Radiant energy – Invisible radiant energy responsive electric signalling – Semiconductor system
Reexamination Certificate
2000-03-14
2002-12-17
Hannaher, Constantine (Department: 2878)
Radiant energy
Invisible radiant energy responsive electric signalling
Semiconductor system
C250S304000
Reexamination Certificate
active
06495835
ABSTRACT:
TECHNICAL FIELD
The invention relates to a device for determining nuclide contents of radioactive inert gases. The device comprises a cylinder with an arbitrary cross section which surrounds a measuring chamber, where the measuring chamber contains the inert gases, a detector for detecting gamma radiation emitted from the inert gases, and calculating members for calculating the nuclide contents in the inert gases starting from the detected gamma radiation.
BACKGROUND ART
A reactor core in a nuclear reactor comprises a plurality of fuel assemblies arranged vertically in the core in a certain spaced relationship to each other. The reactor core also comprises a plurality of control rods which, by being inserted into and withdrawn from the reactor core, control the output power of the nuclear reactor and start and stop the nuclear reactor. A fuel assembly comprises a plurality of vertical fuel rods, each of which containing a stack of pellets of a nuclear fuel arranged in a cladding tube. During burnup of the nuclear fuel, radioactive inert gases, which are normally retained in the fuel rod, are released. Examples of such inert gases are various nuclides of krypton and xenon.
In the event that a failure arises on a cladding tube during operation such that the inert gases may leak out into the reactor core, the inert gases will accompany the off-gases out of the nuclear reactor out of the nuclear reactor. A minor fuel failure which only causes release of inert gases is no obstacle, per se, to continued reactor operation. In the event that no measures are taken, the fuel failure may grow in magnitude by water and steam penetrating into the fuel rod and cause embrittlement of the cladding tube, whereby other fission products, such as iodine and cesium, may leak out into the reactor water. When the fuel failure is really serious, also uranium and/or plutonium may start leaking out into the reactor core. To prevent a serious fuel failure, it is therefore important to detect each fuel failure as early as possible.
In case of a serious fuel failure, the reactor has to be shut down and the failed nuclear fuel be replaced. This is very costly and should be avoided, if possible. Normally, the reactor is shut down approximately once every year for service and refuelling and then failed nuclear fuel may also be replaced. If a minor fuel failure is detected during operation, it need not be acted upon until the next refuelling. To avoid that the fuel failure in the meantime develops into a great and serious fuel failure, the power may be reduced, by the insertion of control rods, into that part of the reactor core which contains the failed fuel. It is, therefore, important not only to detect the fuel failure during operation, but also to locate where in the reactor core the fuel failure has arisen.
One way of detecting a fuel failure is to measure the total content of radioactive inert gases in the off-gases of the nuclear reactor. One problem with this method is that there are two different sources which emit radioactive inert gases, failed nuclear fuel as well as core contamination, that is, radioactive contaminants deposited on surfaces in the reactor core. The contamination successively increases and becomes an increasingly greater source of error. The inert gases which are emitted from the core contamination, however, differ from those which originate from a fuel failure in that they contain a considerably higher proportion of short-lived nuclides. To be able to determine if the inert gases originate from core contamination or from fuel failure, the distribution of short-lived and long-lived nuclides in the off-gases must be known. It is thus necessary to measure the content of each individual nuclide.
The patent document U.S. Pat. No. 5,537,450 discloses a device for measuring, while the reactor is in operation, the contents of radioactive inert gases in the off-gases from the nuclear reactor for the purpose of detecting a fuel failure as well as a method for locating the fuel failure. During the reactor operation, some of the off-gases from the nuclear reactor are continuously passed to a gamma spectrograph which detects the different inert gases which occur in the off-gases as well as measures the activity level of each one of the inert gases. The activity level for each one of the inert gases is a measure of the content of the inert gas in the off-gases. Based on the detection and the measurement of the different inert gases which occur and what activity levels the inert gases have, an assessment is made whether a fuel failure exists or not. When a fuel failure has been detected, it is determined where the fuel failure is located by moving the control rods, one at a time, in a reciprocating manner such that the power increases and decreases in fuel assemblies located adjacent to the control rod while at the same time the activity levels for the inert gases are measured by the gamma spectrograph. When the power is changed in the failed fuel assembly, also the activity levels of the inert gases are changed. In this way, the failed fuel assembly may be located. There are also natural reasons for increased activity levels for the inert gases which have nothing to do with the fuel failures, for example changes in the flow of the off-gases from the nuclear reactor.
The gamma spectrograph which is used in the above-mentioned patent document comprises a measuring chamber with a detector arranged in the centre of the measuring chamber. During measurement, the measuring chamber is filled with off-gases from the nuclear reactor. The detector detects the photons, so-called gamma radiation, which are released from the inert gases in the measuring chamber during a given measurement period. Each photon gives rise to a pulse, the amplitude of which depends on the energy contents of the photon. A calculating member counts the number of pulses within different given energy intervals and, with knowledge of the length of the measurement period, the number of measured pulses per unit of time in the various energy intervals may be calculated. The content of nuclides of different radioactive inert gases may thus be estimated. From
FIG. 2
in the patent document U.S. Pat. No. 5,537,450, it is clear that the detector is a so-called coaxial detector. A typical coaxial detector has the shape of a straight cylinder with a height which may vary between 2 and 7 cm and it measures optimally in an energy range of from 100 keV to about 2000 keV.
Before the location of the fuel failure can be initiated, according to U.S. Pat. No. 5,537,450, the power of the nuclear reactor must be reduced to between 60% and 80% of full power in order not to make the failure worse during the period during which the location of the fuel failure is in progress. Reducing the power of the nuclear reactor is costly and should therefore be done as quickly as possible. A disadvantage of the device described in the above-mentioned document is that it can only make 2-3 measurements per hour.
One of the reasons for a coaxial detector measuring slowly during the detection of the photons which are emitted from the radioactive inert gases is that it gives a high back-ground radiation. The background radiation is caused by interaction in the coaxial detector, for example Compton scattering. The background radiation partially drowns the pulses which are intended to be detected, whereby it becomes necessary to measure for a relatively long time to obtain statistically significant values of the content of nuclides of the different inert gases.
When detecting a fuel failure in a nuclear reactor, Xe-133 is the most important nuclide to measure. Xe-133 is a relatively long-lived nuclide and emits gamma radiation in the energy range around 80 keV. Other nuclides which are of interest to measure, for example Xe-135 and Xe-138, emit gamma radiation with considerably higher energies. Since Xe-133 has a long radioactive half-life in comparison with the other nuclides, it emits much fewer photons per unit of time than these. The coaxial detector which measures
Bjurman Björn
Sihver Lembit
ABB Atom AB
Connolly Bove & Lodge & Hutz LLP
Gabor Otilia
Hannaher Constantine
LandOfFree
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