Experimental study of consistency degradation of different greases in mixed neutron and gamma radiation

Abstract

Many of the moving components in accelerator and target environments require lubrication. Lubricants in such environments are exposed to high fluxes of secondary radiation, which originates from beam interactions with the target and from beam losses. The secondary radiation is a mix of components, which can include significant fractions of neutrons. Lubricants are radiation-sensitive polymeric materials. The radiation-induced modifications of their structure reduce their service lifetime and impose additional facility maintenance, which is complicated by the environmental radioactivity. The study of the lubricants radiation resistance is therefore necessary for the construction of new generation accelerators and target systems. Nevertheless, data collected in mixed radiation fields are scarce. Nine commercial greases were irradiated at a TRIGA Mark II Research Reactor to serve for the construction of new accelerator projects like the European Spallation Source (ESS) at Lund (Sweden) and Selective Production of Exotic Species (SPES) at Legnaro, (Italy). Mixed neutron and gamma doses ranging from 0.1 MGy to 9.0 MGy were delivered to the greases. For an experimental quantification of their degradation, consistency was measured. Two of the greases remained stable, while the others became fluid. Post-irradiation examinations evidence the cleavage of the polymeric structure as the dominant radiation effect. Dose and fluence limits for the use of each product are presented. Apart from the scientific significance, the results represent an original and useful reference in selecting radiation resistant greases for accelerator and target applications.

Radiation Effects in Greases for Use in Facilities Producing Intense Neutron Fields

“…The colour of some of the greases becomes manifestly darker. The grease having a high fluorine content developed acidic gases that corroded the surrounding metallic environment [6,12]. Similar qualitative radiationinduced effects have been reported by R.O.…”

“…For the most radiation-sensitive among the tested greases, 60% consistency increase was reported at 1.6 MGy. G3 and G6 grease samples showed a stable behaviour up to the maximum radiation dose of 9 MGy, reporting less than 5% consistency variation [6].…”

“…Table I reports the most relevant information about the selected grease. These samples were irradiated in the mixed radiation field of the CT [6]. Exposure times for grease samples range from 10 minutes to 10 hours, corresponding to total neutron and photon fluences up to a maximum of about 6.19 • 10 17 n cm −2 and 5.94 • 10 17 ph cm −2 , respectively.…”

“…The hydrogen amount varies from 0.00% for a fluorinated grease to 14.13% for a mineral-oil based grease. The total dose rate for greases varies correspondingly from about 300 kGy/h to 900 kGy/h at the nominal reactor power of 250 kW [6].…”

“…Irradiation and testing methodologies have been developed in the last years, dedicated to the selection of elastomeric O-rings to be used in the SPES project [4,5]. The objective of this study is to examine nine commercial greases for selection for applying to use in ESS and SPES facilities [6].…”