Thick vessels

Another unique consideration is the prevention of nuclear criticality within the cells. In the dissolver and first cycle, criticality is prevented by the presence of the uranium-238, which absorbs neutrons. Later in the process, the plutonium is separated from the uranium. Criticality is prevented by proper design of the vessels and piping. This includes the cell floor and sumps, where materials would collect in case of leakage from the equipment. To prevent criticality, the vessels are limited in either diameter or thickness. Vessels and piping are placed in arrangements designed to avoid a critical array. 

Thick vessels, like a bottle or a mortar, should never be heated because the uneven expansion of the material causes them to break. 

The presence of blood protein molecules, such as albumins and globulins, are critical factors in maintaining the proper fluid balance between cells and extracellular space. Proteins are present in the capillary beds, which are one-cell-thick vessels that connect the arterial and venous beds, and they cannot flow outside the capillary beds into the tissue because of their large size. Blood fluid is pulled into the capillary beds from the tissue through the mechanics of oncotic pressure, in which the pressure exerted by the protein molecules counteracts the blood pressure. Therefore, blood proteins are essential in maintaining and regulating fluid balance between the blood and tissue. The lack of blood proteins results in clinical edema, or tissue swelling, because there is insufficient pressure to pull fluid back into the blood from the tissues. The condition of edema is serious and can lead to many medical problems. 

The ultrasonic system comprises two scanners and one control console and was designed and constructed by Risley Nuclear Laboratories of the UKAEA. The design intention was for defect signals to be reported and sized in accordance with ASME XI code requirements for the 50 mm thick vessel shell. Weld profiles of the main seam welds were used to define probe angles and movements and a novel permeable membrane, irrigated by water at a rate of 0.04 litres / hour was developed to provide the necessary ultrasonic coupling. The main features of the scanners and the control console are as follows. 

A thick vessel is to be designed to withstand an internal pressure of 50 MPa. An internal diameter of 300 mm is specified and steel with a yield stress of 183 MPa is to be used. Calculate the wall thickness using the Tresca and von Mises criteria using a factor of safety of 1.5. 

Shielding Biological concrete 152 cm thick. Vessel 120 cm of bora ted graphite... 

The stress distribution given by Eq. 15.1 is shown in Fig. 15.1 for a vessel with r /fj = 2.2, The maximum stress is in the hoop direction and is at the inner surface where r = r. As the pressure is increased, the stresses increase until they reach a maximum limiting stress where failure is assumed to occur. For thin vessels the ASME Code assumes that failure occurs when the yield point is reached. This failure criterion is convenient and is called the maximum principal stress theory. In thick vessels the criterion usually applied for ductile materials is the energy of distention theory. This theory states that the inelastic action at any point in a body under any combination of stresses begins only when the strain energy of distortion per unit volume absorbed at the point is equal to die strain energy of distortion absorbed per unit volume at any point in a bar stressed to the elastic limit under a state of uniaxial stress as occurs in a simple tension test. The equation that expresses this theory is given by... 

In thick vessels, a steady-state thermal condition gives rise to a logarithmic temperature distribution that can be expressed as... 

Equation 8.1 is shown in Fig. 5.6 as being very similar to Lame s Eq. 5.9 for thick vessels. Disregarding external pressures, s. 5.9 and 5.10 become... 

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