The Tritium Breeding Blanket

The element located after the plasma facing compo­nents is called the blanket (Fig. 4). This component performs several functions:

• First, the blanket provides shielding by signifi­cantly reducing neutron/gamma radiation energy to protect the outer following components (e. g., va­cuum chamber, magnetic system).Its second function is to recover the energy left by the neutrons by heating materials. A coolant circulates in the structure and evacuates the heat generated to conventional equipment for electricity production.

• In the case of a reactor based on the D-T reaction, the blanket must also produce tritium necessary for fusion reactions by neutron bombard­ment on lithium:

n + 6Li-T + 4He + 4,78 MeV

n + 7Li-T + 4He + n – 2,47MeV

One should note that the first of these two reactions is exothermic and, thus, is preferable. This requires 6Li – enriched lithium to be used. This enrichment opera­tion is simple and already mastered in industry (for military purposes). It may also be noted that the blanket is the site of reactions generating energy that altogether account for 20% of the reactor balance. Lithium may be in a solid form (ceramic) or a liquid form (metallic alloy) depending on the blanket design implemented. The presence of a tritium breeding material does not suffice to ensure suitable produc­tion of tritium. Ended, the reaction of a tritium and deuterium atom produces only one neutron, and the reaction of this neutron on lithium produces only one tritium atom. Because neutron losses are inevitable, an adequate amount of tritium cannot be produced under these conditions. To remedy this drawback, a neutron multiplier is introduced into the blanket, allowing the appropriate balance. Lead or beryllium may be used as a multiplier. Blanket tritium-generat­ing capacity is characterized by the self-sufficiency ratio (or tritium breeding ratio). If the tritium breeding ratio is greater than 1, the blanket produces more tritium than is consumed by the reactor.

Numerous tritium breeding blanket designs are available that fulfill the three functions just
described. They differ in terms of tritium breeding material type, coolant type, and structural material types implemented. The combinations adopted are the result of trade-offs regarding compatibility of materials with each other and the allowable opera­tion windows (e. g., operation temperature, swelling resistance). The choice of coolant will also depend on solutions adopted for plasma facing components, with a single coolant being preferable for obvious reasons of simplicity. The choice of structural material is also vital because, in general, it is the structural material that dictates performance levels in terms of efficiency (via its maximum allowable operation temperature) and regarding lifetime of blanket elements. The main criterion is resistance under 14-MeV neutron irradiation, where a max­imum of 150 dpa is usually adopted, corresponding to approximately 5 years operation at full power. Therefore, periodic replacement of blanket compo­nents is provided for in the design. The long-term induced activation of components can be tailored by proper selection of materials to avoid generation of waste that would require deep geological disposal. These materials (so-called ‘‘low-activation’’ materi­als) could be steel in which elements that are penalizing regarding activation are replaced with other elements that are more interesting but compa­tible from a metallurgical standpoint (e. g., nickel, molybdenum) or other material families. These are presented in order of development risk:

• Low-activation ferritic-martensitic steels have been developed to the point where their use in a nuclear component may be possible in the relatively short term. They are notably developed in Japan and in Europe, with the latter having focused its studies on said materials. A particular grade was selected (EUROFER steels) and has already given rise to significant melts (a few metric tons), allowing the manufacture of samples that were subject to numer­ous tests (behavior under irradiation, corrosion, welding, etc.). The operation temperature is approxi­mately 550°C, or even 600 to 650°C, through specific optimizations (e. g., ODS steel, composition optimization). Operation temperature windows lead to efficiency of approximately 35 to 40%. The main question that still remains open concerns the effect of 14-MeV neutrons and, specifically, the effect that the formation of hydrogen and helium produced by transmutation has on embrittlement.

• Vanadium alloys are interesting in that they can be used in up to 700°C. This would allow yields of up to 45%.

• Silicone carbide composites (SiC-SiC) could operate at very high temperatures (1000°C) and, therefore, allow access to efficiency close to 60%. However, their status of development would not allow them to be used in the short term.

Schematically, there are two main types of blankets.

2.4.1 Solid Tritium Breeding Material Designs In this type of blanket, the tritium breeding material is a lithiated ceramic (Li2O, Li4SiO4, Li2TiO3, or Li2ZrO3) and beryllium is used as the neutron multiplier material. These materials are generally in the form of pebble beds (diameter on the order of millimeters). Tritium is extracted by gas circulating (He) that is in contact with the tritium breeding material and transports tritium outside the blanket. The structure is made of low-activation ferritic – martensitic steel, possibly hardened (ODS-type steel) to allow a higher coolant temperature. Coolant may be either pressurized water (typically 320°C and 15 MPa) or helium (typically 500°C and 8 MPa), allowing efficiency of between 35 and 40%. The main difficulties taken into consideration in this type of blanket design are as follows:

* Chemical compatibility of beryllium and water that may induce production of hydrogen and a risk of explosion

* Tritium permeation in water

* Evolution of pebble beds (ceramic and beryllium) when subject to irradiation

* Lower shielding capability if helium is used as a coolant

* Fuel manufacturing costs, the renewal obligation due to consumption of lithium, and fuel reprocessing to recover lithium that has not been consumed

Updated: September 26, 2015 — 1:10 pm