Patent Number: 052176827
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENT The nuclear reactor system shown in FIG. 1 and described in U.S. Pat. No. 4,689,194 contains a principle cooling path with a cooling gas (helium) which flows up through a reactor core in the bottom portion of steel pressure vessel 2 and through a central hot gas conduit to the top portion of vessel 2. The heated gas then flows downward through the principal heat exchangers 9, downward through decay heat exchangers 13 to circulating blowers driven by motors 16 which return the gas flow to the lower part of the reactor core. Decay heat, produced after the reactor is shutdown, can be removed by natural convection if the circulating blowers are no longer available to circulate the gas flow. The principle heat exchanger 9 (only one being shown in FIG. 1) are steam generators with the hot gas from the central conduit flowing through the steam generators 9 from top to bottom whereby the gas temperature is reduced from approximately 700.degree. C. to 250.degree. C. at the outlets of the steam generators. The decay heat exchangers 13 are also traversed by the gas flowing from the top to bottom of the decay heat exchangers after the cooled gas has exited from the outlets of the steam generators 9. The decay heat exchangers 13 are, as a result, exposed to a gas flow at a temperature of about 250.degree. C. during normal operation of the reactor. The decay heat exchangers 13 are connected to an external recooling heat exchanger 22 at a geodetically higher location by two legs 19 and 20 which form a decay heat removal loop 21. A water-steam separator vessel 23 is located in leg 20 between decay heat exchanger 13 and the external recooling heat exchanger 22. The water-steam separator 23 provides for volume equalization in the decay heat removal loop in case of evaporation of the water. The decay heat exchangers 13 are operated, on the secondary side, with cooling water at a pressure chosen such that the cooling water at the outlets of decay heat exchangers 13 does not evaporate during normal operation i.e. when the decay heat exchangers 13 are subjected to a gas temperature of 250.degree. C. from the outlets of steam generators 9. The decay heat removal loop has a low volume of water and is operated, during normal operation of the reactor, by a natural convection flow with shut-off valves 24 being in the open position. If the steam generators 9 are no longer available as a heat sink, they are traversed by hot gas at a temperature of about 700.degree. C. which then enters into the decay heat exchangers 13. This raises the temperature of the decay heat exchangers 13 and leads to evaporation of cooling water in the decay heat removal loops 21 which increases the natural convection flow in loops 21 so that the decay heat is safely removed from the gas flow in the reactor. The increase in natural convection flow in loops 21 when steam generators 9 are not available as heat sinks allows decay heat to be safely removed without incurring a heat loss of the same size during normal operation of the reactor. That increase also happens automatically without the need to actuate any valves, shut-off valve 24 being open during normal operation of the reactor. The decay heat is removed as a result of the rising temperature alone with no additional actuating measures being required. However, with a low water volume in the decay heat removal loops 21, steam will be present in the upper part of hot leg 20 during normal operation of the reactor with water throughout the cold leg 19, the head of water in the cold leg forcing a substantial flow in the loop 21 by natural circulation. This natural circulation flow will result in substantial heat being lost through the decay heat removal loops during normal operation of the reactor. Furthermore, that natural circulation flow can not be restricted during normal operation of the reactor, for instance by an orifice, because it would then be restricted under emergency conditions when it is necessary to safely remove decay heat from the gas flow in the reactor. FIG. 2 shows one proposed system for the removal of decay heat from a CANDU nuclear reactor. The core 41 of a CANDU nuclear reactor has a number of fuel channels 42 extending through the core with cooling water flowing from inlet header 43 via pipes 63 through the channels 42 and via pipes 60 to an outlet header 44. The normal flow of cooling water during normal operation of the reactor is from high temperature outlet header 44 via pipe 61 through a steam generator 46 to a main circulation pump 45 which pumps the cooling water via pipe 62 to low temperature inlet header 43 and back to the reactor core via pipes 63. In this type of system, the main pump 45 will be shutdown when the steam generator 46 is unavailable as a heat sink which may be caused by an accident or when the steam generator is out of service for repairs. The decay heat removal path consists of pipe 14 extending from high temperature outlet header 44 to an inlet of a heat exchanger 11 in a large reservoir 10 of water which forms a heat sink. The tank 10 of water is sufficient large and holds a sufficient volume of water to