Excess coolant inventory in the various circuits which serve to cool the core and the surroundings structures is controlled by the Reactor Draining System. The draining system is designed to perform the following functions:
The reactor draining system consists of three following components:
4.4.1 The Reactor Cavity Draining System
A block diagram of the reactor draining system is shown in Fig. 4.19.
The upper plate of bottom metal structure (6) is a bottom boundary of the reactor cavity (4), where moisture is accumulated in case of leaking control rod channels, radial reflector cooling channels and fuel channels during reactor operation. The 150 mm diameter pipes are welded into each of the bottom metal structure quadrants. Outside the reactor the pipelines merge in pairs into a pipeline, which is welded downstream into an 8 m - high hydrolock (11). The merged pipelines from both reactor halves are equipped with valves (14) for visual drainage monitoring.
The four 50 mm diameter draining pipelines of the upper steam-gas release circuit (1) merging into the 150 mm diameter pipeline of drainage removal from the bottom metal structures into the 8 - meter hydrolock (11). There is a measuring tank (13) with water level indicator in the 8 m hydrolock outlet piping, which allows to monitor the leak rate of water drained from the bottom metal structure and upper steam-gas release circuit pipelines. From the 8 m hydrolock water is further directed to a "dirty" demineralized water tank (17).
Fig. 4.19 Block diagram of the reactor draining system
1 - upper steam-gas release circuit pipelines, 2 - upper interbellow cavity, 3 - reactor interface cavity, 4 - reactor gas cavity, 5 - upper interbellow cavity, 6 - bottom metal structure, 7 - control rod channel bellow, 8 - temperature detector, 9 - upper steam-gas release circuit pipelines, 10 - drainage visual indicators, 11 - 8-meter hydrolock, 12 - heat exchanger, 13 - drainage measuring tank, 14 - valves for visual drainage monitoring, 15 - tank-hydrolock, 16 - draining tank, 17 - "dirty" demineralized water tank, 18 - floor drain water reception tank 19 - peripheral ionization chambers
The control rod channel bellow (7) draining pipelines are located at the lower housing of a control rod channel. The upper part of the housing shows up above the upper plate of bottom metal structure (6). Moisture may appear in control rod channel bellows from the reactor cavity by condensing on colder walls of a control rod channel and flowing downwards. Condensate from every bellow is drained through 10mm diameter pipelines into a 50mm diameter header. Each of the reactor quadrants has a header of its own. Nitrogen or a nitrogen - helium mixture from the reactor gas circuit is pumped through all the draining pipelines. Each control rod channel bellow draining pipeline is equipped with a temperature detector (8), which indicates the appearance of leaks. The signals go to MCR at drainage temperature drop to 160°C. Leaks from each of the reactor quadrant draining headers are drained through drainage visual indicators (10) into a heat exchanger (12) and after cooling are collected in a control rod channel bellow drainage measuring tank (13). In the tank moisture is separated from gas (nitrogen or a nitrogen - helium mixture), the gas being returned into the reactor gas circuit, and the condensate being drained via a hydrolock into the draining tank (16).
The draining pipelines of the lower steam-gas release circuit (9) consist of two outlet 50mm diameter pipes merging into a discharge pipeline of the control rod channel bellow drainage measuring tank into the draining tank (16).
The total value of leakage at the bottom of the reactor is measured by registering of the leakage into the both measuring tanks (13) Bottom metal structure (6), upper steam-gas release circuit pipelines (1) and control rod channel bellow (7) drainage measuring tanks (13) have level meters and motor-driven discharge valves. The system operates in an automatic mode. Signals from level indicators are processed in a summator and self- recorded in terms of cumulative the reactor cavity water leak rate.
An enunciator sends a warning signal in the Gas Circuit Control Room when the leak rate increases to 7 l/h, an emergency alarm is sent to MCR (Main Control Room) when the leak increases to 10 kg/h. Upon receipt of the alarm, standard plant operating procedures require the operator to terminate reactor power by activating the AZ-1 signal. No signals are transferred to the Control Protection System.
4.4.2 Metal Structures and Peripheral Ionization Chambers Draining System
By design some metal structures gas cavities (upper (2) and lower (5) interbellow cavities and reactor interface cavity (3)) have an interface with biological shielding - annular water tanks. The metal structures draining system is designed to monitor status of biological shielding - annular water tanks and drain potential leaks and condensate from these metal structures gas cavity. The leaks are drained through a special 40 mm diameter pipeline from all lowest points of the gas cavity compensating bellows. Each draining pipeline has a valve (14) for visual monitoring of condensate presence. Leak number control was not envisaged by the design. In order to avoid connection between reactor metal structures gas cavities and the atmosphere, the draining pipelines are welded into a tank-hydrolock (15). The content of the tank-hydrolock is drained into a floor drain water reception tank (18).
The peripheral ionization chambers (19) are located in the reactor biological shielding - annular water tanks. A special draining system is provided to monitor the leak-tightness of ionization chambers casings and timely drain potential leaks and condensate. Separate drainage pipes (40 mm diameter) from each chamber are connected to tank-hydrolock (15). The each pipeline from ionization chambers casing is equipped with valves (14) for visual drainage monitoring.