Induced nuclear reactions: processes – systems – and elements – Control component for a fission reactor – Wherein concentration of the reactivity affecting material...
Reexamination Certificate
2001-07-02
2003-09-02
Behrend, Harvey E. (Department: 3641)
Induced nuclear reactions: processes, systems, and elements
Control component for a fission reactor
Wherein concentration of the reactivity affecting material...
C376S327000
Reexamination Certificate
active
06614869
ABSTRACT:
The present invention relates to absorber rods for inclusion in nuclear reactor control clusters. It is particularly usable in reactors that are moderated and cooled by pressurized water, where the core is constituted by fuel assemblies, each having a bundle of fuel rods held at the nodes of a regular array by a skeleton assembly formed by nozzles connected by guide tubes carrying rod spacer grids. Under such circumstances, each cluster is constituted by a spider connected to a control mechanism and carrying rods containing absorber material for the purpose of being inserted to a greater or lesser depth in the guide tubes or even for being completely extracted from the core.
Usually, a reactor has its power adjusted and is stopped by using various groups of clusters of different makeups (such as “black” clusters which are highly absorbent, and “gray” clusters which are less absorbent).
“Black” clusters are constituted by rods containing a highly absorbent material, such as an Ag-In-Cd alloy, also known as AIC, or boron carbide B
4
C, in cladding which is generally made of stainless steel.
Those rods present limitations, in particular when they are for use in reactors operated to “follow load” and/or for very long use. The Ag-In-Cd alloy is subject to creep and to swelling under irradiation. Boron carbide B
4
C presents a large amount of swelling under irradiation which means that it cannot be used in the bottom portions of rods which are the portions that are inserted the most often into the core.
It is also known that hafnium absorbs neutrons and does not creep at the working temperatures and does not swell under irradiation. However it needs to be protected from hydriding if it is in steel cladding and it needs to be protected from wear if it is allowed to rub against guide elements. Welding it to the stainless steel used for making the protective cladding of rods containing boron carbide pellets gives rise to connections that are fragile and sensitive to hydriding, as explained below.
In theory, replacing Ag-In-Cd with hafnium in steel cladding makes it possible to avoid the absorber swelling. But the hafnium must not be allowed to hydride.
Attempts at making rods with hafnium in cladding have encountered difficulties. After the natural oxide film that forms on contact with air during manufacture has been worn away by friction, the hafnium absorbs the hydrogen that passes through the steel cladding and it swells, to such an extent that it can become necessary to change clusters prematurely.
A similar problem arises with “gray” cluster rods containing rods that are less absorbent.
Proposals have already been made (French patent application No. 96/07430, which issued as French Patent No. 2,749,968) to avoid the difficulty by using rods having bottom portions constituted by un-clad bars of HfZr or of hafnium, and replacing the stainless steel cladding in the top portion of the rod with cladding made of an Hfzr alloy containing pellets of (HfZr)B
2
or of HfZr.
To avoid adopting a cladding material that has mechanical characteristics that are very inferior to those of stainless steel, the invention seeks in particular to provide an absorber rod capable of withstanding irradiation over a long period of time in a reactor and also making it possible to retain stainless steel cladding and a stainless steel top connection plug for connection to a spider.
To this end, the invention provides in particular an absorber rod suitable for use in a control cluster and comprising stainless steel cladding closed by plugs and containing a stack of absorber pellets, the rod being characterized in that it also comprises an end bar of hafnium which is advantageously not clad, and which is secured to the bottom plug of the cladding by a surely mechanical connection. The stack of pellets is usually constituted by boron carbide; it could also be constituted by hafnium and zirconium boride, pure or mixed (as described in French patent application No. 96/07430) or indeed of rare earth oxides (europium, dysprosium), pure or mixed with other oxides.
The solid or hollow bar generally constitutes at least 15% of the length of the rod, i.e. of the distance over which the rod can be moved by its mechanism. Frequently, the bar constitutes about 25% of the total length of the rod. It is the bar which is inserted most frequently into the core.
The non-clad hafnium bar comes into direct contact with the pressurized water, it does not hydride, it does not swell, and it does not creep. However, in this use, hafnium that is not clad needs to be protected:
against wear, for the portion thereof that comes into contact with its neighbours; and
against hydriding at the connection between the hafnium and the stainless steel.
The best protection is a layer of oxide which is established on an underlying diffusion layer that is impermeable to hydrogen and that withstands wear. This layer forms naturally in the medium that is to be found in pressurized water reactors (pressure about 150 bars and temperature in the range 280° C. to 350° C.) but only to a thickness that is generally not sufficient to provide effective protection against wear. This layer can also be created, prior to mounting the rod in the reactor, by applying surface treatment such as that described, for example, in document EP-A-0 421 868. In some cases, the thickness still runs the risk of being too little to withstand the highest amounts of wear.
An initial protective layer against wear can also be created by controlled oxidation of the outside surface in an atmosphere of pure oxygen or of oxygen and argon, at a temperature lying in the range 800° C. to 950° C. The layer is advantageously at least 5 micrometers (&mgr;m) thick initially (as described in French patent application No. 96/07430).
Rubbing tests performed in pressurized water have shown that it is desirable to have oxide to a thickness of at least 5 &mgr;m to 10 &mgr;m in order to withstand wear. However, if the thickness of oxide formed by thermal oxidation is too great, then stresses at the metal/oxide interface are high and can lead to the oxide layer spalling off. Nevertheless, even if the oxide does spall off, resistance to wear continues to be provided by the underlying diffusion layer providing it is of sufficient thickness (of the order 12 &mgr;m to 15 &mgr;m).
The oxidizing process advantageously establishes an oxide layer that is sufficiently thick to prevent hydriding (in particular in the zones connected to austenitic stainless steel) and forms a diffused layer that is quite thick and withstands stresses associated with wear.
An oxide layer that is too thick, exceeding 10 &mgr;m, runs the risk of spalling. A good compromise can be achieved by aiming for an oxide layer that is 5 &mgr;m to 10 &mgr;m thick and a diffusion layer having a depth of 15 &mgr;m to 50 &mgr;m, and in particular of 25 &mgr;m to 30 &mgr;m. There is generally no point in attempting to obtain a thicker diffused layer, given the wear stresses that are encountered by absorber rods in pressurized water nuclear reactors.
A compromise can be found by limiting the treatment temperature so as to leave time for diffusion to take place and thus for internal stresses to decrease, and by limiting the rate at which oxygen is supplied either by limiting its partial pressure during oxidization by acting on the total pressure and on its dilution, or by interleaving diffusion stages between the oxidizing stages. The flow speed of the oxidizing gas is another parameter of the process.
Thus, to achieve a density per unit area of included oxygen lying in the range 0.0001 liters per square centimeter (l/cm
2
) of substrate to 0.01 l/cm
2
of substrate, oxidation performed at 860±10° C. for 6 hours at a total pressure in the range 0.1 millibars (mbar) to 0.7 mbar in an argon atmosphere containing 3% to 25% oxygen makes it possible to obtain, on hafnium containing 300 parts per million (ppm) of iron and 300 ppm of oxygen, an oxide thickness of 6±1 &mgr;m and a diffused layer to a depth of 35 &mgr;m to 50 &mgr;m.
A simil
Delannoy Thierry
Duthoo Dominique
Hertz Dominique
Thibieroz Nathalie
Behrend Harvey E.
Kenyon & Kenyon
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