WO2014107214A2

WO2014107214A2 - Introduction of oxygen for prevention of solid formation in a fusion chamber

Info

Publication number: WO2014107214A2
Application number: PCT/US2013/065300
Authority: WO
Inventors: James A. DEMUTH; Daniel L. Flowers
Original assignee: Lawrence Livermore National Security, Llc
Priority date: 2012-11-05
Filing date: 2013-10-16
Publication date: 2014-07-10
Also published as: WO2014107214A3

Abstract

A method of preventing deposition of material inside a fusion chamber includes a step of determining a range of oxygen to carbon ratios where substantially no carbon solids or lead oxides are formed, the range depending upon temperature and hydrogen content of the chamber. Then a step is performed of adding oxygen to the chamber to form carbon monoxide. Next, additional oxygen can be added to the chamber to consume hydrogen in the chamber to form water, and reduce the tritium partial pressure, in addition to forming carbon monoxide. Concentration of additional oxygen levels is limited to levels to preclude lead oxide formation.

Description

CROSS-REFERENCES TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Provisional Patent Application No.

61/722,495, filed November 5, 2012, and entitled: "Prevention of Solid Formation in a Fusion Chamber," the disclosure of which is hereby incorporated by reference in its entirety for all purposes. STATEMENT AS TO RIGHTS TO INVENTIO S MADE UNDER

FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] The United States Government has rights in this invention pursuant to Contract No. DE-AC52-07NA27344 between the United States Department of Energy a d Lawrence Livermore National Security, LLC for the operation of Lawrence Livermore National Laboratory.

BACKGROUND OF THE INVENTION

[0003] The National Ignition Facility (NIF), the world's largest and most energetic laser system, is operational at Lawrence Li ermore National Laboratory (LLNL) in Livermore, California. One goal of operation of the NIF is to demonstrate fusion ignition for the first time in the laboratory. Initial experiments are calculated to produce yields of the order of 20 MJ from an ignited, self-propagating fusion bum wave. The capability of the facilit is such that yields of up to 150-200 MJ could ultimately be obtained. The NIF is designed as a research instrument, one in which single "shots" on deuterium-tritium containing targets are performed for research. A description of the NIF can be found in Moses et al, Fusion Science and Technology, volume 60, pp 1 1 -16 (201 1 ) and references therein.

[0004] There is a rapidly growing need for power, and especially for clean power. At LLNL a project known as Laser inertial-confinemem Fusion Energy, (often referred to herein as "LIFE'^") is working toward introduction of fusion based electric power plants into the U.S. economy before 2030, and in a pre-commercial plant format before that. LIFE technology- offers a pathway for the expansion of carbon- free power around the world. It will provide clean carbon-free energy in a safe and sustainable manner without risk of nuclear

proliferation,

[0005] One challenge w th respect to LIFE, as well as any technology for generating electrical power to be distributed to large numbers of consumers, is reliability. Consumers expect to have extraordinarily high reliability in their electric power supply. The result is that utilities that, provide that electrical power maintain their facilities to assure the required high reliability. Thus, among the challenges wit respect to fusion power, is to provide mechanisms by winch a reliable long-lived fusion chamber can be provided in which the fusion reactions occur, yet which can be maintained or replaced when necessary with minimal downtime for the overall power plant.

[0006] In the technology described herein, a fusion power plant, is provided with a fusion chamber into which targets containing deuterium and tritium fuel are introduced multiple times per second. The targets consist of a hohlraum and a capsule within the hohlraiim. Inside the capsule is the deuterium and tritium fuel. In the preferred implementation the hohiraums are metal typically lead. As the individual targets reach the center of the chamber, banks of lasers fire on the targets, striking the hohlraum, thereby heating and compressing the fuel within the capsule to create a fusion reaction. Heat from the fusion reaction is captured by coolant circulating around the chamber. This heat is then used to generate electricity.

[0007] The Laser mertial Fusion Energy concept is being designed to operate as either a pure fusion or hybrid fusion-fission system. The LIFE fusion chamber subsystem must absorb the fusion energy, produce fusion fuel to replace that burned in previous targets, and enable both target and laser beam transport to the ignition point. The chamber system also must mitigate target emissions, including ions, x-rays and neutrons and reset itself to enable operation at 5-20 Hz. Finally, the chamber must offer a high level of availability. One challenge to the desired availability is the need to keep the chamber free of debris from the fusion targets. This debris can be in a gaseous form, which if unchecked, will reduce the necessary transparenc of the optical path for the laser beams, as well as in condensed form, coating the walls of the chamber, coating associated optical components, and reducing the optical conditions necessary for target ignition. Herein we focus on techniques for maintaining the chamber, and in particular to techniques for assuring minimal contamination in the chamber from left over debris, e.g. lead and other materials, from the fusion targets. [0008] Carbon deposition and hydrogen co-deposition with carbon solids are known to happen in Magnetic Fusion Energy (MFE) systems and present source terms for tritium retention. In the Laser Inertial Fusion Energy system, carbon, lead, and hydrogen are debris products from the fusion reaction in the reaction chamber. The walls of the chamber are sufficiently cool that the lead will condense on them, but hot enough that the lead will remain molten. At these temperature ranges the carbon debris will deposit on the walls and build up over time. Hydrogen, particularly the tritium isotope, must be accounted for at all parts of the LIFE system because tritium is radioactive. Solid formation can also occur from gas phase fusion reaction debris. This solid formation, combined with possible flaking of graphite or other deposits from the walls as it builds up, could cause the laser beam paths to be blocked and prevent subsequent targets from being ignited.

