GB2052133A - Collecting device for melting fuel rods in a nuclear reactor installation - Google Patents

Abstract

In order to safely collect the reactor core 3 in the case of a reactor melt-down, wherein the reactor core escapes through the base 5 of the reactor shell 2, there is provided a chute 6 underneath the reactor shell 2. It deflects the dropping parts of the melting reactor sideways and discharges them into a deposition trough 14. The chute 6 and the trough 14 are rotated relative to each other, so that the dropping parts are spread over the deposition trough 14. The chute 6 is made of a thick-walled corrosion-resistant material 10 which is provided with a surface coating 11 of refractorily vitrified porcelain enamel. In order to safely receive the melting fuel rods 4 arriving through the chute 6, the deposition trough 14 comprises a homogeneous mixture of crushed glass, cement, steel cubes (as impact absorbing elements) and barium sulphate (as filler substance). An exhaust tube 25 serves for controlled removal of gases from the deposition region. <IMAGE>

Description

SPECIFICATION Collecting device for melting fuel rods in a nuclear reactor installation The present invention relates to a collecting device for melting fuel rods in a nuclear reactor.

In nuclear reactors or other nuclear power facilities any radioactive material must strictly be prevented from escaping into the environment. In particular, measures must be taken to prevent melting fuel elements in a "runaway" nuclear reactor from descending and reaching the soil, as this may cause sustained hazardous environmental contamination.

A conventional collecting device is disclosed in German Specification (Offenlegungsschrift) 26 1 4 187 of Westinghouse Electric Corp. It comprises a stationary collecting trough centrally disposed directly underneath -the reactor core. Melting fuel rods would directly drop into a trough, i.e. onto the tips of neutron capturing rods. However, at the very high temperatures possible (about 28000 C) these rods may also melt and lose their effectiveness. The trough is relatively small and has such an extent as to prevent the combination of the different molten reactor portions to a critical mass. However, if the volume of the small trough is not sufficient to collect all the melting fuel elements, then also the chute guiding the elements down has to carry melting material, thereby causing the danger of melting of the chute itself.Therefore, the formation of a critical mass beneath the chute cannot completely be excluded. Even the bed of bricks under the trough gives insufficient security. As a consequence the protection against a possible catastrophe is not sufficient in the known device.

What is desired is a safety device which affords maximum safety against fissionable material to penetrate the soil within the area of a nuclear reactor or other nuclear power facility.

The present invention provides a collecting device for melting fuel rods of a nuclear reactor which comprises chute means arranged underneath the core of the reactor and having an area extending at least over the area of the reactor core; - deposition trough means for receiving said melting fuel rods which have descended through the reactor base; wherein said deposition trough means being sufficiently large in diameter and horizontally disposed beneath the reactor base; and having a raised circumferential rim portion; said chute means having a lower mouth portion located above said raised circumferential rim portion of that deposition trough means; and said deposition trough means and said chute means being movably mounted relative to each other.

In case of a runaway reactor accident causing the fuel elements to descend in an uncontrolled manner, the collecting device will serve to safely convey the fuel elements into the deposition trough by way of the mentioned chute, the trough safely receiving and storing radioactive material, even if the latter is already in the melting stage.

Preferably, the deposition trough is a rotatable trough of a large diameter arranged beneath the reactor base in horizontal alignment, the chute being positioned in such a manner that its mouth portion is located above the circumferential rim portion of the rotatable trough.

In order to uniformly distribute the radioactive material in the rotatable trough, the latter is, at the peripheral area of its bottom, preferably supported by a roiling bearing and is adapted to be slowly rotated by means of a pneumatic or hydraulic driving arrangement.

In a second embodiment the chute is rotatably mounted, whereas the trough is disposed concentrically to the reactor core.

In order to practically exclude the possibility of radioactive material "burning" through the wall of the trough, it is preferably made of a thick-walled corrosion-resistant material, e.g. stainless steel, having a surface coating of refractorily vitrified porcelain enamel.

In order to ensure reception of all fuel elements contained in the reactor core, the width of the chute is preferably selected to correspond to twice the width of the reactor core. Suitably the chute extends, from a location at the reactor base, vertically downward and, after defining a curved intermediate portion, terminates in a horizontal mouth portion slidably supported by a circumferential portion of the deposition trough.

Suitably, the chute has a trough-shaped curved cross section and is made of a corrosion-resistant material, e.g. stainless steel, having a surface coating of porcelain enamel.

