GB2104710A - Standby heat removal system for a nuclear reactor using flow diodes - Google Patents

Abstract

A nuclear reactor using flow diodes (19) to limit reactor core coolant flow through an emergency flow path (12) during normal reactor coplant flow, and to allow unhindered natural circulation around the path with opposite direction when normal reactor coolant flow stops. The emergency flow path may be a separate circuit with a heat exchanger (Figure 3), or may include the core blanket as a heat sink with reverse flow (Figure 4).

Description

SPECIFICATION Standby heat removal system for a nuclear reactor using flow diodes This invention relates to nuclear reactor core heat removal systems, especially emergency or backup heat removal systems for liquid metal nuclear reactors.

Nuclear reactors have a core of fuel assemblies which generate usable energy by means of a nuclear fission reaction. Heat removal systems of relatively large capacity are provided to remove nuclear generated heat during reactor operation.

Due to the nature of the fission reaction, it is not possible to completely shut off heat generation during periods of normal reactor shutdown or even during emergencies. A small percentage of the core thermal power continues to be generated and consequently the core must be provided with a source of cooling to prevent core melting for an extended time after shutdown.

Since an accident such as a pipe rupture could involve a failure of normal operating heat removal, emergency and shutdown heat removal is usually accomplished by an independent and smaller system.

The emergency role played by the backup system suggests that it should be completely automatic, with no external controls or power source and perhaps even with no moving components. A completely passive system is most desirable.

A wide variety of standby core cooling arrangements are known to the art. Some are independent systems while others are modifications of the primary heat removal system intended to mitigate certain accidents or provide emergency core cooling. An example of a passive device used to mitigate a pipe rupture accident is described in "Nuclear Reactor Core Cooling Arrangement," U.S. Patent 4,030,252 by A. H.

Redding.

It is noted that, after shut-down of a nuclear reactor, the fuel region of core continues to produce heat while other parts of the reactor begin to cool down. This causes large temperature differentials also during normal reactor shut-down periods.

It is therefore the principal object of the present invention to provide a reactor in which the coolant remains in circulation after shut-down even if the coolant circulating pumps are not in operation.

With this object in view, the present invention resides in a liquid-cooled nuclear reactor comprising in a vessel a nuclear core including a central fuel region and a circumferential blanket region, and means for circulating a liquid coolant upwardly through said central fuel region and said blanket region to remove heat therefrom during operation of said reactor, characterized in that flow diodes are arranged in the coolant flow path through said blanket region and so oriented that they provide a relatively large flow resistance to upward flow during normal reactor operation but essentially no resistance to downward flow so as to facilitate natural circulation of the coolant upwardly through said central fuel region and downwardly through said blanket region when said nuclear reactor is not in operation.

A hydraulic diode is a device which offers a high resistance to fluid flow in a first direction and a low resistance in a second direction. The device causes flow in the high resistance, first direction to induce turbulence or other frictional phenomenon leading to high resistance. Flow in the second direction is approximately straight through with no vortex motion.

In a first embodiment the flow diode is located in an independent flow loop in which reactor coolant is removed from a region above the nuclear core, passes through external heat removal means such as an atmospheric heat exchanger, and passes through a flow diode or stacked flow diode set into a region below the reactor core. During operation of the standby system, when the main coolant system is shut down, flow through the reactor core and through the standby loop is impelled by natural circulation of the reactor coolant as the heated coolant rises.

Additionally, flow impetus may be augmented by an electromagnetic or other variety of pump. The diode or stacked diode set functions during standby system operation to pass coolant flow through its low resistance direction. During normal reactor operation with the main reactor coolant system on, the standby system is a flow path through which some reactor coolant could undesirably bypass the fuel core. This is greatly reduced by the flow diode to acceptable values of flow since this bypass flow must traverse the diode or diode stack set in the high resistance direction.

