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Fission product release under severe accidental conditions: general presentation of the program and synthesis of VERCORS 1–6 results
Nuclear Engineering and Design 208 (2001) 191– 203 www.elsevier.com/locate/nucengdes
Fission product release under severe accidental conditions: general presentation of the program and synthesis of VERCORS 1–6 results G. Ducros a,*, P.P. Malgouyres a, M. Kissane b, D. Boulaud c, M. Durin d a
Commissariat a` l’Energie Atomique, CEA/DRN/DEC/SECI, Centre CEA de Grenoble, 17 a6enue des Martyrs, 38054, Grenoble Ce´dex, 9, France b Institut de Protection et de Suˆrete´ Nucle´aire, DRS/SEMAR, Centre CEA de Cadarache, Baˆtiment 702, BP 01, 13108, Saint Paul les Durance, France c Institut de Protection et de Suˆrete´ Nucle´aire, DPEA/SERAC, Centre CEA de Saclay, Baˆtiment 389, 91191, Gif sur Y6ette, France d Institut de Protection et de Suˆrete´ Nucle´aire, DPEA/SEAC, Centre CEA de Fontenay aux Roses, BP 6, 92260, Fontenay aux Roses, France
Abstract The French Nuclear Protection and Safety Institute (IPSN) launched the HEVA-VERCORS program in 1983, in collaboration with Electricite´ de France (EDF). This program is devoted to the source term of fission products (FP) released from PWR fuel samples during a sequence representative of a severe accident. The analytical experiments are conducted in a shielded hot cell of the LAMA facility of the Grenoble center of CEA (Commissariat a` l’Energie Atomique); as simplified tests addressing a limited number of phenomena, they give results complementary to those of the more global in-pile PHEBUS experiments. Six VERCORS tests have been conducted from 1989– 1994 with higher fuel temperatures (up to 2600 K) compared with the earlier HEVA tests in order, in particular, to quantify better the release of lower volatile FPs. This paper gives an overview of the experimental facility, a synthesis of FP release from these tests and exhibits, as an example, some specific results of the VERCORS 6 test, performed with high burn-up fuel (60 GWd tU − 1). The on-going VERCORS HT– RT program, designed to reach fuel liquefaction temperatures, is described before conclusions are drawn. © 2001 Elsevier Science B.V. All rights reserved.
1. Introduction Due to the potentially severe consequences of a nuclear accident in terms of radiobiological effects, the international safety authorities initiated * Corresponding author. Tel.: + 33-4-42254572; fax: +334-42254775. E-mail address: [email protected] (G. Ducros).
several experimental programs after the TMI-2 accident in order to improve the understanding and the mitigation of these situations. In France, the Nuclear Protection and Safety Institute (IPSN) launched the HEVA-VERCORS program in 1983, in collaboration with Electricite´ de France (EDF). This program is devoted to the source term of fission products (FP) released from PWR fuel samples during a sequence representa-
0029-5493/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 9 - 5 4 9 3 ( 0 1 ) 0 0 3 7 6 - 4
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tive of a severe accident. The analytical experiments are conducted in a shielded hot cell of the LAMA facility of the Grenoble center of CEA (Commissariat a` l’Energie Atomique), as simplified tests addressing a limited number of phenomena, they give results complementary to those of the more global in-pile PHEBUS experiments. Similar analytical programs have been led in other countries: the HI/VI program in the USA was performed from 1981– 1993 and a recent synthesis report was published (Lorenz and Osborne, 1995), the CRL program is still going on in Canada, with several tests conducted in air conditions (Iglesias et al., 1989; Cox et al., 1991), and the recent VEGA program is beginning in Japan with the objective to reach the fuel melting temperature (Hidaka, 1999). Six VERCORS tests have been conducted from 1989–1994 with higher fuel temperatures (up to 2600 K) compared with the earlier HEVA tests (Leveque et al., 1994) in order, in particular, to quantify better the release of lower volatile FPs. This paper gives an overview of the experimental facility, a synthesis of FP release from these tests and exhibits, as an example, some specific results of the VERCORS 6 test, performed with high burn-up fuel (60 GWd tU − 1). The on-going VERCORS HT – RT program, designed to reach fuel liquefaction temperatures, is described before conclusions are drawn.
2. Experimental apparatus (Ducros et al., 1995a; Tourasse et al., 1996) Conducted with irradiated PWR fuel samples in a shielded hot cell, the tests are aimed at characterizing: the release kinetics and the total release of FPs, actinides and structural materials as a function of fuel temperature and oxidizing/reducing conditions of the environment, the aerosol source as a function of temperature, the chemical behavior of the FPs in the gas phase.
2.1. Fuel sample The test fuel sample is a fuel rod section taken from a nuclear power reactor operated by EDF, it is composed of three irradiated pellets in their original cladding. Two half-pellets of depleted uranium oxide are placed at either end of the sample and held in place by crimping the cladding (Fig. 1). Thus, the cladding is not fully sealed. The fuel sample is re-irradiated at low linear power (B 20 W cm − 1) in the SILOE experimental reactor for around 7 days in order to recreate the short half-life FPs without inducing any inpile release. These short half-life FPs, important for their radiobiological effects, include volatile FPs (iodine and tellurium), gas (xenon) and less volatile FPs (molybdenum, barium, ruthenium, cerium, lanthanum, zirconium…). Since the experimental sequence is performed less than 40 h after the end of the re-irradiation, direct measurement of 99Mo, 132Te, 133I, 135Xe, and sometimes 91Sr is possible, using gamma spectrometry.
2.2. Hot cell lay out The experimental apparatus is shown schematically in Fig. 2. Along the path of gas flow, the following items are found: The main steam and hydrogen injection system. The system supplying helium gas, which is used to protect the graphite or tungsten susceptor from oxidization by steam. A superheater to heat the fluid flow up to 1100 K. The induction furnace itself to heat the fuel up to 2600 K. It comprises, from the inside to the outside, two concentric channels dynamically sealed by a stack of dense zirconia slee6es (the
Fig. 1. VERCORS fuel sample.
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193
Fig. 2. VERCORS apparatus in hot cell.
internal channel containing the sample recei6es the steam and hydrogen flow, the external channel containing the susceptor (graphite or tungsten) is protected by the helium flow with a slightly higher pressure than the internal channel), a double-layer heat insulator (dense zirconia and alumina), a quartz tube, constituting the furnace chamber, and the furnace coils. A junction zone, with a zirconia tube internal channel, linking the furnace to the impactor. A modified Andersen cascade impactor, with five stages, two granular beds and a back-up filter, used to trap the aerosols according to their size, is located in a resistive furnace with an adjustable temperature range from 500 to 1000 K. A filter, heated at 400 K, which traps nongaseous forms of iodine at this temperature, in place from the VERCORS 4 test onwards. A condenser and two dryers (silica gel and molecular sieve) for recovering the steam. A gas capacity to act as a buffer volume for on-line gas gamma spectrometry measurements, in place from the VERCORS 2 test onwards.
