GB2203875A - Seismic protection - Google Patents

Abstract

To ensure that a nuclear reactor 11 or other damage-susceptible installation is, so far as possible, tripped and already shutdown before the arrival of an earthquake shock at its location, a ring of monitoring seismic sensors 14 is provided around it, each sensor being spaced from it by a distance (possibly several kilometres) such that (taking into account the seismic-shock propagation velocity through the intervening ground) a shock monitored by the sensor and then advancing to the installation site will arrive there later than a warning signal emitted by the sensor and received at the installation, by an interval sufficient to allow the installation to trip and shut down, or otherwise assume an optimum anti-seismic mode, in response to the warning signal. Extra sensors 14 located in boreholes may define effectively a three-dimensional (hemispherical) sensing boundary rather than a mere two-dimensional ring. <IMAGE>

Description

Seismic Protection This invention relates to the protection from earthqtlakes of an installation or apparatus having a selectable state or condition in which, for preference, it should be if subjected to an earthquake shock, either because in that state or condition it is more resistant to such a shock or because it will be easier afterwards to deal with the consequences if at the time of the shock it was in that state or condition rather than some other possible state or condition.For example, in the case of a nuclear reactor having a core and control rods and having a shut down condition which is achieved by fully inserting the control rods into the core, it is preferable that if damaged by an earthquake shock the reactor should have been already shut down, with the control rods fully inserted, at the time the damage occurred, since management of the post-accident situation will then, in general, be easier than would otherwise be the case: at the very least, if the control rods are already fully inserted when damage occurs the possibility that the damage will prevent their being fully inserted afterwards is removed.

It is an object of the present invention to provide an installation or apparatus, capable of being damaged by earthquake shock and of a kind having a selectable state or condition which is a preferred state or condition for it to be in the event of being subjected to such a shock, with seismic sensing means arranged to provide advance warning of such a shock and to trigger the installation or apparatus into assuming that preferred state or condition, in response to such warning, before being itself affected by the shock.

In particular it is an object of the invention to provide a nuclear reactor, which is capable of being shut down rapidly in an emergency, with one or more seismic sensors connected to trigger an emergency shutdown, in the event of an earthquake shock being sensed, in advance of the arrival of the shock at the reactor itself.

The invention achieves its objects by providing the nuclear reactor, or other installation or apparatus to be protected, with seismic sensing means spaced from it at such a distance that the seismic-shock propagation time from the sensing means to the installation is sufficient to accommodate first transmission and any desired processing of a triggering signal from the sensing means to the installation and thereafter the initiation and preferably the completion of the emergency shutdown of the latter in response to such signal. The sensing means may comprise a plurality of seismic sensors so disposed in different directions from the installation as to give adequate warning time of seismic shocks incident from various directions.

The invention will be more fully understood from the following description by waxy of illustration of an embodiment thereof, with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic representation of a nuclear reactor installation combined, in accordance with the invention, with a surrounding array of seismic sensors; Figure 2 is a further diagrammatic representation of the relative dispositions of the reactor installation and of one of the sensors, shown in Figure 1, and Figure 3 is a schematic representation of a reactor core of the installation shown in Figures 1 and 2, together with emergency shutdown means associated therewith and operative connections whereby the emergency shutdown means can be triggered by the seismic sensing means represented in Figure 1.

As shown in the drawings a nuclear reactor installation 11 which in known manner incorporates emergency shutdown means of any known and suitable kind (such as an array of emergency-shutdown control rods 12 rapidly insertable into a reactor core 13 of the installation as shown schematically in Figure 3), is provided with seismic sensing means in the form of a surrounding ring of seismic sensors 14a .... 14h each arranged to sense any seismic disturbance of significant magnitude at its own location and each connected by a respective link 15 to a signal receiving and processing unit 16 of the installation 11 and arranged on the occurrence of such a disturbance to provide a signal representative of the disturbance and to transmit the signal to the unit 16 via the respective link 15.Any or all of the links 15 may be in the form of a land line or may, for example, be a radio link with the respective sensor and the unit 16 incorporating respectively a radio transmitter and a co-operating radio receiver. The unit 16, in addition to means 16a for receiving signals via any of the links 15 from any of the seismic sensors 14a .... 14h, may include a signal processing means 16b for performing any desired operations on the received signals, such as discriminating between signals indicating significant and insignificant seismic disturbances, and/or comparing or otherwise combining signals received from different sensors and deriving intelligence therefrom, and is connected via an output line 17 to a trigger unit 18 to which it delivers an output signal in the event that a signal or signals received by the unit 16 are such as to indicate a need for emergency shutdown.Seismic events are only one possible reason why an emergency shutdown of the installation might be required, and the trigger unit 18 may therefore, in addition to the line 17, have other input lines such as lines 19 and 20 by which signals can be fed to it representing other reasons, such as a loss-of-coolant accident or other serious malfunction within the installation itself. On receipt of an input signal, whether from the line 17 or otherwise, the trigger unit 18 delivers a triggering signal over a line 21 to a control-rod release device 22 which normally holds the control rods 12 withdrawn from the reactor core 13 but which on being triggered via the line 21 releases the rods 12 and either actively drives them into the core 13 or allows them to be so driven by gravity or by separate biassing means (not shown).

