Patent Number: 046363505
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates to a gas-cooled high-temperature reactor having a bed or pile of spherical fuel elements and to a process for the control and shutdown of such reactor. More particularly, the invention concerns a gas-cooled high-temperature pebble-bed reactor having cooling gas flowing through the bed from top to bottom with the fuel elements reaching their state of final burnup desired after a single passage through the bed with roof, bottom and side reflectors surrounding the bed, wherein a cavity is provided between the roof reflector and the surface of the pile and with a plurality of absorber rods arranged in the roof reflector, the absorber rods being capable of insertion to a predetermined depth into the space enclosed by the reflectors for the control and shutdown of the high temperature reactor. 2. Background of the Art Gas-cooled nuclear reactors having a core of a bed of spherical fuel elements equipped with core absorber rods and with absorber rods movable in the side reflector, designated hereinafter as reflector absorber rods, are known. An example of such reactor is the High Temperature Reactor THTR-300. In this type of reactor, for startup processes, i.e. for control processes for the reactivation of the reactor following shutdowns, the core absorber rods and the reflector rods are used. The reflector rods are provided for partial or rapid shutdowns and the core absorber rods for full or extended shutdowns. A partial or rapid shutdown of the reactor is defined as a measure whereby the reactor is rendered subcritical immediately by the rapid insertion of negative reactivity into the core of the reactor and maintained in this state for a short period of time (approximately 30 min.). Reflector rods are used additionally for the compensation of excess reactivity in rapid load control processes. In a further nuclear reactor with a bed of spehrical control elements, the installation for output control and for shutdowns consists of a part for shutdowns and a part for output control, wherein the part serving to shut down the nuclear reactor comprises absorber rods capable of being inserted in the bed and the part to control the output is formed by a plurality of plate like absorber elements displaceable within the wall of the roof reflector and within the cavity defined by the pile and the roof reflector and within the cavity defined by the pile and the roof reflector. One such device is described in West German Offenlegungsschrift No. 23 53 653, the disclosure of which is incorporated herein. This nuclear reactor utilizes the strong homogenizing effect of the cavity on the flux of neutrons. If the nuclear reactor is operated such that the fuel elements pass through the bed only once, the absorber elements for output control exhibit an especially high reactivity. A disadvantage of this known nuclear reactor is the fact that two separate arrangements are required for control and for shutdown. The state-of-the-art further includes a nuclear reactor having a single passage of the spherical fuel elements controlled by means of absorber rods moving essentially within the roof reflector. This reactor is described in West German Offenlegungsschrift No. 21 23 894. The shutdown of this nuclear reactor is effected by means of special absorber rods inserted to approximately 1/4 to 2/3 of the height of the reactor core in the bed of fuel elements. Two other nuclear reactors having a bed of spherical fuel elements passing only once through the reactor core have two shutdown systems that are independent of each other. A first shutdown system is used for full or extended shutdowns which are effected in these nuclear reactors by means of core absorber rods. The second shutdown system used for partial or rapid shutdowns may consist either of reflector rods as shown in West German Offenlegungsschrift No. 24 51 748 or of absorber rods moving in the roof reflector or in the cavity defined by the roof reflector and the bed of fuel elements as shown in U.S. Pat. No. 4,148,685, the disclosure of which is incorporated herein. The nuclear reactor disclosed in U.S. Pat. No. 4,148,685 also displays a plurality of reflector rods, but here they are used to control the reactor. To increase the output from a partial load to a full load, selected absorber rods of the second shutdown system (i.e. rods moving in the roof reflector and the cavity) may also be used. The nuclear reactor disclosed in West German Offenlegungsshrift No. 24 51 748 may also be operated with multiple passages of the fuel elements. In this case, reflector rods which are not part of the second shutdown system are performing the control function, wherein they may be assisted by several core absorber rods. If the nuclear reactor is charged with fuel elements in a single passage process, the load control of the nuclear reactor is effected by means of core absorber rods capable of absorbing the excess reactivity necessary for an adequate load cycle range (100-35-100%). Because of the high reactivity effect of absorbing material in the cavity and in the roof reflector of nuclear reactors with a single passage of fuel elements, it is sufficient to move the core absorber rods as a bank within the roof reflector and in the upper part of the cavity. The insertion into the bed of fuel elements is normally not required. In this mode of operation the core absorber rods are, however, exposed to the high flux of thermal and fast neutrons existing in the cavity and the lower part of the roof reflector. Thermal neutrons activate n, .alpha. reactions in the nickel containing steels surrounding the absorbing material which, at temperatures exceeding 500.degree. to 600.degree. C., lead to the embrittlement of the steel. The tips of the absorber rods are particularly susceptible as there is no shading effect at this location of the absorbent material. The progressive embrittlement of the material during extended operating periods has a significant effect on the endurance of the absorber rods. SUMMARY OF THE INVENTION It is, therefore, an object of the present invention to provide a shutdown and control process for the above-described nuclear reactor whereby the exposure to radiation of the core absorber rods due to thermal and fast neutrons is reduced and the life of the rods extended. This object is attained according to the invention by inserting only a portion of the core absorber rods into the cavity for controlling the load while the remaining core absorber rods are held in their upper terminal position in the roof reflector. Depending on the rate of exposure of the core absorber rods used for load control, the rods are replaced by core absorber rods held in their terminal position. The entirety of of core absorber rods are thereby used for shutdown processes only. According to the process of the instant invention only a part of the nuclear absorber rods is exposed during control operations to the high flux of thermal neutrons, while the rest of the rods remain in the roof reflector and are, therefore, extensively protected (at least at the rod tips) against neutron radiation. By means of the interchange of inserted and retained core absorber rods, the mean radiation exposure of the entirety of the core absorber rods may be reduced and the harmful effect on the material of the core absorber rods is correspondingly reduced. This in turn leads to a substantial extension of the endurance of the rods. In order to enable the number of core absorber rods inserted to bind the excess reactivity necessary for an adequate load cycle range, the rods are inserted to a somewhat greater depth into the cavity. The exact depths may be determined from the S curves of the core absorber rods. The effectiveness of the rods are a function of the depth of insertion. It is of advantage for the process according to the invention that the reactivity required for load cycle processes is reduced after extended operations with the increasing poisoning of the reactor core. The process according to the invention utilizes the well known effect whereby during a single passage of the fuel elements the effectivenss of the absorbent material in the roof reflector and in the cavity underneath it is especially high. It is further of advantage to the invention that the average flux of thermal and fast neutrons in the cavity in the axial direction may be considered constant in a first approximation. In a comparison to the known prior art nuclear reactor of this type, the process according to the invention has the advantage that no further apparatus is required for the control of reactor output in addition to the installation used for the shutdown of the reactor, i.e. the core absorber rods. The portion of the core absorber rods used for output control may amount to approximately 25 to 50% of the total number of core absorber rods. With one-half of the rods inserted, the maximum flux of thermal neutrons in the area of the roof reflector is increased by approximately 10% with respect to the flux with all of the core absorber rods inserted. The core absorber rods retained in their upper terminal position (approximately 50 cm above the bottom edge of the roof reflector) receive with 50% of the rods inserted only approximately 50% of the-maximum neutron radiation. In the process, the flux of thermal neutrons is reduced to a lesser extent than the fast flux which is the result of the thermal "peak" in the roof reflector. The effect obtained by the process according to the invention (i.e. the reduction of the radiation exposure of the core absorber rods) may be further improved by setting the upper terminal position of the core absorber rods in the roof reflector of a high temperature reactor controlled according to the invention as a function of the maximum thermal neutron flux. Thus, the maximum flux of thermal neutrons in the area of the core absorber rods inserted is reduced by another 25%, when with 50% of the rods inserted, the upper terminal position of the core absorber rods inserted is moved upwards by 50 cm, compared with the terminal position of the rods in a mode of operation, wherein all of the core absorber rods are inserted to control the reactor output. As shown by investigations, a reduction in the average thermal exposure of approximately 0.6 may be obtained by operating with one-half of the rods inserted to control the output, combined with an upward displacement of the upper terminal position of the rods by 50 cm. The radiation exposure of the core absorber rods may also be reduced without changing the vertical positioning of the rods, by placing neutron absorbing materials in the wall of the roof reflector in the area of the core absorber rods inserted. As mentioned hereinabove, during a rapid shutdown of a nuclear reactor with a pile of spherical fuel elements, the core must be made subcritical immediately by means of the rapid introduction of negative reactivity in the pile and it must be maintained in this state for approximately one-half hour. For a high temperature reactor of the above-described structural type, a concept for rapid shutdown is provided whereby all of the core absorber rods are inserted the same, predetermined depth in the pile. In a high temperature reactor controlled by the process of the invention, this raises the problem that two groups of core absorber rods, i.e. those of the partial number used to control the load and the group of rods remaining in their upper terminal position, must be brought to the same depth of insertion from different initial positions, i.e., it would be necessary to move the two groups with different strokes, thereby requiring complicated control means of the rods. According to an advantageous further development of the process according to the invention, it is, therefore, proposed that for the purpose of a rapid shutdown only the partial number of core absorber rods used for output control and thus located in the cavity be inserted to a predetermined depth in thepile of fuel elements and that all of the core absorber rods be used and inserted in the pile of fuel elements for extended shutdowns only. Calculations indicate that at the depths of relevance to a rapid shutdown, one-half of the core absorber rods already provides approximately 80% of the effect of the entirety of the rods. By choosing a somewhat larger insertion stroke for this group of core absorber rods, the reactivity required for a rapid shutdown can be obtained with this group. The insertion of the second group of rods remaining in their upper terminal position may be effected manually in case of an extended shutdown. The core absorber rods used for rapid shutdowns which as the result of their use for output control may be at different heights depending on the operational state of the core are provided with a constant insertion stroke. The control of the rods is thereby simplified. The employment of only a partial number of core absorber rods for load con-trol and rapid shutdowns results in the fact that these rods are occupying a normal position after an extended operation at a full load to bind the excess reactivity needed for load cycling that is approximately 30-40 cm deeper than in the case of the insertion of all of the core absorber rods. The rod tips may therein be located at approximately the height of the surface of the pile of fuel elements. The stroke for a rapid shutdown with a partial number of rods needs to be larger by approximately 30 cm only than the stroke required when all of the core absorber rods are inserted. Operational conditions following a rapid shutdown are critical to the extent that therein at the tips of the core absorber rods, high temperatures are obtained. These high temperatures in combination with the material behavior of the rods used represent a limitation of the mode of insertion of the rods. The temperatures encountered will be permitted or designed to rise into the vicinity of what is acceptable at the present time. In order to improve the limits of the reactor by extending the life of the core absorber rods, it is proposed in a further advantageous embodiment of the invention to use a mode of inserting the core absorber rods in the case of interference with reactivity caused by the penetration of water or oil for the rapid shutdown of the reactor, that is different from the mode of insertion of the rods normally used. In this manner, the normal insertion procedure may be significantly relieved and an insertion method reducing the exposure of the material achieved. Whether such a case of interference that should be treated separately is present may be ascertained by humidity measurements.