provide a heat sink for several days. The outlet of heat exchanger 11 is connected to pipe 15 and through a check valve 12 to a low temperature inlet header 43. The check valve 12 opposes the main pump head when the main pump 45 is operating to prevent backflow through pipe 15, heat exchanger 11 and pipe 14 during normal operation. The heat exchanger 11 is located at a higher elevation than the reactor headers 44 and 43 so that a natural convection flow can occur from high temperature header 44 to low temperature header 43 when pump 45 is stopped. In this type of system, when the main pumps are tripped, coolant from high temperature header 44 can start a natural convection circulation flow up pipe 14 down through heat exchange 11 and via pipe 15 through check valve 12 to low temperature header 43. This natural convection flow through the decay heat removal path is of a sufficient size to remove heat generated in the reactor core when the reactor is shutdown. 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 that natural convection circulating flow started. This type of system also requires a large volume of heavy water to be present in the decay heat removal path which adds to the cost of the reactor system. FIG. 3 shows an alternative system, according to the present invention, for removal of decay heat generated in the reactor core after a steam generator is lost as a heat sink. This system substantially avoids problems associated with the previously described systems. The normal flow of cooling water in FIG. 3 is the same as in FIG. 2, i.e. from the outlet header 44 via pipe 61 through steam generator 46 to a main circulation pump 45 and via pipe 62 (62') to inlet header 43. However, a heat exchanger 47 is now located between circulating pump 45 and low temperature header 43. The outlet of the secondary side of heat exchanger 47 is connected via pipe 57 to an inlet of a vapor separator 50 whose outlet is connected via pipe 52 to another heat exchanger 54 in a large reservoir 53 of water which forms a heat sink. The reservoir 53 of water is large enough to provide a heat sink for several days. The outlet of heat exchanger 54 is connected via pipe 56 to an inlet of heat exchanger 47 forming a decay heat removal loop which contains a fluid such as normal water rather than heavy water. This provides a substantial reduction in costs compared to the type of system shown in FIG. 2. A fairly large mass of fluid is located in the decay heat removal loop. Heat exchanger 54 is located at a higher elevation than heat exchanger 47 so that a natural circulation flow can occur from the outlet of heat exchanger 47 through the vapor separator 50 and heat exchanger 54 to the inlet of heat exchanger 47. However, during normal operation of the reactor, the natural convection flow is essentially zero because the decay heat removal loop is pressurized to prevent boiling of the liquid on the secondary side of heat exchanger 47. Also the loop is partially filled to keep the vapor/liquid interface 51 above the level 55 of coolant in the heat sink 53. During normal operation, substantial temperature differences exist around the decay heat removal loop i.e. from hot to cold at the vapor/liquid interface near the inlet to the heat sink 53 and from cold to hot at the inlet to the heat exchanger 47. Heat transfer would occur because of these temperature differences but would be insignificant because of the small heat transfer area. The normal heat losses would be small because the temperature differences within the heat exchanger 47 and within the heat exchanger 54 at heat sink 53 would be small. If the steam generator 46 is lost as a heat sink, the coolant temperature at the outlet on the primary side of steam generator 46 increases which raises the temperature of the heavy water coolant entering the heat exchanger 47. This raises the temperature of the secondary liquid in heat exchanger 47 towards boiling. Boiling results in a large reduction in back pressure due to voiding of the hot leg which causes a recirculating flow to develop by natural convection with cold water entering heat exchanger 47 and a hot vapor/liquid mixture entering the heat exchanger 54 in heat sink 53. In this system, decay heat removal would automatically switch from the steam generator 46 to the alternate heat sink 53 when the steam generator is lost as a heat sink without the need for valves being opened or any other type of intervention. The pressure and inventory of water in the decay heat removal loop would be controlled to maintain the required pressure and level in the steam separator 50. The system can then be periodically tested during normal operation by lowering the pressure in the decay heat removal loop and measuring the temperature rise at the entrance to heat exchanger 54. An eventual reactor cooldown to a temperature near 100.degree. C. can be effected by also reducing the pressure in the decay heat removal loop. If it is required to lower the reactor temperature below 100.degree. C., a liquid with a lower boiling point than water can be used in the decay heat removal loop. 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 various other types of nuclear reactor.