Molten salt reactor with molten moderator salt and redox-element

Abstract

Device for producing energy by nuclear fission, and methods of using same. The device comprises a core container of a core container material, which core container encloses an inner tubing of an inner tubing material. The inner tubing and/or the core container has(have) an inlet and an outlet. The device also comprises a molten fuel salt with a fissionable material and a molten moderator salt comprising metal hydroxide(s), metal deuteroxide(s) or a combination thereof and a redox-element having a reduction potential, which is larger than that of the inner tubing material or of the inner tubing material and the core container material. The molten moderator salt is located in the core container, and the molten fuel salt is located in the inner tubing. Alternatively, the molten fuel salt is located in the core container, and the molten moderator salt is located in the inner tubing.

Claims

1. A device adapted for producing energy by nuclear fission, the device comprising: a core container of a core container material, which core container encloses an inner tubing of an inner tubing material, the inner tubing and/or the core container having an inlet and an outlet; a molten fuel salt with a fissionable material; a molten moderator salt; and a redox-element; wherein the molten moderator salt is located in the core container and the molten fuel salt is located in the inner tubing, or wherein the molten fuel salt is located in the core container and the molten moderator salt is located in the inner tubing; wherein the molten moderator salt comprises at least one metal hydroxide, at least one metal deuteroxide or a combination thereof, and water up to 10% (w/w); and wherein the redox-element has a reduction potential, which is larger than that of the inner tubing material or of the inner tubing material and the core container material, and/or wherein the redox-element is a chemical species which controls the oxoacidity of the molten moderator salt and/or the molten fuel salt.

2. The device according to claim 1, wherein the redox-element is a sacrificial material located on a surface of the inner tubing material or on surfaces of the inner tubing material and the core container material.

3. The device according to claim 1, wherein the at least one metal hydroxide and/or the at least one metal deuteroxide comprises a metal chosen from the group of metals comprising alkali metals, alkaline earth metals, or combinations of alkali metals and alkaline earth metals.

4. The device according to claim 1, wherein the concentration of the redox-element is in the range of 1 g/kg to 100 g/kg of the molten moderator salt.

5. The device according to claim 1, wherein the redox-element has a melting point, which is higher than the melting point of the molten salt, and wherein the redox-element is present as a suspension of particles having a size in the range of 0.1 mm to 11 mm.

6. A method of controlling a nuclear fission process, the method comprising the steps of: providing a device adapted for producing energy by nuclear fission, the device comprising a core container of a core container material, which core container encloses an inner tubing of an inner tubing material, the inner tubing and/or the core container having an inlet and an outlet; introducing into the inner tubing a molten moderator salt comprising at least one metal hydroxide, at least one metal deuteroxide or a combination thereof, and comprising water up to 10% (w/w), and a redox-element having a reduction potential, which is larger than that of the inner tubing material, or a redox-element being a chemical species which controls the oxoacidity of the molten moderator salt; introducing a molten fuel salt comprising fluorides of an alkali metal, and a fissile element into the core container; providing a heat exchanger in fluid communication with the inlet and the outlet so as to define a heat exchange loop for removing heat from the molten moderator salt circulating in the heat exchange loop; and circulating the molten moderator salt in the heat exchange loop so as to control the temperature of the fuel salt in the core container.

7. The method according to claim 6, wherein the temperature at the inlet is in the range of 400° C. to 800° C., and wherein the temperature at the outlet is in the range of 600° C. to 1000° C.

8. The method according to claim 6, wherein the fuel salt is a eutectic salt.

9. The method according to claim 6, wherein the fuel salt comprises thorium.

10. The method according to claim 6, wherein the concentration of the redox-element is maintained by supplementing the moderator salt with the redox-element.

11. The method according to claim 6, wherein the at least one metal hydroxide and/or the at least one metal deuteroxide comprises a metal chosen from the group of metals comprising alkali metals, alkaline earth metals, or combinations of alkali metals and alkaline earth metals.

