JPS5815755B2 - upper core grate - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an upper core grid of a boiling water reactor, and particularly to an upper core grid suitable for allowing emergency core cooling water to effectively flow into the reactor core. , when the emergency core cooling system is activated,
The following problems may occur.

The core spray system is activated 30 seconds after the loss of coolant accident occurs, but the strong steam blow-up 7 from the fuel bubble 1 prevents water from flowing into the bundle 8. The moat is called CCFL).

As a result, spray water 11 accumulates in the upper plenum 9, as shown in FIG.

However, as the blowdown progresses, the amount of steam generated 7 from the bundle 1 decreases, and the subcooling of the accumulated water 6 decreases, causing the accumulated water 6 to flow into the bundle 1 all at once (this phenomenon is caused by a CCFL break). (referred to as down).

Regarding this CCFL breakdown, as shown in Fig. 2, the amount of steam generated in the bundle 1 differs in the radial direction of the core due to the burnup of the fuel, as shown in Fig. 3. It is known that this occurs locally in the peripheral bundle 17 of the core, where the amount of steam generated is small.

Furthermore, once such a phenomenon occurs, the circulating flow 14 continues in the core, with steam blowing up the central bundle 16 and cooling water flowing down the peripheral bundle 17.

As a result, the high power central bundle 16 may not be effectively cooled in conventional upper plenum structures.

The present invention provides an upper core grid that effectively cools the high power bundle 16 in the center of the core by preventing uneven distribution of CCFL breakdown.

The present invention secures accumulated water flowing down to the central bundle 16 and prevents the generation of the core circulation flow 14 by making the upper end position 21 of the upper core grid higher around the core and lower at the center of the core. This is what I did.

The configuration and operation of the present invention will be explained using FIGS. 4 and 5.

The fuel is held at the top by an upper tie blind 4, and the fuel bundle 1 is housed in a channel box 2.

The upper part of the channel box 2 is provided with an upper core grid plate 3 to prevent lateral vibration, and its upper end 20 projects into the upper plenum 9.

Spray nozzles 5 are arranged in an annular manner on the side wall of the upper plenum 9, and a stand pipe 10 is installed above the upper plenum 9 as a communication path to the separator.

According to one embodiment of the invention, the height of the upper core grid top 21 increases continuously from the central bundle 16 to the peripheral bundle 17.

In the upper plenum 9 of such a structure, CCFL
As a result, the accumulated water 6 accumulated in the upper plenum 9 becomes CCF.
By the L breakdown, when flowing down to the fuel bundle 1, the flow rate of the falling water 8 is reduced to all the fuel bundles 1.
can be uniformly distributed.

This is because CCFL breakdown progresses inward in order from the bundle 17 around the core where the amount of steam generated is small, but as shown in FIG.
After the upper part 21 is exposed above the water surface, the core peripheral bundle 1
The flow of accumulated water 6 to 7 stops and the remaining accumulated water flows down to the inner bundle.

If the height of the upper core grid upper end 21 is continuously changed,
A certain amount of accumulated water 6 can flow down into each bundle.

Note that even after the upper core grid upper end 21 is exposed, since cooling water is directly supplied from the spray nozzle 5, no problem occurs in cooling the fuel.

Furthermore, in the upper plenum 9 of the above-described structure, the core circulation amount 14 is blocked, as shown in FIG.

Therefore, the CCFL breakdown of the central bundle 16 occurs earlier, which is a special measure for long-term cooling of the entire core.

In the embodiment shown in FIG. 6, the upper end surface of the upper core lattice is made into an inclined surface 22, and the heights with respect to the adjacent upper core lattice are made equal at positions a23 and b24, and equal at positions c25 and d26. The flow rate can be controlled smoothly.

According to the present invention, uneven distribution of CCFL breakdown can be prevented, and the high-power bundle at the center of the reactor core can be effectively cooled.

[Brief explanation of drawings]

Figure 1 is a state diagram of the upper plenum immediately after Tetsushin spray operation, Figure 2 is a distribution diagram of the amount of steam generated in the radial direction of the core, and Figure 3 is a diagram of the state of CCFL breakdown according to the conventional example.
Figure A is a diagram of the upper plenum structure according to the present invention, Figure 5 is a diagram explaining the invention in detail, and Figure 6 is a diagram of an embodiment of the upper plenum structure. 2... Channel box, 3... Upper core grid plate, 4... Upper tie plate, 5... Spray nozzle, 9... Upper plenum, 10... Stand pipe,
11... Spray trajectory, 18... External water surface, 19.
・Internal water surface, 20 ・Top end of channel box, 1
... Fuel bundle, 6... Accumulated water.

Claims (1)

[Claims] 1. An upper core lattice of a boiling water reactor, characterized in that the height of the upper end of the upper core lattice increases in the radial direction of the core toward the periphery.