Ane Håkansson

A facility for studies of neutron-induced reactions in the 50–200 MeV range

A facility for studies of neutron-induced reactions has been built at the upgraded Gustaf Werner cyclotron of the The Svedberg Laboratory, Uppsala, Sweden. Well-collimated, monoenergetic neutron beams are produced with a fairly long distance between the neutron source and the reaction target, to reduce background radiation. A magnetic spectrometer with large angular and momentum acceptance has been constructed to allow measurements of energy and angular distributions of light ions produced in neutron-induced reactions. Initially, studies will be undertaken of isovector monopole and Gamow-Teller resonances, induced by the (n,p) reaction in various nuclei. The performance is illustrated in a measurement of the doubly differential cross section of the 12C(n,p)12B reaction at 100 MeV.

The 54,56Fe(n, p)54,56Mn reactions at En = 97 MeV

Abstract Double-differential cross sections of the 54,56Fe(n, p) reactions have been measured at 97 MeV in the angular range 0–30° for excitation energies up to 40 MeV. The spectra have been decomposed into different multipolarities by a technique based on the use of sample angular distributions calculated within the distorted-wave Born approximation. From the identified Gamow-Teller strength, Sβ+ values were obtained for 54Fe and 56Fe. Comparisons with available shell-model calculations of the GT strength were made. The results are important for models of supernova explosions since electron-capture rates, which are proportional to Sβ+, in 1f2p-shell nuclei affect the dynamics of the star. At higher excitation energies, the spectra are dominated by L = 1 strength in broad distributions with a maximum at about 12 MeV. Microscopic calculations based on the random-phase approximation were performed and compared with the experimental data.

The giant isovector E2 resonance in calcium observed in radiative neutron capture

Abstract The reaction 40 Ca(n, γ 0 ) 41 Ca has been studied in the neutron energy range 20 28 MeV, where the isovector giant quadrupole resonance is expected. Interference between El and E2 radiation gives rise to a fore-aft asymmetry of the emitted γ-rays. The measured asymmetries are compared with calculations based on the direct-semidirect capture model. Good agreement with the experimental data is obtained assuming an isovector E2 resonance located at 32 MeV with a strength exhausting about 35%, of the isovector sum rule. This corresponds to the full strength of the T component.

A Tomographic Method for Verification of the Integrity of Spent Nuclear Fuel Assemblies—II: Experimental Investigation

Abstract A tomographic method for verification of the integrity of used light water reactor fuel has been experimentally investigated. The method utilizes emitted gamma rays from fission products in the fuel rods. The radiation field is recorded in a large number of positions relative to the assembly, whereby the source distribution is reconstructed using a special-purpose reconstruction code. An 8 × 8 boiling water reactor fuel assembly has been measured at the Swedish interim storage (CLAB), using installed gamma-scanning equipment modified for the purpose of tomography. The equipment allows the mapping of the radiation field around a fuel assembly with the aid of a germanium detector fitted with a collimator with a vertical slit. Two gamma-ray energies were recorded: 662 keV (137Cs) and 1274 keV (154Eu). The intensities measured in 2520 detector positions were used as input for the tomographic reconstruction code. The results agreed very well with simulations and significantly revealed a position containing a water channel in the central part of the assembly.

Nondestructive Experimental Determination of the Pin-Power Distribution in Nuclear Fuel Assemblies

A need for validation of modern production codes with respect to the calculated pin-power distribution has been recognized. A nondestructive experimental method for such validation has been developed based on a tomographic technique. The gamma-ray flux distribution is recorded in each axial node of the fuel assembly separately, whereby the relative rod-by-rod content of the fission product 140Ba is determined. Measurements indicate that 1 to 2% accuracy (1σ) is achievable. A device has been constructed for in-pool measurements at reactor sites. The applicability has been demonstrated in measurements at the Swedish boiling water reactor (BWR) Forsmark 2 on irradiated fuel with a cooling time of 4 to 5 weeks. Data from the production code POLCA-7 have been compared to measured rod-by-rod contents of 140Ba. An agreement of 3.1% (1σ) has been demonstrated. It is estimated that measurements can be performed on a complete BWR assembly in 25 axial nodes within an 8-h work shift. As compared to the conventional method, involving gamma scanning of individual fuel rods, this method does not require the fuel to be disassembled nor does the fuel channel have to be removed. The cost per measured fuel rod is estimated to be an order of magnitude lower than the conventional method.