12. The method according to claim 6, wherein the concentration of the redox-element is in the range of 1 g/kg to 100 g/kg of the molten moderator salt.

13. The method according to claim 6, wherein the redox-element has a melting point, which is higher than the melting point of the molten salt, and wherein the redox-element is present as a suspension of particles having a size in the range of 0.1 mm to 11 mm.

14. The method according to claim 6, wherein the molten moderator salt comprises up to 10% (w/w) water.

15. The method according to claim 6, wherein the chemical species which can control the oxoacidity of the molten moderator salt is a gas selected from H.sub.2O, H.sub.2, and HF, and their mixtures.

16. A method of controlling a nuclear fission process, the method comprising the steps of: providing a device adapted for producing energy by nuclear fission, the device comprising a core container of a core container material, which core container encloses an inner tubing of an inner tubing material, the inner tubing having an inlet and an outlet; introducing a molten fuel salt into the inner tubing, which molten fuel salt comprises fluorides of an alkali metal and a fissile element; introducing into the core container a molten moderator salt comprising at least one metal hydroxide, at least one metal deuteroxide or a combination thereof, and a redox-element having a reduction potential, which is larger than that of the inner tubing material or of the inner tubing material and the core container material, and/or a redox-element being a chemical species which controls the oxoacidity of the molten moderator salt; providing a heat exchanger in fluid communication with the inlet and the outlet of the inner tubing so as to define a heat exchange loop for removing heat from the molten fuel salt circulating in the heat exchange loop; and circulating the molten fuel salt in the heat exchange loop so as to control the temperature of the fuel salt in the inner tubing.

17. The method according to claim 16, wherein the temperature at the inlet is in the range of 400° C. to 800° C., and wherein the temperature at the outlet is in the range of 600° C. to 1000° C.

18. The method according to claim 16, wherein the fuel salt is a eutectic salt.

19. The method according to claim 16, wherein the fuel salt comprises thorium.

20. The method according to claim 16, wherein the concentration of the redox-element is maintained by supplementing the moderator salt with the redox-element.

21. The method according to claim 16, wherein the at least one metal hydroxide and/or the at least one metal deuteroxide comprises a metal chosen from the group of metals comprising alkali metals, alkaline earth metals, or combinations of alkali metals and alkaline earth metals.

22. The method according to claim 16, wherein the concentration of the redox-element is in the range of 1 g/kg to 100 g/kg of the molten moderator salt.

23. The method according to claim 16, wherein the redox-element has a melting point, which is higher than the melting point of the molten salt, and wherein the redox-element is present as a suspension of particles having a size in the range of 0.1 mm to 11 mm.

24. The method according to claim 16, wherein the molten moderator salt comprises up to 10% (w/w) water.

25. The method according to claim 16, wherein the chemical species which can control the oxoacidity of the molten moderator salt is a gas selected from H.sub.2O, H.sub.2, and HF, and their mixtures.

Description

SHORT DESCRIPTION OF THE DRAWINGS

(1) This and other aspects of the present invention will now be described in more detail, with reference to the appended drawings showing embodiment(s) of the invention.

(2) FIG. 1 shows a side view of a device of the invention;

(3) FIG. 2 shows a top view of a device of the invention;

(4) FIG. 3 shows a top view of detail of a device of the invention;

(5) FIG. 4 shows a top view of detail of a prior art molten salt reactor;

(6) FIG. 5 shows contour plots of the reactor multiplication factor and the thermal reactivity coefficients of the fuel and NaOH moderator;

(7) FIG. 6 shows the effect of Na in a NaOH moderator;

(8) FIG. 7 shows a potential-oxoacidity diagram for nickel in NaOH—KOH;

(9) FIG. 8 shows a top view of a device with molten fuel salt located in a core container and molten moderator salt located in inner tubing.

