B. Rouben McMaster University EP 4P03/6P03 2016 Jan-Apr CANDU ROP/NOP Systems B. Rouben McMaster University EP 4P03/6P03 2016 Jan-Apr 2016 January

Neutronic Protection Systems CANDU reactors are equipped with protection systems which detect an emergency situation and actuate the safety system(s). There is a separate neutronic protection system for each SDS. Each protection system is triplicated [has 3 separate “logic” (or “safety”) channels] and consists of out‑of‑core ion chambers and in‑core self‑powered detectors. Logic channels are D, E, and F for SDS-1 and G, H, and J for SDS-2. In each protection system, it suffices that 2 of 3 logic channels be “tripped” for the corresponding SDS to be actuated. 2016 January

Out-of-Core Ion Chambers There are 3 ion chambers in each protection system, 1 per logic channel. They are located at the outside surface of the calandria (see next Figure). Each ion chamber trips its logic channel when the measured rate of change of the logarithm of the flux , i.e. the quantity d(ln )/dt, exceeds a pre‑determined setpoint (e.g. 10% per second, i.e., 0.10 s‑1, for SDS-1 in the CANDU 6). 2016 January

Ion-Chamber Locations (eg. in C-6) 2016 January

In-Core ROP (NOP) Detectors There are also fast‑response (platinum or inconel) in‑core detectors in each protection system. In the CANDU 9: 54 in-core detectors for SDS-1, in vertical assemblies, and 48 for SDS-2, in horizontal assemblies (see examples in figure in next slide). The detectors are distributed among the various logic channels: channels D, E and F contain 11 or 12 detectors each, channels G, H, and J contain eight each. The detectors trip the logic channels on high neutron flux: when the reading of any 1 detector reaches a pre‑determined setpoint, the logic channel to which it is connected is tripped. The in-core-detector system is known as the Neutron‑Overpower‑Protection (NOP) or Regional-Overpower-Protection system (ROP). The detector trip setpoints are determined by an extensive analysis of hypothetical loss‑of‑regulation accidents. 2016 January

Location of Some SDS-1 (Vertical) and SDS-2 (Horizontal) NOP Detectors in Bruce A VFD16-R2D VFD20-R5D Assembly 6,16 9,13 17 8,12 4, 20 V F D 3 - R 1 F VFD3-R1F 2016 January

ROP Detector Readings in LOR Suppose there is a Loss of Regulation (LOR), with power rising. As the reactor power increases in the LOR, so will the flux increase at detector locations. The detector reading’s increase will depend on the detector’s location in core, relative to the flux peak, i.e., detectors may “see” the power increase to different degrees. The design of the detector locations, channelization, and trip setpoints is such that, in the event of a global or local power increase from a wide set of anticipated flux shapes, the system logic initiates a reactor shutdown before Onset of Intermittent Dryout (OID) is reached in any fuel channel. 2016 January

Race Between Channel Powers and Detector Readings For each ROP system, the objective is that 2 detectors in different safety channels reach their setpoint before any fuel channel reaches its critical channel power, taken here as the channel power at which fuel dryout is reached. Critical Channel Detector Power Trip Setpoint Reactor Power Increasing in a Loss of Regulation Channel Power Detector Reading 2016 January

The ROP design activity must determine: ROP Design Objective The ROP design activity must determine: The number of detectors required (for each ROP system) Their positions in the various available penetrations, to “see” a very wide range (hundreds) of anticipated flux/power shapes The detector setpoints The detector channelization so that the core is protected against an LOR leading to OID from any of the anticipated flux shapes 2016 January

Triplicated Tripping Logic The tripping logic of each triplicated protection system is as follows (see next Figure): One ion chamber can trip its logic channel on high log rate, or any 1 detector in the logic channel can trip the channel on high flux. Any 2 tripped channels will actuate the associated shutdown system. The triplicated tripping logic reduces the chance of a spurious trip, and allows the testing of the system on-line. 2016 January

Triplicated Tripping Logic for SDS-1 2016 January

END 2016 January