A cold trap (charcoal adsorber cooled by liquid nitrogen) to collect noble gases. The furnace provides a relatively flat temperature profile along the sample (B50 K of gradient), thus insuring similar FP release for the three irradiated pellets in the fuel sample.
2.3. On-line instrumentation and post-test analyses In addition to the conventional instrumentation (flow meters, pressure and temperature sensors), specialized on-line equipment is used: An optical pyrometer, calibrated between 1300 and 3300 K, for measuring the temperature of the crucible base, which was used from test VERCORS 3 onwards. An on-line gas chromatography system located between the two dryers to measure the hydrogen emission kinetics during the cladding oxidization phase. This device is also able to quantify extraneous CO, resulting from partial graphite oxidization, when used as the susceptor material. This was in place from test VERCORS 2 onwards.
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Three complementary gamma spectrometers measure on-line FP release kinetics: one detector is focused on the fuel rod. This unit records the FPs leaving the fuel. It has a low detectability level, around 10% of initial inventory of each FP, given the differential aspect of the release rate measurement at this location and the gamma self-absorption changes during fuel degradation at high temperature. Nevertheless, it is very useful because it is the sole unit able to measure the release of each FP, one detector is focused on the top of the impactor. The advantage of this measurement lies in a high detectability level due to direct measurement of the released fraction, often less than 1% of FP initial inventory. On the other hand, only the FPs that deposit here can be detected, generally the most volatile ones, one detector dedicated to the measurement of noble gases, typically the isotopes of xenon (133Xe, 133mXe, 135Xe, created in the SILOE reactor) and krypton (85Kr created in EDF nuclear power plant). This unit provides a very high sensitivity and excellent measurement dynamics (from 10 − 9 to 10 − 1 of initial inventory per min). It was used from test VERCORS 2 onwards. These three gamma spectrometry units are composed of a portable liquid nitrogen-cooled Ge (High Purity) detector and an electronic device for signal shaping and storage, fitted with a rack unit for correcting pile-up counting losses in order to obtain spectra at a high rate (up to 1 spectrum per min). Compensation better than 5% is guaranteed up to 150 000 pulses per s, the limit of the counting rate defined for the tests. After the test, the fuel is embedded in situ in an epoxy resin and X-rayed. A longitudinal gammascan of the fuel is conducted to measure the final FP inventory in order to calculate the quantitative fractions of FPs emitted by the fuel during the test. All the components of the loop (impactor stages, filters, condenser, dryers, etc.) are then gamma-scanned to measure and locate the FP released during the test and to draw up a mass balance of these FPs. For some tests, a non-de-
structive transversal gamma-scan is carried out for several angles of incidence to determine the spatial location of the FPs remaining inside the fuel and analyze possible interactions of these FPs with cladding components (for instance tellurium or barium trapped in the cladding, masking their emission) or their location inside the corium in case of fuel melting (Ducros et al., 1994). A ceramographic examination is carried out on each pellet of the fuel rod to analyze the changes in the microstructure of the cladding and the fuel. Physico-chemical analyses are then carried out on samples of the loop after dismantling, especially on the impactor plates and on the zirconia sleeves linking the furnace to the impactor. The basic method used is scanning electron microscopy combined with analysis of X-ray emission (SEM/EDS), performed in the LAMA laboratory and in collaboration with the AEAWinfrith laboratories. Some XPS and XRD analyses have been conducted on selected samples.
3. FP release synthesis
3.1. Test matrix parameters The parameters that can be changed are the temperature plateau, the temperature ramp and the burn-up of the fuel sample, the temperature of the impactor, the fluid composition and flow rate (steam and/or hydrogen). Notice that the temperature of the impactor was not modified for the six VERCORS tests, this parameter ha6ing been studied in earlier HEVA tests. Most of these tests have been preceded by an oxidizing plateau with mixed steam and hydrogen flow at a temperature around 1600 K in order to oxidize fully the cladding before the last heating ramp to the final high temperature plateau. The test matrix (Table 1) shows how the program has been implemented. VERCORS 1 and 2 were performed in mixed steam and hydrogen flow up to 2150 K: with low flow injection for VERCORS 1 in order to study the effect of high FP concentration during the aerosol transport phase,
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with higher flow injection for VERCORS 2 and with four intermediate plateaus, between 1070 and 1770 K, in order to quantify fission gas and volatile FP releases in conditions similar to a LOCA.
195
The next four tests were all performed up to higher temperature, around 2600 K, just below fuel collapse, except VERCORS 6, where the high fuel burn-up sample led to liquefaction at this temperature level. The conditions were:
Table 1 VERCORS test matrix and total FP released fraction Test
VERCORS 1
VERCORS 2a
VERCORS 3
VERCORS 4b
VERCORS 5c
VERCORS 6
Date of test Fuel PWR irradiation Fuel burn-up (GWd tU−1) Re-irradiation
11-1989
06-1990
04-1992
06-1993
11-1993
09-1994
Fessenheim 42.9
Bugey 38.3
Bugey 38.3
Bugey 38.3
Bugey 38.3
Gravelines 60
Siloe
Siloe
Siloe
Siloe
Siloe
Siloe
2570
2570
2570
Mixed H2O+H2
Hydrogen
Steam
Test conditions Maximum fuel 2130 2150 temperature (K) Atmosphere (end Mixed H2O+H2 Mixed H2O+H2 of test) Last plateau 17 13 duration (min) Steam flow rate 0.15 1.5 (g min−1) Hydrogen flow 0.003 0.027 rate (g min−1) FP released fraction (%) Sr Y Zr Nb Mo Ru Rh Sb 2 Te 4 I 30 Xe 33 Cs 42 Ba 4 La Ce Eu Np 0.006 Ud Pud a
15
30
30
2620
Mixed H2O+H2 30
1.5
1.5-0
1.5
1.5
0.03
0.012
0
0.03
B6
B6
B6
B3
B4
B4 0.3 79 0.6
17
15
7 18 23 23 30 4
0.016
42 0.36 0.52 69 76 70 77 70 13 B4 B6 0.4
47 7 45 97 100 87 86 93 80 B3 3 B5 6 −2 −0.2
92 6 20 98 \98 93 87 93 55 B3 B3 B3 B4 −2 −0.2
96 97 97 −100 97 29 B3 0.2 B4 0.3
Test with intermediate temperature plateaus at 1070, 1170, 1470, 1770 K for 32, 12, 37 and 30 min. Test under hydrogen, but with oxidizing intermediate temperature plateau under mixed H2O+H2 at 1670 K for 60 min. c Test under pure steam, but with intermediate temperature plateau at 1070, 1270, 1570 K for 30, 30 and 70 min. d Approximate values from ICPOES measurement of aerosols recovered on impactor plates, correlated with 137 Cs measurement. b
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a mixed steam and hydrogen atmosphere for VERCORS 3, pure hydrogen during the last high temperature plateau for VERCORS 4, but after a cladding pre-oxidizing phase, pure steam during the last high temperature plateau for VERCORS 5, but after intermediate plateaus between 1070 and 1770 K, as for VERCORS 2, in order to confirm volatile FP release rates in LOCA conditions, a mixed steam and hydrogen atmosphere for VERCORS 6, like VERCORS 3, but with a 60 GWd tU − 1 fuel sample. The high temperature plateau was maintained 30 min for the last three tests and only 15 min for VERCORS 3 because of a blockage of the loop on the last stage of the impactor.
