Burnable neutron absorber element

A burnable, thermal neutron absorber element is provided with a zirconium alloy elongate container having sealed therein both a burnable absorber and the solid moderator material, zirconium hydride. The zirconium hydride is in a concentration and position to enhance the neutron capture efficiency of said thermal neutron absorber in a light water reactor neutron irradiation environment.

BACKGROUND OF THE INVENTION 
The present invention relates to burnable absorbers (poisons) used in light 
water reactors. It is especially concerned with those burnable absorbers 
which are used in reactors as discrete poison rods (i.e., not mixed with 
fuel in a fuel rod). 
In the past, commercial light water reactors have utilized burnable poisons 
such as boron compounds and gadolinia to extend the fuel cycle by allowing 
higher levels of U.sup.235 to be present at the beginning of the fuel 
cycle. In some designs, gadolinia has been mixed directly with the 
UO.sub.2 to form fuel elements containing pellets composed of UO.sub.2 and 
Gd.sub.2 O.sub.3. In other designs, the fuel pellets may be coated with a 
boron compounds, such as ZrB.sub.2. 
In addition to the foregoing designs in which the fuel rods contained both 
a fissile material and a burnable absorber, the burnable absorber may also 
be dissolved in the coolant and/or be present in separate, stationary or 
mobile, (non-fueled) burnable absorber elements. 
Prior designs of burnable absorber elements (see, for example, U.S. Pat. 
Nos. 3,510,398 and 4,342,722, which are hereby incorporated by reference 
in their entirety) have included a hermetically sealed zirconium alloy or 
stainless steel tubular rod containing a tubular member of borosilicate 
glass as the burnable absorber. Concentrically inside of the borosilicate 
glass was a smaller diameter zirconium alloy or stainless steel tube which 
provided structural support for the borosilicate glass. The remainder of 
the interior of the rod was filled with a gas, such as helium. 
An improvement on the foregoing burnable absorber element has been marketed 
by the Westinghouse Electric Corporation and is known as a WABA (Wet 
Annular Burnable Absorber) rod. The WABA design includes a pair of 
concentrically disposed zirconium alloy tubes having an outer diameter and 
a length essentially the same as a fuel rod. The similarity in size 
permits the WABA rod to be statically positioned in aligned fuel assembly 
grid cells in the same manner as fuel rod or to be used as a movable 
poison element which may be positioned in a thimble guide tube of a fuel 
assembly. Annular burnable poison pellets containing B4C are disposed in 
the narrow annular space between the concentric tubes, which has been 
sealed shut by end plugs welded onto the tube ends. However, the hollow 
portion of the inner tube is not plugged and therefore permits the 
unimpeded flow of aqueous coolant upwardly through the inner tube during 
the time of operation in the water reactor. Examples of WABA rods are 
described in U.S. Pat. Nos. 4,460,540 and 4,474,728. These patents are 
hereby incorporated by reference in their entirety. 
BRIEF SUMMARY OF THE INVENTION 
Significant improvements in the prior art burnable absorber designs may be 
obtained through the use of my invention. In accordance with my invention, 
a burnable thermal neutron absorber element is provided which includes an 
elongated, sealed container holding a burnable thermal neutron absorber 
material and zirconium hydride. Both the burnable absorber and zironcium 
hydride are distributed along the length of the container as needed and 
the zirconium hydride acts as a neturon moderator thereby enhancing the 
neutron capture efficiency of the burnable thermal neutron absorber in a 
water reactor neutron irradiation environment. 
In a preferred embodiment of the present invention, the aforementioned 
container is a tube which has an end plug joined to each of its ends to 
form a hermetically sealed cavity within the tube, and which holds the 
aforementioned zirconium hydride and burnable absorber material. 
Preferably, the zirconium hydride is in the form of generally cylindrical 
pellets stacked on end to form a cylindrical column which rests on one of 
the end plugs. 
It is also preferred that the burnable neutron absorber used herein is a 
boron containing material or compound. 
It is further preferred that the zirconium hydride be in a partially 
hydrided state, preferably with a H to Zr ratio, on an atomic basis, in 
the range of about 1.0 to about 1.8, and more preferably about 1.5 to 1.8. 
These and other aspects of the present invention will become more apparent 
upon review of the following detailed description of the invention in 
conjunction with the drawing which is briefly described immediately below.

