Induced nuclear reactions: processes – systems – and elements – With control of reactor – Reactor start-up
Reexamination Certificate
2000-10-30
2003-08-19
Carone, Michael J. (Department: 3641)
Induced nuclear reactions: processes, systems, and elements
With control of reactor
Reactor start-up
C376S215000, C376S216000, C376S217000, C376S259000, C376S260000
Reexamination Certificate
active
06608878
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to a method and a device for monitoring or limiting a power rise during startup of a nuclear reactor, in which a local power is detected in so-called “power region channels” in individual regions of the core, such as is described, for example, in International Publication No. WO 96/21929, corresponding to U.S. Pat. Nos. 5,875,221 and 5,978,429, and European Patent Application 0 496 551 A1, in normal operation through the use of measuring signals.
During startup of a nuclear reactor, the neutron flux level, which is proportional to the power of the reactor generated by nuclear fissions, is raised from a neutron flux source level at approximately 10
−9
of a rated power initially up to reaching a heating-up power which is, for example, approximately 10
−3
of the rated power. In that intermediate range of the reactor power, the rise in the neutron flux density virtually does not change the thermal conditions in the reactor core, that is to say, in particular, the temperatures of the nuclear fuel. As a result almost no reactivity feedback effects are produced which could influence the rate of rise of the neutron flux density. It is only above that intermediate range, that is to say in the percentage range of reactor power (lowermost power range) that, because of the then sensible heating of the nuclear fuel and because of the energy contribution to the coolant, which results therefrom (and is delayed by the fuel time constant), that a reactivity feedback corresponding to the reactivity coefficients of the reactor core begins. That reactivity feedback generally effects a continuous retardation of the power rise until it comes completely to a standstill.
The rise in the neutron flux density through the many decades of the intermediate range is effected by setting a slightly supercritical nuclear state. An effective multiplication factor of the core configuration k
eff
is therefore raised slightly above 1. That purpose is served by suitable reactivity control elements which, in the case of the boiling water reactor type preferably considered herein, are generally control rods, having an absorption effect for neutrons which is specifically reduced by metered withdrawal from the core. The neutron flux density rises in accordance with an exponential function after a supercritical nuclear state is reached. The rate of rise can be characterized by specifying a so-called reactor period. The reactor period is that period of time in which the neutron flux density in the core changes by a factor e=2.718 . . .
The neutrons released by a nuclear fission are partly “prompt” neutrons, which are released immediately by the split nucleus, and partly “delayed neutrons”, which originate from unstable follower nuclei.
In the case of a normal startup operation, the excess reactivity of the core (that is to say the part of the effective multiplication factor k
eff
exceeding the value 1) is set in such a way that the delayed neutrons retain the determining influence on the rate of rise of the neutron flux density. In order to ensure effective controllability, it is customary to undertake the startup in such a way that the reactor period is more than 30 seconds.
However, because of operating errors or accidents, the excess reactivity during startup of the nuclear reactor can also become so large that the rate of rise of the neutron flux density is exclusively determined by the prompt neutrons with their very fast neutron cycle, and the delayed neutrons lose any influence on the rate of rise. That reactor state is denoted as “prompt critical”. The associated reactor period is far below 1 second. The startup operation then changes into an “excursion”, in the case of which the rated value of the reactor power is exceeded briefly, depending on the excess reactivity, before the power is caught by inherent reactivity feedback. Other than in the case of the below-prompt critical startup operations, in the case of excursions the power rise is not already caught in the lowermost power range.
In the case of the occurrence of an excursion, the task is set for the nuclear instrumentation of the nuclear reactor of automatically actuating an emergency reactor shutdown reliably and in good time in such a way that at least conceivable consequent excursions are suppressed.
A boiling water reactor, a conventional instrumentation system for monitoring and controlling in accordance with the prior art and a typical measuring range are considered below and explained in detail with the aid of
FIGS. 1 and 2
as a preferred example.
In order to also take account of the most dangerous extreme case (“conservative” view), it may be assumed that in terms of order of magnitude the neutron flux level has already risen in that time in the range of the rated power (“primary” excursion). The fuel heating connected therewith has then effected an inherent decrease in the excess reactivity through the promptly acting fuel temperature reactivity coefficient. As a result the reactor power already drops again when the control rods actually begin to be inserted into the core. Thus, viewed conservatively, the emergency reactor shutdown can neither prevent the occurrence of the primary excursion nor decisively dampen its course. However, its aim is to suppress conceivable consequent excursions which could come about due to further extraction of the control rods causing the accident, and through dissipation of the thermal energy determining the reactivity negative feedback.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a different method and device for monitoring the power rise during startup of a nuclear reactor provided with power range channels (diversitary excursion monitoring), which overcome the hereinafore-mentioned disadvantages of the heretofore-known methods and devices of this general type, which monitor consequent excursions in order to further increase reliability with which a suitable countermeasure is initiated and which create an additional initiation of the countermeasure through a monitoring system that is diversitary, that is to say it operates according to a different method and is completely independent of an intermediate range UD system.
Evidently, a power range LD system, which likewise has a redundant layout, does not come into consideration in principle as such a monitoring device which is independent of the UD system and preferably diversitary. That is because the overload limit mark of the LD system has not so far fulfilled the requirements to be placed on the monitoring of reactivity excursions. Specifically, because of the very steep distributions of the neutron flux density in the core, which is typical of such accidents, the LD system detectors situated in the vicinity of the excursion center, which supply the main signal contribution for the LD channels, become saturated and cannot feed the full level of the signal substantially exceeding their measuring range into the LD channel. The otherwise very good track fidelity of LD channels is thus naturally no longer obtained in the case of saturation of assigned LVD signals, and the LD signal does not reach the overload limit mark, although the reactor power is actually much higher. Moreover, the overload limit mark is likewise unable to counteract weaker excursions, which cannot even reach that power.
The invention assumes, however, that the LD system can be used when use is made of a limit value which can be reliably monitored even in the case of an excursion. Thus, use is made for the LD system of the nuclear reactor of a limit value criterion which responds reliably even in the case of an excursion and is diversitary in relation to the RESA triggering derived from the UD system or WD system. The limit at which a countermeasure must be initiated is set within the measuring range of the LD channels at a power which is so low that saturation of the detectors of the LD system either does not yet generally occur there, or in any case can only
Carone Michael J.
Framatome ANP GmbH
Greenberg Laurence A.
Keith Jack
Mayback Gregory L.
LandOfFree
Method and device for monitoring the power rise during... 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 device for monitoring the power rise during..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and device for monitoring the power rise during... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3125581