Technology of immobilization of tritium-containing liquid radioactive waste
Every day the volume increases by 400 m3.
That’s how the station site looks like these – thousands of tritium water barrels.
Heavy-water moderated reactors of “CANDU” type are significantly advantageous compared to other nuclear energy technologies as natural uranium is used there instead of enriched uranium.
But inhibitor (and sometimes – coolant) is heavy (deuterium) water. Some atoms of deuterium catch neutron under nuclear flux and turn into superheavy isotope of hydrogen – tritium.
Each block of CANDU makes over 1.5 mio curie of tritium LRW per year.
But what is the problem?
Why tritium water management is so difficult?
The matter is, there is an attempt to apply processing technologies used for usual liquid wastes for tritium LRW. All such technologies are used for LRW solidification – waste compaction, first (that is, radioactive salts and organic matter separation from water). In case water is “usual”, it is drained without processing followed, and sedimentation either burnt or cemented or placed in some other mineral matrix. In such a way, it is possible to cut the volume of wastes to be processed in 2-2.5.
But in case LRW is dissolved in tritium water this way doesn’t work as separated distillate is radioactive as well.
That’s why its drain into ecosystem is prohibited.
Hence originating of various processing technologies of tritium water itself – they imply tritium concentration in some little water, and the left part of water that is considered to be “clean” is discharged. Next, the water already enriched with tritium is packed into some kind of mineral or metal matrix.
In our opinion, this natural idea is faulty. In fact, chemically and physically heavy water and usual water are identical ones, that’s why their separation is very painstaking. That’s why all the technologies of “tritium concentration” are very nonproductive. The cost of Radium Institute plant (Saint-Petersburg) processing 100 l/h is 12 mio dollars. The processing self-cost is 400 dollars per a ton of wastes.
Our concept implies solidification of LRW “as they are”, i.e. without preliminary dewatering and tritium concentration. For sure, it will result in some volume growth of much less hazardous secondary wastes but it will allow effective managing of the larger amount of cumulated LRW.
So, we do not purify LRW, we do not concentrate tritium but we pack LRW “as it is”.
Crystalline hydrate is a special kind of salts where each molecule binds several water molecules. For example, stable ferric sulphate FeSO4 binds 7 water molecules. It is indicated as follows: FeSO4×7H2O. Molar weight estimation shows that water is about 45% of its weight in this watered crystalline hydrate. But in sodium phosphate Na3PO4×22H2O water is about 57% of its weight.
The idea is as follows: replacing of crystallization water by tritium water in crystalline hydrate.
That is what will bind tritium!
- Crystalline hydrate dehydration.
- Salt tritiation – watering of dehydrated salt powder by tritium LRW right in the same container where such LRW are. After water taking crystalline hydrate consolidates, i.e. transforms into a solid stone.
- Radioactive salts and organic matters non-bound by crystalline hydrate salts are pressed out of the stone within 12-30 hours.
- The space between the stone surface and the upper cover of container is filled with solid mineral compound, for example, based on magnesium oxychloride cement. It’ll bind the separated radioactive impurities firmly.
- The container is closed by the cover and sent to the disposal site.
The most painstaking stage is the first stage. It is quite power consuming stage and can take considerable time. But it is not obligatory to undertake dehydration of crystalline hydrate in the territory where wastes are. It can take place in any territory, in any country. The only point that has to be provide is waterproof package for dehydrated powder. That is why we do not consider time spent for crystalline hydrate dehydration in the total process time.
The next two stages are on behalf of Nature. It is not obligatory that staff supervises consolidation and formation of radioactive impurities at the salt stone surface. Instead the staff can tritiate salt in next containers or prepare mineral compounds for ready containers closing.
Mineral compounds preparation is the process analogous to concrete preparation. In case magnesium oxychloride cement is chosen, even fresh water is not required.
What is left is to close the ready pack with cover and to drive it to a long-term storage disposal site.
According to our estimations, the total time for the staff (1-2 people) spent for packing 200-l barrel (for instance) with 150 liters of tritium LRW is equal to 10-12 minutes.
Devices designed by us in order to implement the described technology provide radiological safety to the staff reliably.
Trials on real tritium LRW showed extremely decreased activity: from initial level 83 mcSv/h to 3.2 mcSv/h for one and the same wastes amount (free and packed).
Research part is completely over at the moment:
- The concept is developed;
- Requirements to the technology and equipment are developed;
- The laboratory-scale plant is manufactured and tried
Key elements of the process and device for its implementation are patented.