SUMMARY OF THE INVENTION

[0009] This invention relates to a fusion chamber for a fusion power plant, and in particular to processes carried out within a fusion chamber as in an inertial confinement fusion power plant.

[0010] This invention provides a solution to contamination of the LIFE chamber by removing all carbon from the system by binding it in a. carbon monoxide or dioxide molecule before it can deposit on the chamber wails, flake off, or form aerosol particles, yet does not form lead oxides. The controlled addition of oxygen to the fusion chamber prevents the formation of solids on the wall of the chamber. Oxygen is added to the chamber to bind with gaseous or solid carbon in the chamber to form carbon monoxide and carbon dioxide, as well as bind with hydrogen to form water, however, excessive oxygen will cause lead in the chamber to form lead oxides. This invention provides a method to add oxygen based on temperature and compositions of oxygen, hydrogen, and carbon to prevent the formation of solid material in the chamber.

[0011] in a preferred embodiment our method of preventing deposition of material inside a fusion chamber includes steps of determining a range of oxygen to carbon ratios where no solid carbon or lead oxides are formed in a fusion chamber, the range depending upon temperature and hydrogen content of the chamber, and then adding oxygen to the chamber to form carbon monoxide. Additional oxygen is then added to the chamber to consume hy drogen in the chamber to form water and carbon monoxide. The addition of oxygen is stopped before lead oxide is formed. The oxygen can be included within the targets themselves, or introduced into the chamber by gas jets. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 illustrates conditions in a fusion chamber without introduction of oxygen;

[0013] Figure 2 illustrates the mass of hydrogen isotopes co-deposited with carbon in the LIFE chamber after 1 year of operation;

[0014] Figure 3a illustrates chamber composition at atomic o gen to carbon ratio of 2: 1; while Figures 3b and 3c are expanded portions of Figure 3a;

[0015] Figure 4 illustrates the operating window for oxygen addition to the fusion chamber for a temperature range of 400-575C;

[0016] Figure 5 illustrates the hydrogen content in various fusion chamber components as a function of temperature;

[0017] Figure 6 illustrates the effect of additive oxygen on the chamber hydrogen content at 400C;

[0018] Figure 7 illustrates the effect of additive oxygen on the chamber hydrogen content at 607C;

[ 0019] Figure 8 illustrates the operating window for oxygen as a function of temperature and oxygen content at reference case hydrogen content,

[0020] Figure 9 illustrates conditions for the formation of PbOH compounds in the fusion chamber; and

[0021] Figure 10 illustrates a technique for introducing oxygen into the fusion chamber.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

[0022] in the environment of the LIFE engine fusion chamber, the target capsule, hohlraum, and ail its constituents cycle in temperature from near absolute zero to thousands of degrees Kelvin and are completely ionized with each laser pulse. The atoms cycle through many states and compounds as they cool before they are finally retrieved and processed by a chamber gas handling system. With each shot the target constituents are driven to complete dissociation and, as they cool, begin to form various compounds, interacting with the debris from previous targets and the chamber walls. Furthermore, the LIFE engine has a high throughput of targets each day depending on the target repetition rate (720k targets per day if injected at 8.33 Hz). After each shot compounds will begin forming as the gas cools. These compounds condense, and could eventually deposit on the walls where the final compounds depend on the initial chemical mix of the hohlraum constituents. The overall LIFE fusion power plant architecture is described in "inertia! Confinement Fusion Power Plant which Decouples Life-limited Components from Plant Availability,'^'' PCX application US 11/59820, filed November 8, 201 1. The fusion chamber is described in "Inertia! Confinement Fusion Chamber," PCX application US 1 i/59814, also filed November 8, 201 1. The fusion chamber gas handling system is described in "Method and System to Remove Debris From a Fusion Reaction Chamber," U.S. Patent application 13/614,831 , filed September 13, 2012. The contents of each of these commonly assigned patent applications are incorporated by reference herein.

Xhree major concerns for the chamber regarding this process are: formation of laser attenuating aerosols, co-deposition of tritium with a condensable material, and solidification and buildup of lead or carbon compounds in the chamber. Timescales inside the chamber are short. Achieving chemical equilibrium under such circumstances is difficult, depending on the region of interest. The outer portion of the chamber, however, will have much longer residence times, and remain at a cooler temperature, facilitating the application of equilibrium chemistry. Additionally, equilibrium solutions can give valuable data both about species after long time scales and to aide in eliminating species if they are altogether