(10) As illustrated in the figures, the sizes of layers and regions are exaggerated for illustrative purposes and, thus, are provided to illustrate the general structures of embodiments of the present invention. Like reference numerals, refer to like elements throughout.

DETAILED DESCRIPTION OF THE INVENTION

(11) The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which currently preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided for thoroughness and completeness, and fully convey the scope of the invention to the skilled person.

(12) Preferred Moderator Materials

(13) As mentioned above, the present invention suggests hydroxides and/or deuteroxides as moderator materials. Metal hydroxides are preferred. The at least one metal hydroxide or deuteroxide may for instance comprise a metal chosen from the group of metals comprising alkali metals, lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium (Cs), carbon (C), silicon (Si) and fluorine (F). Further preferred are fused metal hydroxides of the form X(OH).sub.n, and fused metal deuteroxide of the form X(OD).sub.n. Fused metal hydroxides are compounds generally written as XOH or X(OH)n where X is an alkali or other metal and OH is the hydroxide ion. The integer n equals 1 for monovalent atoms and is an integer>1 for higher valence atoms. The effective moderating effect of fused metal hydroxide lies in the relative high presence of hydrogen in the compound. Fused metal hydroxides have a wide temperature operating window (from melting point to boiling point typically ranging from 300° C. to 1300° C.). The liquid molten salts are pumpable at near atmospheric pressure and therefore do not require a pressurized containment. The fused metal hydroxide moderator may consist of a single chemical compound, such as NaOH, or a mixture of 2 or more metal hydroxides, mixed with other fluids, or embedded into solid materials. Particularly useful metal hydroxides are LiOH, .sup.7LiOH, NaOH and rubidium hydroxide (RbOH). Likewise, particularly useful metal deuteroxides are LiOD, .sup.7LiOD, NaOD and RbOD.

(14) Metal hydroxides such as potassium hydroxide (KOH) and caesium hydroxide (CsOH) as well as metal deuteroxides such as KOD and CsOD are, due to their very high neutron absorption, useful as additively used hydroxides or deuteroxides for adjusting the neutron absorption of the moderator materials in embodiments where the fused metal hydroxide moderator comprises a mixture of two or more metal hydroxides and/or metal deuteroxides.

(15) Rubidium (Rb) and sodium (Na) are both excellent in their natural form. lithium (Li) enriched to 99.95% or more in .sup.7Li has comparable neutronics to Na (higher-enriched Li surpasses Na), while potassium (K) and caesium (Cs) performs worse in terms of neutronics, but is of interest because it can be added to other alkali hydroxides to alter certain physical and chemical properties of the mixture, such as the melting point. Of these, NaOH has the advantage of being very well known as an industrial chemical.

(16) Table 2 below summarises moderating properties of various moderator materials suggested in accordance with the present invention.

(17) TABLE-US-00002 TABLE 2 Moderating effect of various hydroxides. N MFP.sub.ela Σ.sub.abs Material [#] [cm] [1/m] Comments .sup.7LiOH 38.0 1.08 1.385 Compact, low absorption NaOH 43.6 1.13 2.767 Compact KOH 44.9 1.66 5.546 RbOH 45.9 1.76 1.340 Compact fairly low absorption CsOH 46.3 2.38 43.337

(18) The information in Table 2 above leads to the following conclusions. NaOH is a compact moderator and the absorption is comparable to H.sub.2O and polyethylene. RbOH is a compact moderator and has a fairly low absorption. KOH and CsOH are both less suitable as a moderator owing to high absorption.

(19) NaOH, or sodium hydroxide, commonly known as lye or caustic soda, is a well-known industrial product used in soaps, food production, as drain cleaner, in aluminium production and much more. At room temperature and atmospheric pressure, NaOH is solid but melts at a temperature of 318° C. and boils at 1388° C. This makes it a very flexible neutron moderator, as it can be used in either solid or liquid state. Moreover, passive safety features can be designed in which active cooling of solid NaOH is required to keep the moderator in place in the reactor core. In the event of overheating (from power excursion or loss of active cooling), the NaOH would melt and drain out of the core, effectively extinguishing the fission chain reaction.