3.2. FP release results Total FP release of each VERCORS test are summarized in Table 1. Concerning VERCORS 1 and VERCORS 2, performed at 2150 K, approximately the same level as the earlier HEVA tests, similar FP kinetics were measured for fission gases, iodine and cesium; the total released fraction reached 20– 40% for these elements. For VERCORS 1, this fraction is a little higher, due to a prolonged plateau at the final high temperature (17 instead of 13 min) and the use of fuel with a slightly higher burn-up (43 instead of 38 GWd tU − 1). These elements aside, molybdenum, antimony, tellurium (and some trace of barium) had measurable release fractions, which are higher for VERCORS 2, due to more oxidizing conditions for molybdenum and to intermediate plateaus, allowing full oxidation of the cladding, for antimony and tellurium. Due to the more severe conditions, the four complementary VERCORS 3 – 6 tests have led to useful extension of the FP data base. According to their releases at 2600 K, FPs could be classified into four categories (Ducros et al., 1995b): Usual volatile FPs, iodine and cesium and, in addition, antimony and tellurium, with nearly complete release at this temperature level. A delay for the release of tellurium and antimony
have been noticed and identified by trapping in the unoxidized cladding (measured by gamma emission tomography for tellurium), but the release of these two elements reaches rapidly the level of iodine and cesium and seems even eventually to overtake them. Semi-volatile FPs, composed of molybdenum, rhodium and barium, with significant release, about half of the volatile FP release, but with low volatility chemical forms deposited close to the fuel, and with high sensitivity to the oxidizing or reducing conditions; for instance the release of molybdenum is increased in oxidizing conditions due to the formation of volatile oxides MoO3 (92% released in VERCORS 5, instead of 47% in VERCORS 4). On the other hand, the release of barium and rhodium is increased in reducing conditions (respectively, 45 and 80% of rhodium and barium released in VERCORS 4, as opposed to 20 and 55% in VERCORS 5). Low volatility elements, composed of ruthenium, cerium, neptunium and probably strontium and europium, with a low but measurable release, between 3 and 10%, exclusively deposited in the high temperature section of the loop, very close to the fuel. Reducing conditions (VERCORS 4) seem to increase the release of neptunium and cerium. No effect of the atmosphere was noticed for ruthenium in these conditions, an element which is known to have a high release, similar to volatile FP release, in high oxidizing conditions, for instance in air (Cox et al., 1991; Hidaka, 1999). Non-volatile FPs, composed of zirconium, niobium, lanthanum and neodymium, with no measurable release in this temperature range under these conditions. Among these, in more severe conditions up to fuel melting temperature as recently performed in VERCORS HT and RT tests, niobium and lanthanum have been quantified as having a significant release (Ducros et al., 1999). Other elements, like uranium, not measurable by gamma spectrometry, were detected on impactor plates by SEM/EDS, but could not be quantified precisely in terms of released fraction.
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197
Fig. 3. VERCORS 6 fuel collapse.
4. VERCORS 6 results The VERCORS 6 test, the last of the series, was performed with high burn-up fuel of 60 GWd tU − 1; this section gives some results of this test concerning fuel degradation and FP release.
4.1. Fuel degradation Compared with the three earlier tests performed at the same temperature of 2600 K, VERCORS 6 was the first test leading to early fuel collapse during the high temperature plateau. Fig. 3 shows, versus time, the fuel temperature and the total gamma activity signal, measured by the online spectrometer on the fuel. After a continuous decrease during the heat up phase and the beginning of the high temperature plateau, corresponding to volatile FP release, a sudden and large signal increase is noticed after 20 min at 2600 K; it corresponds to the collapse of the fuel and its bulk relocation at the bottom of the crucible (the next decrease of the signal at 18:40 h is the result of a 6oluntary collimator change in order to reduce the count rate which saturated the electronic acquisition equipment).
Fig. 4 compares the X-ray radiographs performed, respectively, on the fuel samples of VERCORS 3, 4, 5 and 6 tests. In the three first tests, the fuel sample has maintained its integrity, even if many cracks could have been observed, especially on the VERCORS 4 sample. On the other hand, the VERCORS 6 fuel sample is severely damaged. The upper part of the radiograph is composed of the upper unirradiated half-pellet, which seems to have maintain its original dimensions, and two irradiated pellets (the upper and central ones), which have become thinner and have partially melted. The lower part of the radiograph is composed of the crucible retaining the remaining part of the fuel sample: the lower unirradiated half-pellet, which seems also to have maintained its integrity, and the lower irradiated pellet, which is totally melted and has interacted with the crucible. Ceramographic examinations of these samples and complementary SEM/EDS analysis have confirmed the significant melting of the original irradiated fuel pellets and the strong interaction of the corium with the crucible, the zirconia sleeves surrounding the crucible, and even the tungsten susceptor.
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4.2. FP beha6ior The volatile FP release, almost complete for the four tests performed at 2600 K, is not particularly affected by the effect of the burn-up, except possibly faster kinetics, an effect which will be soon confirmed. Concerning semi-volatile and lowvolatile FPs, the more severe degradation of high burn-up fuel does not increase their release; the liquid corium phase formation seems even to retain a fraction of them in comparison with solid fuel (respectively, 79, 29 and 0.6% of molybdenum, barium and ruthenium released in VERCORS 6, as opposed to 92, 55 and 6% in VERCORS 5 performed, as a matter of fact, in slightly more oxidizing conditions). Fig. 5 gives the distribution of the main semi and low volatile FPs, compared with nonvolatiles, inside the two samples recovered after the test and along the upper part of the zirconia sleeves, where the deposit of these semi and low-
volatile FPs is significant (22 and 11% of the initial inventory for molybdenum and barium, roughly one-third of the total released fraction). Gamma emission tomography was performed on the sample containing the crucible and the remains of the melted lower pellet. Fig. 6 compares 95Zr and 103Ru distribution, where it can be noticed that 103Ru, possibly associated with tungsten within a metallic phase, is not located within the fuel and oxide phase, represented by 95Zr; it appears to surround the fuel, confirming its specific behavior already evidenced by gamma emission tomography of the PHEBUS FPT1 bundle (Cornu and Giacalone, 1999).
5. VERCORS HT –RT program Since 1996 a new VERCORS HT and RT program has been launched to improve the data base of fission product and actinide releases dur-
Fig. 4. X-ray radiographs of VERCORS fuel samples after the tests.