The bottom of the deposition trough may advantageously be covered, preferably up to twothirds of its effective height, with a layer of an impact-absorbing material and a filler substance, the layer preferably consisting of a homogeneous mixture of 1/6 crushed glass, 2/6 cement, 1/6 steel cubes (as impact-absorbing elements) and 2/6 barium sulphate (as filler substance).

As an additional protective measure, the installation may include an additional protective trough, whose base extends beneath the chute and the deposition trough and whose upper circumferential portion forms a wall extending above the ground plane and encircling all reactor buildings which contain radiactive material The invention will be described further, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a sectional view of a nuclear reactor installation; Figure 2 is a schematic top view of the installation, in which the additional protective trough as well as the soil above the deposition compartment and the chute are not depicted; and Figure 3 is a broken-off partial view in the direction of arrow Ill in Figure 1.

The safety device illustrated in the drawings substantially extends beneath the buildings of a nuclear reactor 1 containing within its shell 2 a reactor core 3 with the rods 4. Connected to the base 5 of the reactor shell 1 is a chute 6 which initially extends downward in a vertical direction, and by way of a curved intermediate portion 7, terminates in a horizontal mouth portion 8. In the area of its curved intermediate portion 7, the chute 6 is supported by base 9. The chute is designed to have a trough-shaped cross section (Figure 3) and has a width corresponding to twice the width of the reactor core 3, beneath which the intermediate portion 7 extends such that it coincides with the area of the reactor core 3 (Figure 2).

The body 10 of the chute substantially consists of corrosion-resistant steel, having a thickness of approximately 10 cm, and is provided with a hard porcelain enamel coating 11 which has been glazed several times and also has a thickness of approximately 10 cm.

The horizontal mouth portion 8 of the chute is supported by way of a slide bearing 12, on the raised circumferential rim 13 of a rotatable deposition trough 14. By means of a ball bearing 1 6, the peripheral portion of the bottom 1 5 of this trough 14 is rotatably mounted on a base ring 17 and can be rotated by means of hydraulic system 18.

By way of example, the rim 13 has a height of approximately 3 m, while the bottom 15 has a diameter corresponding to four times the length of the fuel rods 4.

The trough 14 comprises a main body made of corrosion-resistant steel having a thickness of about 10 cm and is provided with a surface coating 20 of hard porcelain enamel. This coating 20 has been glazed twice as often as the coating 11 of the chute 6.

The interior of the rotatable trough 14 is filled, for about two-thirds of its height, with a homogeneous mixture consisting of 1/6 crushed glass, 2/6 cement, and 1/6 steel cubes measuring lOx 10 cm, as impact-absorbing elements, and of 2/6 barium sulphate as filler substance. The mixture is free of binding agents and is protected against the influence of moisture. In Figure 1, its upper level is identified by the reference numeral 212 Moreover, it is advantageous to add slate slabs with dimensions of about 10 x 10 cm, as well as graphite stone portions of the same dimension, to the mixture.

In the case of a catastrophe, it is the purpose of the mixture to safely absorb the impact of the fuel elements coming down the chute 8, to prevent them from descending further, and in addition, to serve as a shield against radioactive radiation.

The rotatable trough 14 is accessible from the ground surface 23 by way of a shaft 22 via a room, which is also shielded from the environment by a dome-shaped structure 24. An exhaust tube 25, serving to remove radioactive gases, terminates in this dome structure.

These gases may then be conveyed from the upper orifice 26 of the exhaust tube to a safe storage means, e.g. to a gas container, whose interior is coated with barium sulphate, for instance, and which is designed to withstand high pressure.

For additional protection, all of the shown buildings including the safety devies are surrounded by a protective trough structure 27, comprising a trough bottom 28, extending beneath the rotatable trough 14, and a closed side wall 29, which extends above ground level 23 up to 2 m to form an enclosing wall and encircling all reactor buildings containing radioactive material.

The protective trough structure is made of barium sulphate cement, and its walls have the thickness of about 1 to 2 m. Suitably, its bottom is covered with the same homogeneous mixture of impactabsorbing elements and filler substance as described for the rotatable trough 14. It is advantageous to add slate slabs as well as graphite stone portions with a surface extension of about 10 x 10 cm to this mixture.

In order to prevent any release of radioactive material above ground level, entrance and exit gates should be accessible over the protective wall 30 via stairs or ramps.

While the chute 6 and the rotatable trough 14 may be installed in already existing nuclear reactors at reasonable costs, the expenses for subsequently installing a protective trough 27 will be too high. Accordingly, the protective trough will be particularly used in new structures.

