Berkeley Nuclear Research Center

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BNRC Projects

BRNC Projects

Advanced LWR concepts with high conversion ratio and negative reactivity feedback

Advanced LWR concepts with high conversion ratio and negative reactivity feedback

The development and construction of liquid metal cooled breeder reactors is highly desirable for their superior waste managing capability as compared to that of light water reactors (LWR). However, their construction is constrained by, among other factors, (a) Higher construction and operational costs as compared to that of LWR; (b) No presently available industrial infrastructure; (c) Lack of operational experience among the nuclear industry, which instead has accumulated vast experience with light water as coolant for the current fleet of nuclear power plants. Hitachi has developed an innovative Resource-Renewable BWR (RBWR) nuclear reactor core design, which is expected to fission almost all the transuranic (TRU) elements generated in the nuclear energy system. In 2008/2009 UCB was asked to evaluate the ability of the RBWR technology to offer designs that could compete with the corresponding SFR designs. Based on the conclusions of our analysis performed during this work, it appears that the RBWR-AC holds considerable promise to improve the sustainability of nuclear energy. However, it was also found that few concerns still remain on the safe operability of this system, mainly 1) the sign of the void reactivity coefficients and 2) adequacy of the control rod shutdown margin in fully voided conditions. Moreover, the feasibility of increasing the conversion ratio above 1 appears of significant interest. It is our aim to evaluate the feasibility of designing a similar core with negative reactivity coefficients and sufficient control rod shutdown margin in all conceivable conditions. We will first study the performance of a core similar to the RBWR-AC, but using thorium instead of depleted uranium as fertile material. If successful, such a system would address the void coefficient of reactivity problems, while possibly increasing the destruction rate of accumulated TRU and retaining an overall fissile conversion ratio of 1 (or possibly more). It is also possible that the use of thorium would increase the control rod shutdown margin without further system modifications. Although in the past little attention has been devoted by the scientific community to the possibility of light water breeder reactors, it is our believe that the successful accomplishments of the tasks set forth here, will attract the deserved attention to the potential for this technology to improve the sustainability of nuclear power ? by dramatically enhancing the uranium resources while virtually eliminating the actinides from the high level waste stream; all this while using the industrial infrastructure of BWR already in place, thus substantially reducing development cost and time relative to those required for pursuing the SFR route.

Triplet-Triplet Exciton Annihiliation for Pulse Shape Discrimination in Scintillator Detectors (collaboration with J. Siegrist, LBNL)

Triplet-Triplet Exciton Annihiliation for Pulse Shape Discrimination in Scintillator Detectors (collaboration with J. Siegrist, LBNL)

The underlying mechanism enabling pulse shape discrimination of differing particle types in scintillating materials is the production of delayed fluorescence photons.  The intensity ratio of delayed to prompt florescence can be correlated to the energy density deposited by the detected particle with higher energy density resulting in larger delayed florescence intensity.  The observation of prompt and delayed florescence is the result of an allowed transition of the first excited singlet (S=0) quantum state to the ground singlet quantum state or the scintillating molecule.  In addition to the excited singlet states, a number of triplet states (S=1) are populated from the initial energy deposition.  These transition of triplet to singlet ground states is a first forbidden transition and thus occur at a much slower rate making these triplet states metastable to the order of milliseconds.  Within the lifetime of the triplet states, it is possible for the excited state to transfer from one spatial location to another by means of exciton transport mechanisms.  During this period, triplet states may transfer to singlet states by inter-system transfer or may annihilate with other triplet states producing a ground singlet and excited singlet states.  The resulting excited singlet states will de-excite to the ground state producing a delayed fluorescence photon.  Inter-system transfer should not vary with the energy density but triplet-triplet annihilation (TTA) should show higher rates given higher initial energy deposition.  Particle discrimination using pulse shape techniques rely on the understanding of the transport and annihilation of triplet excitons. The goal of this project is to investigate the triplet exciton transport properties under varying amounts of hydrostatic pressure and temperature with the aim of developing a general model for exciton transport. (collaboration with J. Siegrist, LBNL)

Beta-delayed neutron spectroscopy using trapped radioactive ions

Beta-delayed neutron spectroscopy using trapped radioactive ions

These measurements will improve the accuracy for delayed-neutron data for safety and accident analysis for fast reactors and other novel fuel cycle applications.  Currently, much of the delayed-neutron data used in calculations has large uncertainties and in some cases results disagree by factors of 2-3.  A comprehensive program of beta-delayed neutron spectroscopy measurements will be carried out at Argonne National Laboratory using neutron-rich fission-fragment beams from the Californium Rare Ion Breeder Upgrade (CARIBU).  Sophisticated ion-trapping techniques and modern radiation detection systems will be employed to realize these measurements with high efficiency and energy resolutions approaching 1%. (collaboration with E. Norman, LLNL)

Nondestructive Assay using Beta-Delayed Gamma-Ray Signatures to Quantify Fissile Isotopes in Spent Fuel

This research focuses on the temporal dependence of the intensity of the delayed gamma-ray signal in different energy bins and the study of new isotopic-dependent signatures. The ultimate goal is to utilize the spectral and temporal dependence of the delayed gamma rays in the quantitative assay of 239Pu and 235U content in fuel cycle materials. The major aspect of this research is the investigation of the time-response of the delayed gamma energy spectrum within short time intervals (seconds) following an impulse of interrogating thermal neutrons. The fission product population growth and decay depend on the initial conditions after every interrogation pulse and, thus, the differences in fission product yields between various fissionable nuclides project into the resulting decay chains. (collaboration with B. Ludewigt, LBNL, and S. Tobin, LANL)

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