theraiodynamicaily unfavorable. [0024] If left unchecked, we estimate that for the present LIFE baseline design, carbon will deposit on the chamber walls at a rate of 3.45 mm per year based on a 12.4 mg iarget at 8.331 i shot rate on a 6 meter radius chamber. These high levels of carbon deposition could not only form flaky layers that could detach from the wail and block the laser beam lines (which enter the chamber from many different angles around the chamber, but would also provide co-deposition sites for tritium in the chamber. If lead deposited as a solid phase oxide, it would coat the walls at a rate of 15.93 cm/year for 2.47 g/target for the same shot rate. Lead will be liquid at the expected wall temperature, however, if the lead forms higher temperature melting compound such as PbO that would solidify at wall temperatures, it would become a problem. [0025] In the LIFE engine, every shot vaporizes the hohlraum-eapsule target, creating a debris cloud that may fully mix with the entire chamber volume before exiting the chamber. As a result, the steady state gas target constituent concentration will be proportional to the inverse of the chamber gas-clearing rate. The subsequent fate of the target, debris is a concern for three aspects of the fusion process in LIFE: formation of laser attenuating aerosols, co- deposition and solubility of tritium with a condensable material, and solidification and buildup of lead or carbon solid compounds in the chamber. Reference case target compositions show solid carbon formation at temperature below 3590K, however, lead is liquid until its freezing point of 600K. Tritium co-deposition with carbon is a known concern for fusion processes. Collaborative work with others at the assignee has further developed the understanding of tritium co-deposition estimates with carbon in LIFE and, absent appropriate treatment carbon is expected to be a source of tritium retention.

[0026] This invention provides a solution to contamination of the LIFE chamber by removing carbon from the system by binding the carbon in a carbon monoxide or dioxide molecule before it can deposit on the chamber walls, flake off, or form aerosol particles, yet does not form lead oxides. The controlled addition of oxygen to the fusion chamber prevents the formation of solids on the wall of the chamber. Oxygen is added to the chamber to bind with gaseous or solid carbon in the chamber to form carbon monoxide and carbon dioxide, as well as bind with hydrogen to form water. Excessive oxygen, however, will cause lead in the chamber to form lead oxides. This invention provides a method to add oxygen based on temperature and compositions of oxygen, hydrogen, and carbon in the chamber to prevent the formation of solid material in the chamber.

[0027] In our preferred implementation, the composition of the post ignition target gas mixture in the chamber is carefully tuned to have an appropriate oxygen to carbon ratio for a window of operation for the process as described herein. The window of operation is defined as the range of oxygen to carbon ratios where no solid carbon or lead oxides are formed. The range of the window of operation depends on the temperature, and hydrogen content of the chamber, and increases with both. At temperatures of chamber operation, initial addition of oxygen in the chamber from zero causes consumption of carbon, forming carbon monoxide. Further increases in oxygen content cause formation of carbon dioxide and consumption of hydrogen in the chamber to form water, before any lead oxide formation begins. This is the regime of the window of operation. Still further increases in oxygen content cause the formation of various lead oxides. Because hydrogen acts as a buffer species, once the oxygen forms carbon monoxide or dioxide, thereby consuming all the carbon, it must completely form water before it can begin to form lead oxides.

[0028] In addition to removing carbon from the chamber, removal of the hydrogen is also desirable. Removal of hydrogen is desirable to control the tritiu isotope, and exploitation of hydrogen consumption in the chamber and complete removal of carbon from the chamber can significantly reduce the total hydrogen content as would build up over time. [0029] Our approach as described here is not limited to the oxygen-carbon-lead system. In the expected implementation of the LIFE system, lead is used for the hohlraum and carbon based materials are used for the target ablator, i.e. the wall material for the capsule containing the deuterium-tritium (DT) mixture to be ignited. Different target capsule ablators, however, such as aluminum are being considered for this role, and could replace carbon. In such a system we found iodine closely replicated the behavior of the oxygen-carbon-lead system, except the window of operation is significantly enhanced because lead iodide is a gas at chamber wail operating temperatures, and thus large particle/aerosol formation and removal from the chamber is addressed using other processes not described here. [0030] in a hypothetical scenario where carbon deposition on the LIFE fusion chamber wails was nnaddressed, tritium is predicted to co-deposit 229 kg/year of hydrogen isotopes at a 50:50 D:T mixture for surfaces at 400C, and 52.3 kg/year at 600C. Oxygen additives were examined for their effectiveness iti bonding with carbon at high temperatures to prevent solid formation. Nitrogen and hydrogen additives were also considered, but yielded little to no change in carbon deposition. Addition of oxygen caused binding of oxygen with carbon to form carbon monoxide beginning at temperatures above 5000K for oxygen to carbon atomic ratios (OCAR) below 1 iOCAR< S ), and carbon dioxide below 2500K for OCAR>! . Excess oxygen beyond the complete tie up of carbon in carbon dioxide, and hydrogen in water begins to bond with the lead to form lead oxides. A window of viable OCARs based on temperature and hydrogen content was found to allow full consumption of carbon in carbon monoxide and/or carbon dioxide while avoiding any lead oxide formation. This defines what is referred to here as the window of operation. Formation of lead hydrates, hydroxides, and organic compounds was also examined and found to be minimal for the conditions in LIFE. Hydrogen solubility in various materials is examined as a placeholder for tritium. Total hydrogen soluble in the liquid lead did not exceed 1-2 g for a years' worth of shots at 400- 600C respectively.

[0031] We select oxygen as the chemical getter for carbon because of its ability to form bonds at high temperatures while carbon is sti ll in the gas phase. Hydrogen and nitrogen getters unfortunately yield no appreciable change in carbon solid deposition. If oxygen kmetieally bonds with carbon atoms at high temperature, not only are solid deposits prevented from collecting on the chamber walls, but also tritium co-deposition with carbon is prevented.