In the case of a catastrophe, causing fuel elements or the entire reactor core to descend, the fuel elements are smoothly intercepted by the chute 6 and, from its mouth portion 8, evenly distributed and deposited on the surface 21 of the rotatable trough 14, which, in the case of a catastrophe, may be rotated automatically. Until their final removal, the fuel elements may remain in the rotatable trough without disturbing repair work in the reactor 1.

Also, in the case of a catastrophe, an exhaust system, not illustrated, cooperating with the exhaust orifice 26, is rendered operative if the development of radioactive gases is ascertained either in the space enclosed by the shell 2 or above the surface 21 covered with the exorbit fuel elements in the rotating trough 14.

Such gases are then sucked off into a safe room or directly into radiation-proof gas containers for final transport and removal. To avoid the danger of explosion, the exhaust orifice 26 may be opened and closed by hydraulic or mechanical means, but not by electrical means. To prevent release of any gas, the exhaust orifice is provided with safety means which must be actuated, preferably automatically, after every removal.

A person skilled in the art will not need details for designing the chute such that fuel elements falling down the chute 6 will gather a speed sufficient to permit their falling into the rotatable trough 14 when they have passed the mouth portion 8.

In addition, the chute is designed such that it receives descending nuclear plasma, which is in a finely distributed state, and conveys it, in the form of small droplets or spheres, into the rotatable trough.

In a further embodiment, which is not shown in the drawings, the deposition trough which for the rest is designed as described above is stationary and arranged concentrically to the reactor core.

The chute 6 on the other hand, which also is constructed in the above-described way, is rotatably mounted. This is advantageous, since its mass is smaller than that of the deposition trough, resulting in a simpler construction of the bearings and a reduction of the necessary driving power for the rotation. The collection and distribution effect is substantially the same as in the first embodiment described above.

Claims (13)

1. A nuclear reactor installation having a collecting device for melting fuel rods of a nuclear reactor comprising a chute arranged underneath the core of the reactor and having an area extending at least over the area of the reactor core, and a deposition trough for receiving via the chute melting fuel rods which have descended through the reactor base, the chute having its lower end located above a raised rim of the deposition trough, and the deposition trough and the chute being mounted for movement relative to each other.

2. An installation as claimed in claim 1, wherein the deposition trough is rotatably mounted and situated in a position laterally displaced from the centre of the reactor core.

3. An installation as claimed in claim 2, wherein the deposition trough, at the periphery of its bottom, is supported by rolling bearing means, a hydraulic driving system being provided for slowly rotating the deposition trough.

4. An installation as claimed in claim 1, wherein the chute is rotatably mounted and the deposition trough is situated directly below the reactor core.

5. An installation as claimed in any of claims 1 to 4, wherein the deposition trough is of stainless steel or other corrosion-resistant material, and is provided with a surface coating of refractorily vitrified porcelain enamel.

6. An installation as claimed in any of claims 1 to 5, wherein the width of the chute is twice the width of reactor core.

7. An installation as claimed in any of claims 1 to 6, wherein, from a position adjacent the reactor base, the chute extends downwards in the vertical direction and, by way of a curved intermediate portion, terminates in a horizontal portion slidably supported on the said rim of the deposition trough,

8. An installation as claimed in any of claims 1 to 7, wherein the chute has a trough-shaped cross section.

9. An installation as claimed in any of claims 1 to 8, wherein the chute is of stainless steel or other corrosion-resistant material, and is provided with a surface coating of porcelain enamel.

10. An installation as claimed in any of claims 1 to 9, wherein the bottom of the deposition trough is covered with a layer comprising an impactabsorbing material and a filler substance.

11. An installation as claimed in claim 10, wherein the said layer comprises a homogeneous mixture of crushed glass, cement, steel cubes, and barium sulphate, and the layer extends to substantially two-thirds of the effective height of the deposition trough.

12. An installation as claimed in any of claims 1 to 11, wherein the deposition trough is underground and is accessible by way of a protected overground entrance.

13. An installation as claimed in any of claims 1 to 12, including a protected exhaust passage for controlled removal of gases from the deposition region.

1 4. An installation as claimed in any of claims 1 to 13, including an additional protective trough structure whose bottom extends beneath the chute and the deposition trough and which has a rim extended as a wall above ground level and encircling all reactor buildings which contain radioactive material.

1 5. A nuclear reactor installation substantially as described with reference to, and as shown in, the accompanying drawings.