DETAILED DESCRIPTION OF THE INVENTION 
In my invention, zirconium hydride is combined with a burnable poison in a 
non-fueled burnable absorber element. Compared to the prior WABA design, a 
significant advantage is obtained when the burnable poison is a boron 
containing material. This advantage is due to the use of the solid 
moderating material zirconium hydride which contains a significantly 
higher concentration of the moderator, hydrogen, compared to the water 
used in the WABA design. This higher concentration of hydrogen results in 
a more efficient moderation of the neutrons found in a light water reactor 
irradiation environment and thus significantly improves the probability of 
the burnable poison, boron, capturing a neutron. 
FIG. 1 shows a longitudinal cross-section through an embodiment of a 
burnable thermal neutron absorber element 1 in accordance with my 
invention. The element 1 includes an elongated container 3 which is 
preferably a tubular member 5, preferably of circular transverse cross 
section, and which has a top end plug 7 and a bottom end plug 9 welded to 
its ends to form a hermetically sealed cavity 11 within the container 3. 
The materials for the tube 5 and end plugs are preferably selected from 
those stainless steels and zirconium alloys having excellent aqueous 
corrosion resistance in light water reactor environments. Most preferably 
one of the commercial alloys, Zircaloy-2 or 4 is utilized for these 
components. 
Held within the container 3 is the zirconium hydride and the burnable 
poison, preferably boron. Preferably the zirconium hydride is in the form 
of generally cylindrical pellets 13 stacked on end to form a generally 
cylindrical column which is held against the bottom end plug 9 by a spring 
15 or similar means, located between the top end plug 7 and the top pellet 
13 in the column of pellets. 
Each zirconium hydride pellet preferably has a H to Zr ratio, on an atomic 
basis, in the range of about 1.0 to about 1.8, and more preferably about 
1.5 to about 1.8. While it is desirable to maximize the H to Zr ratio to 
maximize the concentration of the moderator H in the element, the hydrogen 
to zirconium ratio should be held below about 1.8 to limit the amount of 
gaseous H that may evolve from the zirconium hydride pellets during the 
reactor usage, since the hydrogen can cause hydriding of the container 3 
material and may adversely affect its mechanical properties. In this 
regard, the inside diameter surface of the tube 5 may preferably have a 
hydrogen diffusion barrier on it. Where the tube 3 is made of Zircaloy its 
inside diameter surface may be preoxidized to limit hydrogen absorption. 
Alternatively or, in addition to preoxidation of the interior surface of 
tube, the atmosphere within the cavity rather than being composed entirely 
of an inert atmosphere, such as helium, may include an oxidizing component 
to oxidize the interior of the tube 5 during use in reactor, as is 
described in my copending application Ser. No. 552,227 filed on Nov. 16, 
1983 U.S. Pat. No. 4,609,524, which is hereby incorporated by reference in 
its entirety. As described in my copending application this oxidizing 
component may be selected from oxygen, carbon monoxide, and carbon 
dioxide, and be present in an amount effective (e.g. 2-3 volume percent 
based on the volume of helium) to form an oxide coating on internal 
surface of the container 3. 
The pellets 13 may be formed by pressing and sintering zirconium hydride 
powder. The zirconium hydride powder may be formed by conventional 
hydriding techniques utilizing zirconium or a zirconium alloy (e.g. 
Zircaloy-2 or 4) stock as a starting material, which is hydrided to the 
desired or higher hydrogen concentration and then committed to zirconium 
hydride powder. 
The starting powder may contain more hydrogen than that desired in the 
final pellet in order to compensate for hydrogen which may be lost during 
pellet sintering. 
The burnable poison, may be incorporated into the element 1 in a number of 
differing manners. For example, the zirconium or zirconium alloy starting 
stock for producing the hydride power may be prealloyed with boron to the 
concentration desired. Alternatively, a particulate boron compound, such 
as B.sub.4 C, may be blended with the zirconium hydride powder, and then 
pressed and sintered to form pellets 13 having the particulate boron 
compound substantially homogenously dispersed through a matrix of 
zirconium hydride. The B.sub.4 C particles may be coated with a diffusion 
barrier material, such as niobium. 
In the foregoing manner, the zirconium hydride and burnable absorber are 
distributed along the length of the container 3 in a location and length 
substantially equal to the location and length (.+-.20%) of the enriched 
fuel pellets in the surrounding fuel elements of the reactor assembly. 
While the concentration of boron is a matter of choice, a concentration of 
0.006 gm B10/(cm of height) is now contemplated. It is further 
contemplated that the tube 5 may have an outside diameter of about 0.381 
inches and a wall thickness of about 0.026 inches, while the pellets 13 
have a diameter of about 0.318 inches, substantially filling the 
container, when viewed in transverse cross section. 
The preceding description has clearly demonstrated the benefits obtainable 
through the practice of the present invention. Other embodiments of the 
invention will become more apparent to those skilled in the art from a 
consideration of the specification or actual practice of the invention 
disclosed herein. It is intended that the specification and examples be 
considered as exemplary only, with the true scope and spirit of the 
invention being indicated by the following claims.