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199
Fig. 5. FP axial distribution after VERCORS 6 test (crucible and zirconia sleeve).
ing the later phases of an accident, in particular up to fuel melting (Malgouyres et al., 1998). Compared with the VERCORS facility, the new hot cell apparatus VERCORS HT (HT for High Temperature) is rather different regarding the furnace, made of thoria, and the instrumentation (Fig. 7). In particular, on-line instrumentation has been improved with: A thermal gradient tube (TGT) 0.7 m long, just downstream of the fuel furnace, devoted to the study of vapor-phase and aerosol deposition; the axial temperature profile decreases from 1300–300 K. The location of the impactor on a branch circuit in order to operate in a more suitable mode during a predefined period of the experiment instead of the full duration. When the impactor is not open, the aerosols are collected in a high capacity filter composed of granular beds. A specific iodine filter, separating the chemical species, in particular its molecular form.
Four gamma spectrometers, instead of three, to give information on five locations: the fuel, the TGT, the impactor, the filter and the gas capacity together. The RT version of the loop (RT for Release of Transuranics) is more compact. The furnace is similar to the HT furnace, but its handling is easier; thus it enables the frequency of the tests to be increased. In this simplified configuration, all FP and transuranic elements are trapped as near as possible from their emission point in a total filter. The release quantification of actinides and pure b FPs is then obtained by post-test chemical analyses, such as ICP-MS, alpha spectrometry, etc. Table 2 gives the test matrix parameters as defined today. HT1 was performed in June 1996 in a reducing atmosphere. The fuel was heated up to 2900 K and collapsed early during the last heating phase at around 2600 K. The final report of this test has been recently released. RT1 and RT2, performed in 1998 without reirradiation of the fuel, were aimed in part at
200
VERCORS tests
HT 1
RT 1
RT 2
RT 5
RT 4
Date of test Fuel
June 1996 UO2
March 1998 UO2
April 1998 MOX
December 1998 UO2
Burnup (GWd tU−1) Re-irradiation Maximum fuel temperature (K) H2 (mg s−1) H2O (mg s−1) He (mg s−1) Main objective
47 SILOE 2900
0.2 0 8 H2 atm., high temperature
47
41
No 2570
No 2440
0.45 25 0 RT reference test
0.45 25 0 MOX fuel
RT 7
HT 3
Beginning 2000 MOX
End 2000 UO2
60
June 1999 November 1999 UO2/ZrO2 debris UO2 debris bed bed 3 cycles 3 cycles
3 cycles
4 cycles
OSIRIS Fuel collapse
No Fuel collapse
OSIRIS Fuel melting
OSIRIS Fuel melting
OSIRIS Fuel melting
1.25
Reducing conditions
Reducing conditions
MOX fuel
Boric acid and SIC injection
0.45 25 0 High burn up
0.4 14.6 0 Phebus FPT4 support
RT 3
1.25 0 Fuel volatilization
G. Ducros et al. / Nuclear Engineering and Design 208 (2001) 191–203
Table 2 VERCORS HT–RT test matrix parameters
G. Ducros et al. / Nuclear Engineering and Design 208 (2001) 191–203
measuring, with high precision the temperature of fuel collapse; RT1 was performed with UO2 fuel while RT2 was performed with MOX fuel (Malgouyres et al., 1999). RT5 was performed in 1998 with re-irradiated high burn-up fuel in order to confirm aspects of the VERCORS 6 test, notably the early fuel collapse. RT4 was performed and RT3 will soon be performed with a configuration of a debris bed to study the late phase of an accident, notably fuel volatilization, complementary to the PHEBUS FPT4 test. RT4 used unre-irradiated fuel (UO2 and ZrO2 fragments) and RT3 will use re-irradiated fuel fragments (only UO2 to achieve higher temperature and quantify, in particular, the fuel volatilization rate). RT7 will be the second test with MOX fuel and the first using a re-irradiated MOX sample. Compared with RT2, it will be performed in reducing conditions, instead of the mixed steam/hydrogen of RT2. Finally, HT3 will be the second HT test. Compared with HT1, it will be performed under reducing conditions and with additional injection of boric acid and SIC (silver, indium and cadmium) within the fluid flow.
Fig. 6.
95
Zr and
103
201
6. Conclusion The VERCORS program represents a significant step forward in knowledge and accuracy of in-vessel source term data. Following the HEVA program, which contributed mainly towards data on volatile FPs, the VERCORS tests have extended the data base up to 2600 K: confirming the nearly total release of volatile species Cs, I, Te and Sb, measuring the release of low volatile species and classifying them in three categories: ‘semivolatile’ (Mo, Rh, Ba), ‘low-volatile’ (Ru, Ce, Np, Sr, Eu) and ‘non-volatile’ (Zr, Nb, La, Nd). The VERCORS 6 test, performed with high burn-up fuel, leads to early fuel collapse and partial liquid corium formation. Nevertheless, FP release did not increase significantly compared with solid fuel; the liquid phase seems even to retain a fraction of some semi and low-volatile FPs. The on-going experiments VERCORS HT and RT focus on the release of low volatility FPs and actinides, and will confirm or extend somewhat the data base at higher temperatures including the consideration of a wider range of parameters such
Ru gamma emission tomography after VERCORS 6 test.
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Fig. 7. VERCORS HT apparatus in hot cell.
as the nature of the fuel (UO2 and MOX), its morphology (intact pellets or debris fragments), its burn-up, the impact of control materials (SIC and boric acid) and the conditions of the accidental sequence (oxidizing or reducing).
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diation Examination Techniques for Water Reactor Fuel, Cadarache. Ducros, G., Andre, B., Tourasse, M., Maro, D., 1995. The fission products and actinide release at high temperature in PWR fuel rod: the VERCORS safety program. In: Proceedings of the IAEA TC Meeting on Behaviour of LWR Core Materials Under Accident Conditions, Dimitrovgrad. Ducros, G., Andre, B., Tourasse, M., Maro, D., 1995. The VERCORS safety program, source term analytical study with special emphasis on the release of non volatile fission products and transuranic elements. In: Proceedings of the CSARP meeting, Bethesda. Ducros, G., Ferroud-Plattet, M.P., Baichi, M., Malgouyres, P.P., Poletiko, C., Manenc, H., et al. 1999. Fission product release on VERCORS HT1 experiment. In: CSARP Meeting, Albuquerque. Hidaka, A., 1999. Current status of VEGA program and a preliminary test with cesium iodide. In: SARJ Meeting, Tokyo. Iglesias F.C., et al., 1989. Ruthenium release kinetics from uranium oxides. In: Proceedings of the ICHMT Seminar on Fission Product Transport Processes in Reactor Accidents, Dubrovnik.
