05 Nuclear fuels (scientific, technical)

Solutions for a flat thermal flux in a two-group diffusion model are found for a range of assumptions, since this is a requisite for both a minimum and a maximum fuel loading in the model. It is shown that a maximum fuel loading then arises when such a region, at unity infinite multiplication factor, is complemented by an outer core to bring the finite reactor critical. Correspondingly, when a core with a flat flux is surrounded by a reflector, the core fuel may be so distributed as to flatten the flux using the 'flux-trap' phenomenon and provide a minimum loading. Critical conditions and necessary fuel density are derived in finite and infinitely thick reflectors. A curiosity is the possibility of observing 'negative' reflector savings. Fuel savings are estimated for a range of models for the minimum loading compared to a correspondingly critical uniform core loading. In some circumstances a saving of up to 70% is indicated with economic and safety implications. It is shown how the reduction of the minimum loading solution from a two- to a one-group model retains the flat flux in the core but fails to satisfy the thermal boundary condition unless a reflector, with the flux-trap, is used so that without it, the two-group minimum fuel loading solution cannot fully transform to a minimum loading in one group. The full solution for a two-group model flat flux core with finite thickness reflector is given. It is shown that as the reflector is reduced it becomes necessary to increase the fuel density at the centre to a point where this exceeds the capability of the chosen fuel, thus providing a secondary criticality equation. The increasing steepness (negative slope) of the flux distribution is required to make the flux trap phenomenon possible in the reducing reflector region. The range in which there may be two reflector thicknesses leading to the same size core, but different fuel distributions observed by Williams, is determined. Constrained solutions that limit either the size of core or the maximum fuel density are considered, generalizing the original work of Goertzel to a practicable core design.

04/02776 Modified APEX reactor as a fusion breeder Sahin, S. and Ubeyli, M. Energy Conversion and Management, 2004, 45, (9-10), 1497-1512. An advanced fusion reactor project, called APEX, with improved effectiveness has been developed using a protective flowing liquid wall for tritium breeding and energy transfer. In the modified APEX concept, the flowing molten salt wall is composed of Flibe as the main constituent with increased mole fractions of heavy metal salt (ThF4 or UF4) for both fissile and fusile breeding purposes and to increase the energy multiplication. Neutron transport calculations are conducted with the help of the SCALE4.3 SYSTEM by solving the Boltzmann transport equation with the code XSDRNPM. By preserving a self sufficient tritium breeding ratio (TBR > 1.05) for a mole fraction up to 6% of ThF4 or 12% of UF4, the modified APEX reactor can produce u ~ t o approximately 2800 kg of 33U/year or approximately 4950 kg of

Pu/year, assuming the same baseline fusion power production of 4000 MWth, as in the original APEX concept. With 6% ThF4 or 12% UF4 in the coolant, the total energy output will increase to 5560 MW~h or 8440 MWth, respectively. For a plant operation period of 30 full power years, the atomic displacement and helium production rates remain well below the presumable limits. The additional benefits of fissionable metal salt in the flowing liquid in a fusion reactor can be summarized as breeding of high quality fissile fuel for external reactors and increase of total plant power output.

04•02777 Novel approach for the bulk synthesis of nanocrystalline yttria doped thoria powders via polymeric precursor routes Ganesan, R. et al. Journal of Nuclear Materials, 2004, 325, (2-3), 134- 140. Three different polymeric precursor routes namely, (i) amorphous citrate process, (ii) Pechini process and (iii) polyethylene glycol assisted process were compared for synthesizing nanocrystalline powders of 7.5 mol% yttria doped thoria (YDT). In each of the processes, parameters such as metal-to-fuel ratio or composition of fuel were varied and the effects were analysed. TG/DTA studies were conducted to identify the ignition temperatures of the precursors. Also, a novel experimental procedure with controlled combustion was devised for the preparation of powders based on the thermal analysis data. All the processes result in phase pure and nanocrystalline powders. The average crystallite size of the powders ranged between 9 and 18 nm. The powder samples were analysed for their carbon content and studied for their sinterability. Densities as high as 99% could be achieved by sintering the compacts of powders obtained from (i) amorphous citrate process with CA/M ratios 2.0 and 3.0, (ii) Pechini processes (independent of fuel composition) and (iii) PEG- assisted process using PEG 4000 at a relatively low temperature of 1500°C for 2 h.

04/02778 Overview of recent experimental works on high energy neutron shielding Nakamura, T. et al. Progress in Nuclear Energy, 2004, 44, (2), 85 187.

