Depressurization valve

Abstract

A depressurisation valve for a cooling system comprising: a main chamber having a main valve, a pilot line having a secondary valve and a blowdown line; the main valve being located to seal a path of the coolant system of the nuclear reactor. The main chamber is connected to the cooling circuit via the pilot line allowing coolant to enter the main chamber, and the blowdown line allows coolant to escape from the main chamber, the pilot line having a lower fluid resistance than the blowdown line. The pressure of coolant in the main chamber maintains the main valve in a closed position, and under elevated temperature and/or pressure conditions fluid is prevented from entering the main chamber via a closure of the secondary valve on the pilot line and reduce the pressure from the valve, moving it to its open position.

Claims

1. A depressurization valve for a cooling circuit comprising: a main chamber having a main valve, the main valve being located to seal a path of the cooling circuit, a pilot line having a secondary valve, a pilot poppet valve located between the secondary valve and the main chamber, and a blowdown line, wherein: the main chamber is connected to the cooling circuit via the pilot line, the pilot line allowing coolant to enter the main chamber, the blowdown line allowing coolant to escape from the main chamber, and the pilot line having a lower fluid resistance than the blowdown line, and in normal operating conditions, the main valve is configured to be maintained in a closed position by a pressure of the coolant, and under elevated temperature or pressure conditions, with respect to the normal operating conditions, the main valve is configured such that when fluid is prevented from entering the main chamber via closure of the secondary valve on the pilot line or via closure of the pilot poppet valve, pressure on the main valve is reduced and the main valve moves to an open position.

2. The depressurization valve as claimed in claim 1, wherein the secondary valve on the pilot line is a magnovalve.

3. The depressurization valve as claimed in claim 1, wherein the secondary valve on the pilot line is a high pressure latching isolation valve.

4. The depressurization valve as claimed in claim 1, wherein the depressurisation valve is located upstream of an automatic isolation valve.

5. The depressurization valve as claimed in claim 1, wherein the blowdown line is provided in a space outside of the main chamber.

6. The depressurization valve as claimed in claim 1, wherein the blowdown line is provided in a space inside of the main chamber.

7. The depressurization valve as claimed in claim 1, wherein the depressurization valve is configured to be used in a nuclear reactor cooling circuit.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Embodiments will now be described by way of example only, with reference to the Figures, in which:

(2) FIG. 1 is a schematic of a first embodiment of the automatic depressurisation valve of the present disclosure;

(3) FIG. 2 is a schematic of a second embodiment of the automatic depressurisation valve of the present disclosure; and

(4) FIG. 3 is a schematic of a third embodiment of the automatic depressurisation valve of the present disclosure.

DETAILED DESCRIPTION OF THE DISCLOSURE

(5) Emergency core cooling systems (ECCS) are provided to ensure the safe shutdown of a nuclear reactor when accident conditions arise. The cooling system is configured to provide a safety mechanism in the event of a variety of accident conditions. There are a number of sub-systems that go into forming the ECCS, each having redundancies, so that the reactor can be safely shutdown even if there is a failure in one of the sub-systems. Of particular interest here are the passive systems such as an Automatic Depressurisation System (ADS), which consists of two valves that open to depressurise the main coolant system and to allow the lower pressure emergency coolant systems to function. Because the low pressure coolant injection systems have larger cooling capacities than the high pressure systems the efficient operation of these in shutting down the reactor is very important.

