Standing in the Qixia Mountain Controlled Nuclear Fusion Research Institute, standing in the laboratory, Xu Chuan looked at the images and data on the display screen.

On one side of the laboratory, there is also an isolated laboratory room.

Inside, scanning electron microscopes, metal in-situ analyzers, mass spectrometers, and other equipment are analyzing the materials in the equipment.

The second limit experiment of the Daybreak fusion device not only set a two-hour high-density plasma operation record, but also a deuterium-tritium raw material fusion ignition operation experiment.

The data and value brought by the real deuterium-tritium raw material fusion ignition operation experiment are not comparable to the operating energy of helium-3 and hydrogen simulated high-density plasma flow.

Although the latter can also be close to the former in terms of temperature and density, it is impossible to achieve fusion after all.

The former, even if it is only a milligram amount, can achieve real deuterium-tritium fusion to release energy, release neutrons, raise the temperature of the plasma, disrupt the operation of the plasma, and so on.

These are all things that cannot be done by helium-3 and hydrogen simulations.

In particular, neutron irradiation damage to the first wall material is the next world problem for controlled fusion relay control of high-temperature plasma turbulence in reactor chambers.

The material of the first wall has to face not only the high-temperature deuterium-tritium plasma in the reactor chamber, but also the neutron beam generated during the fusion of the deuterium-tritium feedstock.

In addition to this, the first wall material may even have to assume the function of tritium self-sustaining.

The two feedstocks for controlled nuclear fusion in DT are deuterium and tritium.

The amount of deuterium on the earth is huge, and about 40 trillion tons of deuterium are stored in seawater alone, and it is relatively simple to produce.

But compared to deuterium, tritium is stored in a very scarce amount on Earth.

The amount of tritium in the world's natural resources is almost negligible, with only about 3.5 kilograms in nature.

At present, the storage of tritium raw materials in all countries does not exceed 25 kilograms in all countries.

On the one hand, tritium will autonomously emit β rays and decay, with a half-life of only 12.5 years.

On the other hand, it can only be prepared by nuclear reactions.

At present, tritium is prepared in industry, mainly using reactor neutrons, using lithium-6 compounds as targets to produce tritium, and then using thermal diffusion method to enrich tritium to more than 99% and then collect and preserve.

The neutron beam is uncontrollable, and the amount produced in the nuclear fission reactor is not large, so the yield is very low.

Therefore, in controlled nuclear fusion technology, how to keep tritium in a self-sustaining cycle is also one of the key issues.

Some people may think that particle accelerators can be used to accelerate the neutron bombardment of lithium materials to make tritium raw materials, but those who have this kind of thinking, to be honest, basically did not study physics seriously in high school.

Neutrons do not carry electrons, and the magnetic field of the accelerator has no effect on them at all.

If the magnetic field could confine the neutrons, the material for the first wall of a controlled fusion reactor would not be so difficult to find.

Fortunately, a large number of neutrons are produced during the fusion process of deuterium and tritium, and if neutrons are used to bombard the target of lithium-6 compounds, tritium can theoretically be maintained for self-sustaining.

In the last operation of the Daybreak fusion reactor, Xu Chuan did such an experiment.

On the first wall, he had various materials such as lithium-6 compound targets, tungsten alloys, molybdenum alloys, graphite, carbon composites, and beryllium alloys installed.

Among them, the lithium-6 compound target material is used to test whether the neutrons released during the deuterium-tritium fusion process can really bombard the lithium material to produce enough tritium raw material as theoretically.

For the other materials, it is to find the most suitable first wall material.

Neutron irradiation is no joke.

At present, it can have a strong transmutation effect on most materials and most metal materials.

This not only destroys the structure of the material, but also acts as a blowing agent, turning the material into an extremely fragile foam.

Imagine what it feels like for a piece of steel as thick as a foam box to be crumbled into slag with a light break by your hand.

Neutron irradiation in controlled fusion reactors can do this.

In fact, this is exactly the case, and even though the last Daybreak fusion device used only one milligram of deuterium-tritium feedstock, the neutrons produced during the fusion process still cause varying degrees of damage to the various test materials deployed on the first wall.

