Primary circuit pipes

Rapsodie (France) displacement monitoring of the vessel inlet and outlet 

PEC ataly) visual inspection of primary piping at reactor shutdown 

JOYO (Japan) sodium leakage monitoring, visual examination, ultrasonic test, displacement measiuement, and test piece surveillance 

BR-10 (Russian Federation) sodium leak monitoring, visual examination 

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A separate works have been carried out in order to develop AUGUR 4 2 procedure for expert ISI of primary circuit piping welds. 

Computer system of defect assessments as applied to primary circuit piping is developed as the tool of expert ISI strength supporting. Results of defect assessments are governed by adequate data on stress distributions and defect geometry. 

After a few years of operation there was a significant increase in radiation fields from the primary circuit piping in the Douglas Point generating station. Other water-cooled reactors around the world experienced similar effects. The principal source of the radioactivity was traced to cobalt-60, formed by neutron absorption in the natural cobalt-59 which arose from hard-facing alloys and was also present as an impurity in boiler materials such as HMDnel, and in carbon steel and other structural materials. The mechanism of this radioactivity transport was found to be corrosion of the cobalt bearing materials, transport... 

Slight chemical activity of LBC enables to realize two-circuit scheme of heat removal, so the RIs become simple and cheaper. The emergency processes associated with tightness loss by primary circuit pipes and intercircuit SG leaks occur without hydrogen yield and any fire and explosion hazards. 

Isolation valves are provided on all primary circuit pipes penetrating the leaktight compartment walls with the exception of the risers from the channel tubes to the steam drums. Steam emitted from these can be at least partially condensed by the ventilation system. 

An accident at a nuclear power plant can be caused by many combinations of anomalous initiating event, malfunction and human error. The types of possible accidental situations are studied in the specific safety analysis of each plant and the safety systems described above are designed to prevent, or mitigate the effects of all the accidents chosen as DBAs. Table 3-1 provides an approximate indication of the effectiveness of various safety systems in limiting external releases in a typical loss of coolant accident (the break of a large primary circuit pipe). The figures are for the release of iodine-131 (often assumed as the reference isotope in indicative evaluations of source terms and for a 1000 MWe reactor). As can be seen, the reduction of the releases caused by the safety systems is very significant and corresponds to a factor of the order of one million. 

At the moment, the probability for primary circuit piping break is evaluated on less than 1.x 10-5 y-1. The result shows, that SGs are in good condition. 

To evaluate the consequences of breaks of another diameters primary circuit pipes, a large scope of activities was performed, by the help of IAEA (Regional Project R 09/002)-WANO (6-months program for KNPP), and Westinghouse (transfer of know-how). 

The elimination of large primary circuit pipes in integral PWRs allows an easing of the containment specification. Pressure suppression systems for PWRs become feasible and several versions have been proposed. 

Valves in the primary circuit piping in combination with self-actuated isolation devices. 

There is no doubt that the generation of radionuclides from the corrosion product elements can only occur in the neutron field, i. e. inside the reactor pressure vessel (RPV). On the other hand, the radionuclides which cause the radiation fields which potentially complicate work during plant normal operation as well as during inspection and repair work are those deposited on the inner surfaces of the out-of-RPV primary circuit piping and components, regions they are transported to by the primary coolant. This means that contamination buildup in the PWR primary circuit is a complex process. It can be roughly divided up into three stages (see Fig. 4.26.), each of which raises its particular questions ... 

DM - Delayed neutron detection (main primary circuit pipes)... 

The studies to apply the LBB concept to primary circuit piping have been initiated on a generic basis. For the Rovno NPP Unit 4 and the Khmelnitsky NPP Unit 2, the programme has not yet been fiilly finalized. The intention is to perform LBB concept application on MCL and on some safety pipings connected to the MCL inside the hermetic zone. 

Therefore, in the BN-800 design, the following principles are implemented to prevent hazardous consequences in the case of primary circuit pipe depressurization ... 

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