Ceramic breeder materials have been tested under various conditions to determine their irradiation

response in terms of tritium production and release and their microstructural, thermal, chemical, and mechanical stability. The nuclear loading of ceramic breeder affects their performance in various ways, of which the most important are the following:

evaluation of the probability distribution of the maxi­mum contact forces for different loading conditions (see Figure 34) 205 A comparison of results obtained by DEM and tomography is shown in Figure 35133: the packing factor distributions agree quite well, and the radial and vertical positions of particles show the same structure as shown in Figure 17(b). Other approaches are found like those of Aquaro and co-

123,199,202,203

workers.

• Lithium burnup and other neutron transmutation reactions gradually change the material composi­tion and affect the chemical and physicochemical interactions.

• Transmutation reactions of major and minor con­stituents gradually change the radioactivity levels relevant for in-tokamak operation, hot-cell opera­tions, and management of waste, including recy­cling options.

•
Atomic displacements are induced by neutron impact, most effectively by fast neutrons, causing significant lattice defects, damage, swelling, and so on.

• Generated tritium affects the physicochemical and mechanical behavior, and the inert, 3He decay product stays within the material if it is not desorbed earlier into the purge gas flow.

• Generated helium resides in the material, and through formation of clusters of bubbles will generate stresses leading to macroscopic effects such as swelling, altered thermal transport, and/or fracture.

Numerous irradiation experiments have been per­formed on ceramic breeder pebbles using material test reactors with thermal, mixed, or irradiation phe­nomena fast neutron spectrum. As the 6Li cross­section, in particular, is much higher for thermal neutrons, many of the irradiation phenomena can be effectively studied in thermal or mixed-spectrum reac­tors, that is, without using 14MeV neutrons. This
section concentrates on the thermal-mechanical behavior, while tritium production and transport are dealt with in the following section.

High fluence and high lithium burnup were achieved in fast reactor irradiations at experimental breeder reactor II (EBR-II) and fast flux test facility (FFTF) facilities, as reported by Hollenberg and cow­orkers.28,134 The bulk of these experiments concerned Li2O, LiAlO2, and LiZrO2, with only a few data on Li4SiO4 shaped as annular pellets and LiZrO3 peb­bles. Good tritium release behavior of Li2O and LiZrO3 has been reported, even for temperatures higher than 1000 °C; pellet thermal conductivity of Li2O and LiAlO2 was decreased at lower irradiation temperatures but appeared fairly unaffected when operated over 400 °C. , , , , ,

AECL tests at NRU and JAEA tests at JMTR addressed the impact of neutron irradiation on pebble-bed properties, such as conductivity. In these cases, the constraints were modest: either higher
burnup with few pebbles in the heat flow direction or low volumetric heat loads and low lithium burnup.135-139 Verrall et a/.138 seem to the first to report on bubble formation in a ceramic breeder. They observed this in Li2O irradiated in NRU up to 1% lithium burn-up up, see Figure 45.

The effective thermal diffusivity of a Li2TiO3 pebble bed was studied in an in-pile irradiation experiment by Kawamura et a/.140 at the JMTR test reactor. The cylindrical assembly of the Li2TiO3 pebble-bed was instrumented with a number thermo­couples to determine the radial temperature profile. The derived thermal diffusivity as function of temperature is shown in Figure 36. The dimension of the pebble bed was 20 mm in diameter and 260 mm in length. The effective thermal diffusivity of the Li2TiO3 pebble bed was found to decrease with increasing irradiation temperature. This tendency remained up to a thermal neutron fluence of 1 x 1024nm~2, while the thermal conductivity at given temperatures also remained constant.

None of these experiments addressed the pebble — bed deformation behavior at the strong temperature gradients envisaged for breeding blankets. Out — of-pile testing is less representative as it requires the use of heater plates, with their specific impact on reduced bed thickness, and running a ceramic — heater interface at the highest temperature. Also, when material is irradiated in a stressed state, there is an additional phenomenon of irradiation — induced creep.

As a major step in the preparation of the European HCPB TBM program in ITER, an in-pile test of pebble-bed assemblies was defined. This experiment was designed to address the neutron-irradiation effects on the thermal-mechanical behavior of a breeder pebble bed at HCPB DEMO representative levels of temperature and defined thermal-mechanical loads.121,130,141 A schematic is given in Figure 37. The core of each test element is a horizontal cylindri­cal bed of ceramic breeder pebbles, either Li4SiO4 (OSi) or Li2TiO3 (MTi), with an outer diameter of about 45 mm and bed thickness of about 10 mm, sand­wiched between two beryllium pebble beds. The breeder and beryllium pebble beds are separated by Eurofer-97 steel plates. The heat flow is managed so as to have a radial temperature distribution in the ceramic breeder pebble bed as flat as reasonably possible. The test element design and test matrix required extensive pretesting, improved pebble-bed modeling, design curves for the material character­istics, and performance analyses allowing in-reactor operation in High Flux Reactor (HFR) Petten121,130,141 (Figure 38).

A specific pretest compaction procedure by press­ing and heating has been developed, in particular to increase the thermal conductivity of the beryllium pebble beds and provide conditions that would result in limited changes during in-pile operation.130,141 The compaction procedure consisted of a subsequent loading of the pressure plate of the total assembly to 3 MPa. The X-ray pictures combined with the

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0 2 4 6 8 10 Thermal neutron fluence (xl023nm-2)

12

Figure 36 The effective pebble-bed conductivity derived from in-pile І_і2ТІОз pebble-bed irradiation in JMTR, as performed by Kawamura and coworkers.140 Reproduced from Kawamura, H.; Kikukawa, A.; Tsuchiya, K.; et al. Fusion Eng. Des.

