Toshihisa Yamamoto

Isotopic ratio of Pu released from fuel cycle facilities — importance of radiochemically pure 236Pu as a tracer

Abstract In order to characterize the possible release of Pu from nuclear fuel cycle facilities, the various isotopic compositions of Pu from different types of reactors have been examined. As a result, for assessment of radiological impact and identification of source of any release, it is shown that precise determination of the isotopic abundances of 238 Pu and 242 Pu will be required. In order to meet this requirement, the advantage of the utilization of 236 Pu as a tracer is emphasized. A technique, using an electron LINAC, was therefore developed to produce radiochemicaUy pure 236 Pu by the bremsstrahlung irradiation of 237 Np. Further, it is demonstrated that 236 Pu can be successfully produced by this method with a 238 Pu impurity of less than 0.06% of the total α-activity of 236 Pu.

Interpretation of actinide transmutation in thermal and fast reactors

Abstract The commonly used transmutation rate of minor actinides in nuclear reactors is decomposed into four components, overall fission rate, Pu production rate, MA production rate, and element production rate. The physical meanings of these factors are described. The transmutation rates of minor actinides in two types of highly-moderated PWRs, a MOX fueled Na cooled fast reactor, and a metal fueled Pb cooled fast reactor are interpreted using the four components. The metal fueled Pb cooled fast reactor can incinerate minor actinides most (79kg/GWth/year), and this amount is about 4 times larger than the thermal reactors. The thermal reactors have large relative overall fission rates for 241 Am and have a potential for the incineration of 241 Am.

Analysis of First-Harmonic Eigenvalue Separation Experiments on KUCA Coupled-Core

The first-harmonic eigenvalue separation, the difference between the fundamental and the first order eigen-values of the higher harmonic neutron transport equations, which were measured at the Kyoto University Critical Assembly (KUCA) has been analyzed. A method was proposed to calculate the first order eigenvalue based on the discrete ordinate method. The 3-D effect, energy group effect, mesh size effect, and transport effect were investigated. Among these effects, the transport effect was significant and when it was taken into account, the calculated eigenvalue separation approached the measured value on the KUCA coupled-core.

A Generalized Approach to Optimize Subgroup Parameters

The comparison of the two subgroup methods, the fitting method and the moment method, was made. It was proved that the both methods are identical under a special condition for the background cross section. More detailed analysis was done from the actual derivation process of the two methods and a generalized formulation was shown that is applicable to the both methods. It became clear that the moment method is regarded as a modified version of the fitting method with a flexible weighting function. It was also concluded that the two methods are identical when the parameter of the moment method takes a specified value (I=1). Two optimization procedures were proposed to obtain the best subgroup parameters by the use of the least square fitting. One of the methods enables to reduce the error in effective cross section from 6 to 2%.

Reaction Rate Calculation in Fast Reactor Blanket Using Multiband Sn Theory

A new method was applied to calculation of reaction rates in blanket of LMFBR using the multiband Sn theory. This procedure leads to the use of direction dependent total micro cross sections, which advances the penetration of neutrons into the blanket. Test calculation with RZ model of a prototype fast reactor shows that reaction rates tend to rise with the penetration into blanket compared to the conventional multigroup calculation: the maximum difference was about 3% for 238U capture, 4% for 235U fission, 4% for 239Pu fission, and less than 1% for 238U fission. This tendency shows the same direction as the difference observed between the continuous energy Monte Carlo method and the conventional method.

Background-Cross-Section-Dependent Subgroup Parameters

A new set of subgroup parameters was derived that can reproduce the self-shielded cross section against a wide range of background cross sections. The subgroup parameters are expressed with a rational equation which numerator and denominator are expressed as the expansion series of background cross section, so that the background cross section dependence is exactly taken into account in the parameters. The advantage of the new subgroup parameters is that they can reproduce the self-shielded effect not only by group basis but also by subgroup basis. Then an adaptive method is also proposed which uses fitting procedure to evaluate the background-cross-section- dependence of the parameters. One of the simple fitting formula was able to reproduce the self-shielded subgroup cross section by less than 1% error from the precise evaluation.

Production of Pure 236Pu Tracer for the Assessment of Plutonium in the Environment

For assessment of the release of Pu to the environment related to fuel cycle activities, the advantage of the utilization of 236Pu as a chemical yield tracer is pointed out. The desired radiochemical purity of 236Pu tracer is proposed. Conditions for the production method using irradiation of 237Np with a bremsstrahlung from an electron linear accelerator (LINAC) was studied to minimize the production of 238Pu as an impurity. It was demonstrated that 236Pu can be successfully produced by this method with a 238Pu impurity of less than 0.06% of the total a-activity of 236Pu.

Two-dimensional cell heterogeneity effect in analysis of fast critical assemblies

Two-dimensional heterogeneity effects on cell-averaged cross sections and diffusion coefficients have been estimated. Both of the heterogeneity effects caused by the flux fine structure and the resonance shielding are taken into account. The accuracy of several one-dimensional cell models is assessed by comparing cell parameters obtained from one- and two-dimensional cell calculations. The two-dimensional cell heterogeneity effects on core performance parameters have been evaluated for the large fast critical assembly ZPPR-9. The Zebra CADENZA plate-pin experiments have been analyzed using the two-dimensional cell model, and the usefulness of the two-dimensional cell calculation is discussed.

Subgroup Parameters based on Orthogonal Factorization

A new methodology to produce subgroup parameters has been developed based on orthogonal factorization of weighting functions. In the existent methods the weighting functions do not appear explicitly, which causes the inconvenience in producing subgroup parameters in some situations. In the present method, the weighting functions are tailored to the required conditions by the use of orthonormal factorization method of which mathematical background is based on the Lanczos method. The obtained weighting functions can be commonly used for any physical quantities in order to produce corresponding subgroup parameters. The superiority of this approach becomes eminent especially when multiple conditions are specified at the same time. The numerical results of two simple examples have revealed the potential of the applicability of the present method to general problems. We have concluded that the present method may be developed into an efficient tool to deal with wide variety of problems using subgroup-method-related methodology.

A Complement Proposal for Optimization of Subgroup Parameters

A new theory and methodology to optimize subgroup parameters were established by the use of Chebyshev approximation. The optimization of the fitting method was realized by the minimax approximation provided by the Remes algorithm. As for the moment method, a new definition for moments was proposed to cure the poor reproducibility of effective cross section by the original moment method. The new moment method uses transformed Chebyshev polynomials as orthogonal bases. This method is mathematically stable and the efficiency was verified by a numerical example. It was also shown that use of preconditioning can furthermore enhance the efficiency. The first dominant term of the new moments can be approximated by old moment definition σα with α= 0.1–0:6, including the case with α=−1/2. This fact can explain the efficiency of the reservation of half-integer moments in the reproducibility of effective cross section that was mentioned by Unesaki.