Patent Number: 046474250
Section: description

DETAILED DESCRIPTION OF THE INVENTION Reference will now be made in detail to the presently preferred embodiment of the invention, with reference to the accompanying Figure. After shutdown of a reactor 100 for maintenance, refueling, or the like the coolant is usually borated to refueling conditions. The RCS is then cooled down using an RHR system 102 which typically comprises a RHR pump 104, a RHR heat exchanger 106 and control valves 108 and 109. The RHR system 102 is connected between a "hot leg" 110 of the RCS which connects the outlet nozzle 112 of the reactor vessel 114 to a steam generator 116. In the Figure, the numeral 118 indicates the reactor vessel head and numeral 120 the reactor vessel flange. The RHR system is an auxiliary cooling system used to cool down the reactor if the main cooling system is isolated as a result of a fault or the like. Since the reactor core 122 continues to generate decay heat for a period of time after shutdown, the RHRS is utilized to take out this residual heat. In accordance with the present invention, selected steam generator(s) 116 and the RHR system are used during degassing. It should be understood that a typical reactor will have on the order of two to four steam generators associated with it, not all of which are necessarily used during the vacuum degassing and refill method described herein. The RCS is drained to the middle of the reactor vessel outlet nozzle 112 which coincides with the middle of the hot leg 110. At the same time, cooling water is set to flow through the shell (secondary side) of selected steam generators 116 via cooling water inlet 122 and cooling water outlet 124. A two-phase pump 126 is used to draindown the RCS to the middle of the hot leg. A two-phase pump 126 is used since, in accordance with the present invention, the RCS is not vented during draindown and will eventually reach saturation conditions resulting in a low available net positive suction head (NPSH). The two-phase pump is required to satisfy the pump suction condition of low pressure. The pump 126 is the only significant structural modification to the existing reactor system necessary in order to practice the present invention. In prior vacuum degassing systems, nitrogen was admitted to the reactor vessel through a nitrogen reservoir via a pressure relief tank. Thus, two-phase pumping was not required and the single, liquid phase reactor coolant would drain by gravity to the reactor coolant drain tank pump 129 to draindown the RCS. As the RCS is drained, the RHR flow is throttled as necessary using valves 108 and 109 to prevent cavitation of the RHR pump 104. The valve 109 is operated by a flow controller FC to bypass the RHR heat exchanger through bypass line 111 whenever the outlet flow rate falls below a predetermined value. Since according to the present invention the RCS is not vented during draindown, steam bubbles will be formed in the reactor coolant due to the low pressure saturation condition existing in the RCS. Essentially, using the present method, the reactor coolant boils during draindown as a result of lowering the pressure at the prevailing temperature. This boiling action enhances degassing the reactor coolant. As alluded to above, in prior art vacuum degassing systems, the draindown operation was performed using a single phase pump 129 and over a slight nitrogen pressure, thus avoiding boiling during the draindown. Because the RHR system is throttled, condensation will occur in the inverted U-tubs 134 of the steam generator 116 whose shell is being cooled by a secondary cooling system 122, 124. This condensed steam will flow back or reflux to the hot leg 110 and be drawn through the RHR system. Thus, the present invention utilizes selected steam generators 116 as reflux condensers to condense steam in the primary side (the side carrying reactor coolant flow) into droplets which form on the inside of the steam generator tubes 134. Thus, using the present invention, both selected steam generators 116 and the RHR heat exchanger 106 are used to cool the reactor coolant and strip away non-condensible gases. As further explained below, the steam generators 116 continue to function as reflux condensers until the vacuum on the RCS is broken thus causing coolant boiling to stop. Radiogas, hydrogen and other gases stripped from the reactor coolant are removed as non-condensibles by the vacuum pump 136 and gas removal system, generally 138, via the pressurizer tank 132. In accordance with a preferred embodiment of the present method, after the reactor coolant level has been drained to the middle of the hot leg 110 and the RHR stabilized, the pressurizer tank 132 is drained and the vacuum pump 136 started. Preferably, the vacuum pump is of the water ring type where water is used as a pump sealant and capable of handling steam. After a vacuum is established in the