Gravity Coring Devices

Free-fall, self-surfacing, ballast release 

Cable deployed and recovered, valve, and core catcher retention 

Bums (1963) described some possible mistakes using information from cores obtained by piston-type cores. A simple experimental setup resulted in some interesting data on different sediment sequences. Fewer mistakes in sediment sequence occurred at full rather than at partial penetrations. 

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The sediments were obtained using short gravity coring devices (Niemistd-corer, 1988 Multi- corer, 2005). From each of the cores, the uppermost slices (1988 0-3 cm 2005 0-2 cm) were separated for analysis, freeze dried, and homogenized. In total, 308 (1988) and 56 (2005) surface samples were investigated. 

Fig. 3.13 Application of a high-momentum gravity corer (Meischner and Rumohr 1974) to obtain samples from marine sediments. The device can also be stationed on smaller vessels and is suited to extract almost unperturbed cores measuring 1 m in length to be applied in pore water analysis.

The reflector above the active core is composed of two layers, a layer of full-height elements over a layer of half-height elements. The top reflector elements channel coolant flow to the active core and provide for the insertion by gravity, of reseirve shutdown material into the active core. They have the same array of coolant holes as the fuel element and the same holes for the insertion of reactivity control devices. 

The reactor trip in emergency is done by simultaneous insertion of the control devices into the core by gravity following de-energization of the drives, which is effected both by signals from the automatic control system, as well as by direct action of the working medium as a result of increased pressure in the reactor or in the guard vessel. 

Insertion of control rod assembly (CRA) by gravity into the core from any position in case of the de-energization of the CRA drives. Design of CRA devices preventing absorber rod ejection from the core in the event of rupture of the sleeve or of the CRA drive mechanism casing. 

Reactor shutdown due to simultaneous insertion of all absorber rods into the core by gravity if the CRA drives are de-energized. CRA drive de-energization is provided automatically or manually in the case of abnormal conditions (e g. in the event of pressure rise in the primary circuit). The number of self-actuated devices corresponds to the number of CRA drives. Their design and location in the GV decreases the possibility of spurious and subversive actions causing the devices to fail. 

Reactor pressure is normally controlled with turbine throttle and bypass valves. When the reactor vessel is isolated from the turbine condenser, an isolation condenser controls pressure. This device was selected because of its simplicity and because it provides high-pressure reactor water inventory control. A failure of the isolation condenser to control reactor pressure, is not expected during the plant live. If such a failure occurs, safety and depressurization valves provide a backup depressurization to the suppression pool which is positioned above the reactor vessel. When the reactor pressure is sufficiently low, check valves open in the suppression pool-to-vessel fill lines and water flows by gravity into the reactor vessel to keep the core covered. The response to a loss-of-coolant accident and transient with failure to scram is similar. 

Control and protection system (CPS) drives that provide for control rod insertion into the core under gravity, in case of a drive de-energization caused by control system signals or instantaneously, by action of a working medium through the self-actuated devices ... 

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