Reply #6: The torus ring acts to help control

[Turbineguy]

reactor vessel steam pressure. Pressure is controlled by sending heat into the torus. It's more efficient than opening a steam dump turbine bypass directly to the condenser, although that exists as well. The control rods deal with controlling the chain reaction and the recirculation deals with the pressure fluctuations. They are separate issues. Here's why: Reactor control is different than conventional fossil fuel boiler combustion control system which can regulate steam pressure fairly precisely because of low hysteresis and low latency.

The Pressure vessel wall thickness is determined by the pressure it has to contain. BWR reactors can run at lower pressures as steam is generated in the reactor and not in a separate secondary pressure vessel such as PWR's do. Hence a thinner shell.

From my trusty Baumeister's (8th edition page 9-122):

"Boiling water reactors have a simpler design and can utilize relatively thin-walled pressure vessels and pipes because they operate at moderate pressures compared with pressurized-water reactors. Fuel cladding temperatures are only slightly higher than steam temperatures and there is an inherent safety factor because steam-void volume increases on a transient power increase.

<snip>

Load changes are accomplished by steam bypass control or rods since adjustment of the turbine throttle and consequent reduction in steam flow will cause an increase in pressure in the reactor, which in turn will cause the collapse of the steam bubbles and increase reactivity. The control system functions to maintain constant reactor pressure, and reactivity control is achieved in part by varying the recirculation rate in the reactor."

There's a lot of information in those two paragraphs that dispute some of the contentions about peak fuel temperatures likely experienced in the Fukushima reactors.

In conventional boilers the collapse and growth of steam bubbles due to pressure fluctuations, which still occur within the parameters of combustion control, result in a phenomenon called shrink and swell. This is where the boiler water level rises when the turbine throttle is opened and drops when the throttle is closed in. Feed water flow is controlled by balancing water level against steam flow with water flow as a corrective feedback. Because of the way steam is generated in the reactor or in boiler tubes, the control of steam bubble size is important as the specific heat of steam and therefore the heat transfer rate is so much less than water that large bubbles create localized and temporary hot spots. In addition, higher pressures result in higher steam density therefore greater heat transfer. Steam is created where ever the heat source is and therefore there's plenty of steam well below the water level.

I hope this helps.