[0032] Because of the high temperature operation of the molten lead, identifying possible formation of any lead hydroxide molecules is a concern. Species Pb(OH)₂ and PbOH were examined to determine if they would form compounds in the LIFE environment. Work by others examined the change as lead hydroxide was heated from room temperature to 600C. They found phase change to PbO for temperatures between 25-170C while flowing either a 2 or 0₂ stream at atmospheric pressure over the sample. While for both streams there was an imtial weight loss from the sample, flowing the O? stream at higher temperatures showed formation of various lead oxide compounds in the process. In this experiment it is not clear if the lead hydroxide is off-gassing H₂ and O?, or a more complex PbOH or Pb(OH2) molecule. Lead hydroxide solids are formed through a precipitate of lead acetate solution to an alkaline solution, and it is unlikely these conditions will occur in LIFE, [0033] Other work examined the kinetics of the PbO(s) + H2 to H20 ÷Pb(s) reaction by flowing argon with 3% hydrogen by volume over the lead oxide specimen. Reaction rates were found to be of first order with respect to the lead oxide concentration, increasing with temperature. This study shows that flowing hydrogen can reduce lead oxide to pure lead. The data fit very well to the expected values leaving little room for any significant anomalous lead hydroxide production because reaction rates were calculated by measured water created by the reaction. This shows that if lead oxide compounds were to form in the LIFE chamber, they could be removed by flowing a gas mixture with less oxygen than hydrogen. Other studies have also shown that bubbling hydrogen gas through molten lead with oxygen impurities can be used as a deoxidation treatment. At 400C a 20-hr bubbling can lower the oxygen content from 30ppm to 5ppm, which can be reduced to 3ppni after 50-hr. This reaction rate increases quickly with temperature and faster deoxidation rates will most likely be seen in the LIFE environment with average temperatures of 500C.

[0034] Our investigation relied upon simulations carried out in Cantera, a python based thermodynamic chemical solver with a MATLAB front end. Thermodynamic data is taken from the NASA gas, and NASA condensed thermodynamic property files as well as from thermodynamic data available from Fact- Web as part of the FactSage thermodynamic database, the Argonne National Laboratory thermochemical database, and various data from the online periodic table element, database. Compound property data is input to the model using the 7 -coefficient NASA polynomial format, which is the format of the NASA gas/condensed property files, if the data is not in the polynomial form, it is fit to

thermodynamic data as a function of temperature. Because xenon, planned to be used as a clearing gas between shots, is believed to radiate to a ~ 6000K stall temperature between shots, xenon pressure is calculated assuming the ideal gas equation from the stalled gas temperature and a density of fmg/cc, [0035] The assumed mass and distribution for a single target used in our investigation are:

[0036] Given a 0.5% chamber clearing fraction per shot, and assuming no gas condenses to the wall, the maximum concentration that will be seen in the gas phase can be predicted by:

where mo is the initial mass of xenon, x is the clearing fraction and Am is the mass of one target. The mass of the steady state number of targets is then mixed with the xenon in the chamber, all in rnonatoniic form (p._xV). The chamber is assumed spherical with a radius of 6 meters. Deuterium and tritium are treated as hydrogen thermodynamically, and are destroyed, creating helium in accordance with an estimated 30% fusion burn-up.

[0037] System temperature was varied between 300K and 5000K, and thermal equilibrium was calculated at each temperature step using constant temperature and pressure.

Equilibrium was achieved by re-distributing the atoms among the molecules in the species list to minimize the Gibhs free energy of the system. The wall temperatures of the chamber are bounded by 400C and 650C. Only condensed phase species for lead and carbon were used, neglecting tantalum and aluminum because of their low concentrations, and in the case of aluminum, excessive system complexity. All reactions are limited to the species list data files given to Cantera.

[0038] Our initial analysis to develop a reference case provided no additional oxygen to the system. Simulations show that the current target composition and a flushing rate of 0.5% will yield a scenario where all the carbon atoms form solid graphite/amorphous carbon layers on the chamber surfaces, and would accumulate at a rate of 3.45mra/year. Fortunately, no lead solids are formed in the reference case, as ail lead will be liquid in the chamber (> 600K). It is unclear if the carbon solids will be able to collect on the walls, or will be swept, away by the lead,

[0039] Figure 1 illustrates the reference case equilibrium in which no oxygen is added to the chamber, that is, an oxygen to carbon ratio of zero. As shown in Figure 1 , as the lead cools it will condense to liquid phase at 1590K, and solidify at 600K. Carbon gas similarly coois until about 3590K below which it begins to condense to graphi e/carbon solids. At 450K it begins to break down, forming methane. The carbon forms bonds with all available oxygen in the chamber at high temperature to form CO gas which begins to pass the oxygen to water at about 700K.

[0040] Hydrogen, being nearly 100 times more abundant than oxygen in the reference case, remains monatomic until about 3000K when it begins to form H₂, and then later methane at 450K. Lead does form a hydrogen compound PbH between 3000K and 1500K, but this is i very minute amounts (about 1 /50,000'^") the overall lead content, and 1/1 ,000™ the overall hydrogen content. For comparison, the maximum mole traction of PbH or CO? in Figure 1 is approximately l appm and scales with number of moles. For example, the maximum ¾ concentration is at about 1 pan per thousand.

