Patent Number: 039649644
Section: description

The present invention will be described as it would be applied to the EBR-II reactor and details of this reactor not found in the present drawing can be found in the ANL reports cited above. It is, of course, apparent that a failed element detection and location system according to the present invention could be incorporated in any fast reactor, although modifications to the reactor might be necessary. Referring now to FIG. 1 of the drawing, the EBR-II is a sodium-cooled, pool-type fast reactor comprising a core 10 including a plurality of vertically disposed fuel assemblies 11 enclosed within a reactor vessel 12 having a movable cover 13, the cover being shown in raised position for fuel handling operations. Each fuel assembly 11 incorporates a bundle of fuel pins consisting of a nuclear fuel enclosed within gastight cladding. Reactor vessel 12 is submerged within a pool 14 of sodium contained within a primary tank (not shown) provided with a cover 15 having a rotating plug 16 therein through which extend control rods 17, reactor vessel cover 13 elevating column 18, and a gripper mechanism 19. Gripper mechanism 19 is capable of raising a fuel assembly 11 completely out of the reactor core 10 whereupon a transfer arm 20 is capable of transferring the assembly 11 to a storage rack (not shown) and ultimately to a transfer port (not shown) where the assembly can be removed from the reactor. Loading involves a reverse series of operations. As shown in FIG. 2 and in more detail in FIGS. 3 and 4, a gas trap consisting of an inverted funnel 21, a normally closed valve 22 at the apex of the funnel and an actuating rod 23 attached to the valve 22 and extending axially upwardly through the upper plenum 24 of the fuel assembly. As in the EBR-II reactor, an upper adapter 25 provided with a locating slot 26 is attached to the top of the fuel assembly. The gas trap is supported in the upper portion of a fuel assembly several feet above the core region by a bracket 27. The diameter of the funnel 21 at its greatest is substantially less than that of the subassembly. A preferred size would be about 1.1 inches since an EBR-II subassembly is .about.2.3 inches in diameter. The diameter could vary between about 0.7 inch and 1.2 inches, this being about 1/3 or 1/2 that of the subassembly. Actuating rod 23 extends upwardly through the adapter 25 and a short distance into locating slot 26 when valve 22 is closed. Spring 28 maintains the valve 22 in normally closed position. Also shown in FIG. 4, mostly in phantom, is the lower end of gripper mechanism 19. This includes gripper jaws 29 and an orientation blade 30, which elements are present in the EBR-II gripper mechanism. Identification of failed fuel elements according to the present invention is apparent from FIGS. 3 and 4. To interrogate a fuel assembly, gripper mechanism 19 is lowered onto adapter 25 as in a normal fuel handling operation, whereupon orientation blade 30 pushes downwardly on actuating rod 23. A downward motion of approximately 1/16 to 1/8 inch of the actuating rod 23 ensues. Any fission gases released in the fuel assembly will rise in the assembly and some will be trapped in the funnel 21, the remainder rising to the reactor cover gas to annunciate the fission product release. When a fission product release has been indicated, the gripper mechanism is lowered onto the adapter 25 of each of the fuel subassemblies in turn as described above. The downward motion of the actuating rod is sufficient to open the valve and release a bubble of fission product gas in every assembly in which a gas release has occurred. Conventional monitoring equipment can be used to detect the increase in reactivity in the cover gas caused by release of the bubble. To estimate the sensitivity of the detection procedure according to the present invention, the change in cover gas signal-to-noise ratio was evaluated for a release yielding an original S/N ratio of 250 (.sup.133 Xe) from a 10 atom percent burnup element. A bubble occupying 1.1 cc at shutdown conditions in the trap would increase the signal 25% 2 days after the original release. The definition of signal-to-noise ratio is: For a given index isotope, .sup.133 Xe, for example, the signal-to-noise ratio is the increase in .sup.133 Xe content in the reactor cover gas divided by the background .sup.133 Xe component in the cover gas from the unavoidable tramp uranium present in the reactor system. PA1 1. The information storage mechanism is almost completely passive and with considered design should present no safety problems. PA1 2. Retrieving the information stored in the gas trap can be accomplished with existing equipment, or at least existing reactor access facilities in an operating LMFBR. PA1 3. any or all subassemblies (with the exception of instrumented subassemblies) can be interrogated without perturbation or removal from the core, thereby eliminating extensive fuel handling operations associated with search and removal efforts. PA1 4. Analysis of the information contained in the gas bubble can be accomplished with existing instruments. PA1 5. The technique lends itself readily to moderate discrete steps in improvement; i.e. minor design changes in the gas trap, improvements in the method and delivery of the bubble to the cover gas, etc., can be performed independently as time and funds permit (for an operating reactor). PA1 6. The technique does not depend on any method, whatever it may be, for inducing a secondary release from the leaking element. The invention described above has the following apparent advantages: