[en] If a graphite reflector of a pebble bed reactor is equipped with insertable absorber rods power control is achieved by less insertion depth of these rods than would be required for conventional designs for the same type of reactor. Therefore the difficulties growing with insertion depth and the danger of damaging fuel elements are reduced. With less stroke, the drives of the control rods are simplified, too. A means for this is doping the graphitic reflector bottom and the lower part of the reflector with neutron-absorbing material. A partial flow of neutrons forced downwards on insertion of the control rods into the upper zone of the reflector will be absorbed in this lower region. A formula is given for this doping. (HP)

[en] The high-temperature reactor is equipped with the usual active engineered safety systems in a simplified form. Due to its inherent safety characteristics and the burst-safe prestressed concrete reactor vessel, activity containment is ensured even without the effect of active safety systems. Even in the event of extremely hypothetical accidents the effect on the environment is low enough so that evacuation or relocation of the population is not required. Therefore large-scale damage of agricultural land and industrially used areas is safely ruled out. Thus the site selection for this type of reactor is not restricted, i.e. it can be constructed near industrial and urban centers. (orig.)

[en] The absorber rod for a pebble bed reactor is assembled from pairs of concentrically arranged cylindrical rod elements, a joint rod tip and joint connectors. The rod elements are cooled by a gas flow, where the inner cylindrical rod element takes over the support function, i.e. takes up and transmits the forces and torques from rod movement when driving in and out. (orig./HP)

[en] The loading of the inside of the reactor with fuel element pellets is done via a central tube. There is a blower in the area of the tube, by which the compressed air and the suction air in the tube can be varied. In this way, the speed of fall of the fuel element pellets in the tube is variable, so that they hit the pebble bed with different speeds of fall. The place at which the fuel element comes to rest on the heap inside the reactor can be determined in this way, within certain limits. (DG)

[en] The device makes it possible to achieve an even design of the edge zone with a small number of supply pipes, without adverse effect on the flow of pellet operating elements into the active core. In order to build up the edge zone, an annular separation reaching to the surface of the pebble bed is fitted to the lower edge of the ceiling reflector, so that an annular gap is formed between the solid reflector jacket and the pebble bed separation, in which the graphite elements of the edge zone are placed and which is continued to the ceiling reflector. The central surface of the edge zone lies above the central surface of the operating pebble bed, preferably inside the part of the annular gap situated in the ceiling reflector. (orig./HP)

[en] An absorber rod is described which is screw-shaped and can be driven in and out by simultaneous longitudinal and twisting movement. In piles of spherical fuel elements, such as occur in high-temperature reactors, the rod acts as a feed screw where unallowed loads on fuel elements and operating elements or reflector devices are prevented. (ORU)

[en] The service life of the absorber rods is extended by 1) exchanging the absorber rods used for controlling the output for absorber rods in the end position, depending on the irradiation dose absorbed, 2) by driving the absorber rods used for controlling the output to the same depth in the fuel element pebble bed, for temporary fast shutdown of the reactor, 3) by inserting all absorber rods into the fuel element pebble bed for long term shutdown. (orig./HP)

[en] In a nuclear reactor of the kind which is charged with spherical reaction elements and in which control rods are arranged to be thrust directly into the charge, each control rod has at least one screw thread on its external surface so that as the rod is thrust into the charge it is caused to rotate and thus make penetration easier. The length of each control rod may have two distinct portions, a latter portion which carries a screw thread and a lead-in portion which is shorter than the latter portion and which may carry a thread of greater pitch than that on the latter portion or may have a number of axially extending ribs instead of a thread

[en] PBMR waste and discharges were characterized mainly through modelling and calculation of the fuel burn-up, the activation of materials inside and around the core and the migration of the radioactivity to various operational and waste handling systems to end up in waste containers or discharges. The waste was quantified and classified according to South African waste strategy requirements and the IAEA classification scheme, which also provides broad management and disposal options. Discharges were quantified for impact assessments and to prove compliance with authorised discharge quantities. The ORIGEN software was used to obtain fuel-sphere fission- and activation-product inventories and decay heat. Characterization of reflector-graphite and control-rod waste involved FISPACT calculations of activation product inventories using neutron fluxes obtained with the MCNP software. Inventories served as source terms to calculate shielding, ventilation and container requirements for storage during the operational phase. Post-closure results served to classify the waste and define possible final disposal options as per waste-strategy criteria including assessed intrusion doses for various institutional control periods in generic waste sites. Normal operational waste was quantified and characterized by means of a compartmental migration model. This followed radionuclides migrating from the helium coolant circuit into the helium purification system and eventually into the liquid and solid waste handling systems to be contained or discharged. The main findings of the characterization exercise are the following: Fuel-sphere and fuel-kernel waste need treatment as HLW requiring deep geological disposal, mainly to ensure an intrusion dose below 100 mSv.a^-1. Heat generation at disposal should be well below 2 mW.m^-3, allowing a relatively high disposal density; The more active reflector graphite and control-rod waste to be replaced during the operational phase will require classification as LILW-LL. Based on assessed intrusion doses, the graphite waste may well qualify for disposal in an intermediate-depth disposal facility; Operational waste qualifies as LILW-SL for disposal in a near-surface disposal facility. (author)

No abstract available