Fausto Franceschini

Silicon Carbide Neutron Detectors

The potential of Silicon Carbide (SiC) for use in semiconductor nuclear radiation detectors has been long recognized. In fact, the first SiC neutron detector was demonstrated more than fifty years ago (Babcock, et al., 1957; Babcock & Chang, 1963). This detector was shown to be operational in limited testing at temperatures up to 700 oC. Unfortunately, further development was limited by the poor material properties of SiC available at the time.

On the Use of Reduced-Moderation LWRs for Transuranic Isotope Burning in Thorium Fuel—I: Assembly Analysis

Multiple recycle of transuranic (TRU) isotopes in thermal reactors results in degradation of the plutonium (Pu) fissile quality with buildup of higher actinides (e.g., Am, Cm, Cf), some of which are thermal absorbers. These phenomena lead to increasing amounts of Pu feed being required to sustain criticality and accordingly larger TRU content in the multirecycled fuel inventory, ultimately resulting in a positive moderator temperature coefficient (MTC) and void reactivity coefficient. Because of the favorable impact fostered by use of thorium (Th) on these coefficients, the feasibility of Th-TRU multiple recycle in reduced-moderation pressurized water reactors (PWRs) and boiling water reactors (BWRs) has been investigated. In this paper, Part I of two companion papers, the analysis is limited to a single assembly, with full-core models presented in Part II. Spatial separation of TRU from bred uranium is found to greatly improve neutronic performance. A large reduction in moderation is necessary to allow full actinide recycle. This will pose thermal-hydraulic challenges, which are discussed in Part II. In addition, the harder neutron spectrum resulting from the reduced moderation also reduces the control rod worth, while there is a neutronic incentive to use increased mechanical shim to maintain a negative MTC. It may therefore be desirable to increase the number of rod cluster control assemblies. Superior burnup is achievable in a reduced-moderation BWR as a larger reduction in moderation is feasible, although the incineration rate is reduced relative to a PWR due to a higher conversion ratio.

Analysis of advanced PWR loading schemes for transuranic incineration in thorium

It is difficult to perform multiple recycle of transuranic (TRU) isotopes in PWRs as the moderator temperature coefficient (MTC) tends to become positive after a few recycles and the core may have positive reactivity when fully voided. Due to the favorable impact on the MTC and void coefficient fostered by use of thorium (Th), the possibility of performing Th-TRU multiple-recycle in reduced-moderation PWRs (RMPWRs) is under consideration. The simplest way to reduce the moderation in a PWR is to increase the fuel pin diameter. This configuration improves the trade-off between achievable burn-up and MTC, but is ultimately limited by thermal-hydraulic constraints. Heterogeneous recycle with the bred uranium (U3) and the TRU are arranged in separate pins was found to be neutronically preferable to a homogeneous configuration. Spatial separation also enables the U3 and TRU to be refueled on different batch schemes. These techniques allow satisfactory discharge burn-up while ensuring negative MTC and fully voided reactivity, with the pin diameter of a standard PWR increased from 9.5 mm to 11 mm. Reactivity control is a key challenge due to the reduced worth of neutron absorbers and their detrimental effect on the void coefficients, especially when diluted, as is the case for soluble boron. It seems necessary to control the core using control rods to keep the fully voided reactivity negative. A preliminary analysis indicates that this is feasible.Copyright

IMPACT OF THE DETAILED FISSION SOURCE DISTRIBUTION ON IRIS SHIELDING ANALYSES

Abstract International Reactor Innovative and Secure (IRIS) is an advanced pressurized water reactor with an integral primary system. It features an integral reactor vessel surrounded by a spherical steel containment 25 m in diameter. Both deterministic and Monte Carlo methods are used to characterize its radiation environment. This paper focuses on the generation of the neutron fission source that is employed as the fixed source in radiation transport calculations. To facilitate radiation shielding analysis, a technique is proposed to synthesize fission source data from the IRIS depletion history into an average and a limiting (maximum) source distribution. The average source preserves the time-integrated, spatially dependent fission neutrons and is suitable for evaluation of long-term irradiation effects, such as the radiation damage on the reactor vessel. The maximum source gives a bounding fission neutron distribution that is suitable for calculation of the maximum instantaneous dose to the personnel. Spatial and spectral effects are also taken into consideration in the source representation. Pinwise axial distributions of the neutron fission source and the associated contribution from primary fissionable isotopes have been generated to allow evaluation of neutron leakage in the critical regions, such as at the core periphery. Less detailed assemblywise axial distributions are also made available to simplify their implementation in the MCNP and TORT models. A comparison of the results obtained with the latter distributions against the reference results (employing the most detailed distribution) will show the impact of simplifications and help identify strategic features and locations where preserving the detailed information is beneficial for meeting specific shielding objectives. The judicious postprocessing and interpretation of the fission source distribution proposed by this approach make the subsequent radiation analysis practical while retaining the critical details needed to achieve high accuracy.

MEASUREMENTS OF THE RECOIL-ION RESPONSE OF SILICON CARBIDE DETECTORS TO FAST NEUTRONS

SiC semiconductor radiation detectors are being developed as neutron energy spectrometers for reactor dosimetry and monitoring applications in high-temperature, high-radiation environments. Detailed pulse-height response measurements have been carried out using isotopic 252Cf fission neutron and 241Am_Be (a,n) neutron sources as well as with electronic deuterium-deuterium and deuterium-tritium sources providing 2.S-MeV, 8.S-MeV and 14.I-MeV monoenergetic neutrons. The SiC energy response characteristics are presented and compared to those of a comparable Si neutron detector. Individual nuclear reaction peaks have been identified and are being used to characterize the energy deposition properties of charged particles from neutron-induced reactions in SiC detectors for a parallel response-modeling effort.