pressurizer tank 132 due to the draindown, relief valve 142 is opened and the non-condensibles, as well as any steam in the RCS, are drawn by the vacuum pump 126 into a gas removal system which may comprise either the existing waste gas removal system including a gas compressor 140 and evacuated gas delay tanks 141 or a dedicated waste gas removal system specifically designed to accommodate any oxygen present in the gas handling portion of the system. After a suitable storage period the evacuated gases may be vented through a vent 143. Vacuum induced gas flow continues until an RHR system sample indicates acceptable radiogas and hydrogen concentration. In a typical reactor, the vacuum degassing operation, when performed according to the present invention, can be accomplished in approximately two hours or less depending upon the design of the vacuum system. After establishing the proper radiogas and hydrogen concentration in the coolant, the vacuum pump 136 is stopped and isolated from the RCS by means of isolation valve 144. Air is admitted into the vacuum system from the air reservoir 146 via a filter 131 and valve 133. This breaks the vacuum and instantly aerates the circulating reactor coolant. The oxygen in the air dissolves in the reactor coolant thus facilitating the solubilization of radioactive material that may subsequently be removed by ion-exchange in a CVCS demineralizer or the like. Both air and hydrogen perioxide are sources of oxygen and either can be used to oxygenate the coolant. It should be appreciated that hydrogen peroxide is a difficult chemical to handle and the sudden inrush of air caused by the breaking of the vacuum is a preferred way to oxygenate the reactor coolant. Purification for solubilized radioactive material removal may be achieved by using a mixed bed demineralizer 180 via feed and bleed through the low pressure purification system of the CVCS. Since condensing stops in the steam generators 116 when the vacuum is broken, RHR flow is increased to satisfy the additional heat load. When adequate purification is achieved, the pressure vessel head 118 is removed and the reactor vessel and refueling cavity is flooded and refueling or other shutdown operations may commence. At the end of the shutdown operations, such as refueling, and after the reactor vessel head 118 is resecured, the vessel is again drained down to the nozzle midplane and the vacuum system is again used to evacuate the vapor space and refill the RCS. Air is evacuated from the steam generator tubes and the vapor space in the reactor vessel using the vacuum pump 136. The system is then refilled under vacuum. The air suctioned off during this evacuation may be vented through the containment vent 152 since it will contain no radiogas. This eliminates the very time consuming operation of jogging the reactor coolant pumps and venting the system multiple times. Importantly, the amount of oxygen previously required to be removed by addition of hydrazine during this operation is also reduced. Air which was previously trapped in the steam generator tubes during the system refill operation and literally squeezed into solution during the prior art jog-vent-fill cycle of refilling is now removed by the evacuation process and therefore very little free oxygen is dissolved in the coolant for removal by hydrazine. Thus, not only is the hydrazine/oxygen reaction time reduced, but also much less hydrazine is required. In this regard it should be appreciated that the jog-fill-vent cycle of the reactor coolant pumps and the removal of oxygen using the hydrazine/oxygen reaction is a very time consuming operation. When refill is completed, the reactor coolant pumps are started once and remain running. Startup then proceeds as normal and the reactor coolant vacuum degassing and refill system is secured. It is important to appreciate that with the method of the present application, the RCS is not vented as it is being drained down by the two-phase pump 126. In accordance with the present method, no nitrogen gas or the like will be introduced into the RCS. This results in degassing during draindown since as the reactor coolant level is lowered, a vacuum is created in the RCS which results in reactor coolant boiling at the prevailing relatively low temperature. In addition, the present method utilizes selected steam generators as reflux condensers by flowing cooling water through the secondary or shell side of the steam generator causing the steam in the primary or reactor coolant side to condense as liquid droplets and reflux back into the bulk of reactor coolant which is flowing through the RHR system. The foregoing description of a preferred embodiment of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and obviously many modifications and variations are possible in light of the above teaching. The embodiment was chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto.