On the structural integrity of the container for a liquid metal spallation target under high power pulsed proton irradiation

摘要

Neutrons are an ideal probe for understanding the microscopic structure and dynamics of the matter and its behaviour. They are mainly produced by the fission chain reaction in reactors or by some accelerator-based reactions such as the spallation. An increase of the neutron flux of reactors for a better instrumental resolution is limited by heat transfer problems. Even if pulsed reactors may partially overcome this limits, a more effective way to produce neutrons seems to be the spallation reaction because the amount of energy released per available neutron is smaller by an order of magnitude. Profiting of the significant advances in the accelerator technology during the past 20 years, a new spallation source has been planned. The specifications given for the European Spallation Source (ESS), a 2 X 5 MW linear accelerator as the power source, two target stations with different pulse repetition rates: Short Pulse Target Station (SPTS) at 50 Hz repetition rate, 1 microsecond proton pulse length, Long Pulse Target Station (LPTS) at 16(2/3)/s to (-1) repetition rate, 2 ms proton pulse length, a peak neutron flux up to 2 X ( 10 to 17) neutrons/(cm to 2 and second) for the SPTS, will, besides assuring the availability of a general purpose neutron source for the research, also enlarge its actual application field. A liquid metal target appeared to be the best choice in order to fulfil the given specifications for the neutron production and lifetime. In order to identify and solve the problems connected with the structural integrity of the liquid metal target within the specified operative conditions the international ASTE (AGS Spallation Target Experiment) collaboration was created. Within this collaboration a liquid mercury target with a simplified geometry was built. In different experiments which took place between 1997 and 2001 various efforts in order to measure relevant quantities as pressure, strain or temperature under realistic conditions were done. Considerable experience was gained concerning the experimental techniques necessary to measure such quantities in a highly radioactive environment . The finite elements simulations of the problem besides giving results in good agreement with the experimental strain data, provided a better insight as far as the pressure measurements in the mercury are concerned. The estimated maximum stress values under the ESS operative conditions in the first critical instants after the beam energy deposition are still within the elasticity limits for the materials under examination. Nevertheless, the modifications in the mechanical properties induced by the irradiation and also by the probable corrosion and cavitation need further investigations.