Induced nuclear reactions: processes – systems – and elements – Testing – sensing – measuring – or detecting a fission reactor...
Reexamination Certificate
2000-06-29
2002-02-05
Jordan, Charles T. (Department: 3644)
Induced nuclear reactions: processes, systems, and elements
Testing, sensing, measuring, or detecting a fission reactor...
C376S249000, C376S250000, C376S251000
Reexamination Certificate
active
06345081
ABSTRACT:
THE BACKGROUND OF THE INVENTION AND PRIOR ART
The present invention refers to a method and a device for evaluating the integrity of the nuclear fuel in a nuclear plant having a reactor enclosing a reactor core comprising a number of fuel cladding members in which said fuel is present.
A core in a nuclear reactor comprises a plurality of fuel assemblies which are vertically provided with a certain distance from each other. The core is enclosed in a reactor vessel and submerged in a liquid, i.e. reactor water functioning as cooling medium as well as neutron moderator and flowing through the reactor vessel. The nuclear reactor also comprises a plurality of control rods which by being displaced into and out of the core regulate the effect produced by the reactor, and shut down and start, respectively, the reactor. A fuel assembly comprises a plurality of fuel rods vertically provided and each containing a staple of circular cylindrical, so called pellets of a nuclear fuel. Each fuel rod comprises a cladding tube which normally is manufactured in a zirconium alloy and which encloses the staple of fuel pellets. During an ongoing nuclear reaction, i.e. during the burn-up of the nuclear fuel, fission gases are formed which comprise radioactive inert gases and which normally are maintained within the fuel rod by means of the cladding tube. Such fission gases comprise different nuclides of xenon and krypton.
During operation it sometimes happens that a defect appears on a cladding tube. The first defect which appears on a cladding tube, for instance as a consequence of mechanical wear, is called a primary defect. The primary defect consists of a relatively small hole or a relatively small crack through which fission gases may be released. Such a primary defect is normally not particularly serious as such. However, after a period of time a primary defect may develop to a larger defect, a so-called secondary defect, which is more serious and consists of a larger hole or a crack extending axially along or essentially perpendicular to the longitudinal direction of the cladding tube. Such a secondary defect may arise by the penetration of water and steam into the fuel rod at the primary defect and by water and steam penetrating into the fuel rod at the primary defect and, by hydration, making the cladding tube brittle which after a certain period of time cracks during thermal or mechanical load or due to tensions caused by the hydrides themselves. By a secondary defect, also other fission products, such as iodine and caesium, are, except the fission gases previously mentioned, released. In case of a really serious fuel defect, for instance due to the fact that a fuel cladding tube is broken, also uranium and plutonium may start to leak out into the core. In order to prevent the occurrence of a more serious defect, it is of course important to detect a fuel defect at an early stage and adapt or interrupt the continuing operation of the reactor.
The defects on the cladding tubes may have many different grounds. In case of a too fast effect increase in the fuel, a combination of tension, due to the thermal extension, and chemical influence on the inner side of the cladding tube may result in the cracking of the cladding tube. This type of defect is called PCI (Pellet Cladding Interaction).
Another primary defect type is a so-called, debris defect, i.e. debris such as metal chips or the like, which causes wear defects on the cladding tubes from outside. Experience has shown that for instance in connection with repair and service of a nuclear reactor, such debris may be introduced and thereafter be carried by the water circulating through the core and give rise to a wear defect in the form of a primary defect on the cladding tube. A further type of defect may depend on a manufacturing failure. Finally, a so-called Dry-Out is also to be mentioned, which means that the liquid film which normally always is present on the outer side of the cladding tubes is evaporated. This leads to a quick local temperature increase in the fuel rod. Such a temperature increase leads to the melting of the cladding tube if the operation of the reactor is not immediately interrupted.
The development from a primary defect which is not serious as such to a secondary defect is influenced and may be accelerated by the manner in which the reactor is operated. If, for instance, the reactor is operated with a high effect or if many control rod movements are performed in the proximity of a defect, the defect may develop faster than it would have done at a lower reactor effect or if the control rods would have been maintained in a position. By stopping the reactor after a primary defect has occurred, the water flow into the defect fuel rod is increased and the degradation, i.e. the development towards a secondary defect, is accelerated.
If a fuel defect occurs and is detected during operation of a nuclear reactor, the people responsible for the operation may choose between shutting down immediately the reactor and replacing the defect fuel rod, or continuing the operation of the reactor, possibly by a locally reduced effect, until the next point of time for service and fuel replacement when the reactor is to be shut down. Normally, i.e. if no serious fuel defects have been detected, such a fuel replacement takes place once a year. The first alternative, i.e. to shut down the reactor, is very costly and ought to be avoided except at a serious fuel defect when the core may be more contaminated than is tolerated by the limits defined. In case of a PCI-defect, it would be sufficient to reduce the effect in the part of the core where the defect fuel rod is located. If the fuel defect is a debris defect, the reactor may be further operated at full effect until the next fuel replacement.
In order to determine if the reactor is to be shut down, if an effect reduction is sufficient or if it is possible to continue the operation at full effect, it is necessary to know the actual type of defect, where the defect is positioned, the size of the defect and the risk of degradation of the defect. These questions are very difficult to answer in a secure and reliable manner by the technique available today. Another problem in this connection is the determination if the primary defect has developed to a secondary defect or if a further primary defect has occurred in the case that higher activity levels of the reactor off-gases have been measured. Today, it is for the people responsible for the chemistry and the operation, in co-operation, to evaluate the type of the defect and to take a decision whether and how the reactor is to be operated further based on the activity levels measured and their experiences.
One way of detecting a fuel defect is to measure the total percentage of the radioactive inert gases in the reactor off-gases. One problem in this connection is that there are two different sources of radioactive inert gases, partly from the fuel due to a defect on the cladding tube and partly due to core contamination, i.e. from fissionable compounds being deposited onto different surfaces in the reactor core. By an increased number of fuel defects, which have occurred previously, this contamination increases and is therefore a growing source of error. However, the inert gases released from the core contamination contain a higher percentage of short-lived nuclides than the gases from a fuel defect.
Devices for detecting the presence of a fuel defect are known, for instance such a device is disclosed in U.S.Pat. No. 5 537 450. In this document, a device is disclosed which detects fuel defects on-line, i.e. during reactor operation, by leading a part of the off-gases from the reactor via a gammaspectrograph which continuously measures the nuclide composition and the activity level of the off-gases. It is also known to determine the position of a fuel defect by a method called flux-tilting, which involves the regulation of one control rod at a time in such a manner that the effect is changed locally in the core at the same time as the activity
Arlebåck Inger
Bjurman Björn
Sihver Lembit
Abbott Yvonne R.
Connolly Bove Lodge & Hutz
Westinghouse Atom AB
LandOfFree
Method and a device for evaluating the integrity of the... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method and a device for evaluating the integrity of the..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and a device for evaluating the integrity of the... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2957390