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Research Papers

# Neutronic Design Features of a Transportable Fluoride-Salt-Cooled High-Temperature Reactor

[+] Author and Article Information
Kaichao Sun

Massachusetts Institute of Technology,
77 Massachusetts Avenue,
Cambridge, MA 02139
e-mail: kaichao@mit.edu

Lin-Wen Hu, Charles Forsberg

Massachusetts Institute of Technology,
77 Massachusetts Avenue,
Cambridge, MA 02139

Manuscript received September 1, 2015; final manuscript received February 11, 2016; published online June 17, 2016. Assoc. Editor: Akos Horvath.

ASME J of Nuclear Rad Sci 2(3), 031003 (Jun 17, 2016) (10 pages) Paper No: NERS-15-1184; doi: 10.1115/1.4032873 History: Received September 01, 2015; Accepted February 11, 2016

## Abstract

The fluoride-salt-cooled high-temperature reactor (FHR) is a new reactor concept, which combines low-pressure liquid salt coolant and high-temperature tristructural isotropic (TRISO) particle fuel. The refractory TRISO particle coating system and the dispersion in graphite matrix enhance safeguards (nuclear proliferation resistance) and security. Compared to the conventional high-temperature reactor (HTR) cooled by helium gas, the liquid salt system features significantly lower pressure, larger volumetric heat capacity, and higher thermal conductivity. The salt coolant enables coupling to a nuclear air-Brayton combined cycle (NACC) that provides base-load and peak-power capabilities. Added peak power is produced using jet fuel or locally produced hydrogen. The FHR is, therefore, considered as an ideal candidate for the transportable reactor concept to provide power to remote sites. In this context, a 20-MW (thermal power) compact core aiming at an 18-month once-through fuel cycle is currently under design at Massachusetts Institute of Technology (MIT). One of the key challenges of the core design is to minimize the reactivity swing induced by fuel depletion, since excessive reactivity will increase the complexity in control rod design and also result in criticality risk during the transportation process. In this study, burnable poison particles (BPPs) made of $B4C$ with natural boron (i.e., 20% $B10$ content) are adopted as the key measure for fuel cycle optimization. It was found that the overall inventory and the individual size of BPPs are the two most important parameters that determine the evolution path of the multiplication factor over time. The packing fraction (PF) in the fuel compact and the height of active zone are of secondary importance. The neutronic effect of $Li6$ depletion was also quantified. The 18-month once-through fuel cycle is optimized, and the depletion reactivity swing is reduced to 1 beta. The reactivity control system, which consists of six control rods and 12 safety rods, has been implemented in the proposed FHR core configuration. It fully satisfies the design goal of limiting the maximum reactivity worth for single control rod ejection within 0.8 beta and ensuring shutdown margin with the most valuable safety rod fully withdrawn. The core power distribution including the control rod’s effect is also demonstrated in this paper.

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## Figures

Fig. 1

Perspective of TRISO fuel particle (top left), horizontal schematic of fuel compact (top right), and horizontal cross section of fuel block (bottom)

Fig. 2

Horizontal (top) and vertical (bottom) cross section of the proposed FHR core; different FHR core components and the depletion zones are illustrated

Fig. 3

BPP kernel radius of 0.026 cm (left) and 0.015 cm (right)

Fig. 4

Variation of atomic density ratio between carbon and uranium as a function of PF in the fuel compact

Fig. 5

Evolution of Li6 atomic concentration along the FHR irradiation

Fig. 6

keff evolution of reference FHR core without BPPs implemented and the cases with different BPPs inventories; F/P represents the fuel-to-poison volumetric ratio

Fig. 7

Core-average neutron spectra of the cases without BPPs and with F/P=70

Fig. 8

keff evolution of the cases with different PFs; for all the presented cases, F/P=70

Fig. 9

keff evolution of the cases with different core heights; for all the presented cases, F/P=70, PF=0.35

Fig. 10

keff evolution of the cases with different BPP kernel radiuses (RBPP); for all the presented cases, F/P=70, PF=0.35, and core height=130  cm

Fig. 11

keff evolution of the cases with (solid lines) and without (dash lines) Li6 depletion taken into account

Fig. 12

Individual control rod reactivity worth with different rod dimensions

Fig. 13

Integral control rod reactivity worth with different maximum inserted positions (control rod radius of 2.6 cm is adopted for the calculations.)

Fig. 14

Distribution of power peaking factors in 1/12th FHR core for BOL state with 52% control rod insertion (left); the black shaded area in the schematic (right) illustrates the presented 1/12th core region

Fig. 15

FHR with NACC power system

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