This International Standard applies to pressurized water reactors (PWRs) with configurations similar to those shown in figures 1 and 2 and presents requirements for the monitoring of adequate cooling within the core during cold shutdown operations.
Adequate core cooling can be achieved only by providing sufficient coolant flow to the core to remove the heat. During cold shutdown operations, core cooling is provided by forced circulation with the residual heat removal system (RHRS). However, in certain shutdown operations when water level in the reactor pressure vessel (RPV) is reduced for maintenance operations, forced circulation may be interrupted, and the core may become overheated.
It is important that plant operators have reliable information to confirm that the temperature and flow of coolant circulated through the RPV is adequate to remove heat from the core. This information includes a reliable measurement of water level in the RPV outlet piping used for circulating the flow from the core to the RHRS, as well as measurements of temperature and flow of the coolant. An unreliable measurement of water level can result in interruption of coolant flow and overheating of the core, an event that has occurred at several PWRs. Annex A, which describes some of these events, identifies conditions which should be considered in the design of core cooling monitoring instrumentation.
Good international practices to be used when designing new or upgrading existing PWR core cooling monitors are summarized in this standard.
Requirements are given in this standard for instrumentation to monitor core cooling for safe operation of PWRs during cold shutdown operations when the coolant temperature is below 100 ℃ (212 °F). Requirements for core cooling monitoring during conditions beyond a design basis accident (DBA), which could be a specific national requirement or consideration, are not covered in this standard.
The core cooling monitoring instrumentation for cold shutdown functions when the reactor coolant system (RCS) is configured for cold shutdown maintenance or refuelling. The circumstances under which these measurement systems need to function are described in this standard. Descriptions of diverse measuring principles and suitable devices are given together with requirements for the following:
- operational conditions;
- installation;
- operator displays;
- testing, calibration and maintenance;
- equipment qualification;
- documentation.