Guard vessel

Figure 3-9. Flow diagram of an Exxon hydrotreating uniF (1) filter, (2) guard vessel to protect reactor, (3) main reactor, (4) gas treatment, (5) fractionator.

Guarded Cold Plate. The guarded cold plate consists of the measuring vessel, which has a capacity of 2.5 liters and a diameter of 6 in. It is completely enclosed (except on the bottom, where it is in contact with the sample during the test) by the ring-shaped, 12-in.-diameter guard vessel (27 liters capacity). The temperature drop through the i-in.-thick bottom plate is estimated not to exceed 1 °F. 

Both guard and measuring vessels have inside copper shields which serve as temperature equalizers to prevent thermal stratification of the liquid at low heat fluxes and as a radiation shield if the liquid level is low in the guard vessel. 

All lines to the measuring vessel have radiation traps in the form of a tee placed in the guard vessel. 

Gas Pressure Control Device. The gas pressure over the cryogenic liquid in the measuring vessel and guard vessel is subject to atmospheric pressure changes. These changes can lead directly to boil-off measurement errors. Calculations show that the pressure above the cryogenic liquid should be controlled within 0.1 torr however, the atmospheric pressure variation may extend over several torrs. 

Reproducibility. The overall accuracy of the thermal conductivity apparatus is estimated to be 10%. This estimate is based on the accuracy of measuring the volume of boil-off gases, controlling the gas pressures above the crypgenic liquids in the measuring and guard vessels, the effect of atmospheric pressure changes on the boil-off rates, and the temperature measurement of the cold and warm surfaces. The estimate is supported by the electrical heater calibration tests. 

Table I. Time Required to Evaporate 10% of the Liquid Hydrogen from the Calorimeter Measuring Vessel and for Total Evaporation from the Guard Vessel (Temperature Gradient 300° -> 20°K)...

A—Time in hours for 10% evaporation from measuring vessel B—Time in hours for complete evaporation from guard vessel... 

During actual tests, concentricity of the sample space was maintained by means of three conically shaped Micarta bumpers which were glued to the guard vessel. With this arrangement the axial displacement did not exceed in. so that the maximum possible error due to that factor was less than 0.25 %. 

Sodium fire prevention and extinguishing measures are mainly provided by the passive means. First of all, these are reactor guard vessel and jackets covering pipeline sections attached to the reactor up to the shut off valves, as well as the main secondary sodium pipeline sections from the IHX to the reactor cell wall. The space between the main and guard vessels is filled with the inert gas preventing sodium from burning in case of leak. 

When there is no coolant flow in the circuit, temperature difference along the main route of sodium is insignificant. However under these conditions, the risk of stagnation zones appearance in the vicinity of the vessel bottom is high. It should be taken into account in the analysis of this phenomenon, that cold sodium from the cold trap enters lower section of the reactor vessel and both reactor pit and reactor guard vessel are cooled permanently by air. 

In the 4S, the decay heat is removed by two systems consisting of the decay heat removal coil installed in the reactor (PRACS) and the natural air ventilation from outside the guard vessel (RVACS). The analysis considers the destruction of PRACS and the RVACS cooling stack by a large falling aircraft. In addition to this extreme severe condition, 50% of the cross sectional area of the RVACS stack is assumed to be blocked. 

Hot Primary Salt Thermal Blanket Cool Salt Annulus Reactor Vessel Guard Vessel Collector Cylinder Reactor Silo... 

Concerning the cooling of the reactor core, loss-of-coolant accident due to piping breech, which is treated as the major accident in high-pressure systems of LWRs, is no more a significant issue in SFRs, since SFR is a low-pressure system. The ECCS, which is significant also for the safety of LWRs, is not required. Instead, a guard vessel is set outside of the reactor vessel to prevent loss of sodium coolant from the core in SFR. 

Security against malicious acts can be enhanced by limiting physical access to potentially sensitive equipment or material and to the rooms where they are housed, e.g. to the control rod drive mechanisms. In addition, the possible consequences of human error or malevolent acts can be mitigated by means of additional safety barriers, such as a guard vessel, pool cover, or remotely activated poison injection system. 

Passive protection a dnst loss of coolant accidents is, in some designs, provided by a second pressure vessel surrounding the reactor n essure vessel. This guard vessel ensures that the core is never uncovered to ensure that this second barrier is never breached, a reliable means of condensing steam should be provided. 

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