Densification behaviour
The behaviour shown in Figure 4.19 shows that effective densification of these composite materials can be achieved via HIPing. The differences in densification observed between the 38.1 and 15.1 mm canisters shows how vibratory packing is much more effective than the tap and press method used to fill the 15.1 mm canisters. The comparison of the 0 wt% loaded canisters shows the canister packing efficiency of the 15.1 mm canister is only 65 % of that seen in the 38.1 mm canister packaged via vibratory packing. The SEM-BSE micrographs in Figures 4.23 - 4.25 also illustrate the effective densification of the composites has occurred as evidenced by the lack of porosity in the ABS-1 matrix. From this densification behaviour and the lack of OPyC oxidation seen in Figures 4.23 – 4.25, we know the processing temperature used in the processing of these samples is appropriate.
Sample Preparation and Sample Cracking
Sample Preparation: As summarised in Section 4.5.1, the sample preparation of the heterogeneous composite materials pose many issues, which in turn affected the ability to analyse the larger samples of the HIPed composites. The major drawback was the inability to open the larger canisters without application of large lateral clamping force to the canister walls and use of a saw without controllable blade rotation or pressure application leading to aggressive sectioning conditions. This resulted in several issues for the analysis of the larger samples including: the complete removal or destruction of TRISO particles from the composites; large regions of fracturing in the ABS-1 matrix; large amounts of material removal and a poor surface finish. This poor surface finish can be seen in Figure 4.21 which shows the 0 wt% loaded canister sectioned using the Abrasimet saw. The aggressive nature of this sectioning can be seen in the burring of the canister edges and visible notches from the cutting plane of the sample surface.
Not only did this sectioning method prevent effective polishing and sample preparation for microscopic study, but the data gleaned from the canister behaviour in respect to the larger scale matrix cracking could not be considered diagnostic, due to the high lateral pressures on the sample during sectioning. The fracture patterns, as shown in Figure 4.21, are characteristic of a load applied at a minimum of two places to the lateral surface of a cylindrical glass tube [227]. This behaviour could be described by either the application of too great a pressure during processing, whilst below the glass Tg or from the clamping forces used during sectioning.
To determine whether this highly detrimental cracking was a result of the processing parameters, 15.1 mm canisters were designed to enable sectioning of the canister with a less aggressive methodology eliminating clamping forces. The decrease in the amount of hard materials within these smaller diameter canisters allowed the use of the hard ferrous abrasive blade and determination of cracking origins.
Cracking Origins: The canisters produced allow for a comparison of the crack morphology and for some speculation on their source. As no external force capable of causing cracking was applied to the 15.1 mm canisters post processing, we know all cracking in these samples is a direct result of the material processing parameters. As the 0 wt% loaded canisters show this fracturing it can be determined that the cracking is a result of the interaction of the canister with the glass and not because of the presence of TRISO particles. Unfortunately, the smaller canisters compacted into a crescent shape, most likely as a result of the lower packing density achieved prior to HIPing. This makes analysis of the cracking patterns more difficult.
Major cracks in both the 0 wt% loaded and 10 wt% loaded samples follow the length of the crescent shape, with an increased degree of cracking in the corners. Despite the complexity of the cracking patterns in the 15.1 mm patterns, some information can be related back to fracturing in the larger samples. It is most likely that the fracturing results from excessive lateral pressure on the canister during processing, whilst below the glass Tg, as discussed above. However, caution should be used in the assignment of this due to the potential for vibrationally induced cracking during sectioning.
Effects of Cracking on Wasteform Quality: Cracking is the major issue precluding the use of HIPing for processing TRISO particles into an ABS-1 encapsulated composite. These cracks propagate from the surface of the canister to the OPyC surface where contacting is poor.
This means fluid could rapidly transfer from a breach in the container, along this crack network to the OPyC layer [228]. This fluid would then fill the gap which exists between the glass and the particle and provide a release mechanism for the radionuclides to the environment.
If this did not already preclude the use of HIPing (using these parameters) then the micrograph shown in Figure 4.24 c) illustrates why the wasteform in its current form should not be considered for use. The cracking of, not just one, but all of the protective layers of the TRISO particle should be considered the greatest possible failure of an encapsulation methodology for TRISO particles. This would open the internal structure of particles to
dissolution and promote the release of the most soluble fission products contained in these layers, as well as promoting the direct dissolution of the TRISO particle fuel core [228].
The parameters utilised in the processing of the HIPed composites were chosen to provide a short dwell period with effective densification, while not exceeding the pressure that TRISO particles are designed to sustain [229-231].
Crack Propagation into the TRISO Particles: Figure 4.24 c) indicates that the cracking of the particles is not a direct result of the pressure applied to the particles, but is instead a propagation of cracking occurring in the ABS-1 matrix. This has already been attributed to the lateral pressures applied to the glass during processing whilst below Tg. From analysis of the HIP data logs it is known that the highest pressure applied to the densified composite canister, after passing below the glass Tg,was at 610 °C with an applied pressure of 82 MPa.
An inference on fracture behaviour can be made from the fracture properties described in Sections 4.3.1 and 4.4.1. These indicate that the ABS-1 has a fracture toughness at the lower end of that seen in sintered SLS glass compositions. The load required to initiate this type of cracking mechanism in isostatically loaded cylindrical SLS glass compositions has previously been measured at between 76.5 MPa and 85 MPa [227]. This could therefore explain the presence of cracking in the sample. However, 76.5 MPa should not be assumed to be the lowest pressure at which cracking is initiated in the ABS-1 glass composition and a study of the HIPing parameters should be performed.
Requirements for Optimisation of Processing Parameters: Processing of the TRISO – ABS-1 glass composites should be readily achieved without application of high pressures to the densified solid matrix. This can be done by simply processing at a lower pressure. 40 MPa should be capable of readily deforming canisters such as those used in this study.
Alternatively if higher pressures were required for the densification of the waste stream, decreasing the pressure below that known to initiate cracking, before passing through Tg, would likely eliminate this crack formation.
Contacting: The fracturing in the TRISO particles has been noted to form as an extension of the fracturing in the ABS-1 matrix. Inferences can thus be made on the origin of the gap which exists between the ABS-1 matrix and the OPyC layers of the TRISO particles. This gap must not exist at the point of crack initiation as the fracture plane in the particles would then not follow the same path as those seen in the ABS-1 matrix. It can therefore be assumed that at processing temperatures where the cracking is initiated, the particles are in contact with
the ABS-1 matrix. The above observation supports the earlier conclusion that this matrix cracking is a result of the excessive pressurisation from the HIP processing fluids below Tg. Evidence supporting this is displayed in Figure 4.25, where it can be seen that micron sized features are present at the OPyC surface which are an exact match for the solidified surface of the ABS-1 matrix. Two mechanisms could account for the creation of this gap. The first would be a non-wetting interaction between the particles and matrix at elevated temperature, followed by contraction of both the ABS-1 matrix and the TRISO particles.
Evidence in support of this theory is presented by the uniform nature of the gap distance noted in section 4.5.1, which suggests the formation of this gap is a result of the materials fundamental properties, such as TECs. contacting, TRISO particles. Evidence for this hypothesis is seen in Figure 4.23, where the sectioned canisters are displayed and the open structure of the cracks can be seen. However, the data is inconclusive as to which of these mechanisms may be responsible for the formation of the gap between the ABS-1 and TRISO particle matrix.