The purpose of this supplement it to make recommendations pertaining to direct cycle boiling water power reactors in addition to those given in IEC Publication 231 and 231A. This supplement covers the general principles for instrumentation of water cooled reactors in which the primary coolant is light water and appreciable boiling may occur, i.e. boiling water reactors. A forced circulation direct cycle reactor is assumed. Steam from the reactor is used to drive the turbine directly without passing through an intermediate theat exchanger. Recommendations are given for neutron flux measurement, temperature measurement, coolant measurements, protection system, and reactor control.