Upon loss of normal reactor coolant flow due perhaps to coolant pump trip, operation of the standby system begins automatically with the onset of natural circulation. The only external, non-inherent requirement could be the switching on of the optional pump, if any such is part of the system.

In a second embodiment, flow diodes or stacked flow diode sets are installed in the base of fuel assemblies and radial shield assemblies, both located in peripheral locations in the nuclear core.

These are together herein termed "blanket assemblies". Blanket assemblies generally experience lower operating temperatures than do central fuel assemblies due to the nuclear characteristics of the core as a whole and other reasons. The flovv diodes are disposed in all or some blanket assemblies to present high flow resistance in the direction of usual reactor coolant flow which is from bottom to top and low flow resistance in the reverse direction. During normal reactor operation the diodes act as an orifice which restrict flow through the blanket assembly.

During cessation of reactor coolant flow, natural circulation is established with coolant flowing upward through central, hot fuel assemblies and then downward through the peripheral, cooler blanket assemblies as permitted by the diode's low flow resistance in that direction. A heat removal system similar to the first embodiment may remove heat from some point in the natural circulation path.

The invention will become more readily apparent from the following description of a preferred embodiment thereof shown, by way of example only, in the accompanying drawings, wherein: Figure 1 is a schematic of a single diode showing forward, high resistance flow; Figure 2 is a schematic of a single diode showing reverse, low resistance flow; Figure 3 is a schematic of a nuclear reactor having a standby cooling system using a single diode; Figure 4 is a schematic of a reactor having blanket assemblies with flow diodes; Figure 5 is a schematic of a detail from Figure 4, showing a stacked diode set; and Figure 6 is a section view from Figure 5.

The operation of a vortex diode is illustrated in Figures 1 and 2. Figure 1 shows the "forward" direction 1 of flow which causes the fluid flow to encounter a high resistance due to the vortex 2 formed, in the schematic of Figure 1 by the circular or otherwise curved barrel 3. During the forward direction of flow as in Figure 1, axial port 4 is the flow outlet.

Figure 2 shows the "reverse" direction 7 of flow for which no vortex is formed and for which only a low resistance is encountered.

Various techniques are used to induce the vortex during forward flow. The barrel 3 may contain vanes or even a propeller-like device for that purpose, such that barrel 3 need not itself be curved.

Diodes may be used in combinations; either in series or parallel arrays.

For a more detailed description of flow diodes, see "Fluidic Diode", H. G. Tucker, U.S. Patent 3,604,442, issued September 14, 1971. The stacking of vortex diodes in parallel and series combinations is taught in "Fluid Vortex Transfer", R. W. Warren, U.S. 3,207,168 (see column 1, lines 60-65) and "Fluidic. . . Element," J.

Swithenbank et al., U.S. 3,631,873. The Tucker, Warren, and Swithenbank patents are incorporated herein by reference.

Figure 3 illustrates the first embodiment as applied by way of example to a liquid metal cooled fast breeder reactor. The normal cooling of fuel core 8 is by a massive flow of liquid sodium which enters via a plurality of inlet pipes 22 (one shown in Figure 4) into an inlet plenum 9. This flow goes up through the core 8, enters a hot plenum 10 and exits via a plurality of outlet pipes 11 (one shown in Figure 3).

The reactor has a standby cooling system (heat exchanger means) comprising an inlet 15, optional pumping means 16, (preferably a battery powered electromagnetic pump if the reactor coolant is liquid metal), a heat exchanger 1 7 (shown in Figure 3 as having radiative heat pipes 18), and a diode 19 (or stacked diode set) on outlet pipe 20 located within inlet plenum 9.

During normal coolant flow, inlet plenum 9 is pressurized by main coolant pumps (not shown) which impel core flow. The standby cooling system passes a backflow 21 which undesirably bypasses core 8. Backflow 21 is small in magnitude (about 1.5% of full flow) because such flow encounters the high resistance forward direction of diddes 1 9.

