JPS645680B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to dry storage of irradiated nuclear reactor fuel and highly radioactive waste.

When the fuel is removed from the reactor, it is highly radioactive and is typically stored in cooling ponds for at least 100 days. The water in this cooling pond contains radioactivity,
It also plays a role in absorbing the thermal energy generated by the decay of nuclear fission products. Water cooling in cooling ponds is not suitable for long-term storage of irradiated nuclear fuel, for example, 50 to 100 years. This is because fuel cladding corrosion occurs and there are many difficulties in maintaining cooling ponds. Another method for long-term storage of irradiated nuclear fuel and highly radioactive waste is to store the fuel or waste in concrete cells and cool them with air circulation. Preferably, this circulation is performed by passive devices, thereby reducing maintenance and reliability problems. In order to completely control radioactive contamination, it is wise to store nuclear fuel in sealed cans made of stainless steel or carbon steel, but over a long period of time, as the natural decay heat decreases, moisture and chlorides in the air There is a problem that non-radioactive contaminants cause corrosion of cans and cell structures.

It is an object of the present invention to provide a means for inhibiting can corrosion in dry storage cells for irradiated nuclear fuel or highly radioactive waste.

According to the invention, irradiated nuclear fuel is stored in a storage cell in which cooling air is flowed through the cell via inlets and outlets with natural draft ventilation to exchange heat with canisters containing fuel or waste. or in a method of storing highly radioactive waste, in which a portion of the thermal energy transferred to the fuel or waste or to the cooling air is recirculated to the air upstream of the fuel or waste, thereby reducing the airflow across the can. It is characterized by raising the temperature above the dew point. By controlling the condensation of vapor in the can and cell structure corrosion is largely avoided and by gradually throttling the air inlets the best air temperature is maintained during long storage periods. A portion of the thermal energy may be recycled by recirculating a portion of the hot air flow downstream of the can upstream of the can.

On the other hand, a portion of the thermal energy may be recycled by a collection of heat transfer tubes extending between the hot air outlet duct and the cold air intake duct. The heat transfer tube assembly provides a completely passive recirculation device that operates throughout the entire cooling period of the fuel or waste.

According to another aspect of the invention, a dry storage cell structure for storing irradiated nuclear fuel or highly radioactive waste comprises a collection of vertical parallel tubes for containing nuclear fuel or waste in cans. the ends of these tubes communicate with opposing chambers for flowing cooling air through the tubes to exchange heat with the can, and for raising the temperature of the air flow across the can above the dew point; Means is provided for recycling a portion of the thermal energy transferred from the fuel or waste to the cooling air to the air upstream of the fuel or waste.

Structural embodiments of the invention will now be described with reference to the accompanying drawings.

The structure shown in FIG. 1 comprises a pair of concrete cells 1, one on each side of an air intake duct 2. The structure shown in FIG. The cell 1 communicates via a duct 3 with a chimney (not shown), so that there is a natural circulation of air through the cell. The fuel stored in each cell is entirely enclosed in a cylindrical steel can with a row of 650 vertical steel cooling channel tubes 4. The lower end of the tube is supported by a perforated concrete platform 5 set above the floor, and the space constitutes an air inlet mixing chamber 6. The upper ends of the tubes 4 are located in a grid matrix with an air space 7 forming an outlet chamber between the top of the tubes and the underside of the slab 8. Cans containing fuel are stacked in pairs in steel channel tubes 4. The residual heat from the fuel is
Natural ventilation is created in the annulus between each can and tube, and the induced convection currents remove heat from the can walls and maintain the enclosed fuel at an acceptable temperature. The air flow leaving the cooling channel of the can mixes in the space 7 above the tube and is discharged by the duct 3.

In order to spin the condensation of vapor from the air stream in the can, means are provided for recycling a portion of the thermal energy transferred from the can to the cooling air to the air upstream of the can, and as shown in FIG. Additionally, thermal energy recycling can be accomplished by recirculating a portion of the hot air flow downstream of the can upstream of the can (as indicated by arrow "A").

The exhaust chimney is designed to operate at temperatures below 80°C and, apart from the concrete surface finish, there are no moving parts or special linings in the cell. The temperature is such that the steel tube is sufficiently heated to avoid condensing moisture from the intake air, conditions that would lead to corrosion. Therefore, there is no need for maintenance or replacement within the cell. Channel cooling tubes without containers do not generate air flow and therefore do not participate significantly in the heat removal process. The tube containing the container is automatically adjusted. The inlet and outlet flow of air is adapted to pass through a shielded duct with dogleg bends made to prevent direct radial flow.