(20) Even if the use of NaOH as a neutron moderator has been rejected in the past, based on the corrosive properties of liquid NaOH as described above, the advantages related to hydroxides in general as moderator materials listed above are especially profound in relation to NaOH, and the present invention, and in particular the suggested measures for corrosion control and choice of materials, make these concerns obsolete.

(21) Preferred Redox-Elements

(22) Exemplary standard electrode potentials are provided in Table 3, where the standard electrode potentials are at a temperature of 298.15° K, an effective concentration of 1 mol/l for each aqueous species or a species in a mercury amalgam, a partial pressure of 101.325 kPa (absolute) (1 atm, 1.01325 bar) for a gaseous reagent, and an activity of unity for each pure solid, pure liquid, or for water (solvent). It is to be understood that a lower negative value for the standard electrode potential corresponds to a more reactive material in the context of the invention. Thus for example, the inner tubing material may be nickel and any element in the Reductant column above nickel can be selected as a redox-element.

(23) TABLE-US-00003 TABLE 3 Standard electrode potentials Oxidant Reductant Value (V) Sr.sup.+ + e− Sr −4.101 Ca.sup.+ + e− Ca −3.8 Li.sup.+ + e− Li −3.0401 Cs.sup.+ + e− Cs −3.026 Rb.sup.+ + e− Rb −2.98 K.sup.+ + e− K −2.931 Ba.sup.2+ + 2e− Ba −2.912 Sr.sup.2+ + 2e− Sr −2.899 Ca.sup.2+ + 2e− Ca −2.868 Eu.sup.2+ + 2e− Eu −2.812 Ra.sup.2+ + 2e− Ra −2.8 Yb.sup.2+ + 2e− Yb −2.76 Na.sup.+ + e− Na −2.71 Mg.sup.+ + e− Mg −2.7 Sm.sup.2+ + 2e− Sm −2.68 No.sup.2+ + 2e− No −2.5 Tm.sup.2+ + 2e− Tm −2.4 Md.sup.2+ + 2e− Md −2.4 La.sup.3+ + 3e− La −2.379 Mg.sup.2+ + 2e− Mg −2.372 Y.sup.3+ + 3e− Y −2.372 Pr.sup.3+ + 3e− Pr −2.353 Ce.sup.3+ + 3e− Ce −2.336 Er.sup.3+ + 3e− Er −2.331 Ho.sup.3+ + 3e− Ho −2.33 Nd.sup.3+ + 3e− Nd −2.323 Tm.sup.3+ + 3e− Tm −2.319 Sm.sup.3+ + 3e− Sm −2.304 Fm.sup.2+ + 2e− Fm −2.3 Dy.sup.3+ + 3e− Dy −2.295 Lu.sup.3+ + 3e− Lu −2.28 Tb.sup.3+ + 3e− Tb −2.28 Gd.sup.3+ + 3e− Gd −2.279 Es.sup.2+ + 2e− Es −2.23 Dy.sup.2+ + 2e− Dy −2.2 Pm.sup.2+ + 2e− Pm −2.2 Ac.sup.3+ + 3e− Ac −2.2 Yb.sup.3+ + 3e− Yb −2.19 Cf.sup.2+ + 2e− Cf −2.12 Ho.sup.2+ + 2e− Ho −2.1 Nd.sup.2+ + 2e− Nd −2.1 Sc.sup.3+ + 3e− Sc −2.077 Am.sup.3+ + 3e− Am −2.048 Cm.sup.3+ + 3e− Cm −2.04 Pu.sup.3+ + 3e− Pu −2.031 Er.sup.2+ + 2e− Er −2 Pr.sup.2+ + 2e− Pr −2 Eu.sup.3+ + 3e− Eu −1.991 Lr.sup.3+ + 3e− Lr −1.96 Cf.sup.3+ + 3e− Cf −1.94 Es.sup.3+ + 3e− Es −1.91 Am.sup.2+ + 2e− Am −1.9 Th.sup.4+ + 4e− Th −1.899 Fm.sup.3+ + 3e− Fm −1.89 Np.sup.3+ + 3e− Np −1.856 Be.sup.2+ + 2e− Be −1.847 U.sup.3+ + 3e− U −1.798 Al.sup.3+ + 3e− Al −1.662 Ti.sup.2+ + 2e− Ti −1.63 Zr.sup.4+ + 4e− Zr −1.45 Ti.sup.3+ + 3e− Ti −1.37 Mn.sup.2+ + 2e− Mn −1.185 V.sup.2+ + 2e− V −1.13 Nb.sup.3+ + 3e− Nb −1.099 Zn.sup.2+ + 2e− Zn −0.7618 Cr.sup.3+ + 3e− Cr −0.74 Ta.sup.3+ + 3e− Ta −0.6 Ga.sup.3+ + 3e− Ga −0.53 Fe.sup.2+ + 2e− Fe −0.44 Cd.sup.2+ + 2e− Cd −0.4 In.sup.3+ + 3e− In −0.34 Tl.sup.+ + e− Tl −0.34 Co.sup.2+ + 2e− Co −0.28 Ni.sup.2+ + 2e− Ni −0.25
In a preferred embodiment the inner tubing material comprises a metal, and the redox-element is a metal having an electronegativity according to the Pauling scale, which is lower than the electronegativity of the metal of the inner tubing material. Pauling electronegativities of a range of metallic elements is provided in Table 4. For example, the metal of the inner tubing and optionally also of the core container may be a Hastelloy, i.e. a nickel-based alloy, and the redox-element may be an alkali metal or an alkaline earth metal.

