| Summary: | The PBMR (Pebble Bed Modular Reactor) is one of the current developments in the field of
nuclear power generators. The control philosophy of the PBMR system relies heavily on the
controlling of valves. The current control valves are subjected to a maximum temperature of
350°C with a pressure difference of 90 bar. Control optimisation can be obtained by including
'hot valves" into the system. The biggest improvement is possible with a bypass-valve after
the low-pressure turbine outlet. This valve will be subjected to a temperature of 720°C with a
pressure difference of 52 bar. PBMR personnel raised the concern that the components of
these valves (valve seat and sealing surfaces) in contact with the hot helium gas could tend to
weld to each other when they are in contact. An investigation was done to establish whether
these surfaces tend to weld together.
As no literature was found on testing for prevention of welding of materials under high
temperature pressurised helium conditions (Chapter 2), a testing facility was designed to test
the hot bypass-valve material (AISI H10) under operating conditions. This included the
design of a pressure vessel according to ASME Vlll Division 1 (Chapter 3) to be able to
simulate the helium operating conditions and a bolted connection (Chapter 4) to simulate the
valve contact conditions.
A finite element analysis was done, using ALGOR FEMPRO software (Chapter 5), to verify
the internal stresses of the pressure vessel based on the maximum allowable stresses for
material UNS NO6230 (Haynes® 230© Alloy), from Appendix 4 of ASME Vlll Division 2.
Firstly, a steady state heat transfer analysis was done to calculate the pressure vessel
temperature distribution. During a static stress analysis, these results were used to assign
the temperature dependent material properties to the various finite element elements. The
helium pressure and external pressure were simulated as uniform surface pressures. Based
on the Tresca effective stress results the maximum allowable 0.2% yield strength of Haynes
230 was exceeded. According to this analysis, the pressure vessel will yield when subjected
to the specified operating conditions. The calculated stresses also exceeded the ASME Vlll
Division 2 - Appendix 4 maximum allowable material stresses.
It is recommended that the same analysis be done with another FEM analysis software
package, to verify the calculated material stresses. This analysis should be incorporated into
the follow-up study, where the water-cooling system must also be designed. Before the
manufacturing of the pressure vessel can commence, a third party inspector must approve
the design. Any design updates necessary from the inspector's report should also be
included in the follow-up study. === Thesis (M.Ing. (Mechanical Engineering))--North-West University, Potchefstroom Campus, 2005.
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