In-pile experiments have the strong advantage that the tritium release characteristics can be studied, as a function of neutron damage and lithium burnup in combination with thermal-mechanical behavior under neutron irradiation. Such in-pile experiments allow steady-state tritium production and release con­ditions to be achieved. In general, such parameters are closer to breeding blanket conditions, as they allow the application of a wide range of temperatures and purge gas conditions and the study of long-term performance issues such as irradiation damage and lithium burnup. At present, such data are limited in terms of fast

Figure 49 Out-of-pile annealing tests for Li4SiO4 with 0.1% H2/N2 sweep gas and 0.1% H2O/N2 sweep gas (amount of breeder, 0.3g); flow rate, 100mlmin~1; irradiation time, 2 min; neutron flux, 2.75 x 1013cm2s% Reproduced from Munakata, K.; Yokoyama, Y.; Baba, A.; Penzhorn, R. D.; Oyaidzu, M.; Okuno, K. Fusion Eng. Des. 2005, 75-79, 673-678.

neutron damage doses (thermal and mixed spectra MTR only).

Several irradiation programs have been executed around the world, involving thermal, mixed — spectrum, and fast reactors [as in Dido (Germany), Siloe (France), EBR-2 (US), FFTF (US), HFR (The Netherlands), JMTR (Japan), and WWRK (Kazakhstan)]. The European irradiation projects under the acronym EXOTIC, for extraction of tritium in ceramics, commenced during the mid 1980s at the HFR in Petten , , , , , , (see Table 4).

The initial series concerned both closed capsule and vented capsule operation. The materials investigated were mainly Li2O, Li2SiO3, LiAlO2, LiZrO3, Li8ZrO6, and Li4SiO4 in the series EXOTIC-1 to-6.26 The objective of the EXOTIC-7 experiment has been to irradiate candidate ceramic breeder materials in the HFR to a high lithium burnup (target ~ 10%) and to determine the effects on the mechanical integ­rity of pellets and pebble-bed configurations, and those on tritium-inventory and-release characteris — tics.33 The experiment concerned 8 capsules, and during 11 HFR cycles (261 FPD), lithium burnups of 6-18% were achieved. The test matrix comprised pellets of Li2ZrO3, Li8ZrO6, and LiAIO2 and pebbles of Li2ZrO3 and Li4SiO4. Two capsules contained a mixture of Li4SiO4 and beryllium pebbles. To obtain a high lithium burnup within a reasonable irradiation time, the target materials were enriched with 6Li to about 50%.

The tritium residence time of Li2ZrO3 pellets a higher temperatures was not changed during the irradiation. After a lithium burnup of about 5%, the residence time at lower temperatures decreases significantly with increasing burnup.

After selection of the HCPB as the single solid breeder concept in the European Blanket Project, the EXOTIC-8 and-9 series concentrated on Li4SiO4 and Li2ZrO3 pebbles and a range of Li2TiO3 pro­ducts.175 The irradiation test program concentrated on two types of experiments:

1. Tritium release to low or moderate lithium burnups

2. High lithium burnup and mechanical integrity

The typical designs for these experiments are given in Figure 55, with the general layout and an example of a cross-section from postirradiation testing.

Figure 56 shows a sample temperature and tritium quantities in the purge gas for a typical irradiation cycle.

Tritium residence time (t):

I

G

Figure 54 Arrhenius plots of the average residence time t. Reproduced from Tanifuji, T.; Yamaki, D.; Jitsukawa, S. Fusion Eng. Des. 2006, 81, 595-600.

I, tritium inventory (Bq); G, tritium production rate (Bqmin-1).

At steady state:

Tritium release rate (R) = Tritium generation rate (G)

Temperature transients are performed: A T Difference in tritium inventory (area): AI = I2—I1 Difference in residence time: At = AI/G Data set is processed to obtain: t(T)

See Figure 57.

