Radioactive Waste

The radioactive waste from a fusion reactor is fundamentally different in origin from that of a fission reactor. This page looks, step by step from intuition to the research frontier, at why fusion produces no long-lived high-level waste, why waste is nonetheless generated, and how far material design can reduce it.

Start with Intuition (High School)

First, let us grasp that the “nature of the trash” is completely different between fission and fusion.

A fission reactor extracts energy by splitting uranium, a heavy nucleus. The fragments that result (fission products) and the very heavy elements such as plutonium that are born by absorbing neutrons accumulate inside the fuel itself. Some of these keep emitting radiation for tens of thousands of years. In other words, in fission the spent fuel itself becomes long-lived high-level waste.

A fusion reactor is the exact opposite. Its fuel is deuterium and tritium, members of the hydrogen family and the lightest elements in the periodic table. Sticking light nuclei together does not create heavy elements like plutonium. So the very phenomenon of fuel transforming into long-lived trash simply does not happen.

Then why does fusion still produce radioactive waste? The key is the neutron that flies out of the reaction. When deuterium and tritium fuse, a very fast neutron is ejected. When this neutron embeds itself in the reactor walls and structures (metals and the like), atoms of the metal that originally had no radioactivity turn into atoms that emit radiation. This is called activation.

An analogy is a sunburn. In a fusion reactor the fuel itself does not become dirty trash. Instead, the “surrounding container” that keeps being bathed in strong light (neutrons) becomes activated, as if getting sunburned. And this sunburn can be made into the “cools down quickly” type if the materials are chosen well. The biggest point is that the character of fusion waste is decided not by the fuel but by what materials the reactor is built from.

Understand the Physics (Undergraduate)

The true nature of activation is nuclear transmutation, in which a nucleus captures a neutron and changes into a different nuclide. The central process is a reaction called neutron capture.

When a nucleus $^A_Z X$ absorbs a single neutron, it becomes an isotope of the same element with its mass number increased by one.

$$^{A}_{Z}X + n \rightarrow {}^{A+1}_{Z}X + \gamma$$

This represents the target nucleus $X$ absorbing a neutron $n$, releasing the surplus energy as a gamma ray $\gamma$, and increasing its mass number from $A$ to $A+1$. If the isotope thus created is unstable, it decays as a radionuclide. When the neutron energy is high, $(n,p)$ reactions (absorbing a neutron and emitting a proton) and $(n,\alpha)$ reactions (emitting an alpha particle) can also occur, sometimes changing the element itself. The 14.1 MeV neutrons produced by the DT reaction are extremely fast, so a wide range of such reactions can occur, which is characteristic of fusion.

The amount of a radionuclide decreases exponentially with time. Writing the number of atoms of a given nuclide as $N$ and the decay constant as $\lambda$, we have

$$N(t) = N_0 \, e^{-\lambda t}, \qquad \lambda = \frac{\ln 2}{T_{1/2}}$$

Here $N_0$ is the initial number of atoms and $T_{1/2}$ is the half-life, the time it takes for the amount to fall to half. A nuclide with a short half-life decays violently but disappears quickly for that reason. Conversely, a nuclide with a long half-life is weak but remains for a long time. The latter is the troublesome one for waste management.

If steel is used for the structural material of a fusion reactor, activation mainly produces nuclides such as $^{55}\mathrm{Fe}$ (half-life about 2.7 years), $^{54}\mathrm{Mn}$ (about 312 days), and $^{60}\mathrm{Co}$ (about 5.27 years). All of these have half-lives of a few years or less, and cooling them for a few decades to about 100 years greatly reduces them. The problem arises when the material contains elements such as nickel, molybdenum, or niobium. Under neutron irradiation these produce long-lived nuclides with half-lives reaching tens of thousands of years, worsening the character of the waste. This is exactly why which elements go into the material matters decisively.

Neutrons do not only deposit energy; they also knock atoms out of their lattice positions and damage the material. The unit for this amount of damage is dpa (displacements per atom, the number of times an atom is knocked out). The first wall of a fusion reactor is estimated to receive damage exceeding 100 dpa during operation, which together with activation becomes a factor determining the lifetime of the material.

Deepen the Theory (Graduate)

To treat activation quantitatively, we need to track a system in which many nuclides repeatedly undergo production and decay under neutron irradiation. The time evolution of the number of atoms $N_i$ of nuclide $i$ is described by a coupled set of ordinary differential equations that add up production and loss.

$$\frac{dN_i}{dt} = \sum_{j} \left( \lambda_{j \to i} + \phi\,\sigma_{j \to i} \right) N_j - \left( \lambda_i + \phi\,\sigma_i \right) N_i$$

The first term is the contribution from other nuclides $j$ turning into nuclide $i$ through decay (decay constant $\lambda_{j \to i}$) or a neutron reaction (reaction cross section $\sigma_{j \to i}$, neutron flux $\phi$). The second term is the contribution of nuclide $i$ itself disappearing through decay or a neutron reaction. Solving this production-loss equation (the burnup equation) simultaneously for hundreds to thousands of nuclides yields the activation inventory (the stock of radionuclides) during and after operation.

What matters here is the neutron spectrum. DT neutrons are born at 14.1 MeV, but they scatter repeatedly within the blanket and structural material and slow down toward lower energies. Because the reaction cross section $\sigma$ depends strongly on energy, the kinds and amounts of nuclides produced depend on how many neutrons of which energy reach the material. Activation assessment is therefore only possible by combining neutron transport calculations (how many neutrons of which energy exist and where) with nuclide-transmutation calculations.

The design philosophy of reduced-activation materials follows naturally from this framework. The goal is to produce as few long-lived nuclides as possible after operation stops. To that end, alloying elements that are the parents of long-lived nuclides are replaced with other elements of similar performance. A representative example is reduced-activation ferritic/martensitic steel (RAFM steel). Japan’s F82H and Europe’s EUROFER97 replace the molybdenum and niobium contained in ordinary steel with tungsten and tantalum, which have good activation characteristics. In addition, impurities that produce long-lived nuclides, such as nickel and cobalt, are managed down to the utmost.

The difference in decay time scales symbolizes the achievement of this design. Spent fuel from fission takes tens of thousands to hundreds of thousands of years to return to a radioactivity comparable to natural uranium ore, because of the transuranic elements. In contrast, fusion structural material made from reduced-activation materials is assessed to have its radioactivity greatly reduced after roughly 100 years of cooling following operation, with much of it approaching a level people can handle. Even under the same phrase “radioactive waste,” the order of magnitude of the time to be managed differs between tens of thousands of years and 100 years. It should be emphasized that this difference is not due to a difference in fuel but is produced entirely by material design.

The physics of reduced-activation materials connects to other material topics as well. The design of the structural material itself is covered in Structural Materials, and the overall comparison with fission in Fission vs. Fusion.

Research Frontier (PhD)

Assessment of the activation inventory is a central research field supporting the safety design and waste planning of fusion reactors. Activation calculation codes have been developed worldwide, a representative one being Europe’s FISPACT-II. These solve the production-loss equations of transmutation and predict, nuclide by nuclide, the post-operation radioactivity, decay heat, and radiation dose. Because the accuracy of the calculation depends on the quality of the input nuclear data (such as nuclear reaction cross sections), the development and validation of dedicated nuclear data libraries such as EAF and TENDL is an ongoing research theme. Uncertainty propagation, which estimates how uncertainties in cross sections carry through to the final inventory assessment, is also being actively studied.

