Water Distribution Networks (WDNs) are vulnerable to accidental or deliberate contamination. Such contamination can be detected and identified by deploying a network of sensors. If the sensor network detects the presence of a contaminant, it is also very important to take corrective response actions to minimize the effects of contamination on the population being served. One possible mitigation option is to prevent the contaminated water from reaching any customer, by shutting down the distribution network using shut-off valves placed in the WDN. The design problem considered in this work is to determine pipes where the shut-off valves can be optimally located such that it is possible to prevent the contaminated water from reaching any demand point, regardless of the source node from where the contamination has originated. We refer to this problem as the actuator network design problem. We map the problem of actuator design into a graph partitioning problem and the minimal set of actuators are identified for ensuring the total shutdown of the network. (C) 2017 Elsevier Ltd. All rights reserved.