2003, 69, 263-267.

Sealing plate: all Eurofer-97

Figure 37 Schematic of HCPB pebble-bed assembly (PBA) test-element for in-pile testing in the High Flux Reactor at Petten, The Netherlands. Each test-element has a cylindrical ceramic breeder section, either Li4SiO4 or Li2TiO3, in between two cylindrical shaped beryllium pebble beds and separated by Eurofer plates.

Figure 38 Picture of PBA test element during assembly: Ceramic breeder section (Li2TiO3 pebbles from CEA) with penetrating thermocouple tubes; note pebble alignment at circumference.

dimensional measurements taken during assembly allowed the determination of actual bed size and compaction values. An example of a postassembly X-ray picture is shown in Figure 39.

The design and safety requirements for in-pile operation required the development of a full-coupled thermomechanical model in the MARC finite-element code. In this way, the pressure buildup and stress relax­ation in the pebble beds could be simulated in detail to guide the required reactor startup profile.121,130,141

The two test elements with Li4SiO4 pebbles were irradiated at nominal temperatures of 600 and 800 °C in the breeder bed, to see any effects ofthermal creep. The other two test elements contained Li2TiO3 peb­bles with different grain sizes and were irradiated at the same temperature, the nominal temperature of 800 °C. The pebble beds were typically purged with a helium-hydrogen mixture of reference composition (0.1% H2). The gas purge entered at the lower beryl­lium bed and exited at the upper beryllium beds.

The PBA has been irradiated for 294 FPD (full power days), achieving lithium burnups of 1.5—2.2% for Li4SiO4 and 2.8—2.9% for Li2TiO3, without enriching in 6Li. The dose in the Eurofer-97 parts

ranged from 2 to 3 dpa.142

Extensive analyses of the in-pile data using FEM calculations showed that the maximum

temperatures in the Li4SiO4 pebble beds of the top and bottom test-elements are about 600 and 800 °C, respectively. The average temperatures in the Li4SiO4 beds are about 550 and 740 °C, respectively.1 2

In postirradiation examinations of both Li4SiO4 samples, a little sintering and a significant amount of cracking or fragmentation have been observed. No significant difference between the lower and higher temperature case was found. Most of the evidence of cracking and fragmentation in the Li4SiO4 pebbles is observed toward the middle of the bed (highest temperature, highest deformation). This is visible in scanning electron microscopy images (Figure 42). There is some evidence of grain growth. Reactions of Li4SiO4 pebbles with Eurofer were found to be very small.

The maximum temperatures in the Li2TiO3 pebble beds of both the two test-elements in the middle are about 780 °C. The average temperatures in the Li2TiO3 beds are about 690 and 720 °C, respectively.142

In both test-elements, Li2TiO3 pebbles showed a significant amount of sintering and necking, which was found most significant in the test-element #3. The average temperature of this test-element was higher by about 35 °C. Almost no fracture or frag­mentation was seen. There appeared to be a small reaction layer, distributed uniformly along the Euro — fer (Figures 43-44).

Figure 40 Neutron radiograph of PBA taken after few irradiation cycles.

In the high burnup irradiation EXOTIC-7 (see details in Section 4.15.5.1.2), the pellet stacks and pebble beds were found to be essentially intact by neutron radiography analyses after irradiation, and except for one capsule containing Li2ZrO3 pellets, three out of five were found intact after unloading. Fragmentation of the 0.1—0.2 mm Li4SiO4
pebbles was also observed but was very difficult to quantify33’143-145 (Figure 46). The pores observed in the images are related to the pebble fabrication method rather than to neutron irradiation.

Chikhray etal. irradiated Li2TiO3 + 5 mol% TiO2 in the Kazakhstan water water research reactor (WWRK) reactor to lithium burnups of about 20% at 760 and 920 K; see details in the next section and Chikhray et a/.82 Pebble crush tests showed reduc­tion of strength’ whereas microhardness tests also revealed ingrowth of soft phases. X-ray diffraction measurements showed traces of LiTi2O4’ LiTiO2’ and Li4Ti5O12; see Chikhray eta/.82 for more details.

Test module #4 Figure 41 Temperature fields in the PBA as calculated with NRG’s fully coupled (cylindrical) pebble-bed thermo-mechanical model.

In an IEA-framed international collaboration’ European Li4SiO4 and Li2TiO3 and Japanese Li2TiO3’ reference pebble materials were tested in a high fluence irradiation project at the HFR in Petten, named high neutron fluence irradiation of pebble stacks for fusion (HICU).146-148’194 The neutronic analyses as reported by Fischer and coworkers149’150 demonstrated that relevant nuclear irradiation para­meters such as the displacement damage accumula­tion’ the lithium burnup’ and the damage production function W(T) are met with the selected neutron shielding and 6Li enrichments chosen. This project is conceived to irradiate ceramic breeder pebble stacks at high temperatures under blanket prototypical ratios of fast neutron damage (dpa) and lithium burnup.

Compared with the PBA experiment, the pebble stacks are smaller’ but capsule dimensions are up to about 10 mm; X-ray tomography was used for detailed mapping of pebble location prior to irradiation.148’151