Demystifying
the hotbox
G
Hulme compares a physical and numerical model of the hotbox of an advanced
gas-cooled nuclear reactor to understand fuel channel flow
The
hotbox region of an advanced gas-cooled nuclear reactor (AGR) receives
coolant gas at high temperature from the 332 fuel channels of the reactor
core. The gas streams recirculate and mix in the hotbox before passing
into the boilers. It is of great interest to know through which boilers
and in what proportions the streams from particular fuel channels pass
in a variety of operating conditions. This region has been studied using
physical scale models such as a 1/8 th scale model but it is desired
to complement the physical models, and possibly to replace them, by
a numerical model. The first step towards this goal has been taken in
the article presented here. The objective was to establish a numerical
model of the 1/8 th scale physical model and to achieve validation of
the numerical model by comparison of predictions with measurements.
The PHOENICS (Parabolic Hyperbolic or Elliptic Numerical Integration
Code Series) CFD code has been used to develop a numerical model for
the analysis of gas transport and mixing in the AGR hotbox. Results
from the CFD model have been compared with a series of measurements
of velocity fields and trace gas concentrations made a physical model
of a hotbox. The work was carried out in two stages. The initial development
and testing was carried out using a model of a single quadrant of the
hotbox. The model was further extended to cover the entire circumference
of the hotbox. This allowed all combinations of asymmetric operating
conditions to be modelled.
Model description
The hotbox region: The hotbox is a disc-shaped region in which the hot
gas from the reactor core is mixed and distributed to the boilers. The
hotbox region has a cylindrical geometry that contains a ring of 12
square section boilers and a central region with a square array of gas
flow channels.The dome of the gas baffle, which separates the hot and
cold gas regions protrudes into the base of the hotbox. 332 standpipes
arranged on a square mesh pass through the gas baffle dome. The standpipes
carry hot gas from each fuel channel of the core. The gas enters the
hotbox vertically through narrow annular nozzles attached to each standpipe.
The tapered outer sleeve is perforated and is known as the tundish.
The hot gas passes from the hotbox downwards into the tops of 12 square
section boilers that are arranged in a ring around the outer part of
the hotbox. A horizontal baffle around the edge of the dome prevents
the downward flow of hot gas around the outside of the boilers. In practice
there is a small exchange of flow with the boiler annulus below the
baffle but this is neglected in the present model. In the 1/8 th scale
model a resistive plate is placed inside the boilers to represent the
flow resistance which is provided by the boiler pipes. Placed above
each boiler are the headers and open tube bundles of the steam reheaters.
In the reactor, heat is transferred from the hot gas to the reheaters,
however in the 1/8 th scale model and in the present version of the
numerical model the flow is treated as isothermal.
....CONTD
