Proposal for the continuation of generic high power target studies

Lead Research Organisation: University of Huddersfield
Department Name: Sch of Applied Sciences

Abstract

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Description Task 1: Power upgrade studies for the ISIS-TS1 target

Introduction

The high demand for neutron yield and beam time for the experiments at ISIS led not only to the construction of a second target station TS2 but also to an upgrade of the previous target TS1. This will lead to an increased neutron flux at the instruments enabling a larger number of experiments to be carried out in a much shorter time. The ISIS accelerator has been upgraded to achieve the increased beam intensity necessary to provide a 10 pulses per second (pps) proton beam to TS2 at the same time as maintaining present intensity to TS1 where the repetition rate is reduced from 50 pps to 40 pps. The ISIS TS1 target is driven by an 800 MeV, 200 µA proton beam equivalent to 0.16 MW beam power.

For the TS1 target the goal is to increase the operating power to 1 MW and beyond therefore our studies focused on a target design capable to cope with an increased heat deposition at 1 MW power and beyond while maintaining the spallation neutron pulse width at the current values.

Findings

For the new target design the Geant4 code was used in our simulations. Validation studies of the neutron yields energy spectra measured at various instruments pointing to the neutron moderators shown an excellent agreement with previous MCNPX predictions. As a result of this, the code was used in target design studies with confidence.

In order to keep the fixed solid target design and to cope with the increased heat deposition due to the additional power on target, more thiner plates are required. Various target plates configurations have been simulated, ending up with a new design consisting of 31 plates instead of the current 12 plates. The thickness of the tungsten plate could be reduced all the way down to 5 mm, however the existing 2 mm tantalum cladding thickness could only be reduced to 1 mm due to limitations imposed by manufacturing conditions. Therefore the new target design have tungsten plates cladded with tantalum with an increased thickness starting from 5 mm for the first 13 plates all the way up to 40 mm for the last plate. The tantalum cladding is 1 mm on all sides.

Initial Geant4 studies suggested that this design can cope with additional power on target plates up to 0.5 MW. However ANSYS studies of thermal analysis and thermal stresses suggested that the 31 plates design can cope up to 0.4 MW beam without modifying the cooling system. Alternatively the cooling efficiency should be increased to more than 3 times the current design value (from 6000 W/m2/C to 20,000 W/m2/C) in order to sustain a 1 MW beam power.

For a direct comparison between the new target design (0.4 MW) and the current TS1 target (0.16 MW), the neutron yields were plotted for each moderator type in the energy range of interest for the neutron instruments. A preselection cut was applied to select only those neutrons which enter the beamlines and which had their final interaction inside one of the neutron moderators in order to eliminate the neutron background. The direct comparison has shown that with the new target plates design the neutron yield is almost three times higher than with the current target design.

Further power increase can be achieved only by a complete redesign of the target either by having a rotating solid target or a molten metal target. Several materials were studies and it was found they will produce a higher neutron yield than tungsten, for the same incident proton beam power. These materials are Pb eutectics: PbAu, PbBi, PbPt, PbTe, PbSb, PbSn. PbTi is the only Pb eutectic which does not produce more neutrons for the same proton beam energy and current compared with the tungsten target.

Above 0.4MW we proposed a compact solid target that is accommodated by the same size pressure vessel used for the current ISIS target. It consists of three novel ideas: apply a beam off-centre offset on thin target discs instead of plates and rotate the discs such that the beam will hit the discs forming an impact ring at a radius given by the beam offset. Third, since the first discs will suffer the greatest thermal shock impact, for the first 10 discs we can use an alternating propeller design such that an incident proton cannot hit two consecutive discs. In this study all the constraints imposed for an upgrade of the ISIS TS1 target were considered (e.g. keeping existing shielding and instruments channels, preserving signal/ background ratio and the neutron pulses time structure and wavelength band, as well as not exceeding the current thermal stresses and peak temperatures in the target plates). Geant4 and ANSYS studies found that the new target can very easily cope with a 1 MW beam, the current target limits being reached only for a 3 MW proton beam.



Publications

These studies are currently in press in two journal publications:

"Power upgrade studies of the TS1 target at the ISIS Neutron Facility" - C. Bungau et al. submitted to Journal of Neutron Research

"Power upgrade design study of the ISIS-TS1 target"- C.Bungau et al. submitted to Physical Review Special Topics Accelerator and Beams

A paper for the power upgrade beyond 1 MW is in work.


Task2: Induced activation in accelerator components

Introduction

The residual activity induced in particle accelerators by high-energy neutrons is a serious issue from the point of view of radiation safety as the long-lived radionuclides produced by fast or moderated neutrons cause problems of radiation exposure for staff involved in the maintenance of accelerators. The long-lived radionuclides also contribute to the radioactive waste at the de- commissioning of the accelerator facility as beam components that becomes radioactive are certain candidates for failure as a result of radiation damage. Our studies were focused on computation of induced activation for the magnets and collimators in the High Energy Beam Transport line of the European Spallation Source due to the back-scattered neutrons from the target and also to the direct proton interactions and their secondaries. An estimate of the radionuclide inventory and induced activation were predicted using the GEANT4 code.


Findings

A complete list of radionuclide inventory present in each of the magnets and collimators of the ESS HEBT- S3 beam was determined. Based on their production and decay rates the induced activity in each magnet and collimator was calculated. All possible factors have been considered: the direct interaction of the proton beam with the elements, the interaction of the backscattered neutrons from the target, and the isotope production as a result of the decay of other isotopes produced in these interactions. The main factor proved to be the backscattered neutrons, the particle collimator placed in front of the target being the worst affected. Dose rates were calculated at 1 m from the element surface for the HEBT-S3 magnets and collimators.The induced activity was calculated both during and after a 365-day beam exposure, reaching a maximum of 100 Sv/h for this particular beam element.

Journal Publications:

These studies were published in 2014:
"Induced activation in accelerator components" - C. Bungau et al., Physical Review Special Topics Accelerator and Beams
DOI: 10.1103/PhysRevSTAB.17.084701
Exploitation Route The ISIS TS1 neutron target can be upgraded based on the new design in order to cope with a higher beam power. However the studies of power upgrade up to 1MW and beyond are not limited to the ISIS target only. The design with propeller discs can be adapted to any spallation source worldwide if a higher power on target is desired.

For the ESS studies, the complete list of radionuclides produced, the activity induced in accelerator components and doses are useful from the radiation protection point of view in the HEBT tunnel. The activation studies we performed were a starting point for a more detailed study that we currently work on for the ESS linac and beam dump. These new studies are funded by ESS directly.
Sectors Other
 
Description The studies that were performed for the ESS HEBT beam line had an impact on the radiation protection issues. As a result of this we received fundings from ESS to perform similar studies on the magnets in the linac and beam dump. Three reports were submitted to ESS.
First Year Of Impact 2016
Sector Environment,Healthcare
Impact Types Policy & public services