Patent Number: 054597684
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the figures of the drawing in detail and first, particularly, to FIG. 1 thereof, there is seen a diagrammatic, sectional view of a nuclear reactor pressure vessel 1 (referred to below as a pressure vessel) of a pressurized-water nuclear power station which is constructed, e.g., for a thermal reactor output of 3765 MW, corresponding to a gross electrical output of 1300 MW. A reactor core 2, which is composed of fuel assemblies of which only a single one 3 is shown, is cooled with light water which enters through inlet ports 4 and flows downward in an annular cavity 5 (shown by flow arrows f1). The cooling water flows upward from a bottom plenum 6 through a perforated lower grid 7, through cooling ducts of the fuel assemblies 3 in which it is warmed, and then flows from an upper plenum 8 through outlet ports 9 and a so-called hot primary-circuit pipe or pipeline 10 connected thereto to a non-illustrated steam generator where it conveys its heat through heat-exchanging tubes to a secondary coolant. The cooling-water flow through the reactor core 2, the upper plenum 8 and the outlet ports 9 is illustrated by flow arrows f2. The cooled cooling water, which is also known as the primary coolant, is pumped back from the steam generator through the non-illustrated so-called cold primary-circuit pipe to the inlet port 4 of the pressure vessel 1, so that a continuous circulation is established in normal operation. In normal operation, the primary coolant which is in the primary circuit and therefore is also inside the pressure vessel 1, is at a pressure of approximately 158 bar, and coolant temperature at the outlet port 9 is approximately 329.degree. C. The reactor pressure vessel 1, with its fittings, is constructed for this pressure and temperature load, including a safety margin. It is formed of a cup-shaped vessel bottom section 1A with a hemispherical dome 11 and a flange ring 12 at its upper end, to which a domed cover 1B having a counter flange 13 is bolted in a sealing manner. Cover bolts which are used are not shown, but bolt passage holes 14 can be seen. Only the most important of the fittings will be mentioned, which are a lower perforated drum 15 and the previously mentioned lower grid 7 above the lower perforated drum 15 which forms a bottom of a core barrel 16. The core barrel 16 is suspended by means of a supporting flange 16.1 on an annular shoulder 17 of the flange ring 12 and has a bottom section in which it accommodates the core 2 with the individual fuel assemblies 3. The core 2 is covered by an upper grid plate 18 on which a guide framework 19 having an upper support plate 19.1 is supported. Control rods 20 which can be lowered or raised by non-illustrated control rod drives disposed above the cover 1B, are inserted into a portion of the fuel assemblies. Four outlet ports 9 and four inlet ports 4 are alternately distributed over the perimeter of the pressure vessel 1 in a plane 21--21, in a four-loop system. The primary coolant, which is held below a supercritical pressure and is therefore liquid in normal operation, not only covers the core 2 but also fills the upper plenum 8 up to about the upper support plate 19.1. Effective cooling is therefore ensured even of those fittings which are subject to so-called gamma heating by virtue of gamma radiation, even though they do not themselves generate heat (such as the fuel assemblies 3). If the water level in the pressure vessel drops, due to an extremely improbable failure of all of the cooling and emergency cooling devices, the temperature of the assembly (normally approximately 400.degree. C.) starts to rise, and heat is increasingly conveyed, especially by radiation and conductance, through the pressure vessel 1, especially if the water level has dropped to the upper grid plate 18 or even slightly below. This overheating is utilized, in the still relatively early stage, by the safety device according to the invention, in order to reliably prevent overpressure failure of the pressure vessel 1 in the case of an inadequate core cooling mentioned above. To this end, a differential pressure-loaded pressure relief valve 22 is set into a coolant conducting surface or conduit which may be a wall of the pressure vessel 1 or a pipeline connected to the pressure vessel (the conduit shown is the hot primary coolant pipe 10). As is seen in FIG. 2, the valve 22 has a closure piece that is mounted in such a way as to be longitudinally displaceable. The closure piece is preferably a differential-pressure piston 23 which is held by a fusible stop 24, 25 in an illustrated closure position thereof so as to provide a seal. When the reactor interior reaches an upper threshold temperature which, for example, is 700.degree. C., the fusible stop 24, 25 is caused to melt due to a threshold temperature heat flow reaching it. It can then no longer withstand the shear forces acting on it, so that the differential-pressure piston 23 is displaced into its open position due to the differential-pressure forces acting on it. The pressure