Cooling thermal shield

The still cooling power gstm, due to the 3He evaporation process, can be relatively large (up to a few mW in big DR). It is thus possible to thermally connect a thermal shield to the still and also to berth and thermalize capillaries and wires. 

Thermal shields connected to heat exchangers cooled by the evaporating gas are used to drastically reduce the radiative input. 

Cryopumping surfaces cannot generally be exposed directly to a source of gas at room temperature because the heat load due to radiation would exceed that due to the condensation of gas molecules. Therefore, the cryogenic surface is protected on the side facing the gas source. As protection against thermal radiation, an optically dense baffle comprising liquid-nitrogen-cooled blackened shields is used often. 

The standard DSC experiment with lipid systems requires degassing the samples and instauring an extra pressure of 1-2 atm N2. The cells and the surrounding adiabatic thermal shield are then equilibrated at least 5 °C below the desired start temperature of the experiment. Heating starts at a pre-established rate (e.g. 1 °C/min) in a region where no transition is expected, so that a baseline is obtained. At a temperature above the phase transition the whole system is cooled down, reequilibrated and re-scanned. Three heating scans are routinely recorded sometimes the first scan differs from the other two, owing to insufficient equilibration of the sample, in which case the first scan is discarded. 

Immediately surrounding the small enriched lattice is a primary reflector %be yIlium metal which is also water cooled. This whole assembly of active i% ice and beryllium reflector is mounted in a tank system through which the er flows and which contains the control rods and their bearings. Outside tank system are a secondary reflector of graphite, a thermal shield, and JSsAatlbiological shield, the whole forming an approximate cube of about 34 ft to side.. 

The approximate overall size is as follows 16 ft in the north-south direction, 14 ft in the east west direction, and 12 ft 6 in. in height. The thermal shield, whose heat generation, rate is approximately 95 kw, is cooled by an induced-draft air system which also serves as the graphite cooling system (see Section 2.7.2). The primary supporting structure is composed of four vertical "WF" columns with the load transmitted to these columns at the four corners of the bottom plate. 

The bottom plate of the lower thermal shield, while resting directly on the steel supporting structure, is in direct contact with the barytes concrete below it. This helps to. cool the concrete by direct thermal contact. The upper plate of the lower thermal shield is in two parts. An inner ring 14 in. wide is welded directly to tank section E (see Fig. 2.E) in order to provide maximum cooling for this section.The outer, section is supported from the lower plate and is air cooled only. 

Air Cooling. The thermal shield and the graphite. are cooled by air drawn through them from the Reactor Building. The amount of heat that, must be removed is.indicated in Chap. 4 the external air system to provide proper air flow is described in Chap. 8. ... 

The major part of the air flows between the side plates of the thermal s,hield and then into the space between the bottom thermal shields. Holes in the upper plate of the bottom thermal shield distribute the air into the pebble zone and into the cooling holes of the permanent graphite. A small amount of this air goes outside the outer side plates of the thermal shield to help cool the concrete. By means of baffles in the inlet ducts some of the air is drawn between the two plates of the upper thermal shield and then through holes in the lower plate into the space above thegraphite. 

Since graphite begins to oxidize (O.OOOS to 0.0009% per clay.) at about 570 F, this temperature was considered to be the maximum, allowable for 45,000-kw operation. With this limit it was found ) that the required cooling in the pebble zone could be obtained by an air flow of 840 Ib/min, which requires a pressure drop of 26 in. of water. This gives an average exit air temperature of 203°F from the pebble zone if the air is heated 15°F in the thermal shield to 90 F. To this air is added the 684 Ib/rnih flow through the permanent graphite and experimental facilities and the 174 Ib/min to cool the top... 

Heatiog and Cooling of the Thermal Shield. Heat production in the thermal, shield has been calculated, and these, caIculations -show a total... 

Briggs. R. B., Cooling of Graphite Reflector for UTR, ORNL CP-50-3-102, March 20, 1950 Temperature in Bottom Thermal Shield Proposed for MTR, ORNL CF-50-1-45, January 10, 1950. 

Subsequent calculations gave the air req.uirements as 1526 Ib/min to cool the graphite, And experimental facilities, and 174 Ib/min to cool the top thermal shield, for a total air flow of 1700 Ib/min. Also, critical experiments conducted at ORNL indicated that the heat generation in the reflector would be somewhat less than that used for the above calculations and the air requirements would therefore be less. However, the design figures were maintained at 2000 Ib/min flow and 55 in. of water pressure drop since it may be desirable and possible to operate the reactor at a power level as high as 60,000 kw at some time in the future. 

The inlet system consists of four sets of filters, plenum chambers, ducts and manifolds, one set in each wall of the reactor structure. The air enters each set through louvers located near the top of each pile face. Back of each set of louvers are six, 2 ft-square filters and a 2- by 2- by 12-ft plenum chamber. Each plenum chamber is joined to the top edge of the thermal shield by five or six 8rin.-square ducts and a manifold which is continuous around the top of the thermal shield. A separate channel with a damper is provided in each of the 8-in. ducts whereby a small amount of air is by-passed for cooling the top thermal. shieId. There is a total of 22 8-in. ducts. Each duct has one 90 elbow and connects to the plenum chamber, and the. thermal shield manifold... 

The reactor rests on a concrete foundation. This foundation, of ordinary concrete, supports the reactor and provides shielding for the rooms and equipment below. Generally, there is an insulating layer between the bottom thermal shield and foundation. Only the K Reactors are provided with foundation cooling pipes. 

The inlet and outlet biological shields are cooled by the water in the process tubes. Because of the annular gaps in the donut system, the heat transfer path is rigorous. Ail reactors except B, D and F have an aluminum reflector between the biological and thermal shields to reduce heat transfer between the two shields. 

Better materials are available for thermal shields. The N Reactor utilizes boron steel plates one-inch thick with the cooling tubes welded in place. This material has all the assets of cast iron plus a lower potential cost. With a 1-1/2 per cent boron inclusion, only a one-inch thick thermal shield is required. This means other cost reductions such as smaller biological shield, shorter rods. etc. 

FIG. 7. Plan view of a liquid metal reactor of submarine factory number 601, showing the approximate layout of the emergency cooling tubes and core. For clarity, individual thermal shields and Pb-Bi coolant channels are not shown. 

FIG. 13. Formation of an annulus from the emergency cooling tubes and corrosion from the outside of the thermal shields in the liquid metal reactors of submarine factory number 601. 

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