The fuel cladding monitoring system is intended to monitor fuel cladding integrity. An indicative parameter of fuel integrity is the radiation level the coolant leaving the reactor core. This is characterized by the level of activity of nuclear fission products in the steam and coolant. The fuel cladding monitoring system serves both safety and operational tasks. As a safety related system it is intended to prevent violation of the design limits or damage of fuel elements and to restrict radiation impact on personnel, population and environment. The purpose of the Fuel cladding monitoring system is to identify fuel assemblies with leaking fuel elements (fuel cladding failure) during power operation, refueling, transients modes and plane preventive maintenance. This system also helps to determine the necessity and scheduling of unplanned reloading of fuel assemblies with leaking fuel elements. In addition, it also provides redundant monitoring of coolant flowrate in fuel channels and a check of the fuel type. A schematic of the system is presented in Fig. 5.17. There are separate group monitoring and individual channel monitoring subsystems, which are structurally independent, but are related.
The group monitoring subsystem diagnoses fuel element failure with a delay no longer than 10 minutes. Upon detection of a leaking fuel element it activates the warning and alarm signaling that a violation of fuel cladding integrity occurred, and also actuates the signal which activates the individual channel monitoring subsystem. This subsystem consists of four identical monitoring trains (A1-A4). Each of these trains includes two sampling lines, a gas removal device (3), a moisture separator (4), a heater (5), and two sensors equipped with electric precipitators (6), which are separated by a delaying tank (7). Sampling lines are used to transport steam from steam lines of each separator drum into the chillier (2), where the steam is cooled and condensed. Radioactive inert gases (isotopes of krypton and xenon) are removed from the condensate by a gas removal device (3). A moisture separator (4) and a heater (5) are used to decrease the absolute and relative humidity of the gas. Condensate from the gas removal device (3) and moisture separator (4) is drained to a special drainage system. Operation of the sensors is based on electronic precipitation of ions of rubidium and cesium on a wire, which is then moved to the scintillation detector to record the beta-activity of the precipitated isotopes. The first sensor is used to monitor the total activity of radioactive inert gases, the second one, the activity of relatively long-lived isotopes. Estimation of the state of fuel cladding is based on the results of measurements of these two sensors. After passing the sensors the samples of gas are removed by a special ventilation system.
Fig. 5.17 Schematic of fuel cladding integrity monitoring system
A - the train of group monitoring subsystem, B - the train of individual channel monitoring subsystem,
1 - separator drum, 2 - cooler, 3 - gas removal device, 4 - moisture separator, 5 - heater, 5 - heater, 6 - sensor with electric precipitation, 7 - delay tank, 8 - row of steam-water pipelines, 9 - individual channel monitoring sensor, 10 - cart, 11 - box
Monitoring of fuel cladding integrity of individual channels is intended to estimate the state of the fuel cladding by recording the gamma-activity of the coolant in each steam-water pipe exiting the core block. In addition the system provides spectral analysis and is capable of identifying weather the design limits of fuel clad damage are violated. It can diagnose the type of fuel channel loading using relative changes of signal amplitude generated by the residual activity of nitrogen-16 in the steam-water piping. The individual channel monitoring subsystem consists of eight monitoring trains (B1-B8), Fig. 5.17. Each of these provides monitoring for two rows (8) of steam-water pipes going to the drum separators (1). Each monitoring train measuring the gamma-activity of the coolant in the
steam-water piping includes two scintillation sensors (9). The sensors are placed in a lead protecting cover with apertures for collimation. The protecting cover is assembled on special carts (10). There are four rows of pipes supplying steam water mixture to the drum separator. Movable enclosures (11) containing the sensor assembly are guided along the rows of steam-water pipes.
The output from the group and individual channel monitoring systems of fuel cladding integrity is provided in an analog mode on paper in recording devices, signal analysis is performed using special algorithms. The design of the fuel cladding monitoring system provides two possibilities for system operation - with and without use of the plant computer system. Measurements from group and individual monitoring subsystems are checked in all ranges of reactor power. If fuel assembly with leaking fuel elements are identified by the fuel cladding integrity system, and if operational limits of fuel element damage are violated (that is activity of I-131 in MCC coolant above 4.0 10-6 Cu/kg, then such a fuel assembly is removed from the fuel channel. The reactor must be shut down immediately, if measurements in corresponding drum separator increase two or more times as compared to the initial value. The reactor must be shut down if the following failures of the fuel cladding integrity monitoring system are not repaired in a specific time: