Bypass flow in a pebble bed reactor

[Maokeng]

I've been trying to get more understanding of bypass flow in a reactor, especially the "hot bypass flow" and how it affects the core state, and its origin. Does anyone know any relevant literature to read or can give an explanation to this.

Thanks.

 

[Astronuc]

Bypass flow would have to be considered in calculations related to the total coolant flow and the thermal energy removed from the core, i.e. one simply subtracts the by-pass flow from the total flow to determine the flow available to cool the core. The by-pass flow is determined by the core design.

Total flow may be determined from the pumps (turbo-compressors for gas flow) and the performance curve and power input.

 

[rainier]

A bypass flow was foreseen in the German AVR pebble bed reactor in order to cool the shutdown rods, which cannot withstand temperatures higher than 700°C and which were guided in separate, so called reflector noses. Unfortunately this essential bypass was forgotten by AVR operators in calculation of the core flow, i.e. the core coolant flow was calculated higher than it was in reality. This led to a temperature increase in the core compared to calculated values by at maximum 40 to 70°C.

This does not explain the observed hot spots in the AVR, which amounted to values of 300°C and are probably mainly due to pebble bed mechanics reasons. Core external bypass flows do not lead to hot spots but to a homogeneous temperature increase.

For more details see:

http://www.nuclear-engineering-journal.com/web/o_archiv.asp?o_id=200804221619-119&task=04&nav_id=