Induced nuclear reactions: processes – systems – and elements – With control of reactor – Reactor start-up
Reexamination Certificate
2000-12-19
2004-02-03
Carone, Michael J. (Department: 3641)
Induced nuclear reactions: processes, systems, and elements
With control of reactor
Reactor start-up
C376S236000, C376S237000, C376S238000
Reexamination Certificate
active
06687323
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to a method for starting up and monitoring a boiling water nuclear reactor and to an apparatus for controlling a boiling water nuclear reactor.
When a nuclear reactor is started up, the neutron flux level, which is proportional to the reactor power output generated by nuclear fission, is initially increased from the neutron flux emission level at about 10
−9
of the rated power output until the heating power output is reached. The latter amounts, for example, to about 10
−3
of the rated power output. In this intermediate range of the reactor power output, the increase in the neutron flux density causes virtually no change in the thermal conditions in the reactor core, that is to say, in particular, the temperatures of the nuclear fuel, so that there are virtually no reactivity feedback effects which could influence the rate of increase in the neutron flux density. It is only above this intermediate range, that is to say in the percentage range of the reactor power output (lowest power output range), that a reactivity feedback corresponding to the reactivity coefficients of the reactor core commences due to the then noticeable heating of the nuclear fuel and to the resulting introduction (delayed by the amount of the fuel time constant) of energy into the coolant and, as a rule, brings about a constant slowing of the power output increase until it has come to a complete standstill.
The increase in the neutron flux density through the many decades of the intermediate range is brought about by the setting of a slightly supercritical core state. The effective multiplication factor of the core assembly k
eff
is therefore raised slightly above 1. This purpose is served by control rods, the neutron absorption action of which is reduced in a controlled manner by metered movement out of the core. The neutron flux density increases according to an exponential function after the supercritical core state has been reached. The rate of increase may be characterized by specifying the so-called reactor period. The reactor period is the timespan in which the neutron flux density in the core changes by the factor e=2.718 . . . .
The neutrons released by nuclear fission are predominantly “prompt” neutrons which are released immediately by the fissioned core. A small proportion consists of “delayed neutrons” which originate from unstable follower decay cores.
During a normal start-up operation, the excess reactivity of the core (that is to say, that part of the effective multiplication factor k
eff
which exceeds the value 1) is set in such a way that the delayed neutrons maintain the determining influence on the rate of increase in the neutron flux density. In order to ensure good controllability, it is customary to carry out the start-up in such a way that the reactor period amounts to more than 30 seconds. The control rods are therefore moved out correspondingly slowly, so that the multiplication factor is always kept only slightly in the supercritical range. To be precise, when the nuclear reactor is being started up, the excess reactivity could otherwise be so great that the rate of increase in the neutron flux density is determined solely by the prompt neutrons, with their very rapid generation sequence, and the delayed neutrons lose any influence on the rate of increase. This reactor state is designated as “prompt critical”. The associated reactor period is well below 1 second. The start-up operation then changes to an “excursion” in which, depending on the excess reactivity, the rated value of the reactor power output may be exceeded briefly, before the power output is absorbed as a result of inherent reactivity feedback. When excursions occur, therefore, the increase in power output is not absorbed in the lowest power output range, unless the start-up operations are subprompt critical.
This gives rise to the general object of controlling and monitoring the outward movement of the control bars during start-up, in such a way that the neutron flux (and consequently the reactor power output) is increased in a controlled manner only and, for example, the probability of excursions is reduced or, if possible, eliminated.
Whether an individual control rod can be drawn quickly, slowly or not at all, without the effective multiplication factor being changed to too great an extent, depends on the current configuration of the control rods and on the neutron flux which is caused thereby and to which the control rod is exposed. If the local neutron flux is low at the location of this control rod, because a large number of control rods in its vicinity are still in the initial position (fully retracted into the core) and therefore shield the respective control rod due to their own absorption capacity, then the control rod is only of little effectiveness and can be moved out, without the reactivity conditions of the core being changed greatly.
If, by contrast, all the control rods in the vicinity of this control rod are in the final position (fully moved out of the core), the control rod has its highest effectiveness.
The active fuel elements used for generating energy are arranged in the reactor core, with longitudinal axes parallel to one another, along a cross-sectional plane of the reactor core which is perpendicular to their longitudinal axes, in a pattern forming a regular grid structure with rectangular or square meshes. At the same time, they stand in the meshes of the grid-shaped pattern in such a way that in each case four fuel elements form a square cell, in the center of which is located a control rod movable in the axial direction. Fuel elements at the edge of the core, which are left over when these square “control cells” are formed from four fuel elements arranged around a control rod, are not taken into account in this pattern. Excursions can occur only as long as the power output is clearly below 5% of the rated power output of the nuclear reactor. The invention affords two restrictive criteria which are adhered to when the control rods are moved out until this upper limit of the start-up range is reached. These restrictions may even be incorporated into the planned program by means of which the control rod drives are activated; there may also be provision, however, for utilizing these criteria in order to monitor the start-up operation. In particular, an apparatus in which these criteria are installed may precede or overrule the activation of the control rod drives.
With these and other objects in mind there is provided, in accordance with the invention, a method of starting up a boiling water nuclear reactor with a reactor core having a plurality of control rods distributed about a cross section of the reactor core in a two-color checkerboard pattern in which each square is assigned a control rod, and wherein the control rods are moved into the reactor core in a shutdown state of the reactor and are moved at least partially out of the reactor core during a start-up of the reactor, the method which comprises:
defining a first configuration, in which simultaneously moved control rods are assigned to squares of different color in the checkerboard pattern, and/or a second configuration, in which virtually half the control rods are moved completely or partially out of the reactor core and fully moved-in control rods are assigned to squares of different color; and
moving the control rods out of the reactor core under the following restrictions in order to reach a reactor power output upper limit limiting a start-up range:
a) move a given control rod completely or partially out of the reactor core only when all directly or diagonally adjacent control rods remain at rest; and
b) as long as the reactor power output upper limit is not yet reached and a predetermined minimum number of the control rods are not yet moved virtually completely out of the reactor core, all the control rods which in each case are directly adjacent to a completely or at least partially moved-out control rod remain moved in com
Carone Michael J.
Framatome ANP GmbH
Greenberg Laurence A.
Locher Ralph E.
Palabrica R
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