(24) TABLE-US-00004 TABLE 4 Pauling electronegativities of selected elements Tin (Sn) 1.96 Silver (Ag) 1.93 Nickel (Ni) 1.91 Silicon (Si) 1.9 Copper (Cu) 1.9 Technetium (Tc) 1.9 Rhenium (Re) 1.9 Cobalt (Co) 1.88 Iron (Fe) 1.83 Gallium (Ga) 1.81 Indium (In) 1.78 Cadmium (Cd) 1.69 Chromium (Cr) 1.66 Zinc (Zn) 1.65 Vanadium (V) 1.63 Thallium (Tl) 1.62 Aluminium (Al) 1.61 Niobium (Nb) 1.6 Beryllium (Be) 1.57 Manganese (Mn) 1.55 Titanium (Ti) 1.54 Tantalum (Ta) 1.5 Protactinium (Pa) 1.5 Uranium (U) 1.38 Scandium (Sc) 1.36 Neptunium (Np) 1.36 Zirconium (Zr) 1.33 Magnesium (Mg) 1.31 Hafnium (Hf) 1.3 Thorium (Th) 1.3 Americium (Am) 1.3 Curium (Cm) 1.3 Berkelium (Bk) 1.3 Californium (Cf) 1.3 Einsteinium (Es) 1.3 Fermium (Fm) 1.3 Mendelevium (Md) 1.3 Nobelium (No) 1.3 Plutonium (Pu) 1.28 Lutetium (Lu) 1.27 Thulium (Tm) 1.25 Erbium (Er) 1.24 Holmium (Ho) 1.23 Yttrium (Y) 1.22 Dysprosium (Dy) 1.22 Gadolinium (Gd) 1.2 Samarium (Sm) 1.17 Neodymium (Nd) 1.14 Praseodymium (Pr) 1.13 Cerium (Ce) 1.12 Lanthanum (La) 1.1 Actinium (Ac) 1.1 Calcium (Ca) 1 Lithium (Li) 0.98 Strontium (Sr) 0.95 Sodium (Na) 0.93 Radium (Ra) 0.9 Barium (Ba) 0.89 Potassium (K) 0.82 Rubidium (Rb) 0.82 Caesium (Cs) 0.79

(25) Thus, based on Table 3 and Table 4, and in light of Reaction (A) to Reaction (D), preferred materials for the inner tubing and/or the core container comprise nickel, copper and cobalt, and preferred metals for the redox-element comprise alkali metals, alkaline earth metals, transition metals, lanthanides and/or actinides.

