A test system comprising: a reactor, the reactor comprising a fluid circuit, a test zone for at least one item under test, said test zone being included in said fluid circuit to expose said at least one item to fluid flowing in said circuit during use, a plurality of control zones included in said fluid circuit at a respective different location, each control zone including at least one control device for controlling at least one parameter of said fluid in accordance with control information, wherein, for at least one of said control zones, said at least one control device comprises a fluid injector, the fluid injector being operable to inject fluid into said fluid circuit; and a control system for controlling the operation of the reactor, the control system being in communication with said control zones to provide said control zones with said control information and wherein said control system is configured to: receive input data specifying at least one test value for said at least one parameter; predict the behaviour, in respect of said at least one fluid parameter, of said fluid using said input data and a mathematical model of the reactor; calculate said control information based on the predicted fluid behaviour; and communicate said control information to at least one of said control zones. a reactor, the reactor comprising a fluid circuit, a test zone for at least one item under test, said test zone being included in said fluid circuit to expose said at least one item to fluid flowing in said circuit during use, a plurality of control zones included in said fluid circuit at a respective different location, each control zone including at least one control device for controlling at least one parameter of said fluid in accordance with control information, wherein, for at least one of said control zones, said at least one control device comprises a fluid injector, the fluid injector being operable to inject fluid into said fluid circuit; and a fluid circuit, a test zone for at least one item under test, said test zone being included in said fluid circuit to expose said at least one item to fluid flowing in said circuit during use, a plurality of control zones included in said fluid circuit at a respective different location, each control zone including at least one control device for controlling at least one parameter of said fluid in accordance with control information, wherein, for at least one of said control zones, said at least one control device comprises a fluid injector, the fluid injector being operable to inject fluid into said fluid circuit; and a control system for controlling the operation of the reactor, the control system being in communication with said control zones to provide said control zones with said control information and wherein said control system is configured to: receive input data specifying at least one test value for said at least one parameter; predict the behaviour, in respect of said at least one fluid parameter, of said fluid using said input data and a mathematical model of the reactor; calculate said control information based on the predicted fluid behaviour; and communicate said control information to at least one of said control zones. receive input data specifying at least one test value for said at least one parameter; predict the behaviour, in respect of said at least one fluid parameter, of said fluid using said input data and a mathematical model of the reactor; calculate said control information based on the predicted fluid behaviour; and communicate said control information to at least one of said control zones.