In the EXOTIC-8 program, the tritium release characteristics and mechanical stability of the refer­ence breeder materials for the European HCPB project have been studied, including the effect of long-term neutron irradiation and high lithium burnup.175 The EXOTIC-8 program started in 1997 and ended in 2002. It consisted of ten experi­ments, named EXOTIC-8/1 to EXOTIC-8/10. The irradiations were carried out in the HFR in Petten in peripheral core positions with the typical neutron fluence rate of about 9 x 10 17 m 2s 2 (fast, En > 0.1 MeV) and 5 x 1017m—2s—1 (thermal). The following materials were used: Li2TiO3 peb­bles produced by agglomeration — sintering and extrusion — sintering, provided by CEA; Li2TiO3 pellets produced by cold pressing, provided by CEA; Li2TiO3 pebbles produced by wet processing and sintering, provided by ENEA; Li2ZrO3 pebbles produced by extrusion — sintering process, provided by CEA; and Li4SiO4 pebbles produced by melt spray process, provided by FZK Karlsruhe.

Two types of experiments were targeted:

1. Major focus on tritium release characteristics by determining differential tritium inventories by thermal transients and achieving low to medium lithium burnups of 1-3%.

2. Major focus on high lithium burnup experiments using pebbles with 50% 6Li enrichment and achieving 11% lithium burnup for Li4SiO4 and 17% for Li2TiO3, at relatively constant tempera­tures, and only few data on release, mostly from postirradiation annealing tests.

Tritium release characteristics are measured both in situ by applying the temperature transients and after irradiation in the TPD setup. The tritium release is characterized by the tritium residence time t with the Arrhenius temperature behavior, as depicted in the summary graph; Figure 58.

A correlation has been established between the pebble density and the measured residence time. The obtained results confirm the understanding that open porosity and small grain size are favorable for faster tritium release.

The corresponding activation energies derived from the temperature transients and from the TPD measurements are in fair agreement. For Li2TiO3 pebbles, Q= 82-93 kJmol—1 and for Li4SiO4 peb­bles, Q= 112-123 kJmol—1. The activation energy for Li2ZrO3 pebbles is derived only from the temperature transients: Q= 84kJmol—1. In TPD experiments, it was shown that the tritium release can involve multiple release processes. Moreover, in some in­stances the release can be limited by recombination at the grain surface, which introduces uncertainties in the measured values of the activation energy.

The results of in-pile tritium behavior during nor­mal operation and during transients in temperature and gas chemistry measured in the latest irradiation experiment from the EXOTIC series, EXOTIC-9/1, are reported in Peeters eta/.180 The Li2TiO3 pebbles produced by extrusion-spheronization sintering at CEA were irradiated in the HFR in Petten (thermal 0.5 x 1018m—2s—1) for 301 FPD and achieved a burnup of 3.8—4.1%. The temperature varied between 613 and 853 K. Based upon the in-pile tritium release measurements and the analysis of the tritium resi­dence time, it was concluded that tritium release in the new batch of the high-density Li2TiO3 pebbles (93.0% TD) is rather slow compared with the ceramics irradiated in the EXOTIC-8 irradiation campaign.174 Thus, the tritium residence time measured at 773 K in the EXOTIC-9/1 experiment was ^30 h, whereas

