As with traditional light-water PWRs, the safety analysis and regulatory requirements drive the safety instrumentation design. The regulatory requirements stem largely from the national regulations as well as several other regulation guides and referenced documents. The safety analysis is the process of evaluating the plant response to anticipated occurrences, accidents, and abnormal events to ensure the safe response of the reactor. The safety analysis defines and models the expected events, steady state outcomes, anticipated occurrences, transients and accident conditions to ensure fuel design limits are not exceeded. It defines and models the reactor vessel pressure boundary limits and the containment pressure boundary limits to ensure they are not exceeded. It also analyzes radiation conditions in order to minimize the radiation dose. The design response to the safety analysis and regulatory requirements is the called the reactor protection system (RPS) and is usually defined as the set of all reactor trips (RTs), engineered safety features (ESFs), and monitoring, required to meet all safety system analyses and regulations in order to provide safe monitoring and safe shutdown of the reactor.

For example, the need for a reactor trip when reactor pressure drops to a certain level is derived from the safety analysis requirement to protect the core from a core damage condition known as departure from nucleate boiling (DNB). This requirement drives the need to measure reactor pressure and temperature in order to know the conditions associated with DNB, and so protect the reactor from this potential accident condition.

The safety system instrumentation must be designed to protect the core from damage and to operate the reactor within safe limits as defined by the safety analysis and applicable regulations. For these reasons the following instrumentation requirements are generally required. Unique iPWR designs may preclude some of the instrumentation measurements itemized below, or may add additional instrumentation, but for the most part the following instrumentation will be required:

• pressurizer pressure and reactor pressure;

• pressurizer level;

• core temperature;

• reactor coolant temperature (wide range, and narrow range, hot leg and cold leg);

• reactor vessel level;

• steam generator level;

• reactor coolant flow;

• reactor water storage tank level;

• feedwater flow;

• main steam flow;

• main steam pressure and temperature;

• reactor power;

• core power flux (power range, intermediate range and source range);

• reactor coolant pump voltage and frequency;

• containment pressure temperature and level.

The measurement requirements listed above are used in the RPS, where RTs and ESF actuations are automatic based on sensed parameter values. Some of the sensed RPS signals are provided downstream, through isolators, to the non-safety NSSS control system, where control actions are taken in automatic or manual to keep the reactor operating within established limits. Additional safety related parameters that may be needed based on some iPWR designs are:

• safety accumulator tank pressure;

• safety accumulator tank level;

• safety valve positions;

• feedwater flow;

• boron concentration, temperature, tank levels and mixing amounts.

The following sections describe the traditional and new technological devices that are used and will be used to measure the parameters described above.