FIG. 2 shows a preferred arrangement for recirculating a portion of the air flow flowing from the outlet air space 7 above the tube to the inlet chamber 6. FIG. The air inlet 2a is provided with an injection nozzle 9 which enters a choke 10 formed in the chamber 6. The chimney of the cell overcomes its own friction and the entry of air into the cell, and the injection nozzle 9
Create sufficient natural ventilation to provide kinetic energy. The temperature increase in the fuel can creates sufficient natural draft to overcome the frictional losses of airflow across the can, and the kinetic energy of the injection nozzle is sufficient to overcome the natural draft generated over the height of the can; Approximately 50% of the outlet cooling air is thus recycled to the inlet chamber 6. As a result, the air inlet temperature to the can is high enough to prevent corrosion.

FIG. 3 shows another injection nozzle arrangement that recirculates a portion of the cooling air stream. Nozzle 11 entering station yoke 12 is energized by an external source of compressed air.

FIG. 4 shows yet another arrangement for recirculating hot air by means of a circulation fan 13 arranged in a duct 14 connecting air space 7 and inlet chamber 6. FIG.

The dry storage cell for irradiated nuclear fuel with the injection nozzle recirculation device shown in FIG. 2 is completely passive compared to the one with the recirculation device shown in FIGS. 3 and 4. There are advantages. This variant has passive natural draft cooling, but the recirculation system is active and therefore requires reliability and extra installation and plant maintenance.

FIG. 5 schematically shows a heat transfer tube assembly 14 extending between the hot air chimney and the cold air inlet duct 2. FIG. The tubes contain a liquid, such as water or a coolant, which circulates by natural convection and transfers thermal energy from the outgoing hot air to the incoming cold air.
The inlet air is kept well above its dew point temperature. The resulting stack temperature is lower than that due to recirculating a portion of the hot air, thus reducing natural draft, but this loss of natural draft is
Compensated with lower flow recirculation energy losses.

Also shown in FIG. 5 is a second valve having a regulating valve 17 arranged to bypass the heat transfer tube (or to bypass the injection nozzle of the structure described in the embodiment of FIG. 2). A cold air intake duct 16 can also be provided. This bypass serves to reduce the overall temperature of the cooling system in case of adverse conditions such as abnormally high ambient temperatures, otherwise the stack temperature would be 80°C.
This would exceed the maximum design temperature.

Structures using heat transfer tubes have the advantage of being less complex than those using injection nozzles or fan circulation devices.

[Brief explanation of the drawing]

FIG. 1 is an elevational cross-sectional view of a pair of typical dry storage cells for irradiated nuclear fuel, FIGS. 2, 3, and 4.
5 shows a structural variation of the air inlet and outlet arrangement for recirculating a portion of the air flow downstream of the fuel; FIG. 5 shows a thermal energy recirculation device. 1... Cell, 2, 3... Duct, 4... Tube, 5... Raft, 8... Slab, 9... Injection nozzle, 11... Nozzle, 13... Fan, 14...
...Heat conduction tube assembly.

Claims (1)

[Claims] 1. Cooling air is flowed through the cell through an inlet and an outlet with natural draft ventilation to exchange heat with a can containing fuel or waste, and from the fuel or waste is transferred to the cooling air. Irradiated nuclear reactor fuel or highly radioactive waste characterized in that a portion of the thermal energy is recycled into the air upstream of the fuel or waste, thereby raising the temperature of the air stream above the can above the dew point. Method of storing in storage cells. 2. A method according to claim 1, wherein a portion of the thermal energy is recycled by recirculating a portion of the hot air stream downstream of the can upstream of the can. 3. In a dry storage cell for storing irradiated reactor fuel or highly radioactive waste, the cell comprises a collection of rigid parallel tubes for containing the nuclear fuel or waste in cans; The end of the tube communicates with an opposing chamber to allow cooling air to flow through the tube and exchange heat with the can, raising the temperature of the air flow above the can above the dew point.
A structure for a dry storage cell characterized in that it is provided with means for recycling a portion of the thermal energy transferred from the fuel or waste to the cooling air to the air upstream of the fuel or waste waste. 4. A dry storage cell structure according to claim 3, comprising means for recirculating a portion of the air flow downstream of the can upstream of the can in order to recirculate a portion of the thermal energy. 5. According to claim 4, the means for recirculating a portion of the air flow downstream of the circuit upstream of the can comprises an injection nozzle arranged to be drawn into the cell by the air flow of the downstream air. Dry storage cell structure. 6 The means for recirculating a portion of the air flow downstream of the can upstream of the can comprises an injection nozzle arranged to draw the downstream air into the cell by a compressed air flow from an external source of compressed air. Structure of a dry storage cell according to claim 4. 7. A dry storage cell structure according to claim 4, wherein the means for recirculating a portion of the air flow downstream of the can upstream of the can comprises a circulation fan. 8. A dry storage cell according to claim 3, wherein the means for recirculating a portion of the thermal energy comprises a collection of heat transfer tubes extending between the hot air outlet duct and the cold air intake duct of the cell. Structure. 9. A dry system according to claim 8, comprising a second cold air intake duct containing a flow conditioning device and arranged to bypass the collection of heat transfer tubes associated with the first cold air intake duct. Storage cell structure.