26.08.2015   The Future of Nuclear Power

Sub-Channel Analysis

Sub-channel analysis is performed to determine the safety margin to boiling in peak rated channels in LWR assemblies. The flow and heat transfer distributions inside a fuel assembly can be analysed by sub-channel codes. The usual reason for analysis is to demonstrate compliance with the ‘Departure from Nucleate Boiling Ratio’ (or DNBR) requirements. The codes calculate the DNB from various channel-averaged parameters. Well-known sub-channel codes are COBRA and VIPRE (Table 16.4).

In such codes, two-phase flow is normally treated via a 3D flow model, which is coupled to a 1D model for fuel rods of different ratings. A detailed model of the heat transfer between the surface of the cladding and the coolant is included. The critical heat flux is calculated with a correlation.

Most fuel bundle designs are complex and it is necessary to consider the effect of such

geometries on the DNBR. Modern fuel bundle designs may include part-length rods and/or large water holes and these are clearly difficult to model. Grids of varying design exist for

Table 16.4. Thermal-hydraulics

Code type

Computer code/model

Sub-channel

Transient analysis system

CFD codes (CFMD in development)

COBRA, VIPRE

TRAC, RELAP5, TRACE, CATHARE, ATHLET, RETRAN CFX, FEAT, FLUENT, CODE-SATURNE, TRIO-U, FLUBOX

Guffee et al., RELAP5/MOD3 Code Manual (1995), Spore et al. (2001), Page et al. (1998), CFX 4.3 (1999), Weiss et al. (to be published), Scheuerer et al. (to be published), Paillere et al. (to be published) and Yadigaroglu (to be published).

support and promote mixing. There is a need to improve sub-channel codes to take account of these features and ensure, in particular, that void distributions are adequately modelled.

New fuel vendors supply correlations for their individual fuel rod designs. These are developed for fresh fuel and generally do not include the effects of burn-up, so their adequacy for highly irradiated fuel needs to be established. In highly irradiated rods, the surface may be significantly oxidised with different thermal-hydraulic performance characteristics. A particular issue may be different boiling characteristics and any influence on critical heat flux needs to be established.