Density lock

Density locks perform a fundamental role in PIUS ensuring core cooling during emergency conditions, and thus the potential for blockages caused by gas collection, material distortion or plugging by detached insulating materials should be analysed in depth. The density lock concept has been used in other new reactor schemes. 

There is another "density lock" arrangement at a high location in the pool, connected to the upper riser plenum - the volume on top of the riser from which the water is drawn into the hot leg pipes. This upper density lock has a similar arrangement of tube bundles and a buffer volume above the hot/cold water interface level. There are also a number of small openings between the riser and the density lock. 

This reactor system configuration - with the two always open density locks - is the basis for the exceptional safety performance of PIUS plants. There is always an open natural circulation path through the core, going from the reactor pool to the lower density lock, to the... 

In a PIUS plant, the core coolant flow rate is determined by the thermal conditions at the reactor core outlet - relative to the reactor pool. The resulting pressure drop across the core and up through the riser must correspond to the static pressure difference between the interface levels in the upper and lower density locks. The main coolant pumps are operated to establish a pressure balance across the lower density lock to keep the reactor system in operation. In case of a severe transient or an acddent, the natural circulation flow loop will be established, providing both reactor shutdown and continued core cooling. 

The hot/cold intraface level in the upper density lock is determined by the total volume of the primary loop water mass, when the position of the interface level in the lower density lock is kept constant. The temperature measurements for the interface level in the upper lock are basically used for reactor pool volume control purposes. (The reactor primary loop volume control utilizes level measurements in the pressurizer.)... 

The inside of the cavity is provided with a stainless steel liner. In addition, there is a second barrier - an embedded steel membrane about 1 m into the concrete - up to a level above the upper density lock to ensure that the reactor pool water volume below this level cannot be lost by liner leakage. Concrete vessel penetrations are not permitted below this level. 

Compared with current commercial LWR designs a number of safety-grade systems have been eliminated the control rods and the safety injection boron system are replaced by the density locks, the automatic depressurization system is not required, the auxiliary feedwater supply system for RHR is replaced by the reactor pool, the containment heat removal and containment spray systems are replaced by the passive cooling of the reactor pool. The safety-grade closed cooling water stem, HVAC sterns, and a.c. power supply systems have been replaced by non-safety-grade systems, allowing major simplification of the plant. 

Density locks (thermal barriers), siphon breakers, and wet thermal insulation Prestressed concrete reactor vessel Long-term passive residual heat removal system Reactivity control without control rods... 

J, Fredell, C, Pind "Summary of theoretical analyses and experimental verification of the PIUS density lock development program", IAEA-TECDOC-677, Progress in development and design aspects of advanced water cooled reactors (A TCM in Rome, Sept 1991), pp 213-219... 

It is an integrated PWR, completely immersed in a large pool of cold boronated on water which builds up on the density lock concept originally proposed by ABB ATOM for the PIUS plant and embodies revolutionary ideas for enhanced passive safety. The ISIS reactor components, on the other hand, are mainly based on proven technology derived from the ANSALDO experience in the field of both LWRs and LMFBRs. 

Hot and cold plena are hydraulically connected at the bottom and at the top of the inner vessel by means of open-ended, vertical, tube bundles referred to in the following as lower and upper DENSITY LOCKS. The Inner Vessel houses the core, the SGU and the primary pumps. 

A variable speed operation is required to control the "hot-cold interface in the lower density lock. The variable frequency electric supply is provided by two generators driven by a common variable-speed turbo coupling wth a flywheel of high mechanical inertia (about 5000 Kg m ). 

Both the IE I C systems (Protection and Safety related monitoring) and the control systons (characterized by the absence of control rods and by the "hot-cold" interface regulation inside the Density Locks) are designed utilizing a proven, distributed microprocessor-based technology. An advanced control room, vrith a wall panel information station and work stations for operators and supervisor is foreseen, taking into account the most recent achievements and improvements in the man-machine interface systems. 

During normal plant operation, the natural circulation of the highly borated water of the intermediate plenum through the lower density lock, the core, the riser, and back to the intermediate plenum via the upper density lock, is kept inactive by the main coolant pumps. In case of LOSSP the pumps coastdown and natural circulation establishes itself causing reactor shutdown and providing continued core cooling. 

The outflow of primary water into the SGU tubing is compensated by pressurizer water. Boronated water enters the primary system from the upper density lock and shuts down the reactor, while depressurizing the primary system and hence stopping the primary water loss. The... 

N13 1 he cold and berated water entering m natural eireulation mlo the primary system through the density locks... 

Reactivity control Boronated water from mtermediate pool Pnmary pump tnp Passive (B) Passive (D) Boronated water enters the primaty system via density locks when unbalance occurs between generated power and extracted power or when forced circulauon is lost... 

This activity should include experimental tests on the Wet Thermal Insulation and the Density Locks for this specific de gn, even though the Doiaty Lock concept has been extensively tested by ABB for the PIUS reactor. 

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