Plant modes of operation

Modes of operation
Reactors have various modes of operation, spanning from start-up, power operation, shutdown and refuelling. The relevance here is that these modes usually have different sets of associated EOP and SAMG. Modes can be different in different countries; examples are here given for widely used modes of operation.
PWR Modes of Operation
Mode 1 is the power operation mode; power is greater than about 5% full power.
Mode 2 is the start-up mode, in which the reactor is critical and power is less than or equal to 5% full power.
Mode 3 is the hot standby mode, in which the reactor is critical and Tave greater than or equal to 165 °C (typical value, actual value is plant-specific). RCS pressure may be above or below the pressure at which accumulators can discharge. Thus, injection from accumulators can be affected.
Mode 4 is the hot shutdown mode, in which the reactor is subcritical and RCS Tave is typically between 165 °C and 93 °C (actual value is plant specific). In this mode, any high pressure injection is shut off, accumulator discharge valves are closed, the RCS pressure and temperature are reduced to the Residual Heat Removal system (RHR) operational conditions, and the RHR is placed in service. When this is accomplished, the steam generators are removed from service, followed by shut-down of the Reactor Coolant Pumps (RCPs).
Mode 5 is cold shutdown mode, in which the reactor is subcritical and RCS Tave is less than 93°C (typical). In this the RCS has been cooled down and steam generators are not available. During this mode, the following applies:
• The RCS is depressurized.
• The steam generators are not available; they may be prepared for outage activities (drained, filled, or placed in lay-up).
• Containment integrity may not be intact (generally only as necessary for outage activities).
• The RCS may be drained down to mid-loop operation, in preparation to removal of the Reactor Pressure Vessel (RPV) head (occurs in mode 6).
• The RCS pressure boundary may be opened (for example, pressuriser manway cover and steam generator primary side manway cover).
Mode 6 is refuelling mode. Transition is made from mode 5 to mode 6 when one or more closure head bolts are less than fully tensioned. RCS level is at mid-loop, with core cooling by the RHR system. The RPV head is removed, the fuel transfer is in preparation for refuelling. Refuelling commences.
BWR Modes of Operation
These are largely identical to the PWR-modes, except that no hot standby is defined and there is no steam generator.
Mode 1 encompasses all operational states associated with power operation.
Mode 2 represents the start-up and return to full-power operation.
Mode 3 represents the hot shutdown operational states. In this state, decay heat is initially rejected by the feedwater/ condensate system. If operators close main steam isolation valves and open safety relief valves to dump steam to the suppression pool, the suppression pool cooling mode of the RHR system is used. After the closure of main steam isolation valves, decay heat removal is accomplished by the RHR system in shutdown cooling mode.
Mode 4 contains all the operational states associated with cold shutdown conditions. Decay heat removal is performed by the shutdown cooling mode of RHR. In this mode of operation, it is permitted to open the containment.
Mode 5 encompasses the operational states associated with reactor refuelling. In this mode of operation, both the RPV pressure boundary and containment are open. The RHR is used exclusively in shutdown cooling mode. Because the RPV water level is drained below the steam lines and steam line plugs are installed, RHR cannot be operated in suppression pool cooling mode. In this state, there can be a number of distinct configurations of relevance to SAMGs, reflecting whether the RPV and Spent Fuel Pool (SFP) are connected as a contiguous water volume and whether the RPV has been defueled.
CANDU Modes of Operation
The CANDU PHWR modes of operation are similar to those shown above for PWR, however, the modes of operations are not itemised into Mode 1, Mode 2, Mode 3, etc. Unlike the PWR, refuelling is on-line, i.e. Mode 1. Note: in CANDU language, a ´steam generator` is usually called a ´boiler`.
The general CANDU modes of operation or operating states are as follows:
• Full Power Operating State – Similar to PWR.
• Zero Power Hot State - Condenser vacuum is still established and is available for cooldown on Condenser Steam Discharge Valves (CSDV). One main boiler feed pump and one main condensate extraction pump are running. The deaerator is still hot and level is controlled at its normal setpoint. Boiler level control is in very low load mode. The Heat Transport (HT) System is in normal mode.
• Various Shutdown States
     o Cold Pressurised Shutdown Cooling State - Condenser is at atmospheric pressure. All main boiler feed pumps      are shutdown. All main condensate extraction pumps (CEPs) and the auxiliary CEP are shut down. All deaerator heaters      are off and the deaerator is cooling down. All HT System pumps are shutdown. Two HT System feed pumps are      running. One Shutdown Cooling pump is in service (depending on heat sink requirements).
     o Cold Depressurized Shutdown Cooling State - Condenser is at atmospheric pressure. All main boiler feed pumps      and the auxiliary pump are shutdown. All main condensate extraction pumps and the auxiliary CEP are shut down.      All HT pumps are shutdown. HT feed pumps are shutdown. Shutdown cooling pump is in service (depending on      heat sink requirements).
     o Drained Guaranteed Shutdown State (GSS) - The moderator drained GSS is established by first establishing the      overpoisoned GSS, then draining the moderator from the calandria, and applying additional condition guarantees      to isolate liquid poison injection and to establish a 'hole' in the moderator system below calandria level; so that any      heavy water entering the moderator system cannot accumulate in the calandria.
     o Overpoisoned Guaranteed Shutdown State - The overpoisoned moderator GSS is established by adding      sufficient neutron absorbing poison to a full calandria to ensure subcriticality, then applying condition guarantees to      prevent poison removal, dilution, and precipitation.
|