Post-test irradiation temperature determination and neutron dosimetry methods

Vande Pitte Jonas

Promoter

Detavernier Christophe, (UGent), christophe.detavernier@ugent.be

SCK•CEN Mentor

Uytdenhouwen Inge
inge.uytdenhouwen@sckcen.be
+32 14 33 31 71

SCK•CEN Co-mentor

Goussarov Andrei
andrei.goussarov@sckcen.be
+32 14 33 85 06

Expert group

Structural Materials

PhD started

2017-10-01

Short project description

Performing high quality neutron irradiations depends strongly on providing well-defined irradiation conditions (i.e. thermal and fast neutron flux, fluence, temperature, …) for fuel and materials. The accuracy with which these conditions can be defined has a direct relation with the accuracy with which material properties and their evolution with neutron dose can be determined in the post-irradiation examinations.  In addition, more extreme environments are sought after, both in the fission (GenIV and SMR reactors) as the fusion field. This means higher temperatures, flux and fluence levels. In most of the research reactors, the neutron flux and energy spectra are relatively well understood and monitored, while the irradiation temperature can be more difficult to determine [1]. However, to be able to set up a more unified database, such as for the construction materials for advanced reactors (e.g. MYRRHA), it would be beneficial to be able to compare microstructural and mechanical properties across different neutron spectra : thermal neutrons, mixed spectra and fast flux.

The tailoring of the neutron spectrum on individual samples in one single irradiation campaign avoids the need to perform multiple irradiations and assures that other parameters are kept identical.  Such tailoring could be performed by applying selective absorber coatings on the specimens. In such a way, the effect of flux, fluence, temperature and spectrum of the neutron irradiation on the microstructure and mechanical behavior can be examined. By using the same coating methods on dosimeters, it would be possible to also assess the spectral distribution of the dose.  Furthermore, by tailoring the dosimeters themselves, it becomes possible to perform energy dependent dosimetry. Present certified materials existing for neutron dosimetry consist out of two types of materials, namely high purity metals and Al-alloys. They have different activation threshold energies to allow the establishment of reactor neutron energy spectra [2].

Deposition techniques today can allow accurate engineering of a material composition to tune the neutronic properties of a coating layer to suit a particular purpose.

In order to address the effect of the temperature, more and accurate knowledge of the capsules heating effect is necessary, ultimately requiring active instrumentation of the rigs. It is not always possible to have full instrumented and active temperature monitors due to the high costs or limited inner volumes of the capsules. In those cases, passive temperature monitors can be used. The most common approach is to use melt wires or paint spots. They are relatively easy but give the disadvantage that they only detect whether a certain temperature limit was exceeded or not. A single temperature monitor that can be used to determine the peak irradiation temperature within a broader temperature range would be more beneficial. High-throughput deposition techniques used in modern material screening allow compositional variations of a multi-element coating to accurately fine-tune properties, such as the melting point.  By deposition of a functionally graded material (FGM) ; continuously varying melting point), it becomes possible to fabricate a passive thermometer. By verifying which compositions have molten, it becomes possible to assess the temperature achieved in an irradiation experiment with a higher accuracy than with melt wires. The technology to make FGMs is already well established in the material science world and is used in a large field of applications (aerospace, medicine, nuclear fuel pallets, fusion reactor plasma facing components, protective coatings in turbine blades, communication fields related to lenses and semiconductors, …). However, as far as the authors are aware, it hasn't been applied yet for temperature monitors in reactors. Ghent University developed a home-built combinatorial deposition system, where they can deposit thin films with a continuous compositional gradient as such that they create binary or ternary phase diagrams on planar substrates [3].

