Recent activities in the field of reactor physics

ABSTRACT

Reactor Physics that treat the essentials of how fission nuclear reactors work fundamentally has played important roles in safe operations and design studies of various types of nuclear reactors. From the latest activities in the field of reactor physics, this report summarizes some outstanding researches and developments published in scientific journals, including the Journal of Nuclear Science and Technology and others.

KEYWORDS:

• Reactor physics
• analysis method
• experimental data and method

In the field of reactor physics, sensitivity and uncertainty analyses for reactor physics parameters, which are essential for criticality safety and rationalization of reactor design, have been paid significant attentions in recent years. Several numerical methods to calculate sensitivity coefficients were proposed, including the efficient approach for prompt neutron decay constant using perturbation theory [1] and the random sampling approach based on a penalized regression ‘adaptive smooth-lasso.’ [2] As cross-section adjustment methodologies, the dimensionality-reduction technique was adopted in Ref [3]. The decay heat uncertainty reduction was studied by the nuclear data adjustment method in Ref [4]. As an alternative to deterministic methods, Monte-Carlo (MC) simulations can be a very powerful tool for sensitivity and uncertainty analyses such as Refs [5,6]. A recent MC method has the capability to compute sensitivities with respect to system dimensions or locations of material interfaces [7]. A useful guidance to analyze uncertainties for density and composition using MCNP6 and SCALE 6 was presented in Ref [8]. Besides these uncertainty analyses, the MC simulations have been significantly progressed in computing kinetic parameters [9] and analyzing supercritical transient in fissile fuel debris systems [10]. In Ref [11], the application of graphic processing unit was proposed to an efficient Monte-Carlo calculation.

While rapidly progressing in the MC methods, deterministic approaches are still useful for design studies for new concepts in advanced reactors from the viewpoint of an equilibrium between calculation precision and computational cost. Following outstanding developments were done: in Ref [12], the GENESIS code was developed implementing the Legendre polynomial expansion of angular flux method; Ref [13] assessed consistent and rigorous precision of various methods for calculating the diffusion coefficients in a two-step reactor core analysis of light water reactors. New resonance self-shielding methods were proposed to treat for radially and azimuthally dependence in general multi-region geometry [14] and for the random medium of the high-temperature reactor lattice cells [15]. Several approaches of nuclear fuel depletion calculations were proposed in Refs [16,17]. The nuclear data processing system from evaluated nuclear data library to any application (FRENDY) was newly developed [18].

Experimental researches using critical assemblies and research reactors play important roles from a different perspective, with the comparison of computational researches. Also, experimental researches provide integral data that can be utilized to validate nuclear data and nuclear calculation methods and to develop and improve experimental technique. Following researches were carried out for the development of advanced nuclear reactors, such as the accelerator driven system (ADS). Integral data to evaluate coolant materials for ADS were measured: sample reactivity worth of lead [19] and bismuth [20] at the Kyoto University Critical Assembly (KUCA), and lead void reactivity worth at the Nevada National Security Site [21]. At KUCA, online subcriticality monitoring system [22] was developed, and effective delayed neutron fractions [23] and reaction rates [24] were measured with spallation neutrons generated by the combined use of 100 MeV protons and a lead-bismuth target. On 14th and 15 February 2019, the first significant attempt was made to demonstrate the principle of nuclear transmutation of minor actinides (MAs) by ADS through the injection of high-energy neutrons into the KUCA core at a subcritical state [25]. Benchmark models for the fission rates of MAs in various neutron spectra measured at FCA of JAEA were developed, demonstrating systematic integral evaluation of their cross sections [26]. Some fundamental researches were carried out to improve measurement techniques as follows: in UTR-Kinki reactor, experimental study of neutron counting in the zero-power reactor driven was carried out by the neutron source inherent in highly enriched uranium fuels [27]. Ref [28] proposed the linear combination method to reduce the higher order mode effect on the measurement of the prompt neutron decay constant by the α-fitting method. A theory of the modified Feynman-α technique for the subcritical system driven by periodically triggered neutron bursts was developed to apply this technique to the subcriticality monitor for ADS [29]. An uncertainty approach for parameters in subcritical measurements was presented and validated with the use of measured data of subcritical experiments [30,31].

Disclosure statement

No potential conflict of interest was reported by the authors.