On the materials side, several directions are being pursued in parallel: oxide dispersion strengthened steel (ODS steel) for using RAFM steel at even higher temperatures, vanadium alloys and silicon carbide composites (SiC/SiC composite) aiming for extremely low activation, and high-purity refining technology that suppresses impurities such as nickel from the smelting stage. All of these tackle the difficult problem of reconciling performance (strength, heat resistance, irradiation resistance) with activation characteristics. Building a powerful neutron source that simulates the actual 14 MeV neutron environment to obtain data, the International Fusion Materials Irradiation Facility (IFMIF, along with successor plans such as Japan’s A-FNS), has also been a long-standing task.

On the waste-disposal side, the concepts of clearance and recycling are being studied. Clearance is a system that removes materials with sufficiently low radioactivity from regulatory oversight and treats them as ordinary resources. “Recycling within fusion,” in which structural material that has fallen to the clearance level through cooling is remelted under shielding and returned to components of a new reactor, is also being examined as a conceptual design. However, how far material recovered by remote operation can be reused, and whether it is economically viable, remain assessment challenges for the future.

Decommissioning after the reactor finishes operation is also an important research theme. Because people cannot approach the activated in-vessel components, dismantling presumes remote handling by robots. ITER has developed large-scale remote operation systems for maintenance during operation, and that technology carries over to decommissioning. A whole sequence of plans is built on the results of activation inventory assessment: how long a cooling period is needed after operation stops for the decay heat and dose to fall to manageable levels, in what order which components are cut out, and how secondary waste from cutting and decontamination can be reduced.

Note that for the radioactive materials of a fusion facility, management of tritium itself is also indispensable. It is another pillar alongside activation of the structural material, and is covered in detail in Tritium Management.

Check Your Understanding

Q1. Why does a fusion reactor not produce long-lived high-level waste (such as plutonium) like a fission reactor does?

A Because the fuel is deuterium and tritium, the lightest elements, and sticking light nuclei together does not create superheavy elements B Because no neutrons are produced at all in fusion C Because fusion reactors use no solid structural material D Because the fuel tritium all turns into stable elements after the reaction

Q2. In the neutron capture reaction that is central to activation, what happens to the target nucleus?

A The target nucleus splits in two and becomes fission products B A neutron flies out of the target nucleus and its mass number decreases by one C The target nucleus absorbs one neutron, its mass number increases by one, and it releases the surplus energy as a gamma ray D A proton in the target nucleus turns into an electron and it becomes a different element

Q3. Which is correct about the relation between the half-life T1/2 and the decay constant lambda, and about why the half-life matters for waste management?

A lambda = ln2 / T1/2, and nuclides with shorter half-lives remain longer, making them harder to manage B lambda = ln2 / T1/2, and long-half-life nuclides remain for a long time even though weak, so the amount of long-lived nuclides produced governs the management period C lambda = T1/2 / ln2, and the half-life is unrelated to the strength of the radioactivity D lambda = ln2 times T1/2, and the longer the half-life the faster the decay

Q4. In reduced-activation ferritic/martensitic steel (RAFM steel), on what idea are the alloying elements chosen?

A Strength is the top priority and alloying elements are chosen without regard to activation characteristics B Molybdenum and niobium, the parents of long-lived nuclides, are replaced with tungsten and tantalum of similar performance, and impurities such as nickel and cobalt are reduced as much as possible C Nickel and cobalt are actively added to enhance heat resistance D Iron is not used, and everything is made of tungsten alone

Q5. How do the time scales for radioactivity to fall differ between spent fuel from fission and fusion structural material made of reduced-activation materials?

A Both take tens of thousands of years or more, with no difference B Spent fuel takes about 100 years and fusion structural material tens of thousands of years, so fusion is longer C Spent fuel takes tens of thousands of years or more, whereas reduced-activation structural material has its radioactivity greatly reduced after roughly 100 years of cooling following operation D Fusion structural material does not lose radioactivity at all even with cooling

Related Topics

• Structural Materials - Design and performance of reduced-activation materials
• Fission vs. Fusion - The overall difference in fuel and waste
• Tritium Management - Handling tritium, the other radioactive material