[0041 ] The reference case illustrated in Figure 1 is representative of the current LIFE chamber set-up, and what would happen were equilibrium achieved at the various temperatures explored with no modifications to the element composition. Element modifications due nuclear transmutation from the fusion reaction are currently ignored as they are expected to be in very small quantities,

[0042] in this reference case with no additive oxygen, and nothing else done to getter the carbon, excessive deposits will accumulate on the chamber walls. The fusion community is familiar with tritium co-deposition with carbon in ITER and other systems. Preliminary estimates of the effect of co-deposition with carbon are estimated to dominate the tritium retention in the fusion chamber in the absence of oxygen. The Hydrogen to Carbon Atomic Ratio (HCAR), as carbon deposits on the walls of the LIFE chamber, increases rapidly with decreasing temperature. At LIFE chamber temperatures, co-deposition of HC AR is estimated to be 0.1 at 700K (427C), and 0.03 at 900K (627C). Baseline system HCAR is 0.383 in the chamber, however this would only be saturated at very low temperatures. Thus, hydrogen in the LIFE chamber that encounters a carbon atom at near wall temperatures, will bind to it, forming a C_yH_x molecule, and then deposit on available chamber surfaces. [0043] Figure 2 illustrates the mass of hydrogen isotopes (50:50 D:T atomic ratio) co- deposited with carbon in the LIFE chamber after 1 year of operation. This co-deposition totals 229 kg/year for surfaces at 400C, and 52.3 kg/year at 600(1 again assuming 30% fusion target burn up. Of the burn up, 60% of the hydrogen by mass is expected to be tritium assuming a 50:50 molar split, of deuterium and tritium with negligible protium.

[0044] Due to the strong affinity for carbon to form solids, and the low pressure/high temperature of the LIFE engine, oxygen can be used to create non-solid compounds with carbon. The addition of nitrogen and hydrogen can form both methane and cyanide compounds at standard temperature and pressure conditions, however, due to the low pressure of the LIFE system, formation of these compounds is unfavorable. The addition of oxygen in equilibrium simulations causes formatio of CO at very high temperatures. At lower temperatures (below 2500K) the most favorable carbon state converts to C0₂ consuming any available oxygen in the process. Additionally, all hydrogen will begin forming water instead of remaining in monatomic or diatomic states. The addition of oxygen process aims specifically at forming carbon monoxide and carbon dioxide molecules, thereby preventing solid carbon formations from occurring in the first place, if carbon deposits can be eliminated from the fusion chamber, the total hydrogen content can be significantly reduced down to the level of that in the gas phase species.

[0045] Figure 3a illustrates the LIFE chamber composition for an atomic oxygen to carbon ratio of 2:1. Figures 3b and 3c are expanded views of portions of Figure 3a with enhanced resolution to show smaller concentrations. At lower temperatures and very hig oxygen concentrations, oxygen will also react, with lead in the chamber to form lead oxides or lead carbonates, which have a much higher melting temperature. As a result, these will plate out on the walls. If these compounds are allo wed to adhere to the fusion chamber walls, solid material will accumulate at the rate of about 13.7 cm/year assuming the density of lead oxide (PbO) litharge/massicot is -9.53 g0/cm^'\ the chamber radius is 6 meters, and the shot rate is 8.333Hz.

[0046] With an Oxygen to Carbon Atomic Ratio =^: 2 (GCAR=2) (See Figures 3a, 3b, 3c, and 4), no solid carbon compounds are formed above the minimum wall temperature, and no solid lead compounds are formed at all. Minute amounts of gaseous PbO are formed, peaking at about 2100K along with formation of OH and O?, but these break down at lower temperatures. At just below 400C (the expected minimum wall temperature), the formation of carbo solids does become favorable, even with OCAR-2, The peak formation temperature of carbon solids depends on the oxygen concentration, and is depressed with increasing oxygen. The formation of carbon solids in a low temperature oxygen environment coincides with the formation of water. In this process C0₂o breaks down, interacting with hydrogen to form ¾G, and C_(S)0 where the peak amount of carbon solids formed is maximum at the chamber hydrogen content. The formation of any lead oxides is predicated by the complete saturation of carbon and hydrogen with oxygen, fully tying up all the carbon as CO₂, and all the hydrogen as ¾0.

[0047] Due to the existence of hydrogen in the LIFE chamber, and the preference of oxygen to bond with carbon before hydrogen, there is a "sweet spot" window of operation where oxygen completely ties up all the carbon to form gaseous carbon compounds, while not bonding with lead to form solid lead oxides. The window of operation for this effect increases with increasing temperature (OCAR (400C) = 0, OCAR (575C) = [1.35, 2.2]}, 0 and increasing hydrogen content in the chamber. A l x increase in atomic hydrogen content in the chamber will result in a lOx increase in the width of the operating window. As described previously, the presence of hydrogen also promotes the formation of carbon solids at low temperature for given oxygen to carbon ratios. Consequently the allowable oxygen to carbon ratio window shifts upward (more oxygen needed for the same effect), and grows wider (wider range in oxygen to carbon ratio variability) with added hydrogen. For example, a 1 Ox increase in hydrogen would cause the window at the minimum temperature (400C) to shift from OCAR 0 ==^: 0 [2.07, 2,2] to [2.6, 3.8], [0048] Figure 4 illustrates the operating window for oxygen addition to the LIFE chamber for a temperature range of 400-575C (673-848K). Notice that as oxygen is added in excess of the window of operation, lead oxides and carbonates begin to form. Oxygen to carbon ratios of up to 60x were examined with the formation temperature of the lead solids to ensure the species formed were unaffected by any further increase in oxygen content. Lead-oxygen species formation saturation occurred at OCAR ratios of 25.2x.