(26) Fuel Salt Composition

(27) The fuel salt (abbreviated FS) in general consists of a non-actinide carrier part (chosen for its thermodynamic properties), and an actinide component ensuring reactor criticality. The actinide component An.sub.i may further be split up in a fuel component and an added fertile component. The fuel salt vector F.sub.i is described by a pre-defined fuel vector which contains an initial plutonium component (typically Spend Nuclear Fuel (SNF) i.e. nuclear waste) along with additional components (some added after chemical reprocessing). The added (fertile) part is defined by the vector A.sub.i which is chosen from its role in the reactor burnup process and will typically consist of added thorium and uranium. The actinide composition is defined by the various fuel vectors and is captured by the following values of merit: F.sub.Pu the fuel plutonium (cation mole) fraction; A.sub.Th the fuel thorium (cation mole) fraction of the added fertile vector; F.sub.A the added (fertile) (cation mole) fraction.

(28) Here the two first fractions refer to the cation mole fractions of the fuel vector and the added fertile vector, respectively. The fuel salt is defined by the various fuel vectors, a carrier salt vector CS.sub.i, along with the following values of merit for the fuel salt: FS.sub.Pu the fuel salt plutonium (cation mole) fraction; FS.sub.Th the fuel salt thorium (cation mole) fraction; FS.sub.CS the carrier salt (cation mole) fraction.

(29) Here “fraction” refers to the cation mole fraction of the combined fuel salt. With these definitions, the fuel salt vector can be written: (FS).sub.i=FS.sub.CS CS.sub.i+(1−FS.sub.CS) An.sub.i. The actinide vector is split up according to: An.sub.i=(1−F.sub.A)F.sub.i+F.sub.A.Math.A.sub.i. Here F.sub.Pu of F.sub.i consists of plutonium isotopes and A.sub.Th of A.sub.i consists of thorium. We note that the following relations exist between the salt parameters:
FS.sub.Pu=(1−FS.sub.CS)(1−F.sub.A)F.sub.Pu; FS.sub.Th=(1−FS.sub.CS)F.sub.A.Math.A.sub.Th
An exemplary fuel salt contains the following fuel salt vectors: CS.sub.i=NaF; A.sub.i=ThF.sub.4. This fuel is summarised in Table 5.

(30) TABLE-US-00005 TABLE 5 A preferred fuel salt composition Fraction cmol % Motivation FS.sub.CS 78 Eutectic point F.sub.Pu 80 Chemical reprocessing f.sup.238U 97.5 Chemical reprocessing A.sub.Th 100 Waste burning F.sub.A ≈90 Optimization study f.sup.238Pu 0.5 Industry waste standard f.sup.239Pu 69 Industry waste standard f.sup.240Pu 25 Industry waste standard f.sup.241Pu 2 Industry waste standard f.sup.242Pu 1 Industry waste standard f.sup.241Am 2.5 Industry waste standard S.sub.Pu ≈2 — S.sub.Th ≈20 —

