Applicability of statistical geometry model to light water moderating systems

Mori, Takamasa; Kojima, Kensuke*; Suyama, Kenya

JAEA-Research 2018-010, 57 Pages, 2019/02

JAEA-Research-2018-010.pdf:6.25MB

In order to estimate applicability of the statistical geometry model (STGM) of MVP/GMVP, a parametric study in infinite geometry and criticality safety analyses for direct disposal of spent fuel in simple finite geometry have been carried out by using the MVP Monte Carlo code. It has been found that calculations with STGM for larger fuel spheres give larger thermal utilization factors and larger infinite multiplication factors compared with explicit random models in the range of fuel sphere packing fraction between 6.5 % and 63.3 %. Substantial differences are not observed between the results with two nearest neighbor distributions (NNDs); that given by the MCRDF code and the analytical expression based on a statistically uniform distribution. It is inferred that the overestimation by STGM is caused by the facts that STGM cannot take account of the surroundings of each neutron, whether a fuel sphere rich region or a water moderator rich one, because STGM always uses an NND averaged over such surroundings and that STGM, therefore, cannot take the effect of consecutive scatterings in the water moderator into account.

Study of the neutron multiplication effect in an active neutron method

Komeda, Masao; Ozu, Akira; Mori, Takamasa; Nakatsuka, Yoshiaki; Maeda, Makoto; Kureta, Masatoshi; Toh, Yosuke

Journal of Nuclear Science and Technology, 54(11), p.1233 - 1239, 2017/11

https://doi.org/10.1080/00223131.2017.1359118

The previous active neutron method cannot remove the influence of the multiplication effect of neutrons produced by second- and subsequent fission reactions, and it might overestimate the amount of nuclear material if an item contains large amounts. In this paper, we discussed the correction method for the neutron multiplication effect on the measured data in the fast neutron direct interrogation (FNDI) method, one of the active neutron methods, supposing that the neutron multiplication effect is caused mainly by third-generation neutrons from the second-fission reactions under the condition that the forth-generation neutrons are much fewer. This paper proposed a correction method for the neutron multiplication effect in the measured data. Moreover we have shown a possibility that this correction method gives rough estimates of the effective neutron multiplication factor and the subcriticality.

MVP/GMVP version 3; General purpose Monte Carlo codes for neutron and photon transport calculations based on continuous energy and multigroup methods

Nagaya, Yasunobu; Okumura, Keisuke; Sakurai, Takeshi; Mori, Takamasa

JAEA-Data/Code 2016-018, 421 Pages, 2017/03

JAEA-Data-Code-2016-018.pdf:3.89MB
JAEA-Data-Code-2016-018-appendix(CD-ROM).zip:4.02MB
JAEA-Data-Code-2016-018-hyperlink.zip:1.94MB

In order to realize fast and accurate Monte Carlo simulation of neutron and photon transport problems, two Monte Carlo codes MVP (continuous-energy method) and GMVP (multigroup method) have been developed at Japan Atomic Energy Agency. The codes have adopted a vectorized algorithm and have been developed for vector-type supercomputers. They also support parallel processing with a standard parallelization library MPI and thus a speed-up of Monte Carlo calculations can be achieved on general computing platforms. The first and second versions of the codes were released in 1994 and 2005, respectively. They have been extensively improved and new capabilities have been implemented. The major improvements and new capabilities are as follows: (1) perturbation calculation for effective multiplication factor, (2) exact resonant elastic scattering model, (3) calculation of reactor kinetics parameters, (4) photo-nuclear model, (5) simulation of delayed neutrons, (6) generation of group constants, etc. This report describes the physical model, geometry description method used in the codes, new capabilities and input instructions.

Void reactivity evaluation by modified conversion ratio measurements in LWR critical experiments

Yoshioka, Kenichi*; Kikuchi, Tsukasa*; Gunji, Satoshi*; Kumanomido, Hironori*; Mitsuhashi, Ishi*; Umano, Takuya*; Yamaoka, Mitsuaki*; Okajima, Shigeaki; Fukushima, Masahiro; Nagaya, Yasunobu; et al.

Journal of Nuclear Science and Technology, 52(2), p.282 - 293, 2015/02

https://doi.org/10.1080/00223131.2014.937781

We have developed a void reactivity evaluation method by using modified conversion ratio measurements in a light water reactor (LWR) critical lattice. Assembly-wise void reactivity is evaluated from the "finite neutron multiplication factor", , deduced from the modified conversion ratio of each fuel rod. The distributions of modified conversion ratio and on a reduced-moderation LWR lattice, for which the improvement of negative void reactivity is a serious issue, were measured. Measured values were analyzed with a continuous-energy Monte Carlo method. The measurements and analyses agreed within the measurement uncertainty. The developed method is useful for validating the nuclear design methodology concerning void reactivity.

