Reactor Coolant System Heatup

The heatup and cooldown of the reactor vessel and the addition of makeup water to the reactor coolant system can cause significant temperature changes and thereby induce sizable thermal stresses. Slow controlled heating and cooling of the reactor system and controlled makeup water addition rates are necessary to minimize cyclic thermal stress, thus decreasing the potential for fatigue failure of reactor system components. 

Heatup and cooldown rates from 0 F/hr ("Isothermal" pressurization) to the design limit of 100°F/hr at 10 F/hr intervals are examined in determining the allowable heatup and cooldown rates as a function of temperature to meet low-temperature overpressure protection (LTOP) requirements for the Reactor Coolant system. LTOP considerations are discussed in Section 5.2.2.10. 

The chemical and volume control system is capable of maintaining a constant volume in the reactor coolant system while the plant is being heated up or cooled down. During a heatup it is... 

Figure 2.4-13 shows the reactor coolant system condition at 6 h. Liquid was being released intermittently through the PORV block valve. Two makeup pumps (HPI pumps) were running, and core heat removal was by heatup of the injected water. Steam generator heat transfer was blocked by hydrogen. 

The secondary-side reactor operator ran to his station and immediately increased the speed of the No. 2 main feedwater pump to compensate for the decrease of feedwater flow from the No. 1 pump. The primary-side operator had already opened the pressurizer spray valve in an attempt to reduce the pressure surge resulting from the heatup of the reactor coolant system due to a decrease in feedwater flow. 

While the operator was away from the primary-side control station, the pressurizer PORV opened and closed twice without his knowledge. The pressure had increased because of the continued heatup of the reactor coolant system that resulted when both steam generators had essentially boiled dry. 

System 80+ Allowable Heatup Cooldown Rates Reactor Vessel Reactor Coolant Pump Steam Generator... 

Plant heatup is a series of manual operations which bring the RCS from cold shutdown to hot standby. The SCS heat exchangers are bypassed to maintain flow through the core without the heat removal effect of the heat ixchangers. Flow can be initiated to the heat exchangers if necessary to control the heatup rate. When the reactor coolant pumps can be run, the SCS pumps are stopped and the system is isolated for the standby mode. 

A. At the onset of the loss-of-load transient, the reactor coolant and main steam systems are at maximum rated output plus a two percent uncertainty. By choosing the highest possible power output, the heatup rate of the primary loop is maximized along with the rate of pressurization. 

Core heatup begins with the start of boiloff of water from the core region. Before this time fuel temperatures are close to the system saturation temperature because there is very little heat transfer resistance between the fuel and liquid reactor coolant. So long as fuel remains submerged, it is not expected to be damaged due to high temperature. 

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