30 Containment systems Depressurization

A transfer vessel is a device that receives the contents of another vessel for emergency or nonemergency purposes. It can be as simple as a vacuum truck or as complex as a hard-piped, dedicated system. For liquids, the system typically consists of a container or containment system located below the protected vessel where gravity will promote a rapid transfer. In the few instances where a transfer vessel is used with gases, it assists in the depressurization of a process. In other instances, it may consist of a spare vessel capable of accepting the contents of a nearby vessel (in case of fire or leak) so that the damaged vessel s entire contents are not destroyed or released (Lees, 1980). In this case, a pump may be used to make the transfer between vessels. 

Safety systems of advanced type (primary system depressurization in PWRs, containment of molten masses on the containment floor, etc.). 

Failure of the pressure containment system (piping, SCWO reactor, post-reactor air cooler, or pressure let-down system) could result in rapid depressurization and dispersal of hot fluids and debris at high velocities. Similarly, failure of the pressure let-down system could result in a large pressure surge that could rupture equipment downstream.. .. The pressure let-down system may be the weak Unk in the full-scale SCWO process chain (NRC, 1998b). 

No pressurization of containment Molten salts do not pressurize containment under accident conditions. This avoids a major energy source for dispersal of actinides and fission products. Gas cooled reactors typically include vented containments to allow escape of the helium if the reactor system depressurizes. 

Figure 2.4-14 TMI-2 scenario Primary system depressurizing and reieasing hydrogen through the pressurizer into the containment...

The only human actions important in more than 50% of the BWR IPEs are manual depressurization, containment venting, initiation of standby liquid control (SLC), and system alignment for decay heat removal. In PWRs, only switch over to recirculation, feed-and-blecd, and the actions associated with depressurization and cooldown are important in more than 50% of the... 

Manifold barriers confine the radioactivity to the 1) ceramic fuel pellet 2) clad 3) cooling water, as demonstrated by the TMI-2 accident 4) primary cooling loop 5) containment and 6) separation from the public by siting. Further protection is provided by engineered safety systems pressurizers, depressurization, low pressure injection, high pressure injection and residmil heat removal systems. 

The AP600 passive safety system includes subsystems for safety injection, residual heat removal, containment cooling, and control room habitability under emergency conditions. Several of these aspects are in existing nuclear plants such as accumulators, isolation condensers as natural-circulation closed loop heat removal systems (in early BWRs), automatic depressurization systems (ADS - in BWRs) and spargers (in BWRs). 

In theory, the application of radon barriers should be adequate to avoid elevated radon levels in houses. In practice, however, a backup radon mitigation system has been found essential for maintaining indoor radon concentrations below 4 pCi/L in most homes studied. In the recent radon-resistant residential construction projects conducted by U.S. EPA and/or private builders, several of the homes designed to be radon resistant have contained radon concentrations above 4 pCi/L. In each of those houses, a backup system consisting of an active (fan-assisted), or passive (wind-and-stack-effect-assisted), SSD system was installed at the time of construction. When mechanical barriers failed to adequately control radon, the soil depressurization methods were made operational. 

The techniques discussed up to now use C02 as the mobile phase for substrates and products. Naturally, this restricts the applications to relatively non-polar and/or volatile components with sufficient solubility in the supercritical medium. An intriguing alternative for processing highly polar substrates are inverted aqueous systems. In this approach, a C02-philic catalyst resides in the non-polar C02 phase, while water-soluble substrates and products are contained in the aqueous layer [58, 59]. A very attractive and unique feature of the scC02/H20 system is that the stationary supercritical phase is never depressurized and hence the large energy input required for recompression is avoided. Furthermore, the aqueous solution is not contaminated with any organic solvent or catalyst residues, which is particularly important if the product is a fine chemical intended for direct further use in aqueous solution. 

Fig. 37. Schematic representation of the in-containment passive safety injection system (PS1S). 1RWST = in-containment refueling water storage tank. PRHR-HX = passive residual heat removal heat exchanger. ADS = automatic depressurization system (four stages). (Westinghouse)...

The extract is collected by depressurization on a column packed with a solid sorbent, in a vessel containing the appropriate solvent, in a collection device connected to a chromatograph, or on combined solid phase-solvent traps [92]. For extraction of volatile compounds, such solvents as acetone, CH2CI2, methanol, or liquid nitrogen are used. Silica gel columns are the most popular way of trapping solids. In this case, the selectivity of the process can be improved by selective elution of the sorbent [88, 92]. SFE can be conducted in a static mode in which sample and solvent are mixed and kept for a user-specified time at a constant pressure and temperature, or in a dynamic mode where the solvent flows through the sample in a continuous manner [56]. The extracted analytes can be collected into an off-line device or transferred to an on-line chromatographic system for direct analysis. 

A simplified flow diagram for a VPSA system to produce oxygen is illustrated in Figure 16. The adsorbent is contained in the three vertical vessels. The beds are cycled so that one is in use in the adsorption mode while another is being depressured and the third is being desorbed or regenerated. Both two- and three-bed systems are used. 

The SC-CO2 drying procedure was as follows Steel containers were loaded onto a metallic support that was put into the high-pressure vessel, which was then closed and filled from the bottom with SC-CO2. When the required pressure and temperature were obtained (200 bar and 35 °C), drying was performed for 4 h with an SC-CO2 flow rate of about 1 kg/h, which corresponded to a residence time inside the vessel of about 4 min. The depressurization time of 20 min was allocated to bring the system back to atmospheric pressure [65]. 

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