Table 4 Overview of ceramic breeder irradiation experiments performed in fast or mixed spectrum fission reactors to relatively high levels of thermal and/or fast neutron fluence. Values for the experimental parameters quoted were taken from the mentioned references. Readers are also referred to the text throughout the whole chapter. Experiment ID Materials Shape Li-6 (%) Li-6 burnup (%) Total Li burn up (%) Fluence thermal (1020 cm 2) Fluence E > 1 MeV (1022 cm 2) EFPD (Days) Temp (°C) Reference FUBR-IA Li2O, LiAlO2, pellets 56, 95 1.1-1.2 1.4-1.45 96.7, [186] Li2ZrO3, Li4SiO4 2.0-2.2 3.9-4.1 177.6, 3.1-3.5 2.5-2.7 274.3 FUBR-IB Li2O, LiAlO2, pellets 0.07,7.5,56,95 3.9-4.3 4.7-5.1 341.5 450-1225 [186] Li2ZrO3, Li4SiO4 BEATRIX-I Li2O, LiAlO2, pellets 0.07,7.5,56,95 6.7-7.4 8.2-8.9 599.3 [186] Li2ZrO3, Li4SiO4 10.3-11.3 13-14 940.8 10.3-11.3 13-14 936.9 MOTA-2A Li2O, LiAlO2, rings, 0.07, 0.2, 1.7, 7.6-9.9 0.2, 7.4-8.8 299.7 550-640 [186] (BEATRIX-II) Li4SiO4, Li2ZrO3 pebbles 7.5, 440-1000 56, 95 MOTA-2B Li2O, LiAlO2, rings, 0.2 1.27, 5.3-7.9 0.2, 7.7-7.9 203.3 530-640 [186] (BEATRIX-II) Li4SiO4, Li2ZrO3 pebbles 85 440-1100 95 CRITIC-3 Li2TiO3 pebbles 1.85 0.9 0.57 334 200-900 [208] COMPLIMENT LiAlO2, Li2SiO3, pellets, 7.5 0.25 178 (HFR) 400-450 [209] (ELIMA-2, Li4SiO4, Li2O, pebbles 1 75 (OSIRIS) 650-700 DELICE-3) Li2ZrO3 In-Pile Pebble- Li2TiO3 pebbles 0.01 JMTR 400-610 [140] Bed EXOTIC-1 LiAlO2, Li2SiO3, pellet 0.06 0.004 0.01 0.013-0.025 25 350-700 [26],[210] (BEATRIX) Li2O 0.6 0.035 7.5 0.28 EXOTIC-2 LiAlO2, Li2SiO3, pellet 0.55-0.6 0,1 0.02-04 0.025-0.05 50 350-725 [26],[210] Li2O EXOTIC-3 Li2ZrO3, Li2O, pellet, 0.55-0.6 0.12-0.13 0.03-0.06 0.03-0.08 75 385-650 [26] Li2SiO3 solid EXOTIC-4 Li2ZrO3, Li6Zr2O7, pellet, 0.55-0.6, 7.7 0.13-0.15, 0.04-0.07 0.05-0.1 97 310-680 [26] Li8ZrO6, Li2O, solid 0.8 Li2SiO3 EXOTIC-5 LiAlO2, Li2ZrO3, annular 7.5 1.8-2.1 0.07-0.13 0.08-0.17 166 325-630 [26] Li4SiO4, pellets Li6Zr2O7, and Li8ZrO6 pebbles

EXOTIC-6 LiAlO2, Li2ZrO3, annular 7.5 3.0-3.2 0.08-0.15 0.1-0.2 199 315-520 [26] Li6Zr2O7, pellets Li8ZrO6, Li4SiO4 and pebbles EXOTIC-7 Li2ZrO3, Li8ZrO6, annular 38-50 5.8-18.1 0.27 0.13 261 410-745 [33] Li4SiO4, LiAlO2 pellets and pebbles EXOTIC-8/1 Li2TiO3 pebbles 7.5 1.86 0.046 201 310-570 [175] EXOTIC-8/2 Li2TiO3 annular 7.5 1.34 0.039 173 420-640 [175] pellets EXOTIC-8/3 Li4SiO4 + 2% pebbles 50 2.6-4 0.003-0.013 201 200-700 [175] TeO2, Be EXOTIC-8/4 Li4SiO4 + 2% pebbles 7.5 1.38 0.033 201 280-590 [175] TeO2 EXOTIC-8/5 Li2TiO3 pebbles 7.5 2.1 0.06 200 500-660 [175] EXOTIC-8/6 Li2ZrO3 pebbles 7.5 2.5 0.071 299 400-640 [175] EXOTIC-8/7 Li4SiO4, Be pebbles 51.5 8-17.5 0.025-0.064 648 400-690 [175] EXOTIC-8/8 Li2TiO3, Be pebbles 50 4.1-10.9 0.11-0.38 450 400-700 [175] EXOTIC-8/9 Li2TiO3 pebbles 7.5 3.51 0.1 448 420-560 [175] EXOTIC-8/10 Li4SiO4 pebbles 7.5 0.09 448 520-695 [175] PBA Li4SiO4, Li2TiO3 pebbles 7.5 1.5-2.9 (for 0.04-0.07 0.11-0.17 294 550-800 [142] natural 6Li EXOTIC-9 Li2TiO3 pebbles 7.5, 30.5 3.1 (for 0.15 0.09 300 340-580 [143] Li4SiO4 7.5, 20 natural 6Li) K-578 (WWRK) Li2TiO3 pebbles, 96 18-23 220 500-900 [212], [82] pellets HICU Li4SiO4 pebbles 0.06, 7.5, 20 0.7-13 1.5 0.7 403 600-900 [211] Li2TiO3 0.06,11,30

Figure 56 Examples of tritium release from the EXOTIC-8 experiment series with a number of programmed temperature transients (including reactor power scram at day 19+20).

the same characteristic measured on the Li2TiO3 pebbles obtained from CEA and ENEA in the EXOTIC-8 campaign was 1.3 and 5.2 h, respectively (Figure 59).

