Stock material or miscellaneous articles – All metal or with adjacent metals – Composite; i.e. – plural – adjacent – spatially distinct metal...
Reexamination Certificate
2000-10-05
2002-01-22
Koehler, Robert R. (Department: 1775)
Stock material or miscellaneous articles
All metal or with adjacent metals
Composite; i.e., plural, adjacent, spatially distinct metal...
C075S621000, C148S538000, C148S672000, C148S407000, C148S421000, C376S339000, C376S416000, C376S419000, C376S447000, C376S457000, C420S422000, C428S615000, C428S636000
Reexamination Certificate
active
06340536
ABSTRACT:
This invention relates to an alloy of zirconium and niobium that includes erbium as a consumable neutron poison.
The invention also relates to a method for the preparation and conversion of said alloy and a component comprising said alloy.
Such an alloy is particularly intended for the manufacture of cladding and/or other elements or structural components of fuel assemblies for nuclear reactors using water as coolant, that is to say, reactors in the French network of the pressurized water (PWR) type. This alloy could also be used in any type of reactor under development or in the future.
In particular, in order to reduce the price of producing electricity, it is of interest to prolong the operating cycle of nuclear pressurized water reactors (PWRs). In effect, an operating cycle which changes from 12 months to 18 months enables an economy to be made of one shut down per three year period which has repercussions, which are far from being negligible, on the overall economic results of the installation.
The prolonging of the duration of a cycle, otherwise referred to as long cycle operation, nevertheless requires the fuel to have a reserve of additional reactivity, that is to say, an increase in the initial fuel enrichment which changes, for example, from about 3.7% to about 4.2% when a third of the core is used for management.
This increase in the reactivity of the fuel must be compensated for by an excess of anti-reactivity at the start of the cycle, that is to say, by an increased reactivity control requirement.
According to current practice in PWRs, the anti-reactivity is provided by soluble boron dissolved at variable concentration, in the coolant of the primary circuit.
The capture of surplus neutrons is provided by the reaction
10
B(n, &agr;)
7
Li so as to maintain a multiplication factor of one during the course of a cycle, which enables the divergence of the fission reaction to be controlled.
However, the increase in the initial soluble boron content in the primary circuit, which is already carried out in practice, has numerous disadvantages, certain of which have an impact on the safety of the installation.
Hence, it is essential that the quantity of dissolved boron be maintained at less than a maximum limit in order to respect the criterion of having a negative coefficient of reactivity for the moderator (&agr;
m
<0) under all conditions of operation of the reactor including cold shutdown.
The risk that the moderator coefficient could possibly become positive is not to be dismissed since the soluble boron, like other poisons dissolved in the coolant, is liable to expand when there is an increase in temperature thereby inducing a positive contribution to the &agr;
m
coefficient.
In addition, the introduction of extra quantities of boron in the form of boric acid H
3
BO
3
increases, on the one hand the problems of direct corrosion (of austenitic alloys) and on the other hand of indirect corrosion associated with the concentration of lithia (zirconium based alloys).
In effect, the increase in the quantity of boron in the form of H
3
BO
3
implies an increase in the quantity of pH control agent, generally
7
LiOH, so as to limit the activation of the circuits resulting from the release of components of the austenitic materials by the activation products, such as
58
Co,
60
Co,
54
Mn,
59
Fe,
51
Cr etc.
The conditioning lithium and still more the recoil atoms of
7
Li arising from the neutron captures of the
10
B cause acceleration of the corrosion of the cladding.
Furthermore, large deposits on the core could restrict the operating conditions and the availability of the installation.
This phenomenon called axial offset is found today in units, above all in the USA, which have a tendency to function with a coolant chemistry that has too low a pH.
The danger of untimely dilution of the boron is on the other hand one of the main initiators of a reactivity accident (RIA).
Such an accident is particularly feared during reloading of the core with withdrawal of the shim rod banks and in the case of an untimely start-up of a primary pump.
Control by dilution of the boron leads to large production of contaminated effluents as well as considerable running restrictions; the speed of dilution being additionally limited by the dimensioning of the installation.
Finally, it is apparent that the soluble boron is generally insufficient to control cores made up of a fuel formed with 100% MOX, given the leakage hardening of the neutron spectrum. This would also be the case for under-moderated reactors which might be considered for the incineration of waste.
Consumable poisons other than soluble boron or used in conjunction with it have therefore been considered.
These poisons are in solid form and do not therefore expand like water when there is an increase in temperature. Because of this, they do not lead to a positive contribution to the reactivity coefficient of the moderator &agr;
m
.
Gadolinium associated with the fuel pellet has for a long time been considered the reference solid poison, but its mediocre thermal conductivity leading to the development of hot spots has lead to a consideration of erbium for the same purposes.
Hence it has been proposed to use erbium as a consumable poison in the form of the sesquioxide (Er
2
O
3
) dispersed in a homogeneous way in certain fuel rods.
Such an arrangement displaces fissile material from the fuel rods and reduces the overall proportion of fissile material that is effectively available to produce energy.
An additional disadvantage to introducing the consumable poison into the fuel is contamination of the production lines.
It has also been suggested that binary alloys of zirconium and erbium be used with from 10 to 90% by weight of erbium as well as Zircaloy®-2 with 0.5 to 2% by weight of erbium as a material going into the composition of the control rods.
However, such alloys have poor resistance to corrosion which makes them unsuitable for use in pressurized water reactors (PwRs).
Particularly in order to remedy these disadvantages, documents U.S. Pat. Nos. 5,241,571 and 5,267,284 propose the introduction of erbium in proportions, in percent by mass, respectively of from 0.05 to 2% and from 0.1 to 0.4% into a zirconium base alloy whose composition specifications are derived from those of Zircaloy®-4 and/or Zircaloy®-2.
In particular, document U.S. Pat. No. 5,241,571 describes an alloy of zirconium, derived from Zircaloy®-4, which contains erbium or gadolinium as consumable poisons, the erbium content being preferably from 0.05% to 2% by weight. This alloy also contains preferably up to 1.4% tin, from 0.2 to 0.5% of iron, and from 0.07 to 0.25% of chromium.
Niobium can also be added in a quantity ranging up to 0.6% by weight, similarly vanadium in a quantity ranging up to 0.5% by weight. The role of the niobium is to increase the mechanical strength and the resistance to corrosion of the alloy.
A preferred range for the niobium content is from 0.1% to 0.3% and Table 1 of this patent indicates specifically a niobium content that is imperatively less than or equal to 0.6% by weight, which corresponds approximately to the solubility limit commonly accepted for niobium in solid solution at 500-600° C. in the &agr; phase (hcp structure) of the zirconium.
Finally, silicon and oxygen may also be present at respective contents of from 50 to 120 ppm and from 1000 to 2000 ppm.
The alloys described in these documents, and particularly in the document U.S. Pat. No. 5,241,571 have numerous disadvantages, defects and limitations. In particular, this document at no time describes the industrial feasibility of the alloys described and no example of use is given, so that the possibility of obtaining, from the alloys in this document, cladding corresponding to the specifications considered, is unreliable. The same comments are applicable to the alloys from patent U.S. Pat. No. 5,267,284.
In effect, it is well known to a man skilled in the art in this field of technology that the incorporation of rare earths, such as erbium, into alloys of the Zircal
Beslu Pierre
Brachet Jean-Christophe
Noe Maxy
Parmentier Philippe
Porta Jacques
Commissariat a l'Energie Atomique
Koehler Robert R.
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