Induced nuclear reactions: processes – systems – and elements – Reactor structures – Circulating fluid within reactor
Patent
1998-06-09
1999-11-02
Wasil, Daniel D.
Induced nuclear reactions: processes, systems, and elements
Reactor structures
Circulating fluid within reactor
G21C 107
Patent
active
059784342
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a pebble bed reactor comprising a core cavity which is filled with spherical fuel elements and bounded by reflector material, and through which spherical fuel elements pass under the effect of gravity and coolant gas flows in a descending or ascending stream.
2. Background Information
Pebble bed reactors are known. For example, a pebble bed reactor with graphite core is described in DE 31 49 794 C1. In pebble bed reactors, a pile of spherical fuel elements known as the pebble bed passes under the effect of gravity through a core cavity bounded by reflector material. Control of the nuclear reactor is achieved by neutron absorbers, for example control rods, which either are moved in bores in the reflector material or are inserted directly into the pile of spherical fuel elements for reactor control.
To replace spent spherical fuel elements, which are removed from the core cavity, the pile is refilled. Pebble outlet tubes are usually provided at the bottom of the core cavity for removal of spherical fuel elements. The fuel elements are fed toward the pebble outlet tubes via funnel-shaped depressions in the bottom reflector. Larger core cavities or core cavities with a central column of graphite in the pebble bed are equipped with a plurality of pebble outlet tubes.
Usually a stream of coolant gas, especially helium, is passed vertically through the pile of fuel elements. The direction of flow of the coolant gas can be the same as that of the direction of movement of the spherical fuel elements in the gravity direction (descending flow), but the coolant gas can also flow through the core cavity counter to the direction of movement of the spherical fuel elements, or in other words upward (ascending flow). For supply and removal of coolant gas, the core cavity is provided in the top and bottom reflectors with coolant-gas inlet and outlet orifices, which usually discharge into respective gas plenums, which are used to make the gas stream uniform, and especially to equalize the temperature in the coolant gas.
As the spherical fuel elements move through in gravity direction, the spherical surfaces become abraded. It is even possible for fragments of spheres to be formed and to accumulate at the bottom of the core cavity. Fragments of spheres are formed in particular when control rods are inserted into the pile and moved in the pile. Such fragments of spheres are mainly removed from the core cavity via the pebble outlet tubes, but they can also stick in the gas penetrations in the bottom reflector and cause fouling. In such cases, the coolant-gas flow can be greatly impaired.
SUMMARY OF THE INVENTION
The object of the invention is to ensure, for pebble bed reactors, that coolant gas can be conveyed through the pile of fuel elements in the core cavity without problems even over long operating times.
This object is achieved in a pebble bed reactor of the present invention. Accordingly, the connections for the coolant-gas lines for supply and removal of coolant gas in the core cavity are attached laterally above the cavity bottom. The coolant gas therefore flows laterally above the cavity bottom, into or out of the core cavity depending on whether the flow is ascending or descending. Lumps broken off from the spherical fuel elements cannot enter such lateral orifices.
If a bowl-shaped or funnel-shaped structure of the cavity bottom is provided for removal of the spherical fuel elements, or if the cavity bottom is provided with a plurality of bowl-shaped or funnel-shaped structures disposed next to each other, it is provided in an advantageous embodiment of the invention that the coolant-gas lines discharge in at least two levels disposed one above the other in the bowl or funnel walls. Preferably, according to another embodiment of the present invention, the coolant-gas lines run horizontally in the reflector wall.
According to a further embodiment of the present invention, when a central column is disposed in the core cavity, the coolan
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patent: 3398051 (1968-08-01), Seltorp
patent: 3420738 (1969-01-01), Grant
patent: 3481831 (1969-12-01), Grant
patent: 4382908 (1983-05-01), Petersen
patent: 4642214 (1987-02-01), Zhong
patent: 4863676 (1989-09-01), Helm
Gerwin Helmut
Scherer Winfried
Forschungszentrum Julich GmbH
Wasil Daniel D.
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