above FP classification. For all the VERCORS tests performed at temperatures representative of a severe accident, the fission gases were released in their entirety. The instantaneous fission gas release kinetics were characterised by successive burst releases at each temperature ramp. Furthermore, a higher release of long half-life FP (85Kr) at low temperature (below 1473e1573 K) was monitored due to the higher gas content inventory located at the grain boundaries for this type of gas [30], compared with short half-life FP (133Xe, 135Xe) [31], as well as the final puff linked to fuel melting and inducing the release of the ultimate gas fraction contained in the intragranular bubbles. Between these two phenomena, fission gas releases are driven by intragranular diffusion and enhanced by pore interconnections. The results obtained for VERDON-1 fall perfectly in line with these observations. Pore interconnections have been observed to begin around 1773 K corresponding to the main burst release observed for Xe and Kr [14]. Even though no general fuel collapse was obtained, a very last puff was recorded at the end of the sequence where complete release was measured.

11 It was shown that actinides can be sub-divided into two categories. The first includes U and Np with a released fraction that can reach 10% and behaviour similar to the low-volatile category; the second (Pu) has a very low released fraction, typically well below 1% and behaving more like non-volatile FP.

In general, volatile FP were released entirely or in great quantities at temperatures of around 2603 K. The nature of the test (fuel type, initial geometry, atmosphere at the end of the test, etc.) essentially affects the release kinetics of these species and has little effect on the released fraction once this temperature level has been attained during the test. I and Cs release rates were somewhat equivalent. All of the iodine was released and practically all of the caesium was released in all the most severe VERCORS tests. In fact, a low but significant amount of caesium remained in the sample for various tests, with released fractions of around 97e98%. Caesium retention in corium was also identified as a result of the TMI2 accident where it was suggested that there were associations with oxides that remain stable at high temperatures: metal oxides of chromium (Cs2Cr2O4) or iron (Cs2Fe2O4), or silicates (Cs2Si4O9) [32]. Apart the complete release of these FP, the impact of a high burn-up on the release kinetics was also highlighted. In order to illustrate this point, Fig. 17 shows a comparison between the release kinetics of caesium during RT1 (considered as the RT reference test) and RT6 (high burn-up test). The 137Cs release kinetics during RT6 were much faster than for RT1, which was conducted in similar atmospheric conditions; for instance, at the end of the “oxidation plateau” (1773 K), the fraction of caesium released was approximately three times higher for RT6 and the corresponding fractional release was greater at every moment during the entire test. A similar increase in the release kinetics was observed for: (1) MOX fuel compared with UO2 fuel (2) “debris bed” configurations compared with rod-like geometry (3) oxidising atmospheres instead of reducing atmospheres. Again the VERDON-1 results prove to be very similar, with little retention of caesium inside the sample (<0.5%) and faster release

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3000 RT1 : UO2 4 cycles

80%

RT6 : UO2 6 cycles

2500

Cs 137 RT6

2000

60% T empérature T (RT1) RT1

40%

1500 Température T (RT6) RT6 Cs 137 RT1

20%

1000

500

0% 16:00

Temperature (°C)

Released fraction (% initial inventory)

100%

16:30

17:00

17:30 18:00 Durée (h: mn)

18:30

19:00

19:30

Time(h) Time

Fig. 17. Release rate of

137

Cs as a function of temperature: Comparison between RT1 (reference test) and RT6 (high burn-up fuel).

kinetics compared with a fuel with a moderate burn-up. For instance, the releases were approximately 60% at the beginning of the 1773 K plateau in VERDON-1, compared with around 20% at the same moment for the VERCORS RT1 test (4 cycles UO2 fuel, Fig. 17). Generally speaking, tellurium and antimony releases were comparable and quasi-total for all of the most severe VERCORS sequences. The main difference lies in the quantities deposited in the hot zones of the experimental loop: much higher for antimony than tellurium. For VERDON-1, the Sb and Te releases were 76% and 96% respectively, including deposited fractions of ~50% and 32% (in the same order) along the furnace column. These results are in good agreement as regards the deposited fraction along the furnace column and also for the total release of Te. On the contrary, the released fraction of Sb was small. In fact, it was remarked that Sb releases were lower for the RT grid tests (80e95%) compared with the VERCORS 3, 4 and 5 tests (97e100%). This behaviour has been attributed to antimony retention in the corium formed after fuel melting. Since VERDON-1 was characterised by a high temperature without fuel collapse, greater releases should be expected. However, this point may be also linked to the low detection limit of gamma spectrometry for these two FP. If this is the case, the released fractions obtained at this stage would correspond to the minimum releases. In other words, the “real” releases would be higher. Based on this assumption, if the detection limit calculated during the data treatment of the crucible zone is considered as a real FP detection limit for Sb (125 and 127 isotopes), an amount of 5e10% in the crucible should be deduced from this approach, which would correspond to a global release (deduced from APS/AVS by the classical differential method) of 90e95%. The different release kinetics observed for fission gases and volatile FP are consistent with previous studies [33]. This can be explained by the speciation mechanisms which come into play in the case of volatile FP: the different chemical reactions between them and the fuel or other FP

can thus delay or accelerate their release. The behaviour of semi-volatile FP is characterised by releases that can be very high, in some cases as much as those of volatile FP, i.e. near total release, but with high sensitivity to the oxygen potential, and giving rise to significant deposits on the furnace column located above the fuel sample. Mo releases are enhanced in oxidising conditions, while Ba releases are enhanced in reducing conditions. Moreover, the release kinetics seem to be faster for high burn-up fuel. In fact, the results obtained for the VERCORS RT6 show a significant increase in the fractional release compared with VERCORS 4 and 5 (Fig. 18); for instance, at T ¼ 2273 K, this was 40% and 100% for Ba and Mo respectively for the VERCORS RT6 instead of 0% (0%) and 10% (70e80%) for the VERCORS 4 (and 5). The VERDON-1 results confirm these observations with a very fast and high Mo release rate in the first part of the test (oxidising H2O/H2 atmosphere conditions). At the same time, no Ba releases were measured. However, during the second part of the test (reducing conditions), the release of Mo stopped and the Ba releases accelerated quickly. After the sequence, no drastic differences were measured but there was a slightly higher Ba release compared with Mo (~65% and ~70% for Mo and Ba respectively) including deposits along the furnace column of ~19% (Mo) and 24% (Ba) of the initial inventory, which correspond to approximately 30e35% of the total release. Low-volatile FP tend to have low, yet significant, released fractions of around 3%e10% on average, but these values can reach 20e40% in the case of some FP under specific conditions, e.g. oxygen potential or high burn-up. In addition, the FP in this category are essentially deposited in the high temperature section of the test loop, i.e. close to the fuel. At this stage of the data analysis, the VERDON-1 results are also very consistent with these observations since no significant releases were measured by the fuel sight gamma station which is known to have a low sensitivity level which

Y. Pontillon et al. / Journal of Nuclear Materials 495 (2017) 363e384

Fig. 18. Release rate of

99

Mo

and 140

383

Ba as a function of temperature: Comparison between RT6 and VERCORS 4 and 5.

makes it possible to accurately monitor releases exceeding 10% [5] of the initial inventory. Zr and Nd did not produce any detectable releases during VERDON-1, as was the case for all the VERCORS tests. 6. Conclusion This paper discusses the VERDON-1 test. The main issue addressed by this first test concerns the behaviour of high burn-up UO2 fuel and the corresponding fission product releases under reducing conditions at very high temperature (up to 2873 K). Moreover, the first part of the test (i.e. up to the end of the oxidation plateau at 1773 K) was performed under the same atmospheric conditions as the VERCORS RT6 test, which was conducted with a very similar UO2 high burn-up fuel in order to check continuity between VERCORS and the future VERDON databases. This experiment was performed in the new VERDON laboratory at the CEA Cadarache centre. During this VERDON-1 test, the good performance of the VERDON loop was clearly demonstrated in terms of leaktightness, thermal-hydraulics, and hafnia ceramic behaviour, as was the effectiveness of the gamma scanning and sighting device. Consequently, it can be asserted that the VERDON facility is technologyapproved. The results, in terms of FP and fuel behaviour, presented in the

previous part have been discussed according to three main axes: (1) melting temperature, (2) comparison with VERCORS RT6 test and finally (3) general FP volatility. Generally speaking, the VERDON-1 test did not result in the global and/or clear loss of the fuel sample's integrity at the end of the test (i.e. up to a temperature of 2883 K). This behaviour must be connected to the final atmosphere of the test (i.e. reducing conditions). Comparison with the VERCORS RT6 has been possible and conclusive. We were able to identify similar FP release kinetics at 1773 K, a high burn-up effect on the release kinetics, and an atmospheric effect on Mo and Ba releases. The “release configuration” of the VERDON loop is thus now considered qualified and VERDON programme ensures continuity with the VERCORS experiments. According to the released fractions measured by the online gamma station and thanks to the information obtained through pre- and post-test gamma scanning, the FP general classification, in relation to their released fractions and specific behaviour, remains as follows: (1) volatile FP (fission gases, iodine, caesium, tellurium, antimony) with an almost total release; (2) semi-volatile FP (molybdenum and barium), with high sensitivity to oxidising-reducing conditions and significant released fractions; and (3) FP that are low- or non-volatile (ruthenium, europium, niobium, cerium, zirconium, neodymium). Two other articles will detail (1) the fuel behaviour in terms of

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the fuel microstructure and chemical changes and (2) the FP speciation during the VERDON-1 test. This series will provide experimental data on the FP and fuel behaviour in a hypothetical severe accident scenario involving significant H2 production. Acknowledgements This work was carried out within the framework of the International Source Term Programme (ISTP) launched by the CEA, EDF and IRSN. The authors would like to thank the European Commission (grant no. 3428-88-07-TP-ISP-F), Tractebel Engineering (GDF SUEZ, Belgium), the Paul Scherrer Institute (Switzerland), the Canadian Nuclear Laboratories, Atomic Energy of Canada Limited (Canada), and the US Nuclear Regulatory Commission (United States) for their support. References [1] J.P. Leveque, B. Andre, G. Ducros, G. Le Marois, G. Lhiaubet, The HEVA experimental program, Nucl. Technol. 108 (1) (1994) 33e44. [2] Y. Pontillon, G. Ducros, P.P. Malgouyres, Behaviour of fission products under severe PWR accident conditions VERCORS experimental programmedPart 1: general description of the programme, Nucl. Eng. Des. 240 (7) (2010) 1843e1852. [3] Y. Pontillon, G. Ducros, Behaviour of fission products under severe PWR accident conditions: the VERCORS experimental programmedPart 2: release and transport of fission gases and volatile fission products, Nucl. Eng. Des. 240 (7) (2010) 1853e1866. [4] Y. Pontillon, G. Ducros, Behaviour of fission products under severe PWR accident conditions. The VERCORS experimental programmedPart 3: release of low-volatile fission products and actinides, Nucl. Eng. Des. 240 (7) (2010) 1867e1881. [5] G. Ducros, P.P. Malgouyres, M. Kissane, D. Boulaud, M. Durin, Fission product release under severe accidental conditions: general presentation of the program and synthesis of VERCORS 1e6 results, Nucl. Eng. Des. 208 (2) (2001) 191e203. [6] M. Schwarz, G. Hache, P. von der Hardt, PHEBUS FP: a severe accident research programme for current and advanced light water reactors, Nucl. Eng. Des. 187 (1) (1999) 47e69. [7] R.A. Lorenz, M.F. Osborne, A Summary of ORNL Fission Product Release Tests with Recommended Release Rates and Diffusion Coefficients, Nuclear Regulatory Commission, Oak Ridge National Lab, 1995. NUREG/CRe6261. [8] T. Kudo, T. Fuketa, A. Hidaka, VEGA: an experimental study of radionuclides release from fuel under severe accident conditions, in: Proceedings of 2005 Water Reactor Fuel Performance Meeting, Kyoto, Japan, 2005. [9] Z. Lui, D.S. Cox, R.S. Dickson, P. Elder, A Summary of CRL Fission Product Release Measurements from UO2 Samples during Post-irradiation Annealing (1983-1992), 1994. COG-92e377. [10] M.S. Veshchunov, V.D. Ozrin, V.E. Shestak, V.I. Tarasov, R. Dubourg, G. Nicaise, Development of the mechanistic code MFPR for modelling fission product release from irradiated UO2 fuel, Nucl. Eng. Des. 236 (2) (2006) 179e200. [11] G. Brillant, C. Marchetto, W. Plumecocq, Ruthenium release from fuel in accident conditions, Radiochim. Acta 98 (2010) 267e275. [12] E. Beuzet, J.S. Lamy, H. Perron, E. Simoni, G. Ducros, Ruthenium release modelling in air and steam atmospheres under severe accident conditions using the MAAP4 code, Nucl. Eng. Des. 246 (2012) 157e162. [13] B. Clement, The phebus fission product and source term international programmes, in: Proceedings of the Annual Meeting on Nuclear Technology Jahrestagung Kerntechnik - Tagungsbericht Proceedings, 2006.

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