G. Ducros et al. / Nuclear Engineering and Design 208 (2001) 191–203 Leveque, J.P., Andre, B., Ducros, G., Le Marois, G., Lhiaubet, G., Oct. 1994. The HEVA experimental program, Nuclear Technology, Vol. 108. Lorenz, R.A., Osborne, M.F., 1995. A summary of ORNL fission product release tests with recommended release rates and diffusion coefficients, ORNL/TM-12801, Nureg/CR6261, July 1995. Malgouyres, P.P., Ducros, G., Ferroud-Plattet, M.P., Prouve, M., Boulaud, D., 1998. The VERCORS HT facility for studies up to molten fuel conditions. In: Proceedings of the
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European Working Group Hot Laboratories and Remote Handling Meeting, Windscale. Malgouyres, P.P., Ferroud-Plattet, M.P., Ducros, G., Poletiko, C., Tourasse, M., Boulaud, D., 1999. Influence of MOX fuel in fission product release up to meltdown conditions. In: Proceedings of the NURETH-9 ANS Meeting, San Francisco. Tourasse, M., Andre, B., Ducros, G., Maro, D., 1996. Overview of the VERCORS program devoted to safety studies on irradiated PWR fuel, Material Chemistry’96, Tsukuba.
Fission product release under severe accidental conditions: general presentation of the program and synthesis of VERCORS 1–6 results G. Ducros a,*, P.P. Malgouyres a, M. Kissane b, D. Boulaud c, M. Durin d a
Commissariat a` l’Energie Atomique, CEA/DRN/DEC/SECI, Centre CEA de Grenoble, 17 a6enue des Martyrs, 38054, Grenoble Ce´dex, 9, France b Institut de Protection et de Suˆrete´ Nucle´aire, DRS/SEMAR, Centre CEA de Cadarache, Baˆtiment 702, BP 01, 13108, Saint Paul les Durance, France c Institut de Protection et de Suˆrete´ Nucle´aire, DPEA/SERAC, Centre CEA de Saclay, Baˆtiment 389, 91191, Gif sur Y6ette, France d Institut de Protection et de Suˆrete´ Nucle´aire, DPEA/SEAC, Centre CEA de Fontenay aux Roses, BP 6, 92260, Fontenay aux Roses, France
Abstract The French Nuclear Protection and Safety Institute (IPSN) launched the HEVA-VERCORS program in 1983, in collaboration with Electricite´ de France (EDF). This program is devoted to the source term of fission products (FP) released from PWR fuel samples during a sequence representative of a severe accident. The analytical experiments are conducted in a shielded hot cell of the LAMA facility of the Grenoble center of CEA (Commissariat a` l’Energie Atomique); as simplified tests addressing a limited number of phenomena, they give results complementary to those of the more global in-pile PHEBUS experiments. Six VERCORS tests have been conducted from 1989– 1994 with higher fuel temperatures (up to 2600 K) compared with the earlier HEVA tests in order, in particular, to quantify better the release of lower volatile FPs. This paper gives an overview of the experimental facility, a synthesis of FP release from these tests and exhibits, as an example, some specific results of the VERCORS 6 test, performed with high burn-up fuel (60 GWd tU − 1). The on-going VERCORS HT– RT program, designed to reach fuel liquefaction temperatures, is described before conclusions are drawn. © 2001 Elsevier Science B.V. All rights reserved.
1. Introduction Due to the potentially severe consequences of a nuclear accident in terms of radiobiological effects, the international safety authorities initiated * Corresponding author. Tel.: + 33-4-42254572; fax: +334-42254775. E-mail address: [email protected] (G. Ducros).
several experimental programs after the TMI-2 accident in order to improve the understanding and the mitigation of these situations. In France, the Nuclear Protection and Safety Institute (IPSN) launched the HEVA-VERCORS program in 1983, in collaboration with Electricite´ de France (EDF). This program is devoted to the source term of fission products (FP) released from PWR fuel samples during a sequence representa-
0029-5493/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 9 - 5 4 9 3 ( 0 1 ) 0 0 3 7 6 - 4
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tive of a severe accident. The analytical experiments are conducted in a shielded hot cell of the LAMA facility of the Grenoble center of CEA (Commissariat a` l’Energie Atomique), as simplified tests addressing a limited number of phenomena, they give results complementary to those of the more global in-pile PHEBUS experiments. Similar analytical programs have been led in other countries: the HI/VI program in the USA was performed from 1981– 1993 and a recent synthesis report was published (Lorenz and Osborne, 1995), the CRL program is still going on in Canada, with several tests conducted in air conditions (Iglesias et al., 1989; Cox et al., 1991), and the recent VEGA program is beginning in Japan with the objective to reach the fuel melting temperature (Hidaka, 1999). Six VERCORS tests have been conducted from 1989–1994 with higher fuel temperatures (up to 2600 K) compared with the earlier HEVA tests (Leveque et al., 1994) in order, in particular, to quantify better the release of lower volatile FPs. This paper gives an overview of the experimental facility, a synthesis of FP release from these tests and exhibits, as an example, some specific results of the VERCORS 6 test, performed with high burn-up fuel (60 GWd tU − 1). The on-going VERCORS HT – RT program, designed to reach fuel liquefaction temperatures, is described before conclusions are drawn.
2. Experimental apparatus (Ducros et al., 1995a; Tourasse et al., 1996) Conducted with irradiated PWR fuel samples in a shielded hot cell, the tests are aimed at characterizing: the release kinetics and the total release of FPs, actinides and structural materials as a function of fuel temperature and oxidizing/reducing conditions of the environment, the aerosol source as a function of temperature, the chemical behavior of the FPs in the gas phase.
2.1. Fuel sample The test fuel sample is a fuel rod section taken from a nuclear power reactor operated by EDF, it is composed of three irradiated pellets in their original cladding. Two half-pellets of depleted uranium oxide are placed at either end of the sample and held in place by crimping the cladding (Fig. 1). Thus, the cladding is not fully sealed. The fuel sample is re-irradiated at low linear power (B 20 W cm − 1) in the SILOE experimental reactor for around 7 days in order to recreate the short half-life FPs without inducing any inpile release. These short half-life FPs, important for their radiobiological effects, include volatile FPs (iodine and tellurium), gas (xenon) and less volatile FPs (molybdenum, barium, ruthenium, cerium, lanthanum, zirconium…). Since the experimental sequence is performed less than 40 h after the end of the re-irradiation, direct measurement of 99Mo, 132Te, 133I, 135Xe, and sometimes 91Sr is possible, using gamma spectrometry.
2.2. Hot cell lay out The experimental apparatus is shown schematically in Fig. 2. Along the path of gas flow, the following items are found: The main steam and hydrogen injection system. The system supplying helium gas, which is used to protect the graphite or tungsten susceptor from oxidization by steam. A superheater to heat the fluid flow up to 1100 K. The induction furnace itself to heat the fuel up to 2600 K. It comprises, from the inside to the outside, two concentric channels dynamically sealed by a stack of dense zirconia slee6es (the
Fig. 1. VERCORS fuel sample.
