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Description
The ESS target is designed with three levels of defence in depth to allow for safe operation of the tungsten target with pulsed proton beams from the linac. Target Safety System (TSS) is a dedicated safety group system against radiological events acting in the last level. The target wheel rotation speed is one of the parameters monitored by TSS to trip the proton beam to target. The wheel is designed with 36 sectors such that each beam pulse is incident on a single sector during rotation. The beam repetition rate from the linac is 14 Hz which gives 23.33 rpm for normal operations. To set the TSS trip point for the reduced wheel rotation, a model based on the expected shift in the beam footprint on the target wheel geometry and associated criteria for the trip are developed in this work (see Figure 1).
Although the design basis safety case analysis of wheel rotation assumes complete loss of rotation, anticipated transients such as loss of power to the are evaluated on allowed oxidation from the tungsten surface during the transient. Tungsten oxidation is dependent on the peak surface temperature which is a function of the heat load from the proton beam pulse and the allowed cooling between consecutive incident pulses. Consequently, trip condition or criterion is based on the allowed overlap between two consecutive pulses measured in terms of RMS value of beam profile. The calculated trip points for different trip criteria include the test data for coast down characteristics of the wheel, the uncertainty in TSS sensors, and the response time of the TSS (see chart in Figure 2). The recommended trip condition for TSS is below 3 RMS overlap (or above 14.26 rpm) which is valid for proton beam with averaged beam power of 5 MW or lower.