THE C-12(N,P)B-12 REACTION AT E(N)=98 MEV

Double-differential cross sections of the C-12(n, p) reaction have been measured at 98 MeV in the angular range 0-degrees-33-degrees for excitation energies up to 35 MeV. The spectra have been analysed in terms of multipole components, using peak fitting and decomposition techniques. The data have been compared with cross sections obtained in the distorted-wave Born approximation, using matrix elements from shell-model calculations. The unit cross section and the related strength of the spin-isospin part of the nucleon-nucleon interaction were determined from the extracted Gamow-Teller strength. Cross sections and fractions of strength exhausted by dipole and higher-multipole transitions were extracted and compared with data available in the literature.

A Device for Nondestructive Experimental Determination of the Power Distribution in a Nuclear Fuel Assembly

Abstract There is a general interest in experimentally determining the power distribution in nuclear fuel. The prevalent method is to measure the distribution of the fission product 140Ba, which represents the power distribution over the last few weeks. In order to obtain the rod-by-rod power distribution, the fuel assemblies have to be dismantled. In this paper, a device for experimental nondestructive determination of the thermal rod-by-rod power distribution in boiling water reactor and pressurized water reactor fuel assemblies is described. It is based on measurements of the 1.6-MeV gamma radiation from the decay of 140Ba/La and utilizes a tomographic method to reconstruct the rod-by-rod source distribution. No dismantling of the fuel assembly is required. The device is designed to measure an axial node in 20 min with a precision of >2% (1σ). It is primarily planned to be used for validation of production codes for core simulation but may also be used for safeguards purposes.

A Tomographic Method for Verification of the Integrity of Spent Nuclear Fuel Assemblies - I: Simulation Studies

Abstract A tomographic method for experimental investigation of the integrity of used light water reactor fuel assemblies has been developed. It is based on spectroscopic measurements of the gamma radiation from fission products in fuel rods. The method utilizes beforehand information about the nominal geometry of both the measured fuel assembly and the measurement equipment. A reconstruction code of the algebraic type has been written. The potential of the technique has been examined in extensive simulations, assuming a gamma-ray energy of either 662 keV (137Cs) or 1274 keV (154Eu). The ability of detecting various configurations of manipulated rods, both single and in groups, has been investigated. Two main types of manipulations have been simulated. First, there is the removal of rods without replacement. The results indicate that all investigated configurations of removed rods in boiling water reactor (BWR) fuel can be reliably detected using 137Cs radiation. For pressurized water reactor (PWR) fuel, the same result is obtained, with the exception of the most central positions. Here, the more penetrating radiation from 154Eu may have to be used. Second, there is the replacement of rods with fresh fuel or fuel-like material. The results clearly indicate that all simulated cases of such manipulation can be most confidently detected. The simulations include various configurations of replaced rods in both BWR and PWR fuel, using both gamma-ray energies.

Gamma-Ray Spectroscopy Measurements of Decay Heat in Spent Nuclear Fuel

Abstract A method for determining the residual thermal power in spent nuclear fuel using gamma-ray spectroscopy is suggested. It is based on the correlation between the residual power and the 137Cs activity, which is nearly linear for fuel with cooling times between 10 and 50 yr. Using available data of calorimetrically measured values of the decay heat in 69 boiling water reactor and pressurized water reactor spent-fuel assemblies resulted in agreement with a standard deviation of 3%.

The isovector quadrupole resonance in yttrium excited by neutron radiative capture

Abstract In order to investigate the properties of the isovector giant quadrupole resonance ( ΔT = 1, ΔS = 0) in the A = 90 mass region, gamma-ray spectra from the reaction 89 Y(n, γ) 90 Y were recorded at several neutron energies in the energy range 12 to 27 MeV at 55°, 90° and 125°. The measured fore-aft asymmetry for the ground-state transition is very small in the low-energy region, but becomes appreciable above a neutron energy of 18 MeV. The observed asymmetry is attributed to interference between radiation from the isovector giant quadrupole resonance and radiation of opposite parity (from the high-energy tail of the giant dipole resonance and direct El capture). The data obtained in the present work, interpreted in terms of the direct-semidirect capture model, indicate that the excitation energy of the isovector E2 resonance in 90 Y is 26 ± 1 MeV. The data are consistent with a resonance width of 10 ± 2 MeV and with complete exhaustion of the energyweighted sum rule for the lower isospin component of the resonance.