HEAT REMOVAL FROM FUSION REACTOR DIVERTORS
ASIF AHMED
Department of Nuclear Engineering, University of California,
Berkeley, CA 94720-1730
NE-161 Project Report
Key words: ITER, ICF, PFC, CFC, CHF, HV
In a International Thermonuclear Experimental Reactor (ITER), plasma facing components (PFC's) will be subjected to high heat flux. Therefore, the protection of these components (PFC) is a very important issue for the design of ITER especially the divertor plates which will be subject to high heat loads. The divertor is a fluid cooled component of a fusion reactor, and there are number of factors which affect the heat transfer capabilities of the divertor. The relatively important ones are :(a) Heat flux distribution , (b) Tube materials and orientation. The development of actively cooled components for the divertor of a fusion reactor which can withstand the high heat flux during transient and normal operation of the reactor is one of the key issues for the development of the fusion reactor. (see K. Wilson article for heat removal from ICF Reactors) ICF.
Impurities in plasmas can lower plasma performance. Therefore it is important to remove these impurities from the plasma. The way of removing these impurities involve the concept of divertor. Therefore the divertor target plates must be able to withstand and remove heat from the hot plasma that impinges on them.
The concept of divertors consist of a metallic heat sink structure containig the coolant tube made of either copper or molybdenum alloy and protected by the heat by an erosion resistant surface material. In this multilayer structure the major engineering concerns are to demonstrate structural integrity at relevant temperature and stress cycles for the design life time. Some of the major concerns for such a concept are:
(a). Joint interface problem because of the significant differences in the physical and mechanical properties of the joining materials which causes thermal stress.
(b). Interaction with run-away electrons is another crucial problem which can cause the melting of the brazing or even the metal tube. It can also cause a sharp rise of a pressure in a cooling tubes.
(c). Because of the high heat flux, the material erosion rate is high which consequently requires a replacement of damaged divertor plates regularly. This limited life time of divertor plates presents a key problem in fusion technology.(see J. Rhoad's article for more on mechanical behavior of materials). Mech. Behavior of Mat.
In the case of a plasma disruption event the hot plasma is dumped along the magnetic field lines to the divertor plates in a very short interval of time, causing a sudden evaporation of a thin layer of the divertor plate material. However, most of the ablated material would be redeposited back to the surface. This erosion and re-deposition processes during plasma-material interaction is a major concern in a design of a PFC's. The erosion of the divertor armor due to the ablation by the incident plasma ions can significantly reduce the life time of the armor.
The deposition of heat by neutrons and impinging high energy particles on divertor is also non-uniform. This uneven heat deposition will result in a uneven temperature distribution. This would lead to an exchange of radiation heat between the various parts of the reactor interior which could have significant effect on the divertor plates.
Development of the PFC is one of the main issues for the construction of the future fusion reactors. Structural material that can be used for PFC have to fulfill the following requirements:
- High resistance against thermal shocks.
- High resistance against thermal fatigue.
- low Z-number.
- High mechanical strength.
- Preservation of the above properties under fast neutron irradiation.
Therefore for the operating conditions of a fusion reactor the following materials are the prime candidate for the divertor structure:
(a). Carbon fiber composite (CFC) is used as a armor tile material, because it is a low Z material with good thermal conductivity and has a superior thermal shock resistance capabilities. It is also used to avoid cracking and fracture of tiles which was observed in graphite tiles due to high heat flux.
(b). Copper and Molybdenum and their alloys as a heat sink material.
Armor tiles made of high conductivity CFC material are required to be metallurgical bonded to the cooling tubes in order to remove the high heat flux on the divertor efficiently
Click here for pictures of Type of Divertors.
Image taken from Ferro Gasparotto Knoepfel, Fusion Technology, 1992
(a). FLAT-PLATE TYPE (F-type): In this type of divertor CFC material is brazed on a copper heat sink that has a cooling tube, but this concept has a problem with high residual stresses after brazing.
(b). SADDLE TYPE (S-type): This concept is an intermediate between the F-type and the M-type divertors. The main advantage of this type of divertor is in situ repair of the damage tiles.
(c). MONOBLOCK TYPE (M-type): A Monoblock divertor consists of a metallic tube inside an armor tile, which is made of a CFC with tubes of molybdenum or copper. These cooling tubes contains a stainless steel twisted tape inserted to enhance the heat transfer into the water resulting in higher critical heat flux (CHF) of the component. Since in this type of divertor CFC block directly surrounds the cooling tube without any break, therefore the thermal stresses are lower than the F-type divertor. But the M-type is hard to manufacture and it is difficult to replace the damage tiles
(d). MACROBLOCK TYPE: In order to avoid the above concerns an alternative divertor concept has been proposed which uses only a single material, CFC, for the whole structure. This would eliminate or significantly reduces the concerns explained above (e.g. joint interface problem, thermal stresses, etc.). This new concept of a long CFC macroblock structure lined with a brazed thin copper cooling tube as a divertor target would avoid the critical issues due to the use of the small size M-type or F-type tiles brazed to a copper or molybdenum tube. CFC is implied to provide both the mechanical structural support for the thin cooling tube and to act as a heat sink. The large volume of the macroblock target would also reduce the thermal heat fluxes on the cooling tube by distributing the peak heat deposited into the volume of the heat sink by diffusion.