difference is calculated as: P=P.sub.1 -P.sub.0, where P.sub.1 =internal pressure, and P.sub.0 =external pressure. The pressure P.sub.1 prevailing in the interior of the pressure vessel 1 can thus be reduced through a connection pipe socket 26 and an open cross section of the annular duct 27 leading into a pressurizer discharge line 28, through a pressure relief flow shown by arrows f3. The fusible stop 24 is disposed between sealing surfaces 23.1 of the piston 23 and associated seating surfaces 29 of the pressure relief valve 22. In the illustrated example, these seating surfaces 29 are formed by internal peripheral surfaces of the connection pipe socket 26 which may be provided with an additional reinforcement in the region of the seating surfaces. The fusible stop is constructed in such a way that at normal temperature it can easily withstand a differential pressure of 160 bar. As an additional safeguard, the further fusible stop 25 is disposed between peripheral piston surfaces 23.2 (at an insertion end of the piston 23) and guide surfaces 30 at an inner periphery of a guide cylinder 31. The piston 23 and the guide cylinder 31 are preferably constructed as hollow bodies, because such a structure causes the heat flow to reach the fusible solder selectively and without major losses. The guide cylinder 31 is held in a centered position in a valve body 220 in such a way that the annular cavity or annular duct 27 remains free as an overflow duct, between the outer periphery of the guide cylinder 31 and the inner periphery of the valve body 220. Vanes 32 which are disposed in the annular cavity 27 hold the guide cylinder 31 in a centered position and are joined to a wall of the valve body. As is shown, the inlet cross section of the annular cavity or overflow duct 2 is sealed by the piston 23 in the normal position of the piston, but is cleared in a release position thereof. In the release position, the piston 23 is completely inserted into the guide cylinder 31. In order to generate the differential pressure at the piston 23, and in order to facilitate the insertion movement, the guide cylinder 31 has a bottom 33 which is provided with a pressure relief orifice 34 therein. As was already mentioned and as is shown, the pressure relief valve 22 is set into the wall of the primary coolant pipe 10, and specifically into the wall of the so-called hot primary circuit, close to the pressure vessel 1. According to an alternative embodiment, the connection pipe socket 26 of the pressure relief valve 22 could be set into the cylinder wall of the pressure vessel 1 at the level of the primary coolant pipe sockets 4, 9 which are shown in FIG. 1, and specifically in a non-illustrated circumferential interspace therebetween. Alternatively, the pressure relief valve 22 may be constructed as a pressure control valve 22' shown in FIG. 3. The pressure control valve 22' has correspondingly smaller cross-sectional dimensions, and instead of the pressure control line 28, a pressure control line 28' is provided which is connected to a servo piston unit 35 of a relief valve 36. This relief valve 36 is connected on the input side to the system pressure P.sub.1, which it normally shuts off from a line 37 running to a non-illustrated pressurizer relief tank. It is only if the control valve 22' responds, in the case of overheating of the pressure vessel 1, that the relief valve 36 be would opened. This relief valve may be the relief valve which in any case is connected to the primary circuit in the vicinity of the pressurizer in conventional nuclear reactor plants. The valve 22' (in its smaller embodiment as a pressure control valve) may alternatively be installed in the interior of the pressure vessel 1, e.g., in the vicinity of the lower grid 7 or the upper grid plate 18, so that it is disposed even closer to possible hot spots and thus responds even more quickly. The associated pressure control line 28' would then be run to the outside, in a pressure-tight manner, in the form of a thin measuring line through the cover 1B or at a point between the inlet and outlet ports 4, 9. Whether a direct pressure relief function (FIGS. 1 and 2) or an indirect pressure relief function in the case of overheating is implemented, both configurations provide increased safety, because the pressure in the interior of the pressure vessel 1 is reduced to values below 30 bar. This ensures that even in the case of a so-called core meltdown, which may possibly be followed by melting of the pressure vessel bottom, the supporting and retaining structure of the pressure vessel 1, like the remaining nuclear reactor building structure, are only subjected, at most, to design forces. Since a core meltdown accident, for the standard pressurized-water reactor design, is an extremely improbable event, the relief valve 22 or 22' can be welded into the pipe socket 10 or the pressure vessel wall. Alternatively, a pressure-resistant flange link may be provided which permits inspection of the fusible points at certain intervals (when the pressure vessel is depressurized in any case because of fuel recharging).