As described above, each of the sensors 14a .... 14h is disposed at a distance from the installation 11, and the minimum required distance will depend both on the time required to achieve shutdown once the release device 22 has been triggered (this time may be about 2 seconds) and on the earthquake shock propagation velocity in the ground between a particular sensor and the installation. If the geology in the vicinity of-the installation 11 is uniform in all directions for a sufficient distance, the sensors 14a .... 14h may all be at an equal distance and thus lie on a circle 23 (see Figure 1) of appropriate radius. Since propagation velocity of earthquake shocks is strongly dependent on the material through which they are being transmitted, and may vary from about 1500 to 6000 metres per second, the required minimum radius for the circle 23 might vary between 3 and 12 Km in order to provide a two-second advance warning (disregarding the relatively short transmission time of signals in the links 15) to the installation 11. If the surrounding geology is non-uniform, the sensors may be located at correspondingly different minimum distances.

For instance, if, as represented in Figure 1, there is a boundary 24 between rock of the kind on which the installation is built and a different rock type in which the earthquake shock propagation velocity is smaller, any of the sensors (as illustrated, the sensors 14a and 14b) which are located on the remote side of the line 24 may be at a correspondingly reduced distance and thus inside the circle 23.

The sensors 14a .... 14h as described above are located on or very close to the earth's surface, and in effect define a defensive ring at surface level. If there is a risk of earthquake shock arriving from deep below the surface and incident at a substantial angle to the horizontal, it may be preferred to provide further sensors located deep below the surface in boreholes provided for the purpose. One such is represented in Figure 3, which is a schematic vertical sectional view of the earth 25 below and in the vicinity of the installation 11. As shown, a bore hole 26 enables a seismic sensor 14j, which may be like the sensors 14a .... 14h, to be located deep below the earths surface and at the same distance (through the earth) from the installation 11 as are the sensors 14a .... 14h which are located on the surface.The sensor 14j has its own link 15 connecting it with the signal receiving unit 16 of the installation 11, this link 15 extending from the unit 16 to a well head 27, at the top of the bore hole 26, and thence down the bore hole. The sensor 14j and others not illustrated but located, like it, down boreholes of appropriate depth at other locations distributed round the installation 11, may then serve to define in effect a hemispherical surface 23', of which the ring 23 is a section, at which earthquake shocks being propagated towards the installation 11, from any direction, are intercepted in time to provide a sufficient advance warning to enable an emergency shutdown of the installation to be triggered and become effective before arrival there of the earthquake shock.

Even an earthquake originating within the hemisphere 23' will result in a partial shutdown if its location is such that it is sensed by one of the sensors 14a .... 14b, 14j etc before it reaches the installation 11; but if desired further partial-shutdown protection against such shocks of nearby origin may be obtained by providing a second set of sensors at a distance of something less than half that of the sensors 14a .... 14h.

The invention may be incorporated, from the beginning, in the design of new nuclear reactor or other installations and, in combination with a choice of site such that there are no known active faults in the zone within the protective ring of seismic sensors, will give a high degree of assurance that the installation will never suffer earthquake shocks except when in a shut-down condition. This should allow savings in the design of the shutdown system, as compared with systems which are required to be able to operate even after earthquake damage has occurred. The invention may also be applied to existing installations, as a valuable addition to overall safety and as a modification whose expense would be justified in terms of the likely resulting reduction of the cost of dealing with a post-earthquake situation if one should occur.

Claims (5)

Claims:

1. A nuclear reactor provided with emergency shutdown means and with seismic sensing means adapted and connected to generate a triggering signal on sensing a seismic shock and to apply the triggering signal to the emergency shutdown means and thereby initiate an emergency shutdown of the reactor, wherein the sensing means includes at least one seismic sensor which is spaced from the reactor at such a distance that the seismic-shock propagation time from the sensor to the reactor is sufficient to accommodate both the time required for transmission and any desired processing of such triggering signal from the sensor to the reactor and thereafter for initiation and at least partial completion of such emergency shutdown.

2. A nuclear reactor provided with emergency shutdown means and with seismic sensing means as claimed in claim 1, wherein the seismic sensing means comprises a plurality of seismic sensors disposed in different directions from the reactor and each spaced from the reactor at such a distance that the seismic-shock propagation time from it to the reactor is sufficient to accommodate both the time required for transmission and any desired processing of a triggering signal from it to the reactor and thereafter for initiation and at least partial completion of such emergency shutdown.

3. A nuclear reactor as claimed in claim 1 or claim 2, wherein the or each sensor of the seismic sensing means is located substantially at ground surface level.

4. A nuclear reactor as claimed in claim 1 or claim 2, wherein the or at least one sensor of the sensing means is located subterraneously at substantial depth and at a substantial angle to the horizontal with reference to the reactor.

5. An installation or apparatus, capable of being damaged by earthquake shock and of a kind having a selectable state or condition which is a preferred state or condition for it to be in the event of being subjected to such a shock and having triggerable means for selecting that state or condition, seismic sensing means spaced from it at such a distance that the seismic-shock propagation time from the sensing means to the installation is sufficient to accommodate first transmission and any desired processing of a triggering signal from the sensing means to the installation and thereafter the triggering of the.

selection and preferably completion of the assumption of the said preferred state or condition by the installation or apparatus.