Changes in the tritium inventory resulting from the variation of the H2 concentration in the purge gas (from 0.1% to 1.0%) appeared to be much smaller
than those resulting from the temperature transients. From this observation, it was concluded that the tritium inventory was determined by the thermally activated processes taking place in the bulk of the material (dissociation from traps, diffusion) rather than by recombination and isotope exchange with hydrogen at the surface.

Figure 57 Determination of differential tritium inventory from a small negative temperature transient in a steady state tritium production/release experiment.

Figure 58 Summary graph of tritium residence time obtained from the temperature transient experiments for different pebble materials used in the EXOTIC-8 program.

1000/T (K)

Tritium recovery characteristics of a binary bed containing 0.3 and 2 mm diameter Li2TiO3 pebbles were studied under continuous (20 h) and pulsed (200, 400, and 800 s) neutron irradiation in the JMTR.1 5 From the temperature transients from 573 to 623 K, the tritium residence time was estimated as 3h (63.2% of the steady-state value). The complete recovery of the steady-state conditions was achieved after 20 h. The tritium recovery behavior under the
pulsed operation was almost the same as under contin­uous operation, except for the modulations introduced by the pulse operation, which did not exceed 20% of the total signal variation (Figures 60-62).

Effects of irradiation temperature, purge gas flow rate, and hydrogen content in the purge gas on the tritium release characteristics of the Li2TiO3 pebbles were studied in the in-pile irradiation experiment in the JMTR.185 The Li2TiO3 pebbles were fabricated

Figure 60 Tritium release rate variation as function of the central temperature of the Li2TiO3 pebble bed irradiated in JMTR-experiment185. Reproduced from Tsuchiya, K.; Kikukawa, A.; Yamaki, D.; Nakamichi, M.; Enoeda, M.; Kawamura, H. Fusion Eng. Des. 2001, 58-59, 679-682.

by the rotating granulation method. The irradiation continued during one cycle of 25 days with the tritium generation rates of 6 x 101°Bqd~1 or 11 x 101°Bqd~1 depending on the position in the core. The following findings were reported: an increase in the purge gas flow rate accompanied by a temporal increase in the tritium release, which was followed by a swift recovery (<10h at 773 K). A decrease in the flow rate produced

cr m ф w ra ф ф E

50

100 150 Elapsed time (h)

200

250

0

Figure 61 Example of change of tritium release on variation of sweep gas flow rate185. Reproduced from Tsuchiya, K.; Kikukawa, A.; Yamaki, D.; Nakamichi, M.; Enoeda, M.; Kawamura, H. Fusion Eng. Des. 2001, 58-59, 679-682.

an opposite effect. The hydrogen concentration in the purge gas varied from 10 to 10 000 ppm. It was found that at hydrogen partial pressures <100 Pa, the tritium desorption is controlled by the surface reactions and at higher partial pressures by bulk diffusion.

Chikhray et a/.82 performed irradiation tests of Japanese Li2TiO3 ceramics with 96% enrichment of isotope 6Li in the WWRK reactor. Three types
of ceramic samples were examined simultaneously using a system for in-pile tritium monitoring: one (pebbles) — under constant temperature of 650 °C, and two (pebbles and pellets) — within temperature change ranges from 500 to 900 °C. Lithium burnup reached 23% for the active ampoule pebbles, 20% for the passive ampoule pebbles, and 18% for the pellets. The tritium measurement system permitted the

Figure 62 The relation found between hydrogen partial pressure and overall rate constant of tritium desorption185. Reproduced from Tsuchiya, K.; Kikukawa, A.; Yamaki, D.; Nakamichi, M.; Enoeda, M.; Kawamura, H. Fusion Eng. Des. 2001,58-59, 679-682.

tritium yield rate to be determined under long-term irradiation of lithium ceramic Li2TiO3. Postirradia­tion testing also included mechanical testing.