(31) Preferred Device of the Invention

(32) A preferred device 100 of the invention is illustrated in FIG. 1, where it is depicted from the side. Specifically, FIG. 1 shows the device 100, which has a core container 20 with a molten moderator salt 2, which core container 20 encloses an inner tubing with a molten fuel salt 1. The inner tubing has two inlets 6 in fluid communication with an inlet manifold 61, which in turn is in fluid communication with the fuel pins 10. The fuel pins 10 communicate with an outlet manifold 62, which collects the flow, in this case of molten fuel salt 1 in a single outlet 7. The direction of the flow is indicated with the symbol “>”. The inlets 6 and the outlet 7 are in fluid communication with an inlet and an outlet of a heat exchanger (not shown) to provide a heat exchange loop. The inner tubing material and the core container material are preferably made from a nickel based alloy. The device 100 may further comprise an additional safety feature 8 comprising an overflow system in addition to the commonly used salt plug system of the prior art. This safety system prevents meltdowns, hinders accidents from human operator error, automatically shuts down in case of out of scope operation conditions, and may flush the fuel inventory to a passively cooled and sub-critical dump tank below the core vessel in case of a loss of operation power.

(33) The reactor size is determined from two conditions; circulation time and negative temperature feedback for both fuel and moderator. In practice the operating power density can be adjusted through physical feedback mechanisms in the reactor core. In particular, the negative temperature feedback of both the fuel salt and the moderator means that the power density can be controlled by adjusting the external energy in-flow. Since core circulation may carry delayed neutrons away from the chain reaction, the mass flow rate through the reactor core should be held constant for optimal reactor control and safety reasons. Rather than changing the internal core flow, it is more desirable to control the power production by varying the mass flow through the external heat exchanger system. In order to attain maximal reactor control, the mass flow rate through the reactor should be chosen so that the change in the reactor reactivity as compared to no circulation is as small as practically possible. In this way, in case of pump failure scenario, the concentration of decaying precursors in the reactor core will only be minimally larger than at normal operation.

(34) FIG. 2 shows a top view of a section of the device 100 shown in FIG. 1. Thus, the fuel pins 10 are distributed in a hexagonal pattern in the core container, which has a cylindrical cross-section with an external cladding 5. The external cladding may also be referred to as a blanket or shielding. A hexagonal pattern is superimposed on the cross-section of the device 100, but this pattern is not intended to represent any specific material.

(35) FIG. 3 and FIG. 4 illustrate and compare the packing of the fuel pins 10 of a preferred device of the invention (FIG. 3) and a prior art MSR (FIG. 4) where graphite 3 is used as a moderator. The superimposed hexagonal patterns show how the metal hydroxide/deuteroxide moderator allows a much denser packing of the fuel pins 10 than available in the graphite moderated MSR thus providing a much smaller form factor F.

(36) FIG. 5 shows contour plots of the reactor multiplication factor and thermal reactivity coefficients of the fuel salt and the NaOH moderator, respectively. The multiplication factor needs to be above a certain threshold to ensure that the reactor can reach criticality. Moreover, the reactivity coefficients should be lightly negative for optimal reactor control and (inherent) safety reasons. The zone of configuration space 9 compatible with all three conditions allows determination of the ranges on the reactor dimensions. Specifically, the fuel content S.sub.Pu is 2 cmol % and the radius r.sub.pin of the fuel pins may be in the range of 1 cm to 5 cm, with the parameter l in the range of 0.5 cm to 1.5 cm.

(37) FIG. 6 shows the dependence of the amount of Na as redox-element dissolved within NaOH as moderator salt on the neutron multiplication factor (shown on the Y-axis); the error bars are one standard deviation. Specifically, FIG. 6 illustrates the detrimental effect on the fission chain reaction from displacing hydrogen atoms in NaOH with atoms of sodium, and thus effectively diluting the moderator salt and decreasing its moderating power. Evidently an upper limit of redox-element in the moderator salt exists from the point of view of neutronics; in practice the amount of redox-element should not be higher than 100 g/kg. The lower the amount of the redox-element the better the moderating effect, but in order to provide the protection against corrosion the moderator should contain at least 1 g/kg of the redox-element.