[0049] Tritium release is a concern of the LIFE fusion plant; hence careful evaluation of all hydrogen isotopes was also studied. Most of the hydrogen in LIFE will flow out of the fusion chamber through the chamber gas handling system. Some hydrogen, however, will exist which is soluble in liquid lead, in steel, and in the carbon, or co-deposited with the carbon. Figure 5 illustrates the hydrogen content in various LIFE fusion chamber components as a function of temperature, expressed as grams of hydrogen in the fusion chamber material for the reference case, i.e. no added oxygen. Only 1-2 g of hydrogen (depending on temperature) will be present in the LIFE chamber materials in steady state. Hydrogen gas content is set for a clearing ratio of 0.5%, i.e. 99.5% of the chamber gas is removed by the gas handling system. The solid phase is allowed to accumulate for one year's worth of deposits with the same solubility as the liquid phase.

[0050] The solubility of hydrogen in liquid lithium is high for the partial pressures in the chamber, especially relative to the 0.1 appm level desired for tritium management. Thus, if hydrogen can diffuse through the steel and any lead or carbon layer that may be on the wall, the lithium will act as a hydrogen sink for the chamber gas. Applying these solubilities to actual mass estimates of carbon, iron, and lead in the LIFE chamber gives initial estimates of tritium inventory in the LIFE chamber. This effect is shown in Figure 5, which counts mass o ver a year's worth of deposition for the reference case composition as a function of temperature.

[0051] Hydrogen content in steel is based upon the iron mass from a i mm thick spherical shell at a 6 meter radius approximating the LIFE system interior first wall. Graphite mass was based upon the assumption that at a given temperature, all carbon solids formed in the chamber went to the walls and stuck over the course of a year. Lead mass was based upon the lead forming a 1 cm thick film on the wall as it flowed out of the chamber. In reality, however, this will be smaller, especially if lead does not wet the walls. At temperatures below the solidification point of lead, the lead mass is instead taken as the total amount of lead that would deposit over the course of a year, where solid lead is assumed to have the same hydrogen solubility as liquid lead.

[0052] The hydrogen content in solid carbon dominates all other sources by about two orders of magnitude in some cases, and especially at the wall temperatures expected in LIFE. Between 400C and 575C (minimum and maximum wall temperatures, respectively ) the carbon content in the chamber varies between 1-2 g, of which half is tritium {0.5-1 g). In this simulation, the chamber clearing rate was again set at 0.5%, however this only affects the gas phase species because the condensed phase assumes a clearing rate of 0%.

[0053] Figure 6 illustrates the effect of the addition of oxygen on the chamber hydrogen content at 400C. The addition of oxygen to the LIFE chamber to remove carbon also lowers the concentration of hydrogen in the chamber and in solution. The addition of oxygen lowers carbon and hydrogen concentrations in the gas phase by forming FLO and COj. Hydrogen co-deposition with carbon is not discussed here, but prevention of carbon deposition will also simultaneously prevent co-deposition of hydrogen.

[0054] At low temperatures, as shown in Figure 6, hydrogen soluble in carbon solids dominates the hydrogen for the chamber. With, the addition of oxygen, water and carbon

S 3 dioxide are formed; consuming available hydrogen and carbon. Once there is enough oxygen to completely consume all available carbon, the hydrogen content in the chamber is then dominated by the hydrogen locked up in water. At 400C, die addition of oxygen to OCAR>2 will lower the hydrogen content i the chamber by up to lOx. Excess oxygen causes lead oxide formation as discussed previously. Figure 7 illustrates the effect of additive oxygen on the chamber hydrogen content at. 607C.

[0055] Figure 8 illustrates the operating window for additive oxygen as a. function of temperature and oxygen content at reference case hydrogen content (for the baseime hydrogen/deuterium/ tritium content of 1.4 mg/^'target, and a total cleaning ratio of 0.5%). The zone to the right of the carbon solids formation temperature curve that is also above the lead oxide solids formation temperature curve is the desired region of applicability. The area under the carbon solids formation temperature curve represents conditions that will form carbon solids, and the area under the carbon solids formation temperature curve represents conditions that will form lead solids, [0056] At higher temperatures, as seen i Figure 8, there is still significant hydrogen in the gas phase after the consumption of the carbon into a gas phase molecule. At high temperatures carbon is saturated in forming CO, OCAR=l , the further addition of oxygen pushes the formation of C0₂o and H 0, ge erating a wide window of operation (where no carbon or lead solids are formed) as an extended intermediary period where hydrogen and carbon monoxide are consumed by the remaining oxygen content. A 607C, the addition of oxygen to the chamber will lower the hydrogen content more rapidly at lower oxygen levels (4x at OCAR=1.2S), but is less effective at higher leveis of oxygen addition, reaching 5x reduction maximum. Figure 8 also shows that at iow temperatures there is a narrow window of operation. As the temperature is increased, however, less o gen is needed to remove the carbon. Finally, above 900C, no lead oxides wili be formed, and the carbon to oxygen atomic ratio need only be greater than 1.