(6) A Passive Depressurisation (PaD) Valve is normally a shut valve, which lies in the discharge pipe lines extending from the reactor circuit. It provides a second and diverse method of isolation from other control system initiated/actuated isolation valves which are located in the same discharge line. The PaD valve is designed to change from a shut state to an open state upon the detection of increasing temperature and/or decreasing pressure upstream of the valve. The advantage of the incorporation of such a system is that the valve will open in the event of a significant LOCA, or elevated reactor circuit temperature to discharge reactor coolant and allow for the injection of fresh coolant under gravity. A schematic example of such operation is shown in FIG. 1. In this PaD valve 100 has a main valve 101 mounted upon a compression spring 102, which is forced down by fluid pressure to be in a closed position. However, when the fluid pressure drops the spring pushes a valve piston coupled to a valve stem 103 up and opens the main valve 101. It is through the movement of the valve piston and stem relative to the coolant system pipeline to which it is connected that causes the main valve to open and close. With the main valve in the shut/closed position the fluid in the upstream reactor coolant circuit passes through the bypass or pilot line 104. Pressure in the system is also high enough to force open a pilot poppet valve 105 and to force the valve piston coupled to the valve stem 103 of the main valve 101 down into a closed position. The system remains in this position whilst the Automatic initiation valve (AIV) 106 is shut, and the main valve spring remains compressed by the reactor circuit pressure. If the AIV is opened by the control and instrumentation (C&I) system, which occurs in the event of low circuit pressure or high reactor fluid temperature, fluid escapes from the main chamber 107 via the blowdown line 108. This escape of fluid will be accompanied by the automatic closure of secondary valve 109, if the fluid temperature exceeds the critical temperature in the magnovalve. The opening of the AIV and the closing of the secondary valve will result in the reduction of pressure in the reactor circuit and ultimately in the main chamber, such that the main valve can be opened, as there is not enough pressure acting upon the valve mechanism spring to keep it closed. The secondary valve 109 can take various forms, for example it may be a magnovalve.

(7) The blowdown line 108 is configured such that the resistance to fluid flow is higher down the blowdown line than it is through the pilot line. In the event of a spurious AIV opening a small slow leak of fluid passes through the AIV, but the flow into the main chamber via the pilot line is greater than out of the chamber via the blowdown line. Thus the pilot poppet spring and the main valve spring 102 remain compressed and consequently the main valve 101 is maintained shut. This state will occur for as long as there is enough pressure in the main chamber to compress these springs. In the event of a LOCA and a subsequent opening of the AIV, where the reactor circuit pressure is not maintained, the force provided by the reactor circuit fluid pressure in the pilot line drops to a level below the pilot poppet spring force resulting in a shutting of the pilot poppet valve. This cuts off the flow to the main chamber from the reactor circuit and the fluid in the main valve chamber escapes via a blowdown line. This results in the force exerted on the main valve spring by the fluid pressure drops, allowing the spring to extend and thus opening the main valve.

(8) In the event of an intact circuit fault and subsequent AIV opening, the reactor circuit fluid temperature rises. Reactor circuit coolant passes through the pilot line, through the main valve chamber and down the blowdown line until the trip temperature of the of the secondary valve 109, in the form of a magnovalve, is reached which causes the secondary valve to shut. Closure of the secondary valve 109 therefore cuts off the flow of water from the pilot line into the main valve chamber 107. As there is no fluid entering the main chamber and the fluid that is present within it is able to escape via the blowdown line and the main valve chamber pressure falls. This allows the spring of the pilot poppet valve to overcome the force of the fluid pressure shutting the pilot poppet valve 105 with the main chamber sealed to incoming water it is able to depressurise causing and the main valve 101 to open. With the main valve 101 open depressurisation of the coolant system can occur.

(9) In this configuration the main valve could be configured to open at temperatures greater than a threshold, i.e. above the normal average operating temperature which is around 312° C. For example, the main valve could be set to open when the coolant temperature reaches about 330° C. It could also open at a higher temperature of about 335° C. or about 340° C. higher. In choosing the main valve opening temperature the design must take into account fluctuations within the coolant temperature and not open during these, but open at a level that would be beyond the normal operating parameters. The poppet valve could be set to close at any suitable pressure below the normal operating pressure of about 155 bar. For example, this could be at about 70 bar. It could also be at pressures higher than 70 bar, such as at about 75, 80, 85, 90 bar, or for pressures lower, such as at about 65, 60, 55, or 50 bar.

(10) In this embodiment the valve arrangement provides a positive means of isolating the reactor circuit from the discharge location until demanded. The inherent design of the main valve and use of a magnovlve and poppet valve allows for the opening of the main valve to be based on both low pressure and high temperature conditions. Such configuration has the benefit that it will not result in a significant hazard in the event of a single spurious AIV valve opening or in a C&I failure.

(11) A second embodiment of a PaD valve 200 removes the requirement for the pilot poppet valve; this embodiment is shown in FIG. 2. This configuration is similar in operation to that of the embodiment shown in FIG. 1. The main valve 201 is maintained in normal operation in a closed position within the circuit. It is maintained in this position by the fluid pressure from the coolant circuit passing through a pilot line 204 into a main chamber 207 at the back of a valve piston coupled to a valve stem 203. The fluid acts by applying a force on the valve piston and forces a compression spring 202 down, such that the main valve is maintained in the closed position. A blowdown line 208 is also provided to maintain fluid flow through the chamber. The blowdown line is narrower than the pilot line so that pressure in the main chamber is maintained. The pilot line is provided with secondary valve 209 for example this can be in the form of a magnovalve, which is thermally activated; this means that if the fluid temperature rises above a certain limit the magnovalve closes and stops the flow of fluid into the main chamber. With the restriction of flow into the main chamber the presence of the blowdown line allows fluid to escape from the main chamber. A magnovalve is a magnetic valve, which when heated above the valve material's Curie Point temperature loses its magnetism and thus allows the valve to move in relation to this change of state, allowing the valve to open and/or close. This therefore reduces the pressure acting on the valve piston, as such the compression spring 202 can expand and the main valve 201 can open to allow the fluid to drain from the main reactor circuit via the AIV 206. The removal of the pilot poppet valve has the benefit of reducing the number of components within the system, but removes the sealing effect of the poppet valve, and the extra pressure control it provides, due to its higher degree of accuracy with respect to the pressure at which the main valve operates at.

(12) An alternative to the use of the magnovalve for the secondary valve 209 would be the use of a fusible plug valve, this makes use of a low melting material in the plug to melt and seal the valve if the temperature of rises. This results in the permanent sealing of the valve and as such is very effective at sealing however they would need to be replaced after operation. Another alternative is the use of a eutectic valve, which also acts as a will melt the eutectic material sealing the secondary valve, which will result in the opening of the main valve.

(13) A third embodiment of a PaD valve 300 of the present disclosure is presented in FIG. 3. In this example the secondary valve 309 incorporates a high pressure latching isolation valve 310 mounted on a pilot line 304; this acts as a pressure sensing line. This line works rather than on a thermal set point for closure of the valve, but rather it shuts down upon the detection of high pressures. The latching valve can be set to close at any suitable pressure that is outside of the normal operation of the cooling circuit. For example, the latching valve could be set to close at 16.5 MPa. The valve could also be set to close at pressures higher than this, for example at about 16.6, 16.7, 16.8, 16.9 17, 17.1, 17.2 17.3, 17.4, 17.5 MPa or higher. Or lower, for example at about 16.4 or 16.3, 16.2, 16.1 or 16 MPa. During normal operation of the reactor the typical pressure of the coolant will cause the fluid to flow along the dotted line. This pressure also retains the poppet valve in the open position. If however, the pressure in a circuit drops as the result of a LOCA event, the lower pressure acting against the poppet valve 305 will not be enough to maintain the poppet valve in the open position. As such the poppet valve will close and fluid can flow down the blowdown line 308 from the main chamber; this resulting in a reduction of the pressure in a main chamber 307. This will in turn reduce the pressure acting upon a valve piston coupled to a valve piston and stem 303, such that a compression spring 302 can expand and the main valve 301 is able to open and fluid can flow down to the AIV 306. If on the other hand the pressure increases, which can be associated with an intact circuit fault, the fluid pressure in the system forces the high pressure latching isolation valve 310 upwards, thus closing the fluid flow path through the pressure release valve. With the lack of fluid flow into the main chamber, the pressure is reduced within this chamber as fluid is able to pass through the blowdown line and reduce the pressure in the main chamber, thus opening the main valve. Although, this is shown as a straight through design, it will be appreciated that this configuration is also suitable for use in an angled design. Like that shown in FIGS. 1 & 2 as well.

(14) Although the above embodiments have been described in light of a nuclear reactor it will be appreciated that the device can be used in any other system where it is important to depressurise a system or discharge the contents of a pressurised system if it reaches a pressure below or above its normal operating pressure. Similarly it can be used in any similar systems where such a discharge/depressurisation is important if there temperature increase above a set point. As the person skilled in the art would appreciate that these valves could be configured to work in different situations from an open to shut scenario with routine modification.

(15) It will be understood that the invention is not limited to the embodiments above-described and various modifications and improvements can be made without departing from the concepts described herein. Except where mutually exclusive, any of the features may be employed separately or in combination with any other features and the disclosure extends to and includes all combinations and sub-combinations of one or more features described herein.