(e). Another technique for removing heat from divertor is the use of a "HYPERVAPOTRON". Hypervapotron (HV) is a water cooled device consisting of a finned surface made of a high thermal conductivity material like copper. Since copper HV can remove heat fluxes in excess of 30 MW/m^2, it can be a potential candidate for the heat removal from a divertor because it has the following advantages over swirl tube cooling:
- Possibility of breakage of twisted tape in a swirl tube, which can not occur in HV.
- Flow blockage or flow reduction can occur in a small channel where as HV has wide channels.
- Twisted tape around the circular pipe increases the distance between the heat flux surface and the heat removal surface.
- In a swirl tube, the burn out occurs as soon as CHF condition is reached at one point whereas in HV CHF condition must be reached on all heat transfer surface before this can happen.
- HV has the same thermal performance as the other concepts.
Two major issues for the design of the divertor due to the high heat flux are:
- Joint technology.
- Thermal hydraulic conditions
Thermal stresses after brazing due to a mismatch in a thermal expansion of a different materials is one of the most important problems in the fabrication of the divertor. Therefore the development of the joint techniques in general and specially in divertor are of paramount importance. There are many different techniques of joint preparation such as diffusion bonding and brazing. High melting temperature brazes are preferred in order to have a reliable margin on the allowed joint temperature in operation. On the other hand too high temperature can degrade the properties of the tube material during the brazing process.
Click here for a gif image of a BRAZING CONCEPT.
Image taken from Ferro Gasparotto Knoepfel, Fusion Technology, 1992
From the thermal hydraulic point of view, it is urgent to predict a critical heat flux (CHF) value and a safety margin for the cooling tube used in the divertor. Although numerous CHF correlations have been developed based on the uniform circumferential heating but none of these are based on the one sided heating. Experiments have done to evaluate the heat transfer abilities and to predict the CHF values at the tube surface for different tube geometries with different flow velocities and pressure under the following design specification:
- Turbulent promoter.
- Tube heating on only one side.
- Subcooled boiling regime.
A turbulent promoter is used to enhance the heat transfer and the CHF to the water. Also, studies has been done at JAERI to evaluate the applicability's of the existing correlations to the experimental data obtained in the experiment based on one sided heating.
PFC's must be protected from the runaway electrons because they can deposit large amount of energy and therefore severely damage those components. Heating by runaway electrons can also lead to over-pressurization of the cooling channels and subsequently to a loss of coolant accident. These structural components can be best protected by the thick carbon tiles or by inserting thin tungsten fiber in carbon tiles, otherwise melting of cooling tubes which are made of copper or molybdenum can occur at high heat flux.
Divertor plates made of CFC/graphite tiles erode because of the high heat concentration at tile edges due to the misalignment. But taper shaping and precise alignment can significantly decrease the local temperature rise and induced sublimation due to the high heat concentration and therefore the erosion of the tiles.
Other techniques, such as sweeping, are also applied to reduce the peak surface heat fluxes and the erosion damages to the divertor because of the high heat fluxes.
Another concern is the development of the structural support for the divertor plates. If the supporting structure for the divertor plate is rigidly fixed to the vacuum vessel, it can cause an unacceptable deformation and stress on the divertor plates due to the difference in the temperature between the high heat flux areas and the low heat flux areas. Therefore, support structures are required to be capable of movement in order to reduce stresses and deformation. In this regard a sliding rail concept has been studied as a support structure.
Several concepts use CFC material to remove power deposited on the PFC's e.g. F-type, M-type and S-type. But the size of these divertors are limited by the problem of brazing processes. In general, M-type divertors are better than the F-type because of smaller residual effects in the M-type.Click here for a gif image of a BRAZING EFFECT ON M- & F-TYPES OF DIVERTORS. Image taken from Ferro Gasparotto Knoepfel, Fusion Technology, 1992. The damaged caused by the runaway electrons can be reduced significantly by using tungsten filaments weaved in a CFC substrate. The erosion of the divertor plates can be reduced remarkably by the precisely alignment of divertor plates and taper shaping of CFC tiles. In addition,further CHF experiments under one sided heating are necessary to evaluate the applicability of existing CHF correlations based on uniform heating conditions.
- T. Ando, et al., Fusion Technology, (1992) 161-165.
- C.B. Baxi, H. Falter, Fusion Technology, (1992) 186-189.
- M. Merola, L. Filipuzzi, R. Matera, Fusion Technology, (1992) 321-324.
- J. Schlosser, et al., Fusion Technology, (1992) 372-375.
