Patent Number: 043436820
Section: summary

FIELD OF THE INVENTION The invention relates to a plant having a feed water heating means utilized for heating feed water introduced into nuclear units during plant start up and a method of operating the plant. BACKGROUND OF THE INVENTION Plants in use utilizing nuclear units for producing a steam supply utilize nuclear reactor coolant pump heat and decay heat from the nuclear fuel within the reactor to bring a nuclear steam supply system up to a maximum temperature before the reactor is made critical to generate nuclear heat. As the water temperature in the nuclear steam supply system increases, steam is generated which is then boiled off to maintain the nuclear steam supply system at desired pressure and temperature conditions. As steam is boiled off, feed water has to be added to the system to compensate for that lost in the boiled off steam. Feed water added to the system is cold and generally at a low temperature while the metal parts making up the nuclear steam supply system are at their maximum operating temperature. The resulting difference in temperature leads to large temperature gradients across the walls of the feed water piping in the system and of the inlet nozzles in the system with the result that the possibility of thermally induced cracks in the feed water piping walls and nozzles is increased. Further introduction of cold feed water into a hot nuclear steam supply system reduces the temperature of the water already in the system resulting in a decrease in volume of the water in the system. This decrease in volume of the water already in the system may lead to possible overfilling of the system as the newly heated feed water becomes heated and expands. It is therefore an object of our invention to provide for a plant incorporating an apparatus and a method of operating the same which will result in a reduction of temperature gradients in a nuclear steam supply system during start up of a plant incorporating such a system and at the same time to provide for a plant which will further minimize overfilling of the system with feed water during start up. GENERAL DESCRIPTION OF THE INVENTION Broadly our invention comprises a plant having feed water heating means for a nuclear steam supply unit where the feed water may be heated during start up procedures. The plant has a nuclear steam supply unit for changing water to steam, a turbine, and a main steam delivery line connecting the turbine with the nuclear steam supply unit. A condenser is connected to a discharge of the turbine and a feed water delivery line extends from the condenser to the nuclear steam supply unit. A feed water pump is included in the feed water delivery line and at least one high pressure heater is positioned in the feed water delivery line between the pump and the nuclear steam supply unit. A conventional high pressure heater steam delivery line extends from a steam extraction point on the turbine to the high pressure heater and a heater operation valve is included in this line. The heater is provided with a drain. In addition to the structure included above all of which is conventional, a start up steam line extending from the main steam delivery line to a high pressure heater is provided and includes therein a start up valve. Opening of the start up valve allows steam produced by the nuclear steam supply unit to pass into the high pressure heater in order that it may heat feed water entering into the nuclear steam supply unit prior to and during plant start up. The result is that temperature gradients in the feed water piping and inlet nozzles leading to the nuclear steam supply unit are reduced thus reducing thermal stress of these parts. The plant may include a plurality of high pressure heaters in the feed water line positioned between the feed water pump and the nuclear steam supply unit. In this event each high pressure heater is connected by a conventional high pressure heater steam delivery line having a heater operation valve therein which leads to an extraction point on the turbine.