Backflow 21 can be further reduced by operation of pumping means 1 6 during normal reactor operation.

When the normal coolent flow ceases due for example to a main coolant pump trip or an accident, the residual heat of the core 8 continues to heat the coolant and natural circulation will begin if a flow path exists. Such a path is automatically provided through the standby cooling system. Natural flow encounters only the low resistance direction of diode 1 9.

One improvement of this standby cooling system over previous systems is the completely automatic and inherent operation made possible by the use of the diode(s).

Refer to Figure 4 which is a schematic of the second embodiment as applied by way of example to a liquid metal cooled fast breeder reactor. The main coolant flow path is identical to that of Figure 3, except that the flow upward through blanket assemblies 12 is decreased due to passage through the high flow resistance path of diodes 1 9. This is desirable due to the nuclear characteristics of blanket assemblies 12.

Upon cessation of main coolant flow, natural circulation begins, with flow upward through the heated fuel assemblies 14 (one shown) in the central region and downward through blanket assemblies 12, passing through the low resistance path of diodes 1 9. This flow configuration is that indicated in Figure 4.

The complete standby cooling system of the second embodiment may be augmented by combination with heat exchanger means similar to that of the first embodiment, as shown in Figure 3, thereby providing for ultimate removal of heat from the reactor. However, even without such combination, the establishment of a pathway for natural circulation in the core serves to redistribute heat from the hottest assemblies to the coo!er blanket assemblies and increase natural circulation flow.

Refer to Figure 5 which is an enlargement and section of one blanket assembly nozzle from Figure 4. In this case, a stacked diode set of three diodes 1 9 is used. The flow arrows in Figure 5 indicate flow during normal coolant flow, which encounters the high resistance path.

Figure 6 is a section from Figure 5 showing a vane 23 disposed in barrel 3 to create the vortex.

Table 1 discloses a preferred design for a stacked vortex set for blanket assemblies.

While Figure 5 shows three diodes in a stacked diode set, one or more could be used. Alternate diodes can be arranged to impart vortexes of opposite directions, perhaps to further increase hydraulic resistance.

Both embodiments described herein could utilize either single diodes, or series or parallel combinations.

Also, the diodes could be located at the top of the blanket assemblies.

This invention was conceived during performance of a contract with the United States Government designated DE-AM02-76-CH94000.

Claims (5)

Claims

1. A liquid-cooled nuclear reactor comprising in a vessel a nuclear core (8) including a central fuel region (14) and a circumferential blanket region (12), and means for circulating a liquid coolant upwardly through said central fuel region (14) and said blanket region (12) to remove heat therefrom during operation of said reactor, characterized in that flow diodes (19) are arranged in the coolant flow path through said blanket region (12) and so oriented that they provide a relatively large flow resistance to upward flow during normal reactor operation but essentially no resistance to downward flow so as to facilitate natural circulation of the coolant upwardly through said central fuel region (14) and downwardly through said blanket region (12) when said nuclear reactor is not in operation

2.A nuclear reactor as claimed in claim 1, characterized in that a heat exchanger (17) is associated with the coolant flow path through said blanket region (12) so as to permit removal of heat from said liquid coolant during reverse downward flow of coolant through the blanket region (12).

3. A nuclear reactor as claimed in claim 2, characterized in that said heat exchanger (17) is associated with said coolant flow path by means of piping (15) and a pump (16) is disposed in said piping (1 5) for augmenting coolant flow through said heat exchanger (17).

4. A nuclear reactor as claimed in claim 1, 2 or 3, wherein said blanket region ) 12) includes a number of blanket assemblies, characterized in that each blanket assembly has at least two diodes (19) arranged together in series combination.

5. A nuclear reactor as claimed in claim 1,2 or 3, wherein said blanket region (12) includes a number of blanket assemblies, characterized in that each blanket assembly has at least two diodes (19) arranged together in parallel combination.