However, the good news is that the lithium-6 compound target did play a corresponding role in the experiment, and the neutron beam produced by the deuterium-tritium fusion collided with it and produced some tritium.

Therefore, theoretically, it is theoretically possible to use lithium-6 compound targets as reactants to solve the problem of tritium self-sustaining.

This is also a major breakthrough.

After all, in the past, no experimental institution or research institution could really use the experimental reactor to test the deuterium-tritium fusion reaction and synthesize tritium raw materials from neutron + lithium materials.

They should be the first time.

But there is good news, but there is more bad news.

The damage degree of the various test materials installed on the first wall material against neutron irradiation is higher than that calculated by Xu Chuan.

Looking at the images on the computer screen, Zhao Guanggui, a professor of materials science on the other side standing beside Xu Chuan, sighed softly and said, "Judging from the experimental data, there are many more problems than we imagined. ”

Xu Chuan looked at the images on the computer and said, "No matter how much you have, you have to solve them one by one, right?" ”

Hearing this, Zhao Guanggui sighed: "That's true, but we have a lot of troubles. And now we have entered a new field, where there is no other research institution or laboratory that can provide us with experience as a reference in the field of controlled nuclear fusion. ”

Hearing this, Xu Chuan smiled and said, "Referring to other people's experiences and ideas can indeed provide us with great convenience, but after all, it is just going on the road of others." In this aspect of scientific research, if you want to make achievements, you must have your own ideas and ideas. ”

"The lazy way may be suitable for other fields, but for us who are engaged in academic research, what to do and how to solve problems requires us to think independently after all."

On the side, Xing Xuexing, a professor of materials science who was transferred from Shuimu University, said with a smile: "Being able to walk in front and expand the boundaries is something that every researcher and scholar hopes for." ”

After a pause, he brought the topic back to the experimental data: "But what Professor Zhao said is right, we have a lot of trouble this time. ”

"Whether it is tritium self-holding or the damage of various neutron-irradiated sample materials, it is much lower than expected before the experiment."

"By bombarding lithium targets with neutrons, tritium can indeed be generated. But the amount generated and the amount we collected is not as much as theoretically possible. ”

"On the one hand, the neutron beam generated by fusion in the chamber does not all act on the lithium-6 compound, and the energy it carries is too high to directly penetrate the target, resulting in a much lower number of reactions than expected."

"On the other hand, the energy level carried by these neutrons is too high, and at a temperature of 120 million degrees, the energy level of the neutron beam released by deuterium-tritium fusion is comparable to that of a medium and large particle collider, which will have a very serious impact on the target and the first wall."

Xu Chuan thought for a while and said, "The first problem is easy to solve, but the thickness of the target can be increased." In addition, it can be made into a full-coverage type, which wraps the reaction chamber as a whole, so that the neutron beam is not wasted. ”

"As for the second one, it's a bit of a hassle."

Controlled nuclear fusion is not nuclear fission, and the temperature of nuclear fission is far less than that of nuclear fusion.

Even if a large-yield nuclear bomb explodes, the core temperature will be in the million-degree Celsius range.

The little boy who was dropped on Hiroshima was only a little over 6,000 degrees Celsius at the heart of the explosion. In contrast, this value is nothing to mention in controlled nuclear fusion.

More than 6,000 degrees, this data is not even a fraction of the temperature of the plasma in which the dawn fusion device operates.

And the temperature at which a nuclear bomb explodes is even lower, even if the nuclear fission effect is used to generate electricity.

Therefore, most of the anti-radiation materials that can be used in nuclear fission reactors cannot be used in controlled nuclear fusion reactors at all.

It is not only the lithium target used for tritium self-sustaining that was damaged during the experiment, but also the other experimental materials deployed in the first wall.

On the side, Zhao Guanggui tentatively said, "How about lowering the fusion temperature a bit?" ”

"The temperature of deuterium-tritium fusion can occur at about 12 million degrees, 120 million degrees, which is a full ten times over."

