Passivating open cooling systems

Nitrite formulations are employed for both hot and cold water closed loops (and also occasionally for open cooling systems). Unfortunately, nitrite is easily oxidized to nitrate and is very susceptible to microbiological attack (by Nitrobacter agilis and other microorganisms). Nevertheless, it is a good low-cost passivating inhibitor. 

Nitrite is suitable as an initial passivator in open cooling systems, but it requires dosing at 1000 to 1500 ppm NaN(>2 to compensate for the loss of nitrite in the system, due to conversion to nitrate. 

A normally isolated, manually-opened flow path is available between the passive contaimnent cooling system water storage tank and flie spent fuel pool. 

Sodium nitrite is incorporated into formulations for both open and closed cooling systems and acts as an anodic inhibitor. It is a good passivator but requires a relatively high dose rate to ensure that all anodic areas within a system are protected from the risk of pitting corrosion. The dose rate has to be increased when high chlorides or sulfates are present. 

The passive contaimnent cooling water storage tank outlet piping is equipped wifli three sets of redundant isolation valves. Failure of a component in one train does not affect the operability of the other mechanical train or the overall system performance. The feil-open, air operated valves require no electrical power to move to their safe (open) position. The normally open motor-operated valves are powered fi om separate redundant Class 1 dc power sources. 

The initial conditions of the experiments were the same as in the case of the emergency core cooling system tests described above. The M-pumps were switched off The dampers were opened partially to avoid sodium temperatures falling below 200°C in the cold legs. The heat removed by natural convection at the air side and at the Na-side amounted to 1.1 MW. This value was consistent with predictions of the given boundary conditions. The dampers of the air coolers, opened by simple manual operation, were introduced as an additional heat sink into the analysis of passive decay heat removal. A maximum temperature of 530°C was then reached after about 10 hours. The demonstrated temperatures lay very close to the normal operating temperatures (Figure 3.2). 

The remaining safety-grade functions are performed by the reactor protection system (it initiates opening of the scram valves to achieve a reactor scram), the containment isolation system (it initiates isolation of the containment by closing isolation valves), the reactor vessel safety valves (based on pressure-activated components), and the passive reactor pool cooling function. These functions are not needed for the protection of the core, however. 

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