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193
Fig. 2. VERCORS apparatus in hot cell.
internal channel containing the sample recei6es the steam and hydrogen flow, the external channel containing the susceptor (graphite or tungsten) is protected by the helium flow with a slightly higher pressure than the internal channel), a double-layer heat insulator (dense zirconia and alumina), a quartz tube, constituting the furnace chamber, and the furnace coils. A junction zone, with a zirconia tube internal channel, linking the furnace to the impactor. A modified Andersen cascade impactor, with five stages, two granular beds and a back-up filter, used to trap the aerosols according to their size, is located in a resistive furnace with an adjustable temperature range from 500 to 1000 K. A filter, heated at 400 K, which traps nongaseous forms of iodine at this temperature, in place from the VERCORS 4 test onwards. A condenser and two dryers (silica gel and molecular sieve) for recovering the steam. A gas capacity to act as a buffer volume for on-line gas gamma spectrometry measurements, in place from the VERCORS 2 test onwards.
A cold trap (charcoal adsorber cooled by liquid nitrogen) to collect noble gases. The furnace provides a relatively flat temperature profile along the sample (B50 K of gradient), thus insuring similar FP release for the three irradiated pellets in the fuel sample.
2.3. On-line instrumentation and post-test analyses In addition to the conventional instrumentation (flow meters, pressure and temperature sensors), specialized on-line equipment is used: An optical pyrometer, calibrated between 1300 and 3300 K, for measuring the temperature of the crucible base, which was used from test VERCORS 3 onwards. An on-line gas chromatography system located between the two dryers to measure the hydrogen emission kinetics during the cladding oxidization phase. This device is also able to quantify extraneous CO, resulting from partial graphite oxidization, when used as the susceptor material. This was in place from test VERCORS 2 onwards.
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Three complementary gamma spectrometers measure on-line FP release kinetics: one detector is focused on the fuel rod. This unit records the FPs leaving the fuel. It has a low detectability level, around 10% of initial inventory of each FP, given the differential aspect of the release rate measurement at this location and the gamma self-absorption changes during fuel degradation at high temperature. Nevertheless, it is very useful because it is the sole unit able to measure the release of each FP, one detector is focused on the top of the impactor. The advantage of this measurement lies in a high detectability level due to direct measurement of the released fraction, often less than 1% of FP initial inventory. On the other hand, only the FPs that deposit here can be detected, generally the most volatile ones, one detector dedicated to the measurement of noble gases, typically the isotopes of xenon (133Xe, 133mXe, 135Xe, created in the SILOE reactor) and krypton (85Kr created in EDF nuclear power plant). This unit provides a very high sensitivity and excellent measurement dynamics (from 10 − 9 to 10 − 1 of initial inventory per min). It was used from test VERCORS 2 onwards. These three gamma spectrometry units are composed of a portable liquid nitrogen-cooled Ge (High Purity) detector and an electronic device for signal shaping and storage, fitted with a rack unit for correcting pile-up counting losses in order to obtain spectra at a high rate (up to 1 spectrum per min). Compensation better than 5% is guaranteed up to 150 000 pulses per s, the limit of the counting rate defined for the tests. After the test, the fuel is embedded in situ in an epoxy resin and X-rayed. A longitudinal gammascan of the fuel is conducted to measure the final FP inventory in order to calculate the quantitative fractions of FPs emitted by the fuel during the test. All the components of the loop (impactor stages, filters, condenser, dryers, etc.) are then gamma-scanned to measure and locate the FP released during the test and to draw up a mass balance of these FPs. For some tests, a non-de-
structive transversal gamma-scan is carried out for several angles of incidence to determine the spatial location of the FPs remaining inside the fuel and analyze possible interactions of these FPs with cladding components (for instance tellurium or barium trapped in the cladding, masking their emission) or their location inside the corium in case of fuel melting (Ducros et al., 1994). A ceramographic examination is carried out on each pellet of the fuel rod to analyze the changes in the microstructure of the cladding and the fuel. Physico-chemical analyses are then carried out on samples of the loop after dismantling, especially on the impactor plates and on the zirconia sleeves linking the furnace to the impactor. The basic method used is scanning electron microscopy combined with analysis of X-ray emission (SEM/EDS), performed in the LAMA laboratory and in collaboration with the AEAWinfrith laboratories. Some XPS and XRD analyses have been conducted on selected samples.
3. FP release synthesis
3.1. Test matrix parameters The parameters that can be changed are the temperature plateau, the temperature ramp and the burn-up of the fuel sample, the temperature of the impactor, the fluid composition and flow rate (steam and/or hydrogen). Notice that the temperature of the impactor was not modified for the six VERCORS tests, this parameter ha6ing been studied in earlier HEVA tests. Most of these tests have been preceded by an oxidizing plateau with mixed steam and hydrogen flow at a temperature around 1600 K in order to oxidize fully the cladding before the last heating ramp to the final high temperature plateau. The test matrix (Table 1) shows how the program has been implemented. VERCORS 1 and 2 were performed in mixed steam and hydrogen flow up to 2150 K: with low flow injection for VERCORS 1 in order to study the effect of high FP concentration during the aerosol transport phase,
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with higher flow injection for VERCORS 2 and with four intermediate plateaus, between 1070 and 1770 K, in order to quantify fission gas and volatile FP releases in conditions similar to a LOCA.
195
The next four tests were all performed up to higher temperature, around 2600 K, just below fuel collapse, except VERCORS 6, where the high fuel burn-up sample led to liquefaction at this temperature level. The conditions were:
Table 1 VERCORS test matrix and total FP released fraction Test
VERCORS 1
VERCORS 2a
VERCORS 3
VERCORS 4b
VERCORS 5c
VERCORS 6
Date of test Fuel PWR irradiation Fuel burn-up (GWd tU−1) Re-irradiation
11-1989
06-1990
04-1992
06-1993
11-1993
09-1994
Fessenheim 42.9
Bugey 38.3
Bugey 38.3
Bugey 38.3
Bugey 38.3
Gravelines 60
Siloe
Siloe
Siloe
Siloe
Siloe
Siloe
2570
2570
2570
Mixed H2O+H2
Hydrogen
Steam
Test conditions Maximum fuel 2130 2150 temperature (K) Atmosphere (end Mixed H2O+H2 Mixed H2O+H2 of test) Last plateau 17 13 duration (min) Steam flow rate 0.15 1.5 (g min−1) Hydrogen flow 0.003 0.027 rate (g min−1) FP released fraction (%) Sr Y Zr Nb Mo Ru Rh Sb 2 Te 4 I 30 Xe 33 Cs 42 Ba 4 La Ce Eu Np 0.006 Ud Pud a
15
30
30
2620
Mixed H2O+H2 30
1.5
1.5-0
1.5
1.5
0.03
0.012
0
0.03
B6
B6
B6
B3
B4
B4 0.3 79 0.6
17
15
7 18 23 23 30 4
0.016
42 0.36 0.52 69 76 70 77 70 13 B4 B6 0.4
47 7 45 97 100 87 86 93 80 B3 3 B5 6 −2 −0.2
92 6 20 98 \98 93 87 93 55 B3 B3 B3 B4 −2 −0.2
96 97 97 −100 97 29 B3 0.2 B4 0.3
Test with intermediate temperature plateaus at 1070, 1170, 1470, 1770 K for 32, 12, 37 and 30 min. Test under hydrogen, but with oxidizing intermediate temperature plateau under mixed H2O+H2 at 1670 K for 60 min. c Test under pure steam, but with intermediate temperature plateau at 1070, 1270, 1570 K for 30, 30 and 70 min. d Approximate values from ICPOES measurement of aerosols recovered on impactor plates, correlated with 137 Cs measurement. b
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a mixed steam and hydrogen atmosphere for VERCORS 3, pure hydrogen during the last high temperature plateau for VERCORS 4, but after a cladding pre-oxidizing phase, pure steam during the last high temperature plateau for VERCORS 5, but after intermediate plateaus between 1070 and 1770 K, as for VERCORS 2, in order to confirm volatile FP release rates in LOCA conditions, a mixed steam and hydrogen atmosphere for VERCORS 6, like VERCORS 3, but with a 60 GWd tU − 1 fuel sample. The high temperature plateau was maintained 30 min for the last three tests and only 15 min for VERCORS 3 because of a blockage of the loop on the last stage of the impactor.
