Advocates of fusion frequently claim that the radioactive waste from fusion reactors not an issue, sometimes saying the waste is safe after 100 years and in some cases saying there is even no radioactive waste. The facts of radioactivity in fusion nuclear reactors are presented below.
In nuclear fission reactors, the dominant source of radioactivity is from the breakup of the uranium nuclear leading to radioactive isotopes. In fusion, the primary reactions leave no residual radioisotopes, apart from unburnt tritium, but the neutrons emitted from the reaction create radioactive isotopes when they hit the surrounding material. This process is called Neutron Activation and the nuclear reactions are illustrated in the figure below.
Neutron activation and subsequent decays
The possibilities for neutron activation are different for each different element, depending on the nuclear physics parameters. For some elements, there are no long-lived radioactive isotopes produced, while for others there are, with varying half-lives. The plot below shows isotopes as a function of the number of neutrons and the number of protons in the nucleus with the showing the isotope lifetimes: black shows stable isotopes and brown/red show those with long lifetimes relevant for the concerns in fusion reactors.
Radioisotopes vs. number of neutrons and number of protons. Colours indicate half-lives: black-stable, brown/red-years, yellow/blue-short.
The difficulties for fusion reactors, arise from impurities in metals which lead to a long-lived radioisotope. Below, are examples which leave long lived radioisotopes.
From the plot of radioisotopes and the nuclear physics, there are no general rules about which elements yield long-lived isotope, they can appear anywhere in the diagram. Furthermore, the appearance of impurities in different metals has numerous causes from the original ore to the refining methods themselves. The enormous number of radioisotopes means that some may be forgotten in theoretical evaluation of risks of radioactivity, one example of this is activation of the dominant oxygen isotope to short-lived nitrogen isotopes in cooling water.
To minimize enduring radioactivity in fusion reactors, there has been extensive R&D on materials for three decades. The necessity is to avoid elements which are activated to isotopes with long lifetimes. This has involved finding alloys which avoid certain elements, e.g. nickel because of their nuclear physics parameters. It has been possible to find special steel alloys which would decay to low level waste criteria in only 100 years, compared to conventional steel which would need 200,000 years.
The levels of radioactivity in fusion reactors are being estimated in reactor design studies and the figure below from, Activation, ... European DEMO concept, calculates the radioactivity levels in part of the vacuum vessel after the end of DEMO operations. The longest lasting radioactivity is due to the nickel radioisotopes and carbon-14.
The figure below from, Waste assessment of European DEMO fusion reactor designs, shows the timescales when the radioactivity in components has decayed to levels of Low Level Waste (LLW). The blanket modules, the divertor and the inner part of the vacuum vessel would remain at the radioactivity levels of Intermediate Level Waste for more than 1,000 years.
To understand the danger of the radiation levels to personnel, it is necessary to understand the relevant units and lethal doses. The table below gives the units.
|Activity||becquerel||Bq||disintegrations / sec|
|Absorbed dose||gray||Gy||Joule / kg|
|Equivalent dose||sievert||Sv||Joule / kg x WR factor|
|Effective dose||sievert||Sv||Joule / kg x WR x WT factor|
Where, WR the radiation weighting factor is 1 for gamma rays; ~10 for MeV neutrons and ~20 for alpha particles and ions and WT the tissue weighting factor.
Safety limits for radiation workers are set at 20 mSv per year. The lethal dose is 5 Sv. The pages, Safety and Maintenance, Breakdowns and Availability, give examples of situations outside the tokamak where dangerous levels of radiation could be encountered by personnel.
There is significant uncertainty in what will be the real radioactivity levels for actual constructions, because of the strong dependence on the impurities in the finally delivered materials. The figures below give an example of tungsten where small impurities (0.001 % Cobalt) adds significant amounts of long lived radioisotopes such as 179Ta, 60Co and 39Ar.
Examples of radioactivity decay curves for different grade of tungsten with different impurities.
Another problematic impurity in fusion reactor material is that of uranium in beryllium. Be is necessary in breeder blankets as a neutron multiplier and the EU-DEMO breeder blankets will contain about 560 t of Be. The U impurity level differs in different Be supplies, but is typically 30 wppm, which would give about 17 kg of U in the blanket set. The plot below, taken from On use of beryllium in fusion reactors: Resources, impurities, shows the activation processes. After 5 years of full power operation, the total amount of plutonium and other fissile isotopes produced would be about 4 kg.
Activation processes for U impurity in Be
To clarify the scale of the radioactive waste issue, in this section we compare the waste inventory of fission and fusion. The table below from, Status and Trends in Spent Fuel and Radioactive Waste Management | IAEA , shows the global waste situation from fission reactors.
Global total volumes of radioactive waste from all world fission reactors. Very Low Level Waste (VLLW), Low Level Waste (LLW), Intermediate Level Waste (ILW) and High Level Waste (HLW).
For fission the high level waste causes most public concern. Public objections to geological waste sites are now fading away and the world's first deep repository, called Onkalo, is ready for use in Finland. After a decade or so, it is possible that all the world's HLW will be buried and safe. It is the ILW which will still pose disposal issues for fission and it is this waste which will be produced in even larger quantities by fusion reactors.
Fusion reactors have much greater amounts of material subjected to a flux of neutron than comparable fission reactors. Correspondingly, the volume of radioactive waste is much greater although with shorter lifetimes. The main long-lived isotopes in fusion waste are given in the table below.
Key nuclides of concern for the waste from nuclear fusion.
The figure below gives a qualitative comparison of waste volumes in fusion and fission, with the US-DEMO (ARIES) reactor having 20 times higher volume of waste than a fission reactor.
An analysis of radiotoxicity of fusion reactors comparing to fission reactors, in Fusion power plants,..., radioactive waste, shows fusion reactors will have higher levels of short-lived radionuclides than fission reactors but the situation reverses over time because of the decay. For instance, after 100 years the radiotoxicity of fusion reactors becomes 100 times lower than fission reactors and after 500 years the radiotoxicity of fusion reactors is close to natural radiotoxicity.