"Although lowering the temperature affects the activity of the deuterium-tritium plasma, which in turn affects the amount of fusion and the amount of energy produced. But sacrificing some heat and energy for the stability of the first wall material is not undesirable. ”

Xu Chuan thought about it, shook his head and said, "It's not very feasible." ”

"Although thermal motion can cause inelastic collisions of neutrons, and the higher the speed of thermal motion, the greater the impact on matter, but the energy level of the neutron beam in a fusion reactor does not come from temperature alone."

"Its main source is the energy propulsion produced during the fusion of deuterium-tritium nuclei, each of which produces a 14.1 MeV neutron, which is partly predestined in high-energy physics, and lowering the temperature only reduces part of the external force."

Zhao Hongzhi nodded and said, "Well, from this point of view, it is basically impossible to reduce the temperature and thus reduce the damage of neutrons to the first wall material. ”

"According to the material analysis data after neutron irradiation, materials such as molybdenum, tungsten and graphene are less affected by neutron irradiation in the first tier, while Austrian steel and ceramics are in the second tier and others are worse."

On the side, Professor Xing Xuexing of Mizuki University shook his head and said: "Molybdenum is not good, this Mizuki has done research before, and molybdenum will transmute into radioactive elements when it is irradiated with neutrons." As for molybdenum alloys, more attempts are needed. ”

"It's tungsten, and tungsten alloy may still be a little promising. At present, the tungsten alloy used in the first wall material of ITER and EAST has good heat resistance, and the transmutation products are osmium and rhenium, and there is no radioactivity problem. ”

Xu Chuan shook his head and said, "Tungsten probably won't work." ”

"There is nothing wrong with the heat resistance and transmutation products of tungsten, but the difference in its physical plasticity and thermal expansion coefficient, as well as the accumulation of thermal stress, can cause cracks to occur inside the material."

"This is fatal for controlled nuclear fusion reactors."

Hearing Xu Chuan's veto of tungsten alloy, the laboratory fell silent again.

The material problem of the first wall is indeed very troublesome, so troublesome that the world cannot find a suitable one at present.

After all, in a controlled fusion reactor, the first-wall material is strongly affected by the high-energy neutrons emitted from the plasma, electromagnetic radiation, and high-energy particles (deuterium, tritium, helium and other impurities).

Theoretically, a commercial tokamak reactor should have a neutron wall load of at least 5MW/m2 or more.

Neutron wall load is a design metric related to the power density of a fusion reactor, which is numerically equal to the product of the intensity of the fusion neutron source and the neutron energy per unit area of the first wall material.

The vast majority of heat-resistant materials simply fall short of these extremely demanding property challenges.

But then again, if this problem really had to be so easy to solve, it wouldn't have been left until now.

After all, controlled nuclear fusion is something that the whole world is capable of doing, and all kinds of technical problems and material problems in it must have been discussed countless times.

Staring at the data on the computer monitor, Xu Chuan pondered for a while and then said, "I think that the material selection of the first wall, maybe we have to change our thinking." ”

Hearing this, everyone else in the lab looked over.

Zhao Hongzhi asked, "What do you say?" ”

Xu Chuan thought for a moment, organized his words, and then said, "Every D-T fusion produces a neutron of 14.1 MeV. Since the neutrons are not charged, they cannot be confined by a magnetic field, and they will be directly bombarded on the first wall material and cause damage. ”

"14.1 MeV is a very large energy, you must know that the atoms bound in the material are all kinds of chemical bonds, and the bond energy is about 1~10 eV."

"In other words, a 14.1 MeV neutron carries enough energy to break millions of ordinary chemical bonds, which will undoubtedly cause irreparable damage to the material."

"In a fusion reactor, high-energy neutrons are like bullets fired at the material, constantly hitting metal atoms, breaking the chemical bonds around them, forcing the atoms to leave their original positions, thus destroying the regular atomic arrangement."

"If it is simply to resist neutrons, perhaps a structure made of materials such as beryllium gold, graphite, graphite and uranium-238 can do it, and nuclear fission reactors use these materials for neutron reflection."

"But in a controlled fusion reactor, it doesn't work."

"The reason is simple, because we need neutrons to do tritium self-sustaining, otherwise the tritium raw materials currently stored simply cannot support the commercial use of controlled nuclear fusion."

"So I personally think that instead of looking for a resistant material in a metal material, why not look at other materials?"