A feasibility study of a liquid shutdown system for a single-fluid molten salt fast reactor

https://doi.org/10.1016/j.nucengdes.2021.111053Get rights and content

Highlights

Abstract

The present study exposes a feasibility analysis for a nuclear reactor shutdown system by means of a liquid neutron absorber. The presented system has been designed for single-fluid Molten Salt Fast Reactors (MSFR) but can be easily extended to other Molten Salt Reactor (MSR) designs and even to liquid metals (LM) reactors.

Most of the safety shutdown concepts for MSRs designs rely on their intrinsic negative temperature coefficient to make the reactor subcritical. In case of emergency, most MSRs concepts make use of the widely accepted drainage system by means of frozen plugs or other passively activated mechanisms that drive the fuel to an also passively cooled set of containers below the reactor. In this study, an alternative shutdown system is described, consisting of filling a set of pipes inside the reactor with a liquid neutron absorbing material. The system may be used as the main shutdown system, but in the present work it is studied as a complementary system to a central shutdown/control rod.

A simplified single fluid external indirect cooling MSFR model with axial and central reflectors has been used to test the system. The only purpose of this model is to check the feasibility of the system and not to show an actual working system in a reactor design. Therefore, a pre-conceptual design is presented. Several representative analyses including neutronics, hydraulics and a mechanical assessment, have been carried out to evaluate the feasibility of the concept.

This feasibility study aims to be a decided step towards a safe passive shutdown system for Generation IV (GenIV) reactors with special focus on MSRs.

Introduction

The next Generation IV (GenIV) reactors are meant to use more disruptive technologies and allow for new shutdown strategies complementary to control rods or drums. It must be highlighted that different passive shutdown systems are being or have been developed for fast advanced reactors.

The present study exposes a feasibility study for a nuclear reactor shutdown system by means of the rapid insertion of a liquid neutron absorber. The presented system has been designed for single-fluid Molten Salt Fast Reactors (MSFR) but can be easily extended to other Molten Salt Reactor (MSR) designs and even to liquid metals (LM) reactors.

MSRs are one of the proposed designs for the GenIV. It was first developed during the 60s at Oak Ridge National Laboratory (ORNL) and tested in the Molten-Salt Reactor Experiment (MSRE) (Haubenreich and Engel, 1970). This reactor used a set of draining tanks as a complementary shutdown system. This set of passively cooled tanks were connected to the core of the reactor through a frozen plug of salt actively cooled during normal operation. In case of emergency, the solid plug melted and the molten salt fuel was drained from the core into the drain tanks by gravity, adopting a sub-critical configuration. This system was tested and proved to be reliable for the MSRE.

There exist many different passive shutdown systems for fast reactors as exposed in Passive Shutdown (2020), among others, and references therein:

  • Lithium expansion modules

  • Lithium injection modules

  • Curie point latches

  • Thermostatic switches

  • Lyophobic capillary porous systems

  • Flow levitated absorbers

  • Cartesian divers

  • Levitated absorber particles

  • Enhanced thermal elongation of control rod drive lines

  • Gas expansion modules

  • Autonomous reactivity controls

  • Travelling wave reactor thermostats

  • Thermo-siphon based passive shutdown systems

  • Static absorber feedback equipment

  • Flow actuated passive shutdown systems

  • Temperature actuated passive shutdown systems

The proposed shutdown system, presented as a pre-conceptual design, uses a liquid neutron absorber reservoir which is discharged by the force of gravity into a network of pipes or ducts inside the core of the reactor. This system, hereinafter the Liquid Shutdown System (LSS), is designed to be activated passively (see definition in Safety Related Terms (1988) Annex A Categories A to C) in case of emergency and to be reversible. Thus, the reactor operation can be quickly restored to the initial state without damaging any system or altering the fuel composition. This reversibility is a key and disruptive feature of the system. Note that the system is not totally passive like those based in natural circulation. However, it does not use any active component to be actuated in case of emergency and relies on gravity.

The main advantages of the proposed system against control rods are as follows:

  • Safety.

  • – Avoidance of buckling if slender structures are inserted.

  • – Better distribution of the neutronic absorber.

  • – No possible jamming of the neutronic absorber.

  • – No neutronic absorber swelling.

  • Operational cost (maintenance).

  • – Simple drive system with no moving components.

  • – Modular (e.g., a set of independent network pipes).

  • – Possibility to be removed for maintenance.

A simplified single fluid external indirect cooling MSFR model with axial and central reflectors was used to test the model. It must be noted that the only purpose of this model is to check the feasibility of the system and not to show an actual working system in an actual reactor design. Several representative analyses including neutronics, hydraulics and a mechanical assessment, were carried out to evaluate the pre-conceptual design.

This feasibility study aims to be a decided step towards a safe shutdown system in case of emergency for next generation reactors with special focus on MSRs.

Section snippets

LSS system overview

The proposed system is designed to shut down the reactor by injecting a neutronic absorber into a network of pipes or ducts placed within the reactor core. It must be noted that the LSS can be designed to be the primary shutdown system, but in the present work it is exposed as complementary to a central control rod, i.e. as an additional layer of safety. This redundancy is not limited to control rods, but could be also applied for other shutdown systems such as the frozen plug.

Shutdown system

Methodology

A methodology which consists in a multi-disciplinary analysis is proposed.

A very simplified model of a single-fluid indirect externally cooled MSFR is designed including the LSS, a radial reflector, a central reflector and a central shutdown rod.

A criticality search is used to size the reactor (including the LSS components) by means of the probabilistic code OpenMC (Romano et al., 2013). Different materials candidates are also tested to assess their performance.

A hydraulic analysis is carried

Neutronic analysis

A simplified reactor parametric model was built to size the reactor design. The proposed model can be seen in Fig. 9. It is important to remark that the neutronic model is independent of the type of bundles used in the LSS, only the size and position of the straight pipes of the bundles are important for the present study. The manifolds of the LSS have not been modelled.

In addition to the selected materials relevant for the LSS, a radial and central reflectors made of tungsten carbide (WC) are

Hydraulic analysis

The hydraulic design of the LSS must guarantee that the filling of the internal pipes with the neutronic absorber is achieved in a short time, fulfilling the design criterion of safely shutting down the reactor. The LSS shall also act passively in a power loss event, so the gravity driving force of the absorber must be enough to compensate for the pressure drop in the piping.

Analytical models were developed to generate a simplified hydraulic model of the LSS and calculate the filling times. The

Reactor kinetics

The calculated discharge times together with the temperature coefficients were used to assess the reactor kinetics by means of PYRK (Huff, 2015), an open source point kinetics code. A simple model for a fast reactor spectrum with 6 neutron precursor groups was set up and run for a shutdown scenario.

A summary of the simulation parameters are shown in Table 5. Note that geometric parameters are the same as those in Section 4.

A step reactivity insertion corresponding to the central rod, followed

Mechanical assessment

A mechanical evaluation of the proposed concept was carried out to detect possible safety problems and to propose solutions or mitigation actions.

Conclusions

The present work exposed a feasibility study of the LSS, a reactor shutdown system based on a liquid absorber for MSFR that can also be applied to other advanced reactor concepts.

The LSS has been analysed for several liquid metals as absorbers. The neutronic analyses showed that it was capable of introducing a negative reactivity in the reactor with the necessary worth (17,500 pcm < worth < 5000 pcm depending on the absorber) to shut down the reactor. The hydraulic analysis, by means of an

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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