BWR Fuel Assembly Question -- Why do BWRs have shrouds?

[Kirk Truax]

Hey all,

I was wondering if anyone could help me with a basic BWR question. Why do BWRs have shrouds?

From what I've been able to find, this is a for "uniform cooling" but that's about it.

My guess is that there are some thermal hydraulic benefits around maintaining nucleate boiling. Is there a pressure gain over the fuel due to boiling that can help prevent the transition to film boiling?

Any thoughts on this?

Kirk

 

[Astronuc]

BWR assemblies have channels that provide for vertical flow in the core. Different assemblies will have different power levels, usually because a cell of four assemblies have two fresh and two once-burned, or two once-burned and two twice-burned, or some other combination. At different power levels, the void fraction profiles will be different, and the nucleate boiling and transition boiling (to bulk boiling, or annular flow) will occur at different elevations. The channels prevent loss of coolant due to lateral flow, which would starve the higher elevations of a fuel assembly. Dryout is to be avoided.

The channel also provides a stiff structural member between the lower and upper tie plates, which provides for seismic stability. In addition the channels of four assemblies help guide the cruciform control rods as they are inserted from the below the core.

There is no pressure gain from bottom to top of the assembly. There is a pressure drop due to drag on the coolant. The pressure equilibrates across the bottom of the core and separately across the top of the core. The coolant flow between adjacent assemblies can be different, depending on the power of each assembly.

Some notes on boiling in BWR fuel:
https://ocw.mit.edu/courses/nuclear...0/lectures-and-readings/MIT22_06F10_lec13.pdf

 

[jim hardy]

Why do BWRs have shrouds?

https://www.nrc.gov/docs/ML1215/ML12158A330.pdf
page 8 of 15

nice sketch on page 13

 

[krater]

Is it fair to say then that a core shroud assembly is only a negligible consideration with respect to radiation shielding along the narrow axis of the reactor in the core region?

 

[Astronuc]

Why do BWRs have shrouds?

Based on Jim Hardy's response, perhaps we better understand if 'shrouds' refers to the channels on BWR fuel assemblies or to core shrouds, which surround the core.

With respect to core shrouds, both BWRs and PWRs have core shrouds, although in PWRs the shroud is comprised of the core barrel and baffle, and supporting structures.
PWR - https://www.nrc.gov/reading-rm/doc-collections/gen-comm/info-notices/1998/in98011-fig01.gif
https://www.nrc.gov/reading-rm/doc-collections/gen-comm/info-notices/1998/in98011-fig02.gif

BWR - http://www.ndt.net/article/wcndt00/papers/idn268/fig1.gif

The BWR core shroud sits between the core and pressure vessel, and it forms an annulus for the feedwater to accumulate. In the annular regions are pumps. In the case of traditional GE design, the pumps are jet pumps which draw water from the annulus and force it into the lower plenum where the water turns up into the core. The ABWR uses more conventional pumps, which are located at the bottom of the annulus. ABB and Siemens BWRs used such pumps from the beginning.

The PWR core barrel and baffle provide the same function as the BWR core shroud.

PWR fuel assemblies are open, without channels, whereas BWR fuel assemblies use channels. The channels prevent cross-flow among adjacent assemblies, which is important because they can have different amounts of boiling or void fraction/quality.