GB2527140A

GB2527140A - Improved fuel salt chemistry and fission rate control in a molten salt nuclear reactor

Info

Publication number: GB2527140A
Application number: GB1410659.5A
Authority: GB
Inventors: Ian Richard Scott
Original assignee: Individual
Current assignee: Individual
Priority date: 2014-06-15
Filing date: 2014-06-15
Publication date: 2015-12-16
Also published as: GB201410659D0

Abstract

A fuel salt for a molten salt reactor which minimises volatile fission products and corrosivity of the fuel salt by adding a sacrificial metal such as chromium to the fuel salt or fuel tube and/or adding a silver salt, chromium salt or zirconium salt to the fuel to react with and convert any iodine or tellurium produced into a non volatile form. Also provided is a means of flattening the fission rate across the reactor core by use of a radial, central or axial neutron reflector that partially moderate the energy spectrum of the reflected neutrons and thereby increase the fission rate induced by those reflected neutrons. Furthermore, there is a means of maintaining a uniform fission rate across a reactor core by changing the geometry of the core so that the rate of movement by the fuel tubes changes as a function of the core in a way defined by the geometry.

Description

IMPROVED FUEL SALT CHEMISTRY AND FISSION RATE CONTROL IN A

MOLTEN SALT NUCLEAR REACTOR

BACKGROUND

A novel design for a molten salt based nuclear reactor was disclosed in UK patent application number 1402908.6 entitled "A practical molten salt fission reactor". The basis for the design was to place the molten salt flssilc material in static tubes from which heat was transferred to a coolant liquid by a combination of conduction and convection.

A consequence of this design is that control of the chemistry of the fuel salt held within the fuel tubes is preferably an intrinsic property of the fud salt and fuel tube and does not rely on outside intervention.

Two desirable properties of the fuel salt/fuel tube combination are that volatile species evolving from the fuel salt be limited as far as practical to the noble gasses and that the fuel salt remain non corrosive to the fuel tube as its composition evolves due to accumulation of hssion products and/or generation ol new elements in the 1ue salt by neutron induced transmutation. This application sets out the means to achieve these ends.

DESCRIPTION OF THE INVENTION

Fission of actinides in the form of halogen salts can lead to a net release of halogen as the valence of the fission products is not sufficient to capture all the release halogen as fission product halides. For example, fission of uranium chlorides leads to a net release of chlorine.

That released halogen is highly corrosive and will attack most metallic fuel tube materials.

The corrosive effect can be minimised by using trihalide as the major fuel salt, for example uranium trichloridc. Released halogen then reacts with the trihalidc producing a tetrahalidc which is much less corrosive than the free halogen. Yet further improvement can be made by making a sacrificial layer of a more reactive metal available to react with the actinide tetrahalide regenerating the trihalide. The sacrificial metal can be coated on the inside of the fuel tube either on the entire surface or over some fraction of that surface, possibly in a location in contact with the fuel salt and the vapour above the fuel salt hut not in the areas of maximum neutron flux. Alternatively the sacrificial metal can he added to the fuel tube as powder or granules. Suitable sacrificial mctals include zinc, nickel and chronuum with chromium being preferred.

Fission produces a wide range of fission products, some of which are volatile. It is advantageous that the gas released from the fuel sail he limited to the noble gasses and in particular that iodine and tellurium not he permitted to contaminate those noble gasses to a substantial degree. Iodine released within the fuel salt forms volatile compounds including zirconium iodides, actinide iodides and elemental iodine. Addition of a molar excess of silver chloride to the fuel salt ensures however that the concentrations of iodine and iodides other than silver iodide are very low. Iodine is thus substantially trapped within the fuel salt in a non volatile form.

Tellurium can also form volatile species in fuel salt, especially in combination with iodine.

While incorporation of silver chloride substantially blocks formation of tellurium iodides, yet further reduction in volatile tellurium compounds and elements can be achieved by incorporation ol zirconium or chromium salts in the luel salt. In practice, the zirconium sails are generated as fission products themselves and the chromium salts can be generated through the action of excess halogen on a sacrificial chromium layer in the fuel tube.

A further improvement to the novel molten salt reactor described in 1402908.6 relates to flattening the fission spectrum across the core. Addition of fresh fuel to the core can be achieved by removing spent fuel tubes from the centre of the core and replacing them with fresh fuel tubes on the periphery.

This means that there is a gradual decrease in fission reactivity of the fuel tubes as they migrate towards the centre due to a decline in fissile isotope concentration and increase in fission product concentration. Achieving a stalie flat fission rate across the core requires balancing these factors. This is made very much simpler by adding a neutron reflector around the core which partially moderates the reflected neutrons by. for example, including carbon or beryllium in the material of the reflector. This has the effect of allowing the fission rate at the periphery of the core to be increased in a very controlled manor without increasing the fissile isotope concentrations to impracticaly high levels. Further improvement in uniformity of fission rate can he achieved by putting a similar shghtly moderating reflector at the centre of the core to compensate for the lower reactivity of the tubes as they approach the centre of the core with their fissile concentration reduced and their fission product concentration increased. Finally, the rate at which tubes migrate towards the centre of the core can be varied by changing the shape of the core. At one extreme, a cylindrical core results in fuel tubes spending most of theft life at the periphery and moving towards the centre of the core increasingly rapidly as they approach that centre. At another extreme, a rectangular core would allow fuel tubes to move towards the midline of the rectangle at a completely uniform rate. This latter arrangement is particularly attractive as it allows a very simple mechanism to move the fuel tubes in just one direction to be used. There are other shapes possible that allow a wide range of speeds of fuel tube migrations to be achieved.

Improved uniformity of fission rate within a single fuel tube along its major axis can also be achieved by positioning moderating neutron reflectors above and below the fuel salt so that the inevitably lower neutron flux towards the upper and lower bounds of the core still creates a similar fission rate due to the higher fission cross section of neutrons of lower energy.

The use of a moderating reflector around the reactor core allows the core to he rendered non critical by physical movement of all or part of that reflector away from the core. In this way the reflector can replace or augment the use of conventional neutron absorbing control rods to control the reactivity of the reactor core.

Claims

CLAIMS1) An improved fuel salt for a molten salt reactor constituted to minimise volatile fission products and corrosivity of the fuel salt by adding a sacrificial metal such as chromium to the fuel salt or fuel tube and/or adding a silver salt or chromium salt or zirconium salt to the fuel to react with an convert to non volatile form any iodine or tellurium produced as fission product.
2) Flattening the fission rate across the reactor core by use of radial, central or axial neutron reflectors that partially moderate the energy spectrum of the reflected neutrons and thereby increase the fission rate induced by those reflected neutrons.
3) Maintaining a uniform fission rate across a reactor core by changing the geometry of the core so that the rate of movement of the fuel tubes changes as a function of distance from the centre of the core in a way defined by the geometry,