Thermal column

This NAA technique is based on the nuclear reactions 23Na(n,7)24Na and 41K(n/y)42K. Half-lives of the activated products are 15.0 hrs and 12.4 hrs, respectively. For Na analysis, the samples were irradiated in a specially designed thermal column to suppress the fast neutron reaction of 27Al(n,a)24Na which interferes with the reaction for Na. For K analysis, the proplnt samples were irradiated at a standard irradiation position of the reactor. For the Na irradiations, the neutron flux in the thermal column was in the order of 1010, whereas for the K assays it was approx 1012 neutrons/cm2-sec... 

A typical procedure for Na assay is One gram proplnt samples sealed in polyethylene vials are irradiated for at least seven hrs in the thermal column. In order to minimize any thermal neutron flux gradient, the samples are rotated uniaxially at 60 rpm. Following irradiation, the samples are allowed to decay for approx 15 hrs to permit all short-lived radioisotopes to decay to insignificance. The 1.369 MeV 7-photopeak of each sample is then counted for 10 min with a 7.6cm x 7.6cm Na iodide scintillation detector coupled to a 400-channel pulse-height analyzer... 

Wiggins et al. [456] used neutrons from the thermal column of a 10 kW pool-type research reactor and from a 120 pg Cf source to study the prompt photon emission resulting from neutron capture in magnesium nodules (ter-romanganese oxides) from the ocean floor. Spectra were recorded with a Ce(Ii) detector and a 1024-channel analyser. Complex spectra were obtained by irradiation of seawater, but it was possible to detect and estimate manganese in nodules in a simulated marine environment by means of the peaks at 7.00, 6.55, 6.22, and 6.04 pV. 

Neutron diffraction patterns of powder samples were taken on a neutron diffractometer (X = 1.085 A) mounted on the thermal column of a WR-SM nuclear reactor [3]. The DBW-3.2 program for the Rietveld neutron diffraction line shape analysis was used in calculations and structure refinement [4]. A DRON-3M X-ray diffractometer (CuK - radiation) was used to measure X-ray powder diffraction patterns. 

Some research reactors are equipped with a so-called thermal column of about 1 m X 1 m X 1 m, consisting of blocks of graphite and installed near the reactor core. Due to the moderator properties of graphite, only thermal neutrons are present in such a column. 

Self-shielding of resonance neutrons can be minimized by irradiating in the graphite-loaded column (thermal column) of the pile. Here the contribution by neutrons of energies greater than thermal is considerably reduced. 

The more important of these reactions will be considered below. Reactions 3, and 4 are brought about by the fast neutron flux in the pile, which flux Mellish et al. 59) calculated to be 0.17 of the slow flux in the center of the Harwell pile (BEPO). Fortunately, however, the cross sections for these reactions are usually considerably lower than those for normal (TO,y) reactions induced by thermal neutrons. Contributions brought about by n,p) and (to, ) reactions can often be greatly reduced, as mentioned previously, by irradiating in the thermal column of the reactor, with some loss of sensitivity. Reaction 4, in,2n), is produced by... 

The determination of sodium in pure aluminum by the reaction Na (7i,y)Na is complicated by the reaction Al (n, )Na brought about by the fast component of the neutron flux spectrum. Salmon (77) determined the contribution to the sodium content by this reaction in BEPO by irradiating identical samples and standards normally and in the thermal column. The difference in the sodium content determined by these two methods gives the apparent sodium content derived from the (n.,o) reaction. Salmon found this value to be 81 ppm under the conditions of the experiment. 

Fission Product Xenon and Krypton Isotope Abundances for Uranium Irradiated in NR.X Thermal Column ... 

Fenger (12) used the dosimeter for gamma-flux measurements in the thermal column of the DR 2 reactor and calibrated the system in the Riso Co-60 facility. He found for a 50 mM oxalic-acid solution a G-value of 4.6 0.3. 

Iso-thermal column temperature and time during thermal desorption... 

Small fires had coalesced into larger fires and, greedy for oxygen, had sucked air from around the coalescing inferno and fanned further fires there. That created the wind, a thermal column above the city like an invis-... 

The east wall of the reacjtor has. access to a. thermal c.pl m . which is provided with nine horizontal and two vertical access holes. Depending upon future demands, space is available through the west waT-l- for a shielding facility or another thermal column. 

Two important experimental facilities, the thermal column and the shielding facility (see Sections 3.1 and, 3.2j, require that a section approximately 6 by 6 ft be removed from.the east and west thermal-shield side plates. Two components are added to the thermal-shield design by.these facilities (1) the neutron window, and (2) the neutron-absorbing curtain. The locations of these components may be visualized by referring to Figs. 2.B and C. When the 6- by 6-ft hole is cut in the thermal shield, it becomes necessary to replace... 

Neutron Curtain in the HTR. A 6 ft 6 in. square sheet of -in.-thick boral mounted in an aluminum frame will serve as the neutron curtain. This curtain will be used during operation, if desired, to cover the face of the thermal column and reduce the number of thermal neutrons entering the column. It will have rollers at its four corners that will travel in vertical guides mounted in the 3-in. space between the two 4-in. thermal shield plates. 

The Neutron Window. The neutron window is a 3-in. sheet of lead placed between the permanent graphite and the thermal column to absorb gamma radiation but permit easy neutron passage. The lead sheet is mounted in a steel frame and pl aced in the approximately 6- by 6-ft hole in the outer thermal shield. 

Lane, J. A., Thermal Column Graphite. ORNL Cf-50-1-20, January 5, 1950. 

T-2—V- l and T-2---V-2 are thermal column holes, 12M by 12H in. , through the top of the thermal column liner..- Each of- the GBF graphite plugs is suspended by a X-in. steel rod from the barytes plug. This rod goes through... 

Figure 3.3.A is a cut-away view of the. thermal, column with the graphite removed. The graphite stacking is shown in Fig. 3.3.B. In Fijj. 3.3.A, the steel liner adjacent to.the concrete serves to absorb radiation from the thermal column in a-manner analogous to the thermal shield around the reactor proper. The cadmium- sheet next to the steel absorbs impinging slow neutrons so that the steel liner will exhibit a minimum of induced radioactivity after pile shutdown.

The lead door indicated in Fig. 3.3.A is designed to reduce the radiation from the thermal column to the same-level as that through the concrete biological shield. It can be raised with the overhead crane and held in an elevated position by lever-actuated pawls. 

Thermal Column.Bsperimemtsl Pnclllties. The thermal-column experimental facilities include one large horizontal hole and 10 small holes, as follows ... 

In some respects the HG-9 hole should not be considered as part of the thermal column since it provides access to the fast-neutron flux directly from the active lattice through sealed empty cans in the beryllium. However, with... 

The holes normal to the direction of the thermal column are expected to be extremely useful in providing very pure fields of thermal neutrons although at somewhat reduced intensity. 

As a result of the above decision, the MTR biological shield now contains a 6- by 6-ft hole which will be filled, for the present, with,closely packed barytes blocks. However, in the hope that eventually this facility may be used for a thermal column or other experiments, the following steps have been taken to permit removal of the concrete blocks without danger of.overexposure to radiation ... 

Thomas, T. H., Materials Testing Reactor Report on Thermal Column, ORNL CF-50-2 28, February 6, 1950. 

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