Abstract
A fast flux irradiation device (FFID) was developed for the Texas A&M Nuclear Science Center's one-megawatt (I-MW) TRIGA reactor, primarily to service the fields of geochronology and thermochronology with unprecedented irradiation characteristics for the Ar40/Ar39 dating technique. The FFID consisted of a dry vertical tube port that extended from the reactor core to the reactor support bridge. A boron-shielded rotisserie, inserted into the vertical tube port, was used to house the samples in the core region, being stabilized radially from below the rotisserie by a bayonet and above the rotisserie by polyethylene plugs. The polyethylene plugs also served as shields to minimize the gamma and neutron dose rates at the reactor bridge level. The rotisserie employs a 0.71 rpm motor for continual rotation during irradiation. The FFID was intended to be operated at full reactor power (one-megawatt thermal). The first four irradiations revealed that the FFID provided nearly ideal irradiation characteristics for the dating technique. The extent of the thermal neutron interference reaction K40(n,p)Ar4O was reduced to levels too small to be measured by the existing standards (K salts and K glass) used by the experimenters. Additionally, the boron shield reduced the quantities of thermal neutron-induced activation products by two to three order s of magnitude. This reduction allowed decay periods to be decreased from several weeks to as little as three days. The radial stability provided by the FFID reduced circumferential flux gradients to below 0.4%, far less than the 2.4% to 3.4% observed at other reactor facilities. The one major drawback in the FFID design was the evidence of temperatures in the sample region in excess of 600' F, indicated by the melting of cadmium metal during characterization. This problem could be reduced by either decreasing the reaction rate (operating at a lower reactor power), or instituting a heat removal system involving the continual flow of a gas that does not easily activate.
Brightwell, Michael Shane (1995). Minimizing the production of unwanted activation products in the AR40/AR39 dating method. Master's thesis, Texas A&M University. Available electronically from
https : / /hdl .handle .net /1969 .1 /ETD -TAMU -1995 -THESIS -B746.