[0057] We also investigated the possibility of forming lead hydroxide compounds. Based on experimental data for various hydroxide compounds found in literature, we correlated the estimating energies of formation, specific heats, and entropy for unknown monohydroxide and dihydroxide molecules by analogy with those of the corresponding halides. This hydroxide data was then used to calculate potential dispersion of fission products, and data for the lead hydroxides was also included in the study. Using this data for entropy and tree energy, combined with experimental data from others for the enthalpy of dissociation and enthalpy of formation, data for the NASA 7-coefficient polynomial fits was created, the species were added to the Cantera database, and the LIFE simulations re-run. Figure 9 illustrates the results of this analysis.

[0058] As shown in Figure 9, only very small amounts of PbOH are formed, peaking at 1600K, and no Pb(OH>2o is seen above 0.132 appm. The results there were developed for an OCAR of 4 to induce the formation of lead oxides. Also added in this analysis are various other indicated charged species. The PbOH has a peak concentration at I600K at 5,281 appm that drops to 90 appm at 750K. Pb(OH)₂ concentrations (not shown, in the figure) peak at 1250K at 0.132 appm. There is uncertainty, however, as to if these compounds will even actually form at all since the data is primarily theoretical. Additionally, when examining the free energy of the lead hydroxide reaction, it is unfavorable by a small margin. This reaction could proceed in either direction in certain conditions due to the lack of our understanding.

[0059] The oxygen process described above supports a viable LIFE chamber design by eliminating the formation of carbon solids, and thus the primary source for tritium retention in the fusion chamber. For the current LIFE target gas composition, oxygen is the primary viable method of preventing the formation of carbon solids in the chamber. To achieve complete tie-up of carbon atoms in CO or C0₂, there is a window of operable oxygen to carbon atomic ratio that can be used, and which will not simultaneously form lead oxides. The size of this window increases with both hydrogen content in the system, and wail temperature. Previous work by others shows that even if lead oxides do form, they can be removed by flowing hydrogen gas over them (i.e. run the process in an oxygen lean mode).

[0060] Based on equilibrium calculations, concentrations of lead organics, in particular tetraethyl lead are on the order of 10-1000 or smaller at any temperature or oxygen content. Thus, formation of these lead organic molecules appears thermodynamically unfavorable with the current target composition, temperatures, pressure, and clearing rate. Lead hydroxide molecules were also studied and found to be in very low concentrations, although there is still doubt as to their formation potential, primarily as a result of the size of potential errors in available data.

[0061] The formation of methane is minimal due to the low operating pressures, and only appears at low temperatures due to this effect. Tins pressure/density effect also explains why hydrogen is ineffective as a carbon scrubber in the LIFE environment. Other studies at LLNL show that, in the absence of a blast wave, the LIFE chamber will be very thoroughly mixed with the vacuum vessel gas through natural convection. Additionally, simulations also show significant mixing of 5-30% between the chamber gas and that of the vacuum vessel . Thus, with the two extremes of the blast wave showing large levels of mixing, it is likely that all the conditions in-between will also exhibit those same effects.

[0062] Figure 10 is an illustration of propagation of a debris cloud 830 in a fusion chamber in which gas is introduced to force advection of the debris cloud from the fusion chamber. Figure 10 is taken from one of the patent applications referenced above, that is, "Method and System to Remove Debris From a Fusion Reaction Chamber," U.S. Patent application 13/614,831. The figure is a simplified schematic diagram illustrating how one or more jets can assist a debris advection process. By modifying the jets described in that patent application to also introduce oxygen, in a manner in accordance with the invention described here, the undesired carbon in the residue or debris can be removed. As illustrated in Figure 10, three gas jets provided from inlets at the origin of the triangles are used to help force debris from chamber, and to also introduce oxygen.

[0063] Debris 830 from the first target is illustrated near an exit port of the chamber. The next target 832 is illustrated as approaching the entry port of the chamber. Although three jets are illustrated in Figure 0, but this is not required by the invention. In other

embodiments, a different number of jets are utilized, for example, one jet, two jets, four jets, etc. According to some embodiments, the jets are injected in such a manner that they propagate toward the chamber center or toward the expected position of the debris cloud at the time the oxygen is introduced. With appropriate sensing devices, the jets may even be steered to a location of maximum effectiveness.

[0064] The timing of the oxygen gas jet injection and the fusion target in jection are coordinated so that the fusion target is free from the jet at the chamber center in preparation for the fusion ignition, thus avoiding having the laser beams used for compression traverse thermal gradients associated with the jet. In Figure 10 the illustrated jets add momentum to the system, which assists in the debris removal process. The timing of the oxygen injection is made to assure minimal influence to the laser beams that will be used to ignite the hydrogen in the next arriving target 832.

[0065] Another, more preferred technique for introduction of oxygen to the fusion chamber is to add oxygen to the initial (pre-fusion) composition of the target. This allows the oxygen to be introduced with each target 832, It also allows the oxygen to mix more thoroughly with the undesired carbon and residual hydrogen from each shot.

