What is Nuclear Waste?
In colloquial use, “nuclear waste” refers to all waste products with increased radioactivity. Not all of these come from nuclear power plants: Industry, mining, oil drilling, research and medicine leave behind substances the radioactivity of which may be more or less above the natural background. Depending on the radiation’s intensity (measured in Becquerel; 1 Bq = 1 decay/second), three groups of waste products are recognized:
- High-Level Waste: > 1014 Bq/m³, typical values 5 * 1015 — 5 * 1017 Bq/m³. Needs both cooling and shielding.
- Medium-Level Waste: 1010 — 1015 Bq/m³, needs shielding, cooling may or may not be needed.
- Low-Level Waste: < 1011 Bq/m³, needs neither cooling no shielding.
According to estimates by the Federal Ministry for Radiation Protection (Bundesamt für Strahlenschutz), of all the ionizing radiation released by the entire amount of nuclear waste stored in Germany by 2040, only around 2% will be emitted by mid- and low-level waste, 98% by high-level waste, among these predominantly by fuel irradiated in light-water reactors.
The light-water reactor is extremely inefficient: less than 1% of natural uranium’s heat content is utilized, at maximum 5% of the enriched uranium’s in the fuel. Little surprise that a lot of waste remains! After utilization in the reactor, the fuel is still made up of 95% uranium 238, 1% unused uranium 235, 3% of fission products and 1% of plutonium and higher transuranics. The accompanying image shows the involved volumina compared to the Brandenburg Gate in Berlin.
Fission products are initially highly radioactive, but decay relatively fast. Most of them will have been transformed to stable isotopes after a few centuries, after 300 years the mixture’s radiotoxicity falls below that of natural uranium. There are some long-lived nuclides among them, but they are very weak beta emitters. It should be noted that half-life and and activity of unstable substances are inversely proportional: The slower the nuclei decay, the less decay events take place per second, the weaker the radiation. Neither extremely long-lived isotopes with half-lives on the order of millions or billions of years (which radiate weakly), nor those with very short half-lives (as they decay rapidly), but those with medium half-life pose the most difficult disposal problem. These are the transuranics, the half-lives of which may be on the order of thousands of years and which also belong to decay chains. Currently, it is planned to store them practically indefinitely in stable rock formations or salt domes. But there is a better option: the Dual Fluid Reactor plus its Pyrochemical Processing Unit.
The Pyrochemical Processing Unit (PPU)
Most people associate the term “reprocessing” with facilities like La Hague, Sellafield or Wackersdorf and thus implicitly with PUREX: Plutonium-Uranium Recovery by Extraction. This process was developed in the course of the Manhattan Project in the 1940s: Solid fuel is dissolved in an aqueous solution. PUREX is extremely expensive and inefficient, but nonetheless the only reprocessing technology that has been put to commercial use. Nowadays, it is rarely used — nearly all nations worldwide operate their nuclear plants in a “once-through-cycle”, that is, the solid fuel stays in the reactor until the fission product concentration has risen too much, afterwards, geological storage is planned.
One of the main advantages of liquid-fueled reactors is the possibility of “on-site, online” reprocessing: While the reactor is running, a plant which is set up directly on the power plant premises removes fission products and adds fresh actinides constantly.
The DFR uses a so-called (Pyrochemical Processing Unit; PPU) is used for this purpose.
The PPU’s purpose is the element-clean separation of the fuel liquid using temperature differences: It is nothing more than a distillery! Production of spirituous liquors, but also many other industrial fields — such as titanium production, oil refining, seawater desalination — are based on the principle of steam rising in a column in which there is a temperature gradient, causing the different components to rain out at certain locations. The PPU adapts this well-known technology to nuclear fuels.
As metals have too high boiling points, for the DFR/m a prestage is needed turning the melt into a chloride salt (afterwards, the chlorine must be removed again).
In the PPU, the liquid salt is first “bubbled”, that is, argon gas bubbles are blown into it. This removes volatile fission products such as krypton, xenon and iodine, which are separated and stored for some weeks to months in order to decay. Noble metals among the fission products, which bind only weakly to chlorine, as well as the isotope molybdenum 99, which is important for medicine, deposit on the bubble surfaces.
The remaining mixture of substances gets evaporated in the distillation column and rains out at different heights — nearly element-clean. Depending on type, different treatments are applied:
- The actinides are transferred to storage containers with dosing valves and either get re-injected into the loop or re-routed to be utilized in other power plants. They are completely unsuitable for weapons production, being contaminated with plutonium 240, which surpresses nuclear detonations (by igniting the explosive too early due to spontaneous fissions, causing it to deflagrate rather than exploding).
- The short-lived fission products emit a great amount of heat, which need not go to waste: By storing them inside the lead loop (but outside of the reactor) they can add up to 20 MW to energy production.
- Long-lived fission products are portioned element-clean, encapsulated and stored in the reactor building.
- Very long-lived fission products such as Tc-99, Se-79 and I-129 are already transmuted inside the reactor and remain in the loop.
During operation, the DFR consumes exclusively actinides — such as depleted uranium, of which there are 2 million tons available worldwide: enogh, to satisfy the current consumption of electricity for 500 years, without additional mining! One could carry the weakly radioactive metal manually to the power plant, if its density wasn’t so exceedingly great (20 kg per liter).
Irradiated fuel from light-water reactors can be fissioned in the DFR, too, most of all the transuranics (plutonium and higher atomic weights), which are the main cause for the waste’s long term radiotoxicity.
The only waste stream are the fission products. Half of these will decay rapidly, meaning that per year only around 500 kg need to be stored in the reactor building (including encapsulation 15 tons). After 50 years part of them can be removed, after 100 years 90%, after 300 years at the latest all nuclides have decayed. These timeframes could be still further reduced substantially with a specific transmutation cycle. The outstanding neutron economy of the DFR enables transmutation of its own long-lived fission products as well as those contained in light-water reactor waste.
The DFR-PPU-Combo thus shortens the storage time of nuclear waste by a factor of 1000. The final repository problem becomes irrelevant! Even if it is decided to run a PPU without a reactor, a great simplifaction of the problem has been achieved: By separating the transuranics the necssary repository volume is massively reduced (see adjacent graphics).