Thermal fatigue durability is a necessary but not sufficient prerequisite for the Be/CuCrZr alloy joint. Neutron irradiation is also expected to affect the high heat flux durability of these joints. This is particularly true for the first-wall components, where the joint will experience rather high neutron fluence.

To investigate the effects of neutron irradiation on the joint properties there are two possible ways. The first method is postirradiation testing, that is, irradia­tion of small scale mock-ups in fission reactors and postirradiation testing in heat flux test facilities. The second method is to try to reproduce more faithfully the situation in ITER and to apply a cyclic heat flux during irradiation. This consists of in-pile heat flux testing of small mock-ups in fission reactors. Given the technical difficulty of achieving simulta­neously representative values of heat flux and neu­tron irradiation and to determine exactly what the heat fluxes are within a reactor, it is suggested that the effect of neutron irradiation on the mock-ups be determined by pre — and postirradiation heat flux tests on mock-ups. This method is probably conser­vative as the postirradiation tests are performed on materials and joints having accumulated damage corresponding to the total neutron irradiation dose. However, this should be supported by analysis and material testing.

Most of the tests conducted in the past were done for DS-Cu as a heat sink. Studies of neutron irradia­tion effects on the durability ofthe Be/Cu-alloy joints have been performed in at least two of the ITER parties: Europe,150,162 and the Russian Federation.173

For the Russian experiment, the irradiation con­ditions were 0.3 dpa at 350 °C. The irradiated Be/Cu mock-ups were then tested in the JUDITH facility. The results of the postirradiation high heat flux test­ing of the different Be/Cu mock-ups are presented elsewhere.174 On the basis of the results of these tests, it was concluded that the effect of the neutron irradi­ation is not critical for the joints. Metallographic inspections did not show any significant changes in the braze joint after neutron irradiation. For the CuMnSnCe, a small intermetallic phase was observed in the middle of the braze layer for unirra­diated and irradiated samples. No crack formation was found in the intermetallic. In addition to thermal fatigue tests, shear tests were conducted and it was found that for CuMnSnCe the shear strength decreased after neutron irradiation from 200 to 155 MPa, whereas for the InCuSil braze no irradia­tion influence was observed (^300 MPa). Neverthe­less, the thermal performance of the joints during high heat flux tests was very similar to the perfor­mance of unirradiated mock-ups.

For the European experiments, the first irradia­tion campaign (named Paride 1 and Paride 2) of small scale Be/Cu mock-ups and Be/Cu joints took place in 1996-1999. Mock-ups were fabricated from a sin­gle 10-mm thick Be tile (grade S-65C) of dimensions 22 mm x 60 mm, HIPped on a 20-mm thick CuAl25 substrate (grade IG1) with a drilled 10-mm diameter cooling channel (Figure 14). HIPping was done at 830 °C for 2 h with the use of a 50 pm Ti interlayer. One mock-up was neutron irradiated in the test reactor high flux reactor (HFR) and then high-heat flux tested in JUDITH. The neutron irradiation was done at about 200 °C up to a neutron dose of about 0.6 dpa in the Be material. The neutron dose expected at the end of life in the Be armor of the first-wall panels is about 1 dpa (Be). The irradiated mock-up was tested for 1000 cycles at 1.6 MW m-2 plus 100 cycles at 1.9 MW m-2 plus 100 cycles at 2.4MW m-2 plus 1000 cycles at 2.8 MWm-2 plus 100 cycles at 3.3 MW m-2 without any sign of failure. It failed dur­ing the first cycle at 4.25 MW m-2 with a partial detachment of the Be tiles on one end (Figure 14). An unirradiated mock-up was tested for 1000 cycles at

1.5 MW m-2 plus 1000 additional cycles at 3 MW m-2 without any sign of failure but a detachment of the Be tile occurred during the first cycle at 4.5 MW m-2. It was therefore concluded that no significant degrada­tion of the Be/CuAl25 joint was observed up to a neutron dose of about 0.6 dpa. Neutron irradiation test experiments are ongoing or in preparation with Be coated first-wall mock-ups made from CuCrZr alloy to confirm the above result with this Cu alloy.

The first experiment was a joint European/Rus — sian irradiation test campaign. It was prepared by the Efremov Institute of St. Petersburg. The original objective was to perform thermal fatigue testing of two first-wall mock-ups at about 0.5 MW m~2 simul­taneously to neutron irradiation. A failure of the surface heating system made from graphite after about 5000 cycles resulted in the discontinuation of the thermal fatigue testing and a continuation of the campaign with only neutron irradiation. The irradia­tion campaign has been stopped with the achievement of a neutron dose of 0.75 dpa. The first wall mock-ups, one made with Be tiles HIPped at 580 °C and a Cu interlayer (Figure 15) and another with brazed Be tiles with STEMET 1108 braze alloy, will then be high heat flux tested together with unirradiated reference first wall mock-ups. Two other test cam­paigns are in preparation at the NRI of Rez (Czech Republic) and at Petten (The Netherlands) with the objective of thermal fatigue testing three first wall mock-ups in parallel to neutron irradiation.

The question as to whether the correlation between fusion and fission neutron spectra assumed in many of the above measurements is valid or not needs to be discussed. Comparison of changes in the mechanical properties, especially at low temperature, needs to be made with the same He to dpa ratio to ensure that the results will be valid for ITER.

Be/Cu alloy mock-ups have been tested in an electron beam for 1.5 s under a deposited energy density of 60 MJ m~2(132) to simulate Be damage dur­ing a VDE. For a 6 mm thick Be tile, the melt layer was ~1.5mm, while that calculated for the same

condition is 1.25 mm. A cross section of the Be tile after the simulated VDE event is shown in Figure 16. The embrittlement of the Be due to neutron irradi­ation increases the loss of material particles, espe­cially at low irradiation temperature. A clear pore formation (which is expected to be He filled) has been observed in the melt layer of all neutron irra­diated specimens after thermal shock loading (see Figure 16). An area of possible concern is the small surface cracks that form when molten metals reso­lidify. These resolidification cracks could serve as thermal fatigue crack initiation sites and accelerate this type of damage. While this effect has not been extensively studied because of the difficulty of simulating disruptions in the laboratory, it may not be a critical issue as thermal fatigue cracks form after a few hundred cycles in most materials and they grow only to depths where the thermal stress level is above the yield stress.175

High heat flux tests of neutron irradiated mock — ups conducted in the past did not reveal any damage in Be and in Be/Cu joints,176 although the irradiation conditions were not fully ITER relevant (damage dose of ^0.3 dpa instead 1 dpa for the end of life and also lower He per dpa). An increase in crack formation and erosion rate has been observed in the surface of irradiated Be at 350 and 700 °C.177 The S-65C grade presented the lowest damage after

irradiation. High heat flux tests (at more severe con­ditions than needed for the first wall) of the cracked unirradiated Be did not reveal any detrimental behav­ior and loss of material due to cracking.178 From an engineering point of view, to avoid possible crack formation and delamination of the brittle Be, it is recommended to use Be tiles without any stress concentrations.