[0066] Because of the difficulty of measuring the temperature of the fusion chamber, in a preferred implementation of the process of this invention, the fusion chamber walls would be renioieiv inspected, or a scraping taken from them remotely, after a period of operation. This allows the oxygen addition levels to be adjusted appropr ately based on lead oxide or carbon solid deposits, in addition, by monitoring the exhaust stream from the fusion chamber, data informing the control of oxygen injection can be gained. As suggested above, the resulting information can be used to alter the oxygen present in the targets or the gas streams injected into the chamber.

[0067] The examples and embodiments described herein are for illustrative purposes only. Various modifications or changes in light thereof will be apparent to persons skilled in the art. These are to be included within the spirit and purview of this application, and the scope of the appended claims, which follow.

Claims

WHAT IS CLAIMED IS: 1. A method of preventing deposition of material inside a fusion chamber comprising:

defining a range of oxygen to carbon ratios where substantially no carbon solids or lead oxides are formed, the range depending upon temperature, pressure, and hydrogen conten of the chamber;

adding oxygen to the chamber to form carbon monoxide:

adding more oxygen to the chamber to consume hydrogen in the chamber to form water, in addition to forming carbon monoxide; and

controlling addition of oxygen to the chamber to avoid lead oxide formation.

2. The method of claim 1 wherein the step of defining the range of oxyge to carbon ratios comprises:

for a first range of oxygen to carbon ratios determining a first range of temperatures below which lead oxides will form; and

for a second range of oxygen to carbon ratios determining a second range of temperatures below which carbon solids will form.

3. The method of claim 2 wherein the method further comprises operating the fusion chamber at a temperature for every carbon to oxygen ratio which temperature is above the higher o f the first range and the second range of temperatures for that carbon to oxyge ratio.

4. The method of claim 1 wherein the temperature of the fusion chamber is at least 400 °C.

5. A method of preventing deposition of material inside a fusion chamber comprising;

defining a range of an active gas to impurity ratio where substantially no impurities are formed, the range depending upon temperature, pressure, and impurity content of the chamber;

adding the active gas to the chamber to form compounds which can be removed from the chamber during operatio of the chamber; and

controlling addition of the active gas to avoid formation of materials other than the compounds which can be removed from the chamber.

6. The method of claim 5 wherein the active gas comprises iodine. 7. The method of claim 5 wherein the active gas comprises oxygen. 8. The method of claim 7 wherein the impurities comprise carbon, hydrogen and lead. 9. The method of claim S wherein the step of defining a range of an active gas comprises:

defining a range of oxygen to carbon ratios where substantially no carbon solids or lead oxides are formed:

adding oxygen to the chamber to react w th carbon to form carbon monoxide and carbon dioxide and to react with hydrogen to form water; and

controlling addition of oxygen to the chamber to avoid forming lead oxide. 10. The method of claim 9 wherein the step of defining a range of oxygen to carbon ratios where substantially no carbon solids or lead oxides are formed comprises: determining levels of lead oxides or carbon solids formed on the chamber wall after a period of operation;

determining the levels of gaseous lead oxide, oxygen, carbon monoxide, carbon dioxide, hydrogen, water, and methane in exhaust gas from the chamber, and

selecting an amount of oxygen to add based on composition of the exhaust gas and the levels of lead oxides or carbon solids formed on the chamber wails after the period of operation. 11. The method of claim 9 wherein the step of adding oxygen comprises adding oxygen to fusion targets injected into the chamber, 12. The method of claim 9 wherein the step of adding oxygen comprises adding oxygen directly into the chamber using a gas jet. 13. A system for preventing deposition of material from a fusion reaction comprising:

a fusion chamber;

a first pathway into the chamber for injecting fusion targets into the chamber; a second pathway out of the chamber to provide for ejection of debris from fusion reactions of the fusion targets in the fusion chamber; and wherein 7 at least some of the fusion targets include a reactive material which reacts

8 chemically with the debris to produce compounds which can be removed from the fusion

9 chamber during operation of the fusion chamber.

1 14. The system of claim 13 wherein the temperature of the fusion chamber

2 is at least 400 °C.

1 15. The system of claim 13 wherein the reactive material comprises

2 oxygen.

1 16. The system of claim 13 wherein the fusion reactions produce debris

2 that comprises lead, carbon and hydrogen.

1 17. The system of claim 16 wherein an amount of oxygen is added to the

2. fusion chamber, and the amount of oxygen added comprises an amount of oxygen sufficient

3 consume carbon and hydrogen, but insufficient to form lead oxide.

1 18. The system of claim 17 wherein the amount of oxygen added to the

2 fusion chamber is determined by monitoring exhaust gases from the fusion chamber.

1 19. ^'The system of claim 17 wherein the amount of oxygen added to the

2 fusion chamber is determined by:

3 determining levels of lead oxides or carbon solids formed on a wail of the

4 fusion chamber after a period of operation; and

5 determining levels of gaseous lead oxide, oxygen, carbon monoxide, carbon

6 dioxide, hydrogen, water, and methane in exhaust gas from the fusion chamber.

1 2.0. The system of claim 1 wherein the amount of oxygen is injected as an

2 additive gas into the fusion chamber using at least, one injection port in the fusion chamber.