Metal treatment – Stock – Titanium – zirconium – or hafnium base
Reexamination Certificate
1999-09-13
2003-01-07
Sheehan, John (Department: 1742)
Metal treatment
Stock
Titanium, zirconium, or hafnium base
C148S672000, C148S714000, C376S457000
Reexamination Certificate
active
06503346
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to a cladding tube for a fuel rod which is or can be used in a fuel element of a boiling water reactor, the cladding tube, between its inner side (the side facing toward the nuclear fuel enclosed in the cladding tube) (inner surface) and its outer side (outer surface), comprising a zirconium alloy with a practically constant chemical composition, but at these two surfaces having a different microstructure.
Such a fuel rod is illustrated in
FIG. 1
, in which the two ends of the cladding tube
1
are closed by means of metal stoppers
2
and enclose a column of fuel pellets
3
. At least at one end (usually the top end), a spring
4
provides a gas collection chamber, while in the state immediately after it has been produced, there is a gap
5
between the pellets
3
and the cladding tube
1
, which gap closes gradually, however, when the cladding tube is compressed by the pressure of the boiling water while the reactor is operating and the pellets swell. To ensure good heat transfer from the pellets to the cladding tube and the cooling water, the tube generally has a helium atmosphere of a few bar.
FIG. 1
also shows the pellets in the state
3
a
immediately after they have been produced and in the state
3
b
when the reactor has started to operate and the pellets have burst due to the high thermal loads.
In view of the fundamental requirement that in light water cooled nuclear reactors the cladding tubes for the fuel rods should exhibit low neutron absorption, the cladding tubes are made from a material which predominantly comprises zirconium of a purity which is standardized for tubes used in nuclear applications (e.g. R60001). However, in addition to the neutron absorption, a multiplicity of chemical, mechanical and other physical conditions which impose demands on the material and its production have to be observed, and some of these requirements are not compatible and, moreover, vary for different types of reactor (boiling water reactor or pressurized water reactor). When used for long periods in water or steam, pure zirconium is not sufficiently corrosion-resistant and must therefore be weakly alloyed with additions which have to be adapted according to the type of reactor.
Thus, the nuclear reaction causes iodine and other gaseous fission products to be formed in the nuclear fuel, leading, on the one hand, to an increase in volume of the fuel and, on the other hand, to an aggressive atmosphere on the inner side of the cladding tube. The pellet fragments
3
b
(
FIG. 1
) may lead to punctiform pressure and substantial local stresses on the inner surface of the cladding tube and, at the same time, the aggressive fission products are directed onto the inner surface through the fractured surfaces. In zircaloy, which is the standard material for cladding tubes, this combination of local stresses and an aggressive atmosphere results in stress cracks beginning to form from the contact points, along which stress cracks intensified corrosion propagates, initiated primarily by the iodine. These stress corrosion cracks grow through the entire wall thickness of the cladding tube and lead to perforation of the cladding tube (so-called “pellet cladding interaction”, PCI).
Pure zirconium (e.g. “sponge zirconium”, which is the standard commercially available form of reactor-purity zirconium) is less susceptible to PCI, since pure zirconium has a higher ductility than zircaloy, so that the local stresses are partially absorbed by plastic deformation of the zirconium and are therefore unlikely to reach the threshold which is critical for PCI. However, pure zirconium is too soft in terms of the high mechanical stability required of such cladding tubes (diameter: approx. 1 cm, length approx. 4 m, wall thickness approx. 1 mm!). For this reason, so-called “liner cladding tubes”, in which a tube made from zircaloy has a thin lining of pure zirconium on the inner side, are frequently used. Since the introduction of such liners, punctiform damage caused by PCI is scarcely ever observed on the corresponding cladding tubes.
Zircaloy is a standardized alloy (e.g. US standard R60802) which has as far as possible been optimized in terms of stability by the addition of tin and in terms of corrosion by the addition of iron, chromium and, if appropriate, nickel.
However, PCI damage has been observed practically only in boiling water fuel elements, but not in pressurized water fuel elements, even though the high pressures in the pressurized water reactor press the cladding tube onto the fuel over the course of time (the so-called “creep” phenomenon). However, the particular way in which boiling water reactors are controlled results in particularly high loads. The most common cause of damage in pressurized water fuel rods is chemical corrosion from the water which attacks the outer surface and/or mechanical corrosion caused by friction in the fuel element (so-called “fretting”). In this case, aqueous corrosion acts practically uniformly on the entire surface of the cladding tube, which is therefore attacked uniformly (uniform corrosion), this corrosion behavior being considerably intensified by the high operating temperature and the chemical composition of the pressurized water in the pressurized water reactor.
Due to the lower operating temperature and the water in the boiling water reactor containing more oxygen, in practice the corrosion observed on the outer surface of the cladding tubes in such reactors is not uniform, but rather is characterized by punctiform, locally delimited oxide blisters (so-called “nodular corrosion”), which are not observed in the pressurized water reactor. While individual blisters are often tolerable, a denser covering with these blisters may lead to deposition (so-called “crud”) of contaminants and dissolved metals (e.g. copper) from the boiling water, an effect which reduces the cooling of the fuel rods and, in extreme cases, uniform corrosion may also be considerably accelerated as a result of overheating of the fuel rod.
Nowadays, the cause of the nodular corrosion is considered to be the fact that the alloying elements iron, chromium and nickel are deposited as secondary phases in zirconium alloys, i.e. as particles (“secondary phase particles”, SPPS) which are distributed throughout the entire grain structure of the material and the number, size and spacing of which are determined by the manufacturing process. If these SPPs have become too large owing to high manufacturing temperatures, they initiate nodular corrosion under the aqueous-chemical conditions of the boiling water reactor. For this reason, cladding tubes for boiling water reactors are manufactured in a “low-temperature process” (LTP).
However, advances in reactor engineering have led to the fuel containing ever more fissile material, i.e. having a higher energy content, thus allowing a longer service life (so-called “burn-up”) of the fuel rods and also leading to somewhat higher fuel-rod and operating temperatures. It is therefore necessary even in boiling water reactors to take into account uniform corrosion of the cladding tubes, which according to current knowledge is promoted if the size of the SPPs is too small. Therefore, there is a need for manufacturing processes which allow optimization between nodular and uniform corrosion.
Further damage to cladding tubes is formed by cracks which have a considerable extent in the axial direction. Although these extensive cracks are significantly less common than the PCI defects mentioned above, they also lead to significantly greater disruptions to operation, since significant quantities of the fuel rod contents can be washed out through these cracks. Since these cracks occur considerably more often in liner cladding tubes than in liner tubes which consist entirely of zircaloy (so-called “solid-wall tubes”), there are increasing objections to the use of the pure zirconium liner. Moreover, in the case of the liner tubes, it is necessary to ensure, by means of meticulous quality te
Locher Ralph E.
Oltmans Andrew L.
Sheehan John
Siemens Aktiengesellschaft
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