Reactor physics

Okajima, Shigeaki; Kugo, Teruhiko; Mori, Takamasa

Genshiryoku Kyokasho "Genshiro Butsurigaku", 258 Pages, 2012/03

http://ci.nii.ac.jp/ncid/BB08754491

This textbook is prepared for a lecture of the reactor physics in Nuclear Professional School of The University of Tokyo. In Chapter 1, the characteristics of nucleus and the interaction between nucleus and neutron were described. In Chapter 2, the fission reaction, its chain reaction and a concept on criticality were described. Chapter 3 described about the fundamental study on a diffusion equation to treat the neutron spatial distributions in a substance. In Chapter 4, the application of a diffusion equation was shown to learn the feature of neutron distribution in a substance with homogeneous composition. Chapter 5 described the physics of neutron slowing-down in a substance and Chapter 6 investigated the neutron slowing-downed to become in thermal equilibrium state in the substance. Chapter 7 considered the effect of the practical heterogeneous composition from the homogeneous one to the criticality. The JENDL-4.0 library was referred for the nuclear reaction cross section data.

A Conceptual design study for active nondestructive assay system by photon interrogation for uranium-bearing waste with MVP code and evaluated photonuclear data

Sakurai, Takeshi; Kosako, Kazuaki*; Mori, Takamasa

Progress in Nuclear Science and Technology (Internet), 2, p.318 - 329, 2011/10

https://doi.org/10.15669/pnst.2.318

Measurement and analysis of reactivity worth of Am sample in water-moderated low-enriched UO fuel lattices at TCA

Sakurai, Takeshi; Mori, Takamasa; Suzaki, Takenori*; Okajima, Shigeaki; Ando, Yoshihira*; Yamamoto, Toru*; Liem, P. H.*

Journal of Nuclear Science and Technology, 48(5), p.816 - 825, 2011/05

https://doi.org/10.3327/jnst.48.816

Calculation of effective delayed neutron fraction with Monte Carlo perturbation techniques

Nagaya, Yasunobu; Mori, Takamasa

Annals of Nuclear Energy, 38(2-3), p.254 - 260, 2011/02

https://doi.org/10.1016/j.anucene.2010.10.020

Alternative methods are proposed to estimate the effective delayed neutron fraction in Monte Carlo calculations: the eigenvalue methods jointly used with the differential operator sampling and correlated sampling techniques. In particular, the eigenvalue method with the differential operator sampling technique has a distinct feature that it gives a theoretically exact value. To verify the proposed methods, Monte Carlo calculations are performed for several systems with simple geometry. It is found that the results obtained with the proposed methods agree with the reference deterministic results within sufficiently small statistical uncertainties. The perturbed source effect must be taken into account to estimate an exact value.

Estimation of reactivity worth with differential operator sampling method

Nagaya, Yasunobu; Mori, Takamasa

Proceedings of Joint International Conference of 7th Supercomputing in Nuclear Application and 3rd Monte Carlo (SNA + MC 2010) (USB Flash Drive), 6 Pages, 2010/10

Nuclear reactor analysis requires calculations of reactivity worth such as control rod worth, void reactivity worth, sample reactivity worth, etc. It is, however, difficult to perform such calculations with the Monte Carlo method if the worth is small. In the present work, we extend the differential operator sampling method such that the derivative terms up to the fourth-order and the perturbed source effect up to the fourth-order can be estimated, and examine the applicability of the fourth-order differential operator sampling method to reactivity worth calculations. As a benchmark calculation, we perform the reactivity worth calculation for Godiva. The perturbation is introduced by decreasing the density in the central region of a radius of 1 cm. The result with the fourth-order differential operator sampling method agrees well with the reference one. As a practical calculation, we also perform Np sample worth calculation for TCA. The calculated sample reactivity worth with the differential sampling method approaches to the reference value as the higher-order effect is taken into account up to the fourth order. However, there still exists a discrepancy of 17%. It is, thus, found that many more higher-order effects must be taken into account or another method must be applied in this case.

Comparison of resonance elastic scattering models newly implemented in MVP continuous-energy Monte Carlo code

Mori, Takamasa; Nagaya, Yasunobu

Journal of Nuclear Science and Technology, 46(8), p.793 - 798, 2009/08

https://doi.org/10.3327/jnst.46.793

In order to investigate the impact of resonance elastic scattering models on the Doppler reactivity effect, the exact and the constant cross section models with the thermal motion of target nucleus were newly implemented into the MVP-2 continuous-energy Monte Carlo code with and without consideration of energy-dependent resonance cross section, respectively, and the UO pin-cell Doppler reactivity benchmark calculations were carried out with the modified code and the JENDL-3.3 library. The present study has revealed that the exact model gives more negative Doppler reactivity coefficients by 711% than the conventional asymptotic model, while the constant cross section model gives slightly less negative coefficients than the conventional asymptotic model. Furthermore, it is found that the impact of resonance elastic scattering models is considerably large around the resonances where elastic scattering has relatively high contribution, whereas capture-dominant resonances have no significant impacts on the Doppler reactivity coefficient.