Induced nuclear reactions: processes – systems – and elements – Fission reactor material treatment – Impurity removal
Reexamination Certificate
1999-07-26
2001-01-16
Jordan, Charles T. (Department: 3641)
Induced nuclear reactions: processes, systems, and elements
Fission reactor material treatment
Impurity removal
C376S352000, C376S444000
Reexamination Certificate
active
06175606
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a filter for capturing foreign matter transported with the coolant in a light-water nuclear fuel assembly, more precisely a nuclear fuel assembly of boiling water type.
BACKGROUND ART
A fuel assembly in a boiling water nuclear reactor comprises a long tubular container. The container is often made with a rectangular or square cross section and is open at both ends to make possible a continuous flow of coolant through the fuel assembly. The fuel assembly comprises a large number of equally long tubular fuel rods, arranged in parallel in a certain definite, normally symmetrical pattern. The fuel rods are retained at the top by a top tie plate and at the bottom by a bottom tie plate. To allow coolant to flow along the fuel rods in the desired way, it is important that these be spaced from each other and prevented from bending or vibrating when the reactor is in operation. For this purpose, a plurality of spacers are used, distributed along the fuel assembly in the longitudinal direction.
Experience has shown that in connection with repair and service of nuclear reactors, foreign matter may enter the coolant of the nuclear reactor. The foreign matter then moves with the coolant which circulates through the reactor core. The foreign matter may, among other things, consist of metal chips formed in connection with repair of, for example, a steam separator, pieces of metal wire or oxide flakes which have been torn loose from, for example, the fuel rods where they have been formed. The foreign matter may give rise to wear damage which may result in serious consequences if the damage arises on parts which are particularly fragile, such as on the fuel rods.
To avoid damage of the above kind, it is known to design various kinds of filters close to the lower part of the fuel assembly. The filter prevents the foreign matter from reaching the fuel rods. It is known, for example, to design the bottom tie plate, to which the fuel rods are attached, with a large number of through-holes. In this way, a strainer is provided, which prevents foreign matter from reaching the fuel rod bundle with control-rod guide tubes and spacers. It is also known to arrange various types of helical springs in a so-called bottom support in which the fuel bundle is arranged to rest. The helical springs are adapted to prevent debris from passing with the coolant flow through the bottom support and up through the bottom tie plate.
By arranging additional structural parts in the flow path of the coolant through the fuel assembly, the pressure drop across the fuel assembly increases. The pressure drop may be increased within certain limits. Too high a pressure drop may, in the worst case, lead to so-called dryout. To minimize the risk of dryout, the fuel assembly is formed with a flow of coolant which with a fixed margin, the so-called dryout margin, exceeds the coolant flow where dryout occurs under the present conditions.
A disadvantage of arranging loose parts in the bottom tie plate or in the bottom support for capturing any foreign matter is that these may be set into vibration because of the high pressure of the coolant. During the vibration, wear arises, whereby the loose parts may become detached. It may also result in flakes of the material, against which the wear occurs, loosening and accompanying the coolant.
SUMMARY OF THE INVENTION
According to the present invention, a debris catcher is designed in a bottom support arranged below a normally substantially vertically arranged fuel bundle. The fuel bundle may possibly be divided into four sub-bundles. The bottom support controls the fuel bundle in a transition piece which, in turn, is intended to be arranged in a so-called assembly supporting plate in a reactor core. The transition piece guides the coolant, flowing upwards through the core of fuel assemblies, into the respective fuel assembly through the bottom support. The bottom support comprises either a unit which is separate from the transition piece, or a part which is integrated into the transition piece.
To give the bottom support a filtering effect, the filter is designed with a plurality of continuous, substantially straight continuous, holes forming a strainer. The bottom support is substantially designed as a parallelepiped with a square flat side. The parallelepiped is provided with a downstream and an upstream flat side and four narrow sides. The flat sides and the narrow sides surround a cavity and form a parallelepiped. The downstream flat side is formed with one or more openings to receive the lower ends of a fuel bundle or a plurality of fuel bundles. The substantially straight holes are arranged in the upstream flat side, forming a filter through which the coolant is forced to pass before it reaches the fuel rods arranged in the fuel bundle or in the fuel bundles.
The advantage of the invention is that the filter is formed as an integral part of a structural part, namely, the bottom support or the transition piece. Because of its shape as an integral part of the bottom support, or the transition piece, the filter may be given a very high strength, the risk of vibrations arising thus being very small.
The above-mentioned shape of the bottom support, or the transition piece, implies that that pressure drop across this becomes higher than if the upstream flat side is designed with one single large through-opening as in the normal case. By arranging the filter integrated with the bottom support, or the transition piece, instead of in the bottom tie plate, the bottom tie plate may, however, be given a more open structure. In this way, the increased pressure drop, caused by the integral filter, is compensated such that a dryout margin set for the fuel assembly may be maintained.
By arranging the through-holes in the bottom support, or the transition piece, instead of in the bottom tie plate, greater freedom of choice is also provided as regards the location of the holes. When locating the through-holes in the bottom tie plate, also the location of the fuel rods must be taken into consideration. Another advantage of arranging the through-holes in the bottom support instead of in the bottom tie plate is that the filter may be made with a considerably greater strength.
Another important factor when designing a filter is that the risk of clogging thereof is minimized. According to one embodiment of the present invention, this problem is solved by arranging so-called bypass holes in each of the corner portions of the bottom support. More particularly, the bypass holes are formed in the respective corners of the upstream flat side. The bypass holes have a size which is considerably larger than that of the through-holes. This location of the bypass holes is advantageous in the event that clogging of the filter should occur. In that case, the upwardly flowing flow, when it reaches the bottom support, is forced to be deflected out towards the corners of the bottom support for passage through the bypass holes and is then deflected again to be able to pass upwards through the through-openings in the bottom tie plate. By forcing the flow of coolant to change direction, the risk that foreign matter, which is possibly oriented such that it may pass through the bypass holes, should not be capable of being re-oriented such that it may also pass through the bottom tie plate, but adhere to the upstream side of the bottom tie plate, is reduced.
The advantage of the present invention is thus that an efficient filtering of the coolant may be achieved with an ensuing low pressure drop and that the risk of clogging is minimized.
REFERENCES:
patent: 5219517 (1993-06-01), Nylund
patent: 5361287 (1994-11-01), Williamson
patent: 5471514 (1995-11-01), Soderlund
patent: 5473649 (1995-12-01), Olsson et al.
patent: 43 32 678 A1 (1994-04-01), None
patent: 0 655 746 A1 (1995-05-01), None
patent: 0 656 630 A1 (1995-06-01), None
patent: 0 710 961 A1 (1996-05-01), None
patent: 465 191 (1991-05-01), None
ABB Atom AB
Jordan Charles T.
Mun Kyongtark K.
Pollock, Vande Sande & Amernick RLLP
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