Patent Number: 052020835
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS The core 1 of a CANDU nuclear reactor has a number of fuel channels 2 extending through it with cooling water flowing from an inlet header 3 via pipe 12 through the channel 2 and via pipe 9 to outlet header 4. The normal flow of cooling water during operation of the reactor is from high temperature header 4 via pipe 10 through a steam generator 6 to main circulation pumps 5 which return the cooling water to low temperature header 3 and back to the reactor core. To prevent overheating of the reactor core when the steam generator cooling is lost due to an accident, a decay heat removal path is also connected between outlet header 4 and inlet header 3 to remove decay heat from the reactor core. Such decay heat removal paths are normally provided with a pump to circulate the coolant to a heat exchanger. However, pumps rely on electrical supplies. Instead the heat exchanger could be located, as in FIG. 1, at an elevation such that a natural convection flow will develop, precluding any reliance on electrical supplies. The decay heat cooling path consists of pipe 13 extending from high temperature header 4 to an inlet of a heat exchanger 8 in a large tank 7 of water which forms a heat sink. The outlet of heat exchanger 8 is connected to pipe 14 and through check valve 15 to low-temperature inlet header 3. The check valve 15 opposes the main pump head and prevents backflow through pipe 14, heat exchanger 8 and pipe 13 when the main pumps 5 are operating. The heat exchanger 8 is located at a higher level than the reactor headers 3 and 4 so that a natural convection flow can occur from header 4 to 3 when pump 5 is tripped. A further heat exchange coil 9 which is connected to service water by lines 16 and 17 can be provided on the other side of a partial divider 19 in tank 7 to remove heat from the water. However, the large tank 7 of water provides a heat sink for several days should service water, via pipes 16 and 17, be unavailable. In this type of system, when steam generator cooling is lost, the main pumps 5 would be tripped, and coolant from high temperature header 4 can start a natural convection circulation flow up pipe 13 down through heat exchanger 8 and via pipe 14 through check valve 15 to low temperature header 3. This natural circulation flow through the decay heat cooling path is of a sufficient size to remove decay heat from the shutdown reactor. However, in a CANDU reactor, the header to header pressure drop is close to zero and can even be in the wrong direction which creates problems in getting the natural circulation flow started in the decay heat cooling path. FIG. 2 shows an alternative system, according to the present invention, for inducing a natural circulation flow in the decay heat cooling path. The system contains the same elements as shown in FIG. 2 with the addition of a pump 20 in the coolant flow pipe 14 before the check valve 15. This pump 20 is normally running, along with main pumps 5, to provide a small flow through the decay heat cooling path. That small flow, from the high temperature header 4, is in the intended direction of the natural convection circulation flow and maintains a temperature difference within the decay heat removal path that provides a buoyancy force to immediately start a natural circulation flow in the decay heat cooling path if all the pumps 5 and 20 are shutdown. The inclusion of pump 20 in the circuit provides the additional advantage of a controllable flow in the shutdown cooling circuit when the steam generator is out of service for repairs. The decay heat cooling path pump 20 will need to have a head which matches that of the main pumps 5. The flow in the decay heat cooling path, during normal operation of the reactor, needs to be controlled so that it is as small as possible and at the same time maintains sufficient buoyancy force to start a natural circulation flow. This will avoid wasting thermal power from the reactor. As shown in FIG. 3 control valve 25 may be used to control the small flow in the decay heat cooling path but speed control of pump 20 is preferred since a flow control valve may stick. For greater reliability, it may be desirable as also shown in FIG. 3 to provide two decay heat cooling path pumps in parallel 20' and 20" each having its own check valve 15' and 15", respectively. If only a single pump 20 as in FIG. 2 were located in the decay heat cooling path and that pump stops, then the buoyancy force necessary to start a natural circulation flow would disappear as the hot leg of the decay heat cooling path cools down. The loss of a single decay heat cooling path pump may, as a result, necessitate tripping the reactor and main pumps before the hot leg cools down in order to ensure that a natural circulation flow is started in the decay heat cooling path. With two pumps 20' and 20" in parallel, the other pump can continue to maintain the low flow in the decay heat cooling path when one of them fails. A decay heat cooling path pump may be lost due to, for instance, shaft failure or a bearing seizure. Another reason for using two pumps in parallel is the resistance to a natural circulation flow that would be created by a seized pump if only one is used in the decay heat cooling path. An alternative means, to pump 20, of providing a small flow in the decay heat cooling path is to continuously bleed the flow from a location 21 through a valve 22 as shown in dotted lines in FIG. 2. Location 21 is positioned after the outlet from heat exchanger 8 and before check valve 15. This could then serve the dual purpose of supplying coolant for purification to purification unit 23 in which case the heat exchanger 8 would serve as a purification cooler. The flow could be returned to the heat transport system upstream of pump 5 to minimize the required pump head. Various modifications may be made to the preferred embodiments without departing from the spirit and scope of the invention as defined in the appended claims. For instance, although the preferred embodiments have been described with respect to a CANDU reactor, similar systems may be used in other types of nuclear reactors wherein a pump, or other means, would be operating during normal operation of the reactor to move a small flow from the primary cooling path through a decay heat cooling path and return that small flow to the primary cooling path, so as to permit a natural convection flow to be rapidly established, when required, in the decay heat cooling path.