Prior Art Examples

(38) Since the moderating power of carbon is less than that of sodium hydroxide, graphite moderated reactors in general display a larger form-factor than sodium hydroxide moderated ones. For reference, we now provide a couple of examples of a simulated graphite moderated reactor (FIG. 4) with the same geometry and the fuel salt composition of Table 5.

(39) A MSR with Small Pins

(40) Fuel pin radius r.sub.pin=2 cm. Cladding thickness δ lies in the range of 0.05 to 0.5 cm and parameter l (half the distance between neighbouring pins) is in the range l=3.0 to 6.0 cm. Within these ranges the form-factor lies in the approximate range F=5 to 15 and the core volume lies in the range V=8 m.sup.3 to 45 m.sup.3. The core radius and height are in the range H=2.0 m to 3.6 m, R=1.1 m.sup.3 to 2.0 m.sup.3 while the total number of fuel pins is in the range of 600 to 800.

(41) A MSR with Large Pins

(42) Fuel pin radius r.sub.pin=6 cm. Cladding thickness δ lies in the range of 0.05 to 0.5 cm and the parameter/(half the distance between neighbouring pins) is in the range/=10.0 to 14.0 cm. Within these ranges the form-factor lies in the approximate range F=6 to 10 and the core volume lies in the range V=8 m.sup.3 to 32 m.sup.3. The core radius and height are in the range H=2.0 m to 3.2 m, R=1.1 m.sup.3 to 2.0 m.sup.3 while the total number of fuel pins is in the range of 50 to 120.

(43) Exemplary Devices of the Invention

(44) We refer to FIG. 5 which shows the optimum configuration zone (defined by reactor criticality and negative fuel and moderator temperature feedback) for a NaOH moderated reactor with the geometry depicted in FIG. 1 and FIG. 2 and at SPu=2 cmol %. For this plutonium fraction the allowed pin radius lies in the approximate range rpin=1 to 5 cm. We note that this range widens as the plutonium fraction grows. We proceed to give subranges for the dimensions of the reactor lattice element. The overall dimensions of the reactor core depend on the power density, form-factor and total power output of the reactor. In order to give ranges on the core dimensions it is therefore needed to assign ranges to these three parameters. We take the total power output to be 300 MW. Reasonable ranges on the power density in the fuel salt is P=100 to 200 kW/l while ranges on the form-factor are given above.

(45) Small Pins

(46) Fuel pin radius r.sub.pin=1 cm. Cladding thickness δ lies in the range of 0.05 cm to 0.5 cm and the parameter/(half the distance between neighbouring pins) is in the range/=0.5 to 1.5 cm. Within these ranges the form-factor lies in the approximate range F=2.5 to 10 and the core volume lies in the range V=4 m.sup.3 to 30 m.sup.3. The core radius and height are in the range H=1.5 m to 3.0 m, R=0.9 m.sup.3 to 1.8 m.sup.3 while the total number of fuel pins is in the range of 8,000 to 12,000.

(47) Large Pins

(48) Fuel pin radius r.sub.pin=5 cm. Cladding thickness δ lies in the range of 0.05 cm to 0.5 cm and the parameter (half the distance between neighbouring pins) is in the range/=1.0 to 2.5 cm. Within these ranges the form-factor lies in the approximate range F=1.6 to 2.5 and the core volume lies in the range V=2 m.sup.3 to 8 m.sup.3. The core radius and height are in the range H=1.2 m to 2.0 m, R=0.7 m.sup.3 to 1.2 m.sup.3 while the total number of fuel pins is in the range of 300 to 700.

(49) Thus, by using the metal hydroxide based moderator with the redox-element a much smaller and more efficient MSR is obtained. The same is contemplated by impressing an electric current on the molten moderator salt where corrosion protection is also obtained.

(50) The person skilled in the art realises that the present invention by no means is limited to the preferred embodiments described above. On the contrary, many modifications and variations are possible within the scope of the appended claims.

(51) Additionally, variations to the disclosed embodiments can be understood and effected by the skilled person in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage.