BS IEC 61888:2002 pdf free download

BS IEC 61888:2002 pdf free download Nuclear power plants — Instrumentation important to safety — Determination and maintenance of trip setpoints
5 Determination of setpoints
Trip setpoints in nuclear safety system instruments shall be selected to provide sufficient allowance between the trip setpoint and the analytical limit to account for uncertainties. Detailed requirements for safety system instrument setpoint relationships are given in the following clauses, as illustrated in figure 1 .
The importance of the various types of safety system setpoints differ, and as such it may be appropriate to apply different setpoint determination requirements. For automatic trip or actuation setpoints associated with systems important to safety, for example, those required by the plant safety analyses and directly related to reactor protection, emergency core- cooling, containment isolation, and containment heat removal, a stringent setpoint methodology should consider all of the items noted in 5.1 to 5.4.2. However, for setpoints that may not have the same level of stringent requirements, for example, those that are not credited in the safety analyses or that do not have limiting values, the setpoint determination methodology could be less rigorous. In general, all uncertainty terms for a particular setpoint methodology may not be required for all setpoint calculations. The methodologies utilised shall be documented and appropriate justification shall be provided.
5.1 Safety
Physical barriers are designed to prevent the uncontrolled release of radioactivity. Safety limits are chosen to maintain the integrity of these physical barriers. For this standard, design limits for engineered safety features are treated in the same way as safety limits. Safety limits can be defined in terms of directly measured process variables such as pressure or temperature. Safety limits can also be defined in terms of a calculated variable involving two or more measured process variables. An example of a calculated variable is the departure from the nucleate boiling ratio.
5.2 Safety analysis
The safety analysis establishes
a) an analytical limit in terms of a measured or calculated variable, and
b) a specific time after that value is reached to begin protective action.
Satisfying these two constraints will ensure that the safety limit of definition 3.1 7 will not be exceeded during anticipated operational occurrences and design-basis events.
Analytical limits are derived from the safety limits through the study of accident cases. The margin between these two values allows to take into account the time dependency of the process itself and the response time of the whole instrument channel protection (sensor, signal process, actuator, etc.). Analytical limits (AL) represent a value that should not be exceeded prior to accomplishing the prescribed action. In establishing the analytical limit for a setpoint the overall function initiated by setpoint action shall be carefully considered. Generally, the analytical limit is provided through design documentation or other calculations. If an analytical limit is not available, the need to prepare a setpoint calculation should be carefully considered.