Residual overpressure

After this draining and a further draining of two residual bulk of sodium, the amount of residual sodium let in the primary vessel was estimated to 70 kg of metallic sodium and 100 kg of aerosols and sodium oxides. An endoscopic examination of the primary vessel confirmed this first estimation. The primary vessel was kept under argon cover gas with a residual overpressure to prevent from any ingress of air. 

Rupture disk released combustible vapors from reactor after it overpressured due to cleaning vapor evolution from residual heat in the vessel. Vapors ignited causing an explosion which severely damaged the facilities. 23,000,000... 

Residual air only now produces the required overpressure and opens the discharge valve, but the vapor has already condensed and the droplets are precipitated in the pump. 

The 3/4-in. venting system should include a solvent flush for rinsing residual Grignard reagent from the feed lines. This will ensure safe opening of the quick-connect junctions typically found in such systems. A pressure relief valve is also needed to prevent overpressurization of the drum. 

A measure will in general not eliminate a failure cause completely, but will reduce the probability of a certain failure cause. For example, the installation of an additional high level detector coupled to an alarm will reduce the probability of overpressure due to overfilling the tank. However, it will not completely eliminate the failure cause, since there is a possibility that the high level detector will fail, or the alarm will not be responded to. This so called residual risk has to be determined as part of the methodology. 

During a startup of the Biblis A plant on December 17, 1987, the first check valves in the pipe from the reactor coolant system to the residual heat removal (RHR) system had to close for overpressure protection. The position indication and a plant computer alarm annunciated a non-closed position of the first check valve in one train of the RHR system. This was overlooked by the operator. Therefore plant startup was continued with the check valve in the open position. The leakage through the first check valve caused the actuation of a relief valve, releasing primary coolant to the chemical and volume control system. 

Reactor coolant system overpressure protection during power operation is provided by the pressuriser safety valves, discussed in Section 6.3.2. The provided overpressure protection is compliant with the requirements of the ASME III Paragraphs NB-7300 and NC-7300 for pressurised water reactor systems. Low temperature overpressure protection is provided by a relief valve in the suction line of the normal residual heat removal system, as described in Section 6.5.3.3. 

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