Patent Number: 046363505
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The high temperature reactor described in FIG. 1 comprises a core with a bed 1 of spherical fuel elements 2. This reactor has a capacity of 1637 Mw.sub.th. The reactor is charged according to the principle of the single passage of the fuel elements. The bed 1, through which the cooling gas is flowing from top to bottom, is surrounded by a roof reflector 3 and a cylindrical side reflector (not shown), together with a bottom reflector. A cavity 4 is located between the roof reflector 3 and the surface of the bed 1. The radius of the reactor core is 4.15 m. The roof reflector 3 has a thickness of 2 m and the cavity 4 has a height of 1 m. The spherical fuel elements have diameters of 6 cm; they contain on the average 10.343 g thorium and 10.285 g uranium with an enrichment of 93%. The filling factor of the bed 1 is 0.61. The shutdown and control installation contains 108 core absorber rods 5, arranged in the roof reflector 3 and capable of direct insertion in the bed 1. The high temperature reactor further has 42 absorber rods that may be moved in the wall of the side reflector. The core absorber rods 5 may be retracted into an upper terminal position 6 in the roof reflector. FIG. 1 shows a section of the rod grid of the 108 core absorber rods 5. To shut down the high temperature reactor, all of the rods 5 are inserted to a predetermined depth into the bed 1. For the control of the load, however, only a part of the core absorber rods 5 is inserted. The rods 5 forming the partial amount 5' at this point are indicated by black circles. The rest of the core absorber rods 5 located in their upper terminal position 6 constitute the group 5". In order to obtain a uniform exposure of all of the core absorber rods 5 following an extended period of operation, the rods belonging to groups 5' and 5" are mutually interchanged. In the embodiment chosen, the partial number 5' is formed by 30 core absorber rods 5. They have an effectivity of 4.8% .DELTA. K/K when inserted to the surface of the bed 1 of fuel elements (the entirety of all of the core absorber elements 5 would yield an effectivity of approximately 7% .DELTA. K/K when inserted to the same depth). In contrast, the load cycle of 100%-35%-100% requires a reactivity of 3.9% .DELTA. K/K and the load cycle of 100%-35% a reactivity of 2.6 .DELTA. K/K. It is, therefore, sufficient for a load cycle to insert the 30 core absorber rods of the partial number 5' within the cavity 4. According to calculations, the reactivity of 2.6% K/K required for a 100-35% load cycle is provided by the 30 core absorber rods of the partial amount 5' when the rods are inserted to a depth of 15 cm in the cavity 4 (measured from the lower edge of the roof reflector 3). The use of all of the 108 core absorber rods 5 for this load cycle process would result in a depth of insertion of 0 cm. The thermal neutron flux (E&lt;1.9 eV) is approximately equal in both of the modes of insertion and amounts in the area of the rod tips to approximately 0.19.times.10.sup.15 (1/cm.sup.2 sec.). A further comparison of the two modes of insertion shows that the control of loads by means of 30 core absorber rods 5 results in a thermal exposure per unit time of all of the rods that is approximately one-half of the exposure incurred in the mode of insertion using the bank of 108 rods. The comparison is even more favorable in relation to the exposure to fast neutrons. It is lower by an approximate factor of 3 per unit time for the method using 30 rods than with 108 rods. A further advantage of the method using only 30 core absorber rods for load control is obtained in the determination of step dimensions for the method using 30 core absorber rods 5. As the maximum reactivity rise with the 30 rod bank is lower than with the 108 rod bank, the minimum step size may be correspondingly larger. The second example as shown in FIG. 2 illustrates the rapid shutdown of a high temperature reactor with spherical fuel elements according to the invention. The reactor has a core radius of 4.87 m and a capacity of 2250 MW.sub.th. The ceramic installations are similar to those of the reactor described in the first example. The roof reflector has a thickness of 2 m and the cavity a height of 1 m. The shutdown and control apparatus includes 150 core absorber rods arranged in an ideal triangular grid in the roof reflector. Additionally, 48 reflector rods are available. These rods are moved in the wall of the side reflector and are in principle fully inserted in the case of a rapid shutdown. In calculating the shutdown reactivity, initially a disturbance reactivity due to water penetration of 2.2 .DELTA. K/K must be assumed. With consideration of the temperature equalization from a full load to zero load and of the cooling of the reactor core by approximately 300.degree. including an uncertainty allowance of 10% and a minimum subcriticality of 0.5% .DELTA. K/K, the maximum shutdown requirement is 6.0 .DELTA. K/K. The necessary rod effectivity amounts to 7.5% .DELTA. K/K wherein the loss of the two most effective rods and a 10% uncertainty deduction is included. Of this rod effectivity, 0.8% .DELTA. K/K is provided by the 48 reflector rods so that the core absorber rods are required to furnish additionally 6.7% .DELTA. K/K. If, as planned heretofore, all of the 150 core absorber rods are used for load control, the tips of these rods are located after an extended operation at a full load prior to a rapid shutdown at a depth of 60 cm in the cavity measured from the lower edge of the rod reflector. The shutdown insertion required is 175 cm. After the rapid shutdown, the tips of the rods are thus inserted to a depth of 135 cm in the fuel element bed. If one-half of the core absorber rods are used both for load control and rapid shutdown, the rods are inserted prior to the rapid shutdown to a depth of 100 cm in the cavity and thus are touching the surface of the bed. The shutdown insertion now requires amounts to 200 cm so that the rod tips are immersed after the rapid shutdown to a depth of 200 cm in the bed of fuel elements. If as proposed hereinabove the disturbance reactivity required in the case of water penetration is provided by means of a special shutdown procedure, the requirement for a "normal" rapid shutdown is only 3.5% .DELTA. K/K for which a rod effectivity of approximately 4.5% .DELTA. K/K is necessary. If all 150 core absorber rods are used in a rapid shutdown, a shutdown insertion of 125 cm is needed so that the rod tips are inserted to a depth of 85 cm in the bed of fuel elements. An insertion of 119 cm is determined when using 75 core absorber rods and the depth of immersion of these rods in the bed thus amounts to 119 cm. In the case of a rapid shutdown with one-half of the core absorber rods, it is necessary to insert the latter only 34 cm deeper in the bed when all of the core absorber rods are used.