3.2. FP release results Total FP release of each VERCORS test are summarized in Table 1. Concerning VERCORS 1 and VERCORS 2, performed at 2150 K, approximately the same level as the earlier HEVA tests, similar FP kinetics were measured for fission gases, iodine and cesium; the total released fraction reached 20– 40% for these elements. For VERCORS 1, this fraction is a little higher, due to a prolonged plateau at the final high temperature (17 instead of 13 min) and the use of fuel with a slightly higher burn-up (43 instead of 38 GWd tU − 1). These elements aside, molybdenum, antimony, tellurium (and some trace of barium) had measurable release fractions, which are higher for VERCORS 2, due to more oxidizing conditions for molybdenum and to intermediate plateaus, allowing full oxidation of the cladding, for antimony and tellurium. Due to the more severe conditions, the four complementary VERCORS 3 – 6 tests have led to useful extension of the FP data base. According to their releases at 2600 K, FPs could be classified into four categories (Ducros et al., 1995b): Usual volatile FPs, iodine and cesium and, in addition, antimony and tellurium, with nearly complete release at this temperature level. A delay for the release of tellurium and antimony
have been noticed and identified by trapping in the unoxidized cladding (measured by gamma emission tomography for tellurium), but the release of these two elements reaches rapidly the level of iodine and cesium and seems even eventually to overtake them. Semi-volatile FPs, composed of molybdenum, rhodium and barium, with significant release, about half of the volatile FP release, but with low volatility chemical forms deposited close to the fuel, and with high sensitivity to the oxidizing or reducing conditions; for instance the release of molybdenum is increased in oxidizing conditions due to the formation of volatile oxides MoO3 (92% released in VERCORS 5, instead of 47% in VERCORS 4). On the other hand, the release of barium and rhodium is increased in reducing conditions (respectively, 45 and 80% of rhodium and barium released in VERCORS 4, as opposed to 20 and 55% in VERCORS 5). Low volatility elements, composed of ruthenium, cerium, neptunium and probably strontium and europium, with a low but measurable release, between 3 and 10%, exclusively deposited in the high temperature section of the loop, very close to the fuel. Reducing conditions (VERCORS 4) seem to increase the release of neptunium and cerium. No effect of the atmosphere was noticed for ruthenium in these conditions, an element which is known to have a high release, similar to volatile FP release, in high oxidizing conditions, for instance in air (Cox et al., 1991; Hidaka, 1999). Non-volatile FPs, composed of zirconium, niobium, lanthanum and neodymium, with no measurable release in this temperature range under these conditions. Among these, in more severe conditions up to fuel melting temperature as recently performed in VERCORS HT and RT tests, niobium and lanthanum have been quantified as having a significant release (Ducros et al., 1999). Other elements, like uranium, not measurable by gamma spectrometry, were detected on impactor plates by SEM/EDS, but could not be quantified precisely in terms of released fraction.
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197
Fig. 3. VERCORS 6 fuel collapse.
4. VERCORS 6 results The VERCORS 6 test, the last of the series, was performed with high burn-up fuel of 60 GWd tU − 1; this section gives some results of this test concerning fuel degradation and FP release.
4.1. Fuel degradation Compared with the three earlier tests performed at the same temperature of 2600 K, VERCORS 6 was the first test leading to early fuel collapse during the high temperature plateau. Fig. 3 shows, versus time, the fuel temperature and the total gamma activity signal, measured by the online spectrometer on the fuel. After a continuous decrease during the heat up phase and the beginning of the high temperature plateau, corresponding to volatile FP release, a sudden and large signal increase is noticed after 20 min at 2600 K; it corresponds to the collapse of the fuel and its bulk relocation at the bottom of the crucible (the next decrease of the signal at 18:40 h is the result of a 6oluntary collimator change in order to reduce the count rate which saturated the electronic acquisition equipment).
Fig. 4 compares the X-ray radiographs performed, respectively, on the fuel samples of VERCORS 3, 4, 5 and 6 tests. In the three first tests, the fuel sample has maintained its integrity, even if many cracks could have been observed, especially on the VERCORS 4 sample. On the other hand, the VERCORS 6 fuel sample is severely damaged. The upper part of the radiograph is composed of the upper unirradiated half-pellet, which seems to have maintain its original dimensions, and two irradiated pellets (the upper and central ones), which have become thinner and have partially melted. The lower part of the radiograph is composed of the crucible retaining the remaining part of the fuel sample: the lower unirradiated half-pellet, which seems also to have maintained its integrity, and the lower irradiated pellet, which is totally melted and has interacted with the crucible. Ceramographic examinations of these samples and complementary SEM/EDS analysis have confirmed the significant melting of the original irradiated fuel pellets and the strong interaction of the corium with the crucible, the zirconia sleeves surrounding the crucible, and even the tungsten susceptor.
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4.2. FP beha6ior The volatile FP release, almost complete for the four tests performed at 2600 K, is not particularly affected by the effect of the burn-up, except possibly faster kinetics, an effect which will be soon confirmed. Concerning semi-volatile and lowvolatile FPs, the more severe degradation of high burn-up fuel does not increase their release; the liquid corium phase formation seems even to retain a fraction of them in comparison with solid fuel (respectively, 79, 29 and 0.6% of molybdenum, barium and ruthenium released in VERCORS 6, as opposed to 92, 55 and 6% in VERCORS 5 performed, as a matter of fact, in slightly more oxidizing conditions). Fig. 5 gives the distribution of the main semi and low volatile FPs, compared with nonvolatiles, inside the two samples recovered after the test and along the upper part of the zirconia sleeves, where the deposit of these semi and low-
volatile FPs is significant (22 and 11% of the initial inventory for molybdenum and barium, roughly one-third of the total released fraction). Gamma emission tomography was performed on the sample containing the crucible and the remains of the melted lower pellet. Fig. 6 compares 95Zr and 103Ru distribution, where it can be noticed that 103Ru, possibly associated with tungsten within a metallic phase, is not located within the fuel and oxide phase, represented by 95Zr; it appears to surround the fuel, confirming its specific behavior already evidenced by gamma emission tomography of the PHEBUS FPT1 bundle (Cornu and Giacalone, 1999).
