WP 4: Improved cross sections through neutron metrology
Background
For detailed performance assessments of new generation nuclear power plants and their fuel cycles, accurate modelling and simulation tools must be developed [1]. Such tools rely critically on nuclear data. A recent sensitivity study performed by a working party of the OECD Nuclear Energy Agency established target uncertainties for neutron interaction data on the basis of target uncertainties for reactor and fuel cycle performance parameters, such as the effective multiplication factor at beginning of life, the reactivity swing, peak power, Doppler and voiding coefficients and end of life inventories. Comparing the target nuclear data uncertainties against the currently established uncertainties resulted in a number of high priority nuclear data requests [2,3]. These requests are often very challenging, being at or beyond the state-of-the-art in neutron measurements. Therefore new avenues need to be explored to try to arrive at measurement uncertainties which meet the requirements of energy applications.
State of the art
Neutron metrology is presently concerned mainly with accurate and traceable measurements of fluence and fluence spectra for the calibration and characterisation of detectors used in radiation protection. A number of well characterised primary and secondary instruments have been developed at NMIs and DIs, which are accurate over a wide energy range of interest to radiation safety for power plants. Quality control is implemented through intercomparisons organized by CCRI(III) and EURAMET. Currently, these standards are not adapted for practical use in cross section measurements. More generally, neutron metrology for cross section measurements is not an established discipline.
Aims of the work package
This work package aims at improving neutron interaction data measurements beyond the present state-of-the-art, which is set by self-normalizing methods and the neutron data standards used at laboratories where the data are measured. Advantage will be taken of NMI experience in neutron metrology, particularly fluence measurements, to improve cross section measurements of interest to new generation power plants and fuel cycles.
The programme of work includes:
The establishment of an easy-to-use secondary fluence standard (A Frisch-gridded 235U ionization chamber) at the JRC (IRMM) GELINA facility along with a procedure for reliable determination of the efficiency of fluence measurement devices (ionization chambers) used in neutron data measurements at JRC (IRMM) and elsewhere. The target uncertainty is at the level of 1 % uncertainty on the ratio of the efficiency of the measurement device relative to the secondary standard, and the uncertainty on the fluence obtained from the secondary standard should ideally be less than 2 %. This secondary standard is an essential tool to reliably calibrate fluence normalization devices (ionization chambers) used in neutron time-of-flight cross section measurements.
The demonstration of the potential of these fluence measurement devices for the determination of cross sections of actual interest (fission cross section ratio measurement with uncertainty better than 3 %).
The demonstration of the potential of these fluence measurement devices for the improvement of nuclear data standards (Au(n,g) standard, keV region – unresolved resonance range, uncertainty better than 3 %).
Transfer of these improvements in measurement uncertainties to other labs where such data are measured (dissemination).
The secondary fluence standard foreseen for the GELINA facility will be a Frisch-gridded ionization chamber with a 235U deposit of a thickness in the range from 50 μg/cm2 to 100 μg/cm2. This chamber will be placed at a 10 m flight path of the GELINA facility in a fixed mount suitable for reproducible positioning. The thin deposit and Frisch-gridded ionization chamber minimize corrections for fission fragment loss in the deposit resulting in events with pulse amplitudes below the detection threshold. The deposit will be prepared by vapour deposition in vacuum of highly enriched 235UF4. The amount of 235U which is deposited will be determined by alpha counting in combination with isotopic mass ratio measurements, with both measurements performed in a traceable manner. The target final uncertainty on the 235U mass per unit area is in the 0.2 % range. For this purpose homogeneity checks will be made through scans with an alpha counter and a mask over the deposit.
This proposed secondary standard should allow measurements of the fluence with uncertainties close to the uncertainties of the 235U(n,f) standard cross section for most of the energy range. To establish that this is indeed the case, intercomparisons with the primary fluence standards of PTB and NPL will be carried out for thermal neutrons and at energies above 1 MeV.
A procedure will be established for the use of the secondary standard at GELINA.
Chambers characterised against the secondary standard will be applied to both cross sections of direct interest to advanced reactors (the 240, 242Pu(n,f) reactions)and to the determination of the Au(n,g) standards cross sections. The latter are of interest to advanced reactor cross section measurements (extension of the energy range for the 197Au(n,γ) is of interest for the normalization of fast spectrum capture measurements). One fission cross section measurement will be made versus chambers calibrated with the secondary standard, if time allows the same cross section will be measured against the primary standards of PTB and NPL at a limited number of energies in order to establish 1) consistency of the results, 2) consistency of the uncertainty budgets.
The relation of the values and uncertainties for cross sections determined along metrological principles in this JRP will be compared with those established by more conventional techniques. The latter will be available from state-of-the-art measurements of the ANDES collaborative project of the European Commission. Thus, a direct link is established to applications and dissemination of the proposed activities is guaranteed.
Scientific tasks
4.1 Preparation of a Frisch-gridded ionisation chamber that will serve as a secondary standard at GELINA and establishing and demonstrating of a protocol for its use (JRC (IRMM), PTB)
4.2 Establishing the fission chamber as a secondary standard (PTB, JRC (IRMM), NPL)
4.3 Demonstration of the use of fission and boron ionization chambers calibrated against the secondary standard (JRC (IRMM), PTB, NPL)
Selected references
[1] Strategic Research Agenda of the Sustainable Nuclear Energy Technology Platform, H. Aït Abderrahim, coordinator, www.snetp.eu (2008)
[2] Uncertainty and target accuracy assessment for innovative systems using recent covariance data evaluations, M. Salvatores, coordinator, Working Party on Evaluation Cooperation of the OECD Nuclear Energy Agency, Volume 26, www.nea.fr/html/science/wpec/volume26/volume26.pdf (2008)
[3] High priority request list for nuclear data, A. Plompen coordinator, OECD Nuclear Energy Agency www.nea.fr/html/dbdata/hprl (2004 - 2008)