Host: The Japan Society of Mechanical Engineers
Name : The 30th International Conference on Nuclear Engineering (ICONE30)
Date : May 21, 2023 - May 26, 2023
The control rods moved into and out of the reactor core by means of Control Rod Drive Mechanisms (CRDMs). During reactor trips, the loss of power to the CRDMs will drop all control rods into the core quickly, thereby shutting down the nuclear reaction. The Rod Drive Motor-Generator Sets provide 260V three-phase power to the CRDMs through the reactor trip switchgear. There are two MG sets with flywheels. During normal operating conditions, both MG sets operate in parallel and equally share the total load demand. Each MG set is capable of supplying the entire load requirements when the other set is out of service. However, it is not uncommon the simultaneous failure of two MG sets resulted in loss of power to the CRDMs.
Normally, the nuclear power plant sets the shutdown protection signal with a Power Range High Negative Neutron Flux Rate. Therefore, if the control rods moved into reactor core quickly, the signal will be generated to trigger the shutdown signal, and then the turbine trip signal will be sent. However, because the passive nuclear power plants are equipped with a Rapid Power Reduction System, it is impossible to set a Power Range High Negative Neutron Flux Rate signal, which can trigger the shutdown signal. If both MG sets fail unexpectedly in passive nuclear power plant, CRDMs will lose power supply, and the control rods will fall into the reactor core under the action of gravity , and the reactor automatically shuts down. However, no trip signal will be generated during this process, which cannot trigger the turbine trip. The safeguards actuation caused by the low steamline pressure unexpectedly in a short time.
For the above problems, the following two solutions are proposed. Solution 1: some undervoltage relays are set on the outlet power supply cable of the MG sets to monitor the status. During the power operation, if both MG sets fail at the same time, the reactor trip signal will be triggered according to the low voltage signal, and triggers the turbine trip signal will also be triggered after a delay of 5 seconds. Solution 2: referring to other conventional nuclear power plants, reactor trip protection with power range high negative neutron flux rate could also be adopted in passive nuclear power plants. Of course, necessary optimization is indispensable.
