Patent Number: 042004918
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The invention comprises a passive system which can be used to detect the presence of radioactive lanthanum-140, the short-lived residual product of radioactive barium-140. the distribution and concentration of the radioactive lanthanum-140 indicated by the intensity of high-energy gamma radiation provides an accurate map of fission product deposition. Lanthanum-140 is characterized in that it emits gamma rays at a relatively high level of about 2.5 mev. In order to selectively produce a measurable neutron flux which corresponds to the level of gamma radiation, a threshold sensitive converter material is employed. Particular substances, such as beryllium and deuterium, exhibit a characteristic photoneutron reaction which has insignificant cross-section to radiation below an energy threshold. Above the characteristic threshold level of radiation, a photoneutron flux is created which is sufficiently high to activate materials for measuring neutron flux. The photoneutron reaction threshold of beryllium is known to be at 1.67 mev. For deuterium, the corresponding threshold energy level is 2.22 mev. One material commonly used to measure neutron flux is the activant gold. Gold and other activant materials such as silver, lutetium and manganese are known to create isotopes exhibiting a long-lived radiation emission characteristic which directly corresponds to the level of incident radiation. This isotope radiation emission characteristic can later be measured by conventional low level radiation detectors. By way of explanation and definition, the term activant is used herein to indicate materials suitable as a neutron flux detecting element. In particular, an activant is a material which, upon exposure to neutron flux, produces an activated isotape emitting radiation with a relatively long half life. For the purposes of the invention, the activant is a material having a neutron activation cross section of greater than about ten barns (10.times.10.sup.-24 cm.sup.2) and whose activated isotope emits gamma or beta radiation with an energy greater than about 50,000 electron volts with a half life of greater than about one hour. Turning now to the figure, there is depicted one practical embodiment of a device operative according to the invention for detecting and recording emissions of lanthanum-140, indicating the recent history of power distribution of a nuclear reactor fuel element. The device comprises a wand 10 consisting of a longitudinally extended tubular casing 12 filled with a sheathing material 14, such as a beryllium oxide (BeO) compound, in which is embedded a centrally disposed longitudinally extended activant 16. The casing 12 is preferably circularly cylindrical. The activant 16 may be a filament such as gold wire or another of the above named materials. A gold wire on the order of 2 mm diameter is sufficient in size. The diameter of the finished cylindrical wand 10 is sufficiently small to permit it to be introduced into the spaces between individual fuel rods of a nuclear reactor (not shown), for instance, through the holes and guide tubes used for the introduction of control rods or pins. A wand diameter of 10 mm should satisfy these requirements. The casing 12 may be a metal such as stainless steel or Zircaloy which is transmissive of gamma radiation. The casing 12 is hermetically sealed by a suitable process such as swaging or soldering to enclose the sheathing material 14 and the gold wire 16. The gold activant 16 may be on the order of 2 mm thick extending the length of the detector wand 10. The detector in cylindrical form may be manufactured in a manner similar to that used for the manufacture of thermocouples or other solid state neutron detectors. For instance, a tubular casing 12 may be swaged or drawn in a manner generally well-known in the arts. The invention need not be limited to tubular configured wands, however. A sheet detector might also find useful application. For example, a sheet may comprise sheaths 14 of neutron flux converter material sandwiching an activant foil 16 which is similarly sensitive to the neutron flux generated by the sheaths 14. The technique for use of the wand 10 illustrates the method of the invention. After the termination of the operation of the nuclear reactor, one or more wands 10 may be introduced into each fuel element whose power distribution is to be measured. The high energy gamma radiation of the short-lived lanthanum-140 activates the beryllium insulative sheath 14 which produces a secondary radiation of neutrons. The neutrons further interact with the gold to create long-lived low level radiation isotopes. The wands 10 are left in place until the neutron flux-sensitive activant 16 (the gold wire) is adequately exposed. This may normally take on the order of a few hours to several days. After the period of exposure is completed, the wands 10 are removed from the reactor fuel element for further analysis. The activant can then be analysed by passing each wand 10 through a collimated low level radiation detector which would identify the location and intensity of power production in the fuel element so that the actual power distribution history can be identified and mapped. The present invention has a number of advantages over the prior art. In particular, the invention can be used with fuel elements after discharge from the reactor to indicate power distribution during the most recent exposure history of the reactor. Secondly, the present invention stores information which can be read by an off-line system a point at a time, as for example, by means of a data collection apparatus 18 coupled to receive signals indicative of the residual radiation through a detection head 20, thereby avoiding any necessity of attempting to read all infomation simultaneously or to account for decay resulting from the time differential of detected primary radiation. A particular advantage of the present invention is that a shielded protector system is not required. Only low levels of radiation are generated in the final read-out involved in this invention. Moreover, the final read-out can be made at a relatively slow pace with substantially no concern over the time-dependent decay of primary radiation of the relatively short half-lived fission products. The invention has been described in conjunction with specific embodiments. Other embodiments incorporating the same or substantially identical features of the present invention will be suggested to those of ordinary skill in the art in view of this disclosure. It is therefore not intended that this invention be limited, except as indicated by the appended claims .