5. VERCORS HT –RT program Since 1996 a new VERCORS HT and RT program has been launched to improve the data base of fission product and actinide releases dur-
Fig. 4. X-ray radiographs of VERCORS fuel samples after the tests.
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199
Fig. 5. FP axial distribution after VERCORS 6 test (crucible and zirconia sleeve).
ing the later phases of an accident, in particular up to fuel melting (Malgouyres et al., 1998). Compared with the VERCORS facility, the new hot cell apparatus VERCORS HT (HT for High Temperature) is rather different regarding the furnace, made of thoria, and the instrumentation (Fig. 7). In particular, on-line instrumentation has been improved with: A thermal gradient tube (TGT) 0.7 m long, just downstream of the fuel furnace, devoted to the study of vapor-phase and aerosol deposition; the axial temperature profile decreases from 1300–300 K. The location of the impactor on a branch circuit in order to operate in a more suitable mode during a predefined period of the experiment instead of the full duration. When the impactor is not open, the aerosols are collected in a high capacity filter composed of granular beds. A specific iodine filter, separating the chemical species, in particular its molecular form.
Four gamma spectrometers, instead of three, to give information on five locations: the fuel, the TGT, the impactor, the filter and the gas capacity together. The RT version of the loop (RT for Release of Transuranics) is more compact. The furnace is similar to the HT furnace, but its handling is easier; thus it enables the frequency of the tests to be increased. In this simplified configuration, all FP and transuranic elements are trapped as near as possible from their emission point in a total filter. The release quantification of actinides and pure b FPs is then obtained by post-test chemical analyses, such as ICP-MS, alpha spectrometry, etc. Table 2 gives the test matrix parameters as defined today. HT1 was performed in June 1996 in a reducing atmosphere. The fuel was heated up to 2900 K and collapsed early during the last heating phase at around 2600 K. The final report of this test has been recently released. RT1 and RT2, performed in 1998 without reirradiation of the fuel, were aimed in part at
200
VERCORS tests
HT 1
RT 1
RT 2
RT 5
RT 4
Date of test Fuel
June 1996 UO2
March 1998 UO2
April 1998 MOX
December 1998 UO2
Burnup (GWd tU−1) Re-irradiation Maximum fuel temperature (K) H2 (mg s−1) H2O (mg s−1) He (mg s−1) Main objective
47 SILOE 2900
0.2 0 8 H2 atm., high temperature
47
41
No 2570
No 2440
0.45 25 0 RT reference test
0.45 25 0 MOX fuel
RT 7
HT 3
Beginning 2000 MOX
End 2000 UO2
60
June 1999 November 1999 UO2/ZrO2 debris UO2 debris bed bed 3 cycles 3 cycles
3 cycles
4 cycles
OSIRIS Fuel collapse
No Fuel collapse
OSIRIS Fuel melting
OSIRIS Fuel melting
OSIRIS Fuel melting
1.25
Reducing conditions
Reducing conditions
MOX fuel
Boric acid and SIC injection
0.45 25 0 High burn up
0.4 14.6 0 Phebus FPT4 support
RT 3
1.25 0 Fuel volatilization
G. Ducros et al. / Nuclear Engineering and Design 208 (2001) 191–203
Table 2 VERCORS HT–RT test matrix parameters
G. Ducros et al. / Nuclear Engineering and Design 208 (2001) 191–203
measuring, with high precision the temperature of fuel collapse; RT1 was performed with UO2 fuel while RT2 was performed with MOX fuel (Malgouyres et al., 1999). RT5 was performed in 1998 with re-irradiated high burn-up fuel in order to confirm aspects of the VERCORS 6 test, notably the early fuel collapse. RT4 was performed and RT3 will soon be performed with a configuration of a debris bed to study the late phase of an accident, notably fuel volatilization, complementary to the PHEBUS FPT4 test. RT4 used unre-irradiated fuel (UO2 and ZrO2 fragments) and RT3 will use re-irradiated fuel fragments (only UO2 to achieve higher temperature and quantify, in particular, the fuel volatilization rate). RT7 will be the second test with MOX fuel and the first using a re-irradiated MOX sample. Compared with RT2, it will be performed in reducing conditions, instead of the mixed steam/hydrogen of RT2. Finally, HT3 will be the second HT test. Compared with HT1, it will be performed under reducing conditions and with additional injection of boric acid and SIC (silver, indium and cadmium) within the fluid flow.
Fig. 6.
95
Zr and
103
201
6. Conclusion The VERCORS program represents a significant step forward in knowledge and accuracy of in-vessel source term data. Following the HEVA program, which contributed mainly towards data on volatile FPs, the VERCORS tests have extended the data base up to 2600 K: confirming the nearly total release of volatile species Cs, I, Te and Sb, measuring the release of low volatile species and classifying them in three categories: ‘semivolatile’ (Mo, Rh, Ba), ‘low-volatile’ (Ru, Ce, Np, Sr, Eu) and ‘non-volatile’ (Zr, Nb, La, Nd). The VERCORS 6 test, performed with high burn-up fuel, leads to early fuel collapse and partial liquid corium formation. Nevertheless, FP release did not increase significantly compared with solid fuel; the liquid phase seems even to retain a fraction of some semi and low-volatile FPs. The on-going experiments VERCORS HT and RT focus on the release of low volatility FPs and actinides, and will confirm or extend somewhat the data base at higher temperatures including the consideration of a wider range of parameters such
Ru gamma emission tomography after VERCORS 6 test.
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Fig. 7. VERCORS HT apparatus in hot cell.
as the nature of the fuel (UO2 and MOX), its morphology (intact pellets or debris fragments), its burn-up, the impact of control materials (SIC and boric acid) and the conditions of the accidental sequence (oxidizing or reducing).
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G. Ducros et al. / Nuclear Engineering and Design 208 (2001) 191–203 Leveque, J.P., Andre, B., Ducros, G., Le Marois, G., Lhiaubet, G., Oct. 1994. The HEVA experimental program, Nuclear Technology, Vol. 108. Lorenz, R.A., Osborne, M.F., 1995. A summary of ORNL fission product release tests with recommended release rates and diffusion coefficients, ORNL/TM-12801, Nureg/CR6261, July 1995. Malgouyres, P.P., Ducros, G., Ferroud-Plattet, M.P., Prouve, M., Boulaud, D., 1998. The VERCORS HT facility for studies up to molten fuel conditions. In: Proceedings of the
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