Induced nuclear reactions: processes – systems – and elements – Testing – sensing – measuring – or detecting a fission reactor... – Flux monitoring
Reexamination Certificate
2000-06-21
2002-08-06
Barefoot, Galen L. (Department: 3644)
Induced nuclear reactions: processes, systems, and elements
Testing, sensing, measuring, or detecting a fission reactor...
Flux monitoring
C376S255000
Reexamination Certificate
active
06430247
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a method and system for monitoring at least one operating parameter of the core of a nuclear reactor and in particular a pressurized water reactor.
BACKGROUND OF THE INVENTION
Nuclear reactors such as pressurized water reactors have a core consisting of fuel assemblies which are generally straight, of prismatic shape and juxtaposed with their longitudinal axis vertical, i.e. in the direction of the height of the core.
It is essential to ensure at all times that the reactor is operating perfectly and in accordance with general safety conditions set by regulations and standards.
In particular, it is necessary to determine if the production and spatial distribution of the flow of neutrons and the spatial distribution of the power generated in the core conform to conditions corresponding to normal and satisfactory operation of the core.
This requires calculation of operating parameters of the core of the nuclear reactor, such as the spatial distribution of power in the core, the form factors of the neutron flux or the critical heating ratio. These parameters are determined by measuring the neutron flux in the core to determine the distribution of the neutron flux throughout the core in three dimensions.
The parameters characteristic of the state of the core in normal operation which are derived from the neutron flux measurements must not at any time be outside ranges determined during the process of designing the nuclear reactor.
It is necessary to trip an alarm and to implement various measures concerning the control of the nuclear reactor if any parameter characteristic of the operation of the core is found to exceed a limiting value.
For effective monitoring of the operation of the core of the nuclear reactor it is necessary to determine the operating parameters of the core and therefore the distribution of the neutron flux in the core in as short a time period as possible.
The neutron flux measurements in the core needed for continuous monitoring of the nuclear reactor in operation are generally provided by chambers outside the containment vessel of the reactor which are generally referred to as “excore” chambers.
These chambers include multiple measuring stages (for example six stages) distributed in the direction of the height of the core and are generally used to perform measurements in four areas at the periphery of the core of the nuclear reactor which are symmetrical about two axial planes of symmetry of the core at an angle of 90° to each other.
The staged excore detector chambers provide flux measurements at various heights in the core and in these four circumferential areas around the core. However, these external systems provide only approximate values of the neutron flux within the core and an approximate representation of the neutron flux distribution. Accordingly, the monitoring parameters are obtained in a relatively imprecise manner and, for safety reasons, greater margins must be provided for critical values of these parameters that must not be reached or exceeded.
To obtain a more exact representation of the neutron flux distribution within the core, additional neutron flux measurements are conducted within the core, at regular but relatively long time intervals, for example in the order of one month, using very small measuring probes, referred to as “incore” probes, which generally take the form of fission chambers. Each incore probe is fixed to the end of a flexible cable referred to as a teleflex cable for moving it inside a measuring channel of the instrumentation of the nuclear reactor. Each measuring channel opens at one end into an instrumentation area in the bottom part of the reactor building. The fission probes are moved inside the measuring channels from the instrumentation area. Each measuring channel includes a fuel assembly instrumentation tube inside the core of the nuclear reactor and a glove finger inside the instrumentation tube in which the fission probe moves. The neutron flux is measured in a set of fuel assemblies distributed throughout the section of the core.
In the case of a core with
177
fuel assemblies,
56
measuring channels are generally used, for example. Similarly,
58
measuring channels are used for a core with
193
fuel assemblies,
50
measuring channels for a core with
157
fuel assemblies and
60
measuring channels for a core with
205
fuel assemblies. The neutron flux measurements are carried out as the incore probes are moved at slow speed over the full height of the core. Many neutron flux measurement points closely spaced along the height of the core are obtained in this way. An image which is sufficiently representative of the neutron flux is obtained in the form of a flux map, given the distribution of the instrumented fuel assemblies within the core and the symmetry of the core. However, the incore probes consisting of the fission chambers cannot be used for very long periods inside the core of the nuclear reactor. An accurate flux map of the core is determined only from time to time and therefore cannot be used for continuous monitoring of the operation of the core of the nuclear reactor.
Also known in the art are neutron flux measuring probes which can remain in the core of a nuclear reactor throughout the operation of the nuclear reactor. These neutron flux measuring probes, which can take the form of “collectrons”, are generally assembled in the form of measuring rods in a vertically aligned arrangement with a constant spacing between two successive probes, to constitute flux measuring detectors distributed throughout the height of the core of the nuclear reactor. Each rod is inserted into a glove finger normally used for measurements by a mobile probe and itself inserted into the instrumentation tube of a fuel assembly. Each of the flux measurement detectors or measurement rods, whose length is almost equal to the height of the core, can include eight measuring probes in the form of collectrons, for example.
In the case of a nuclear reactor core with
177
fuel assemblies, it has been proposed to place
52
measuring rods or detectors in
52
instrumented assemblies of the core of the nuclear reactor distributed throughout the section of the core.
An instrumentation system of the above kind, which has 8×52 measuring points distributed throughout the core, is able to provide an accurate three-dimensional image of the flux distribution in the core of a nuclear reactor.
However, given the response times necessary for monitoring the core of the nuclear reactor, processing neutron measurements produced by instrumentation remaining permanently in the core while the nuclear reactor is operating takes a long time, which is hardly practicable for monitoring operating parameters of the core.
Thus there is no accurate and fast method of obtaining the spatial distribution of the flux and of obtaining parameters for monitoring the core of the reactor from the flux distribution.
Nuclear power stations generally have a plurality of units or “tranches” each consisting of a nuclear reactor in a reactor building and conventional electrical power generation part. In this case, core monitoring concerns the nuclear reactor of each tranche.
BRIEF DESCRIPTION OF THE INVENTION
The object of the invention is therefore to propose a method of monitoring an operating parameter of the core of a nuclear reactor of a tranche of a nuclear power station consisting of a plurality of fuel assemblies juxtaposed along the height of the core using a set of neutron flux measuring detectors introduced into at least some of the fuel assemblies of the core and each including a plurality of fixed neutron flux measuring probes distributed along the height of the core, the method enabling fast and accurate determination of the flux distribution in the core, fast and accurate determination from the flux distribution of an operating parameter of the core, and substantially real time monitoring.
To this end, while the nuclear reactor is operating, at particular time intervals:
the spatial distribut
Louedec Norque
Mourlevat Jean-Lucien
Barefoot Galen L.
Connolly Bove & Lodge & Hutz LLP
Framatome
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