Endurance of Accelerator Beam Window
Investigations of the Irradiation and Thermomechanical Endurance of the Beam Window of ADS Neutron Generating Target
Tech Area / Field
- PHY-PFA/Particles, Fields and Accelerator Physics/Physics
3 Approved without Funding
FEI (IPPE), Russia, Kaluga reg., Obninsk
Project summaryThe main objective of the Project is evaluation, based on calculations, analyses and experiments, of the irradiation and thermomechanical endurance of proton beam window materials for proton energies 600-1500 MeV and beam currents of 1-20 mA. The window ensures entry of the beam into the neutron generating target of an accelerator-driven system based on a sub-critical fast neutron blanket and aimed for transmutation of long-lived radio-nuclides.
Long-lived radio-nuclides, minor actinides (MA) first of all, determine the whole set of complex problems related to storage, reprocessing and final removal from the biosphere of hazardous radwaste of nuclear power industry. The development of industrial methods of MA transmutation based of accelerator-driven technologies meets with numerous R&D problems. One of the most complicated is creation of the neutron generating target to feed the sub-critical blanket. The activity is now in a decisive stage on the ISTC Project 559 with the aim to design and make a prototype target facility cooled by lead-bismuth eutectics for 1 MW beam of 800 MeV protons. It was clear before Project 559 went under way and was substantiated during the work on the Project that the beam window is a crucial component of the target facility determining its lifetime, reliability and most important operation parameters. The window separates the accelerator vacuum beam path from the target material where neutrons are knocked out of nuclei in various nuclear reactions. The target material also serves as coolant in the target under consideration. Diverse and complicated physical processes of nuclear, atomic and macroscopic levels are going on in a structural material of the window. These processes determine the window lifetime which, in its turn, influences decisively on safety and economic parameters of the whole transmutation facility.
The most important of those processes are:
Heat release produced by both the primary beam protons and secondary particles as well as by products of nuclear reactions induced by those particles; this heat release is intricately distributed over the window volume and varies with time due to the beam pulse structure and beam trips.
Radiation damage of structural materials.
Piling up of high concentrations of gas products of nuclear reactions, helium and hydrogen first of all, the diffusion of the gases, the formation of voids and/or bubbles with resulting changes in material’s properties.
High activation of the window material and of the surrounding construction materials induced by primary and secondary particles.
Complicated thermomechanical changes, both monotonous and cyclic, under the influence of the factors listed above.
In the framework of the Project detailed comprehensive theoretical investigations of all those processes are planned with the all available knowledge and world experience accumulated as a result of operation of existing proton accelerators with the beam power about 1 MW taken into account as well as the problems and difficulties realized in the process of designing the Project 559 lead-bismuth target and in conceptual studies of targets for higher levels of the beam power. Following data should be calculated:
1. Rates of all primary processes of the microscopic level to be taken into account: ionization, elastic and inelastic scattering, nuclear reactions.
2. Spatial distribution of heat release in the window material under the influence of all kinds of radiation in various assumptions on the beam current density profile for primary proton energies in the range of 600-1500 MeV.
3. Temperature fields generated by this heat release
4. Rates of displacements of atoms in the window material induced by protons, neutrons, alpha-particles, fission fragments of lead and bismuth; the processes of void formation and growth and /or gas bubbles under irradiation should be modeled numerically as well as the evolution of the material microstructure at operation temperatures and during annealing at high temperatures.
5. Intensities of irradiation of the internal window surface by fission fragments of lead and bismuth as a possible source of fragment corrosion known in liquid fuel reactors.
6. Rates of hydrogen and helium accumulation in the window material as well as of gaseous fission fragments giving rise to the gas bubble formation and growth and to the release of the gases from the material.
7. The influence of swelling and microstructural changes on mechanical properties of the window material (strength, plasticity).
8. The kinetics of accumulation of radio-nuclides in materials of the target induced by beam protons and by neutrons coming from the blanket in various assumptions on the neutron spectrum.
9. Mechanical stresses arising in the window material under the influence of the factors listed above and of cyclic fatigue.
In addition, some experiments will be done on the accelerator implantation of gaseous ions into specimens of the window material and the migration of those gases at different temperatures will be investigated.
The results will be used as a base for an evaluation of the window material degradation rate and of its lifetime before replacement for various beam power levels and beam configurations. The most powerful existing accelerators operate at beam power levels of about 1 MW. The behavior of beam window and target materials in this case is being investigated in details in several laboratories. Two main specific directions of activity in the proposed Project are new:
· Extension of the research to cover power range up to 10-20 MW.
· Taking into account the irradiation of the window by a high flux of fast neutrons emerging from the multiplication blanket.
Precisely these features make the difference between the ADS beam window and the windows of existing accelerators, even of those with record parameters.