Since the early 1960s, SiC has been used as a post-irradiation temperature monitor. They have been used in constant-temperature irradiations from 200 to 1000°C. Knowledge on defect formation in SiC is increasing due to the fact that SiC is also proposed as a cladding material in Accident Tolerant Fuels, as a coating material for high temperature gas-cooled reactor fuel elements and was even proposed as a structural material for fusion reactors. However, still some issues remain such as the use of SiC at lower temperatures due to amorphization; while the high temperature region is limited due to non-saturatable swelling. The temperature memories of SiC monitors can be retrieved from a variety of techniques after isochronal annealing. Most common techniques used are length measurements, x-ray line broadening to detect lattice parameter changes, thermal diffusivity, density, swelling and electrical conductivity. The electrical resistivity [4] and length measurements [5] show the best accuracy. SCK.CEN has an international collaboration with INL (Idaho National Laboratory) to develop and expand irradiation testing and post-irradiatoin examination (PIE) capabilities. INL has the knowledge and experience in-house to conduct post-processing research related to SiC temperature monitors and is currently funded through NSUF (Nuclear Science User Facilities).

References
[1] Rempe J.L., Condie K.G., Knudson D.L., Snead L.L., "Silicon carbide temperature monitor measurements at the high temperature test laboratory", INL/EXT-10-17608 report (2010)
[2] Genthon J.P. and Rottger H., "Reactor dosimetry: dosimetry methods for fuels, cladding and structural materials", vol. 1, Springer (1985)
[3] http://www.cocoon.ugent.be/content/combinatorial-deposition-thin-film-materials-libraries
[4] Snead L.L., Williams A.M., Qualss A.L., "Revisiting the use of SiC as a post irradiation temperature monitor", Effects of Radiation on materials, ASTM STP 1447, M.L. Grossbeck, Ed., ASTM International, West Conshohocken, PA (2003)
[5] Campbell A.A. et al, "Method for analyzing passive silicon carbide thermometry with a continuous dilatometer to determine irradiation temperature", Nuclear Instruments and Methods in Physics Research B 370 (2016) 49-58

Objective

This Phd thesis involves on the technological side the development and optimization of post-test irradiation temperature methods and evaluation. Furthermore, it also addresses the optimization of different coating materials on specimens to tailor the neutron spectrum and in dosimetry to perform spectrally resolved dosimetry.

On the other hand, a strong scientific part related to the understanding of defects in SiC as a function of flux, fluence and temperature and the effect of transmutation (P doping) will be necessary. At the same time, the effect of the neutron spectrum by the coatings should be examined and understood on the mechanical and microstructural properties.

In detail, three different paths will be investigated.

  1. Single temperature monitors:
    - SiC monitors can be used to detect a whole range of temperatures that occurred during irradiation. The damage and defects created and accumulated strongly depend on the peak temperature during irradiation. The appropriate annealing and measuring methods (such as resistivity, swelling, length, …) to find the peak temperature during irradiation will be investigated and calibrated. The temperature range is limited to 200-1000°C due to amorphization (lower limit) and extensive void swelling (high limit). The defect formation and effect of transmutation should be understood and a calibration/correlation with the real temperature will be set-up as a function of flux, fluence and temperature level. Particularly the fluence level is currently considered to have a lower limit of 1dpa, which is relatively high.This is mainly related to the detection methods and should be assessed.For this different post-mortem techniques with in-situ annealing (resistivity, length, …) will be used as well as microstructural tools (SEM, PALS, XRD, …)
    - Based on a mechanistic understanding of the defect phenomena making SiC a suitable material for this application, a screening for potential alternative materials for SiC will be investigated, particularly focusing on avoiding the temperature and dose limitations.

  2. Melt temperature and neutron dosimetry monitors:
    - Instead of single wires, a functionally graded material (FGM) consisting of thin layers deposited with a variable chemical composition will be made in collaboration with the university, in order to have a melting point variation across the sample.
    - Studies related to the re-solidification state as a function of temperature (reference, irradiated state) will be performed and examined with microstructural tools (SEM, XRD, …)
    - In addition, the FGM deposition technique can be used to also perform neutron dosimetry, potentially combined with a spectral dimension as developed under 3)

  3. Selective absorber coatings:
    - A study on possible coatings to tailor the neutron spectrum on materials will be conducted. The effect on the spectrum induced by the coating will be examined through their effect on the microstructural and mechanical properties.