Mission statement
Provide the means to measure, quantify, and characterize the neutron flux and spectrum within the TREAT reactor and within experiment test devices.
System description
The reactor metrology program supports experimental activities performed at TREAT through use of high-purity monitor wires, which serve as neutron activation targets to estimate the total neutron flux and energy-dependent neutron flux (neutron spectrum) within the reactor and within test devices. They are also used to estimate what the fission rate will be within experiments. These tests are performed by inserting the monitor wires into specially designed calibration devices that mimic the true experiment capsules. The calibration devices have central access slots that allow for the insertion and removal of the wires for different test conditions expected during the experimental transients. The term wire usually refers to a wire-shaped item, but can also refer to thin foils or films. There are two general categories of monitor wires, namely, flux wires and fission wires.
Flux wires are composed of non-fissile isotopes that are irradiated during steady state and transient operations of the reactor. The flux wires become activated by neutrons during their irradiation, producing activation products as a result of (n,g) and other nuclear reactions. The flux wires are analyzed non-destructively, through gamma-ray spectrometry, or destructively, through radiochemistry. In either case, the analysis determines the activity of the activated product(s) at some time, t0, and then uses knowledge of the reactor spectrum and reaction cross-section(s) to estimate the reactor flux during the sample(s) irradiation. Interpretation of data from multiple wire materials having different energy-dependent cross-sections, or analysis of pairs of wires irradiated with and without energy-dependent neutron filters, allows for estimation of the neutron energy spectrum within the reactor at the irradiation location. Neutron activation reactions and products that are measured include 54Fe(n,p)54Mn; 93Nb(n,γ)94Nb; 59Co(n,γ)60Co; and 46Ti(n,p)46Sc. Additional materials may be assayed to provide more elaborate analyses of the total neutron energy spectrum within the reactor.
Fission wires are composed of fissile isotopes that are irradiated during steady state and transient operations of the reactor. Typical fission wires are composed of either pure uranium or uranium zirconium alloy. The uranium enrichment used in a particular experiment depends upon specific details of the measurement case and can vary from depleted uranium to highly enriched uranium. Other actinides may also be used. Following irradiation, either steady-state or transient operation, the fission wires are analyzed in a similar manner to the flux wires. Fission wire analysis provides information on the fission rate in the sample location, in terms of fissions per gram U. Correct interpretation of fission wire data depends upon information related to the neutron spectrum in the reactor and accurate assay data on the wire prior to irradiation. As with the flux wires, energy-dependent neutron filters may also be used.
Monitor wires can be analyzed non-destructively, for example, via gamma-ray spectrometry, and destructively, for example, using radiochemistry methods. Non-destructive analyses take place within the Applied Radiation Measurements and Systems Laboratories at INL's Research and Education Campus (REC). Destructive analyses take place using facilities at both the Materials and Fuels Complex and at REC.
Cross sections for isotopes used in flux wires
Applications
Measurements supported by the Reactor Metrology Program include:
- Neutron flux determination
- Neutron spectrum determination
- Neutron flux mapping
Analyses determine the following values, as needed:
- Wire mass
- For pre-irradiation analyses:
- Wire primary elemental composition, in terms of weight percent
- Wire trace elemental composition, in terms micrograms per gram
- For post-irradiation analyses:
- The number of atoms of key activation isotope(s) of interest, in units of atoms, age corrected for the end of irradiation
- The number of atoms of key fission-product isotope(s) of interest, in units of atoms, age corrected for the end of irradiation
- The estimated number of fission events that occurred within the wire, as determined for one or more fission-product isotopes of interest, using thermal-fission yield values taken from the Evaluated Nuclear Data File (ENDF) ENDF/B-VIII.b4 data library