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heat transfer test facility
The PBMR Heat Transfer Test Facility
The Heat Transfer Test Facility (HTTF) at Potchefstroom near Johannesburg, provided PBMR with a basis to test the behaviour of high temperatures in conditions similar to the actual PBMR reactor's conditions. The HTTF was used between 2000 and 2004 to perform heat transfer testing. Temperatures in the reactor must be safe up to 1600° C, which is far beyond the melting point of most metals. This means that it is very important to design the PBMR reactor so that it is able to handle these temperatures sufficiently and safely.
To understand the Heat Transfer Test Facility, one needs to first understand the three basic mechanisms of heat transfer found in the reactor. These are conduction, convection and thermal radiation.
Conduction:  When you touch a hot surface such as a stove plate or a coffee mug, you instantly feel the heat. What actually happens is that the heat is conducted from the hot surface to your cooler hand when you establish contact. Conduction is in other words movement of heat through mostly solid objects from the hot to the cold side.
Convection:  When you put your hand above a fire, you feel a hot stream of air across your fingers. What actually happens is that the fire heats the surrounding air and the air starts to flow upwards. As this air passes over your hand, heat is transferred to your hands from the warm air to your colder hands. This is called convection heat transfer and happens when a liquid or gas passes over an object and heat is transferred from the one to the other.
Thermal radiation:  This you normally experience when you feel the heat from a red hot heater, when you feel the rays of the sun or when you stand in front of a fire. Heat is transferred by means of invisible electromagnetic waves.
Back to the HTTF. Another important fact about the heat transfer test facility was that it mimicked the internal area of the PBMR reactor, which is filled with graphite fuel spheres (a kind of “mini-me”). The internal area of the PBMR reactor is a tall ring-shaped space which looks a bit like a barrel or a pipe (also called an annulus) and is built with graphite bricks. The annulus area is filled with thousands of fuel spheres (almost the size of tennis balls) which provide the heat necessary to generate power through the fission process from the low enriched uranium in these fuel spheres. Once these thousands of fuel spheres are stacked on top of each other in the annular space, it is difficult to predict how heat is distributed or transferred through the different components, or parts, and spaces, by means of the 3 basic heat transfer mechanisms which we discussed above. This is because, just as is the case with a container filled with BB-gun pellets, there are voids or gaps between the particles. These voids are in general irregular and of different sizes, depending on the position in the annular space. Where spheres press against the side of the reactor, the voids might, for instance, be larger than in the middle of the annulus where a sphere is surrounded by five or more other spheres.
It is normally relatively easy for engineers to calculate heat transfer for individual components of parts and materials. but the fact that the reactor consists of thousands of pebbles stacked together with varying voids in between, makes it much more difficult to predict the heat transfer behaviour. The main function of the HTTF is therefore to provide a facility which can be used to create similar conditions to that of the PBMR reactor core. This enables engineers to check their calculations of heat transfer and to make sure that they are on the right track.
The HTTF consisted of two test units, the High Pressure Test Unit (HPTU) and the High Temperature Test Unit (HTTU). Both of these mimic specific conditions found in the PBMR reactor.
The HPTU uses high pressure and low heat to look at each of the three heat transfer mechanisms in isolation at different positions in the reactor annular space.  The HPTU, which is the smaller of the two facilities, is essentially a controlled close-flow-loop providing specified flow conditions to a main test section, very much the same as a wind tunnel. The facilitiy produced a range of flow conditions in order to create meaningful correlations of whatever is inserted into the main test vessel. Twelve interchangeable test sections were designed to provided PBMR the opportunities to test the different heat transfer mechanisms in different ways.
The HTTU used high temperatures to mimic the actual conditions found in the reactor. It also provides a full annular space as found in the reactor and tests all three heat transfer mechanisms together at different conditions.
The HTTU resembles a 1.2 m tall section of an annular reactor core, complete with graphite central column, graphite bricks and 28 000 graphite pebbles. Heat is provided by nine graphite tube heaters with a capability of reaching approximately 1 650° C. Because the temperatures are so extremely high for this test, safety precautions are put in place to control the boundary conditions. A water jacket surrounds the side reflector to keep the unit cool and high-tech insulation materials guard the top and bottom of the bed. By measuring the temperature in the facility at hundreds of strategic positions, a 3-dimensional heat transfer profile of the bed can be obtained at different conditions.
The testing phase of the Heat Transfer Test Facility programme has been completed and final data processed. All tests in the originally specified programme were successfully completed on time and within budget. This concluded over three years of HTTF operations, during which the team achieved nearly 50000 accident free manhours and produced 18 test reports and over 150 technical memos. Certain results (e.g. pressure drop across packed beds) have been published and presented in various forums (HTR, SACAM, ENEN). The facility is currently is a care and maintenance state.
The major components at end of lifespan, the facility will be dismantled.
Last Updated: 16 May 2017
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