Yoshihisa Tahara

Feasibility study on epithermal neutron field for cyclotron-based boron neutron capture therapy.

To realize the accelerator-based boron neutron capture therapy (BNCT) at the Cyclotron and Radioisotope Center of Tohoku University, the feasibility of a cyclotron-based BNCT was evaluated. This study focuses on optimizing the epithermal neutron field with an energy spectrum and intensity suitable for BNCT for various combinations of neutron-producing reactions and moderator materials. Neutrons emitted at 90 degrees from a thick (stopping-length) Ta target, bombarded by 50 MeV protons of 300 microA beam current, were selected as a neutron source, based on the measurement of angular distributions and neutron energy spectra. As assembly composed of iron, AlF3/Al/6LiF, and lead was chosen as moderators, based on the simulation trials using the MCNPX code. The depth dose distributions in a cylindrical phantom, calculated with the MCNPX code, showed that, within 1 h of therapeutic time, the best moderator assembly, which is 30-cm-thick iron, 39-cm-thick AlF3/Al/6LiF, and 1-cm-thick lead, provides an epithermal neutron flux of 0.7 x 10(9) [n cm(-2) s(-1)]. This results in a tumor dose of 20.9 Gy-eq at a depth of 8 cm in the phantom, which is 6.4 Gy-eq higher than that of the Brookhaven Medical Research Reactor at the equivalent condition of maximum normal tissue tolerance. The beam power of the cyclotron is 15 kW, which is much lower than other accelerator-based BNCT proposals.

Engineering Design of a Spallation Reaction-Based Neutron Generator for Boron Neutron Capture Therapy

An engineering design of an epithermal neutron generator for boron neutron capture therapy (BNCT) has been completed, which utilizes the spallation reaction by protons accelerated to 50 MeV. The critical issues for realization of the neutron generator are the mechanical structure of a target with cooling capability and its integrity under operating conditions with powers as high as 50 MeV × 300 μA. The integrity of a target structure design has been confirmed by thermal and stress analyses with a finite element method code ANSYS. Moreover, a target replacement strategy is also studied based on a radioactivity evaluation performed by the IRACM code system. In addition to the target structure design, the neutronics design has been optimized with the Monte Carlo code MCNPX. A high epithermal neutron flux of 1.8×109 cm−2.s−1 has been achieved at the aperture of the collimator, which allows a RBE dose of over 30 Gy-eq to be delivered to a brain tumor within 5.9 cm in phantom depth for a therapeutic time of 31 min.

Two-Dimensional Baffle/Reflector Constants for Nodal Code in PWR Core Design

Currently nodal codes are widely used in three-dimensional core calculation. For nodal calculations, in addition to fuel assembly homogenization constants, baffle/reflector homogenization constants (B/R constants) have to be generated. Due to the complexity of its geometrical structure, the baffle/reflector region is usually represented by the two regions, which are called flat edges and corner edges. B/R constants are generated using an equivalent one-dimensional model for each region. However, errors of 3–4% appear for fuel assemblies along core corner when one-dimensional B/R constants are used. Therefore, in order to improve the accuracy of power distribution calculation based on the nodal method, B/R constants need to be calculated by modeling the geometrical configuration of the baffle/reflector region in greater detail. For this purpose, a method of calculating two-dimensional B/R constants that reflects the geometrical configuration has been developed, in which the geometrical configuration ouside the core is treated explicitly using a two-group fine-mesh diffusion code. The two-dimensional B/R constants thus obtained have reproduced assembly power from heterogeneous calculation within 0.5%, error regardless of fuel loading patterns.

Reactivity Effect of Iron Reflector in LWR Cores

One of the ways of improving fuel cycle cost is to reduce neutron leakage from a core using a reflector. For this purpose, experiments were carried out to investigate the reactivity effect of an iron reflector in a light-water-moderated core using the critical assembly, TCA. The experiment showed that iron reflectors of 15 cm thickness made the core more reactive than water and that the increasing the thickness from 2.2 cm to 15 cm produced 1.8%Δk/k core reactivity gain. The experiment was analyzed with the two-dimensional transport code PHOENIX-P and the continuous energy Monte Carlo code MVP. From the analyses, it has been found that, if the ENDF/B-VI data for iron isotopes are used, the calculated reactivity effect gives good agreement with the experiment and that the epi-thermal and thermal capture reaction rate distributions measured with gold wires and the resultant spectral index distribution are also well reproduced. The experiment and calculations have revealed that increasing a baffle plate thickness in PWRs above 2.2 cm can increase the core reactivity and contribute to fuel cycle cost saving. Based on this result, a stainless steel radial reflector has been employed in the APWR, and it has been shown that a reduction of about 0.07 wt% 235U enrichment can be achieved with the use of this radial reflector. Lastly, through the experimental analyses, the reactivity effect of the iron reflector has been found to give a benchmark useful for the evaluation of iron nuclear data. We expect that the experimental data described in this paper will be used to verify new iron data.

Transport Equivalent Diffusion Constants for Reflector Region in PWRs

The diffusion-theory-based nodal method is widely used in PWR core designs for reason of its high computing speed in three-dimensional calculations. The baffle/reflector (B/R) constants used in nodal calculations are usually calculated based on a one-dimensional transport calculation. However, to achieve high accuracy of assembly power prediction, two-dimensional model is needed. For this reason, the method for calculating transport equivalent diffusion constants of reflector material was developed so that the neutron currents on the material boundaries could be calculated exactly in diffusion calculations. Two-dimensional B/R constants were calculated using the transport equivalent diffusion constants in the two-dimensional diffusion calculation whose geometry reflected the actual material configuration in the reflector region. The two-dimensional B/R constants enabled us to predict assembly power within an error of 1.5% at hot full power conditions.

Evaluation of Delayed Neutron Data for JENDL-3.3

Delayed neutron data of 235U, 238U and 239Pu have been evaluated and recommended for JENDL-3.3. Adjustment of Vd was carried out on the basis of the ßeff measurements at FCA, MASURCA and TCA using the JENDL-3.2 data as the initial guess. Through this adjustment the vd value for 238U below 3.5 MeV was decreased by about 3 % from 0.0481 to 0.0466. The vd values of 235U and 239Pu were also determined in this way. Further appropriate six group constants, the decay constants λi and the group yields αi, were determined from experimental data of the delayed neutron emission rates, which were collected and compiled by Spriggs through the SG6 activity of WPEC. Applicability of the resultant group constants was validated through the analyses of the reactivity measurements based on the period or the rod-drop methods.

Development of Innovative Nuclear Reactor Technology to Produce Protected Plutonium with High Proliferation Resistance: Requirements and Validation of Nuclear Data

The Protected Plutonium Production (P3) project has been initiated. The first stage of this project (P3‐A) is based on the idea of protecting plutonium by 238Pu isotope generated from 237Np doped in the fresh uranium fuel. The present status of nuclear data relevant to the P3‐concept reactor design is surveyed and data requirement is presented.

Verification of Iron Cross-sections based on Reactivity Effect of Iron Reflectors in a LWR Core

Iron reflects fast neutrons well and a thick iron reflector makes a core more reactive than a water reflector. Based on the fact, an advanced pressurized water reactor (APWR) employs a stainless steel radial reflector. Verification of iron cross- section data is important from a reactor core design viewpoint. Therefore, iron cross-sections of ENDF/B-VI, JENDL-3.2 and the preliminary version of JENDL-3.3 were tested against a critical experiment that demonstrates reactivity effect of iron reflectors in a light-water-moderated core