Several experiments on high energy neutron shielding have recently been performed using medium- to high-energy accelerators of energies above 20 MeV. Below 100 MeV, the benchmark experiments have been done using 25 and 35 MeV p-Li quasi-monoenergetic neutrons at the Cyclotron and Radioisotope Centre, Tohoku University, Japan, 43 and 68 MeV p-Li quasi-monoenergetic neutrons at the Azimuthally Varying Field Cyclotron Facility, T IARA of Japan Atomic Energy Research Institute. Above 100 MeV, the neutron shielding experiments have been done using 800 MeV protons at ISIS, Rutherford Appleton laboratory, England, 400 MeV/nucleon carbon ions at the heavy ion medical accelerator facility, HIMAC of National Institute of Radio- logical Sciences, Japan, 500 MeV protons at the spatlation neutron source facility, KEK spallation neutron source facility of High Energy Accelerator Research Organization, Japan, 500 MeV protons at the accelerator facility, TRIUMF, Canada, 1.6 to 24 GeV protons at the Alternating Gradient Synchrotron, Brookhaven National Laboratory, USA, 28.7 GeV electrons at the Stanford Linear Accelerator Centre, USA, 800 MeV protons at the Los Alamos Neutron Science Centre, Los Alamos National Laboratory, USA, 120, 205 GeV/c protons and 160 GeV/nucteon lead ions at the European Organization for Nuclear Research, Switzerland, 230 MeV protons at the Loma Linda University Medical Centre, USA, 200 MeV protons at the Orsay Proton Therapy Centre, France. In this review paper, the outlines of these deep penetration experiments are summarized together with the neutron detection techniques.

04/02779 Progress in real4ime feedback control systems in RFX Barana, O. et al. Fusion Engineering and Design, 2004, 71, (1-4), 35-40. Major modifications of the RFX load assembly and power supplies are in progress to allow extensive active control schemes, such as equilibrium and plasma position control and innovative control of the MHD modes. The digital control system is implemented in VME64 using a distributed architecture. The use of a 'stable' operating system that is likely to survive some generations of processors can help coping with evolution of technology. PowerPC and Pentium processors were thus considered as candidates and tested and the first one has been selected due to the better performance in floating point computation. Wind River VxWorks has been chosen as real-time operating system. 100 Mbit switched Ethernet has been evaluated for real-time communication by using the user datagram protocol (UDP). Measure- ments have been executed on a prototype system to assess data transfer latency, jitter and reliability and the results confirm that the solution is suitable for the application. The paper describes in detail the reasons for the choice in the hardware components. Results from several tests comparing the performance of different solutions are also provided.

04/02780 Rapid optimization scheme for Poloidal Field design of tokamak with divertor configuration Srinivasan, R. Fusion Engineering and Design, 2004, 70, (3), 269-278. The Poloidal Field (PF) coil system in a tokamak must be optimized, taking into account the desired flexibility in plasma equilibria, and practical constraints on the number of coils, their locations and dimensions. Such optimization requires the generation of a large number of magetohydrodynamics (MHD) equilibria spanning the desired equilibrium space. Given the large parameter space, it is necessary to use a fast method for determining the PF coil system required for a given plasma equilibrium. In this work, an inverse Grad- Shafranov solver is coupled with a PF coil current optimizer, which incorporates various design constraints. The method can handle shaped tokamak plasma equilibria with a divertor configuration. This method is found to be considerably faster than conventional free-boundary codes, and the calculated PF coil currents are in reasonable agreement with those from free-boundary solutions. This method has been applied to PF optimization of SST-1 for maximizing the operational envelope.

04/02781 Reduction of EPZ area for APR1400 and its public acceptance Lee, Y. W. et al. Progress in Nuclear Energy, 2004, 44, (2), 75-84. Since advanced light water reactors (ALWRs) are anticipated to provide an enhanced margin of safety and newer nuclear power plants have a higher standard of severe accident safety performance than the existing plant designs, recent efforts aimed at simplifying the concept of EPZ for ALWR standard plant designs had been made by a group of utility companies through the EPRI in the USA. In Korea, similar efforts have been made for the APR1400, which is a 1400 MW-sized PWR of an advanced concept developed in Korea. This is possible due to a large enhancement in the performance of various accident mitigation features adopted in its design. The technical assessment shows that radius of the EPZ area of the APR1400 can be reduced to 700 in. However, even though the reduction in the EPZ area is well accepted through an extensive technical assessment process, it has been found that public acceptance appears to be a completely different matter. Therefore, this study performed a public poll to assess the

394 Fuel and Energy Abstracts November 2004