[en] A feature of the Steam Generating Heavy Water Moderated Reactor is the provision, of a spray cooling system on which major reliance is placed to limit fuel cladding temperatures during a loss-of-coolant accident. The LOCA MK II computer program which has been written to compute SGHWR fuel temperatures in a loss-of-coolant accident therefore models conditions in which blowdown cooling is only of marginal interest and where the major sources of heat removal are spray mechanisms and radiation. The report shows how a model of the SGHWR fuel cluster may be built up and the temperatures calculated. (author)

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[en] 1 - Description of problem or function: A feature of the Steam Generating Heavy Water Moderated Reactor is the provision of a spray cooling system on which major reliance is placed to limit fuel cladding temperatures during a loss-of-coolant accident. LOCA-MK-2, which has been written to compute SGHWR fuel temperatures in a loss-of-coolant accident, therefore models conditions in which blowdown cooling is only of marginal interest and where the major sources of heat removal are spray mechanisms and radiation. LOCA-MK-2 models the fine details of the fuel cluster, and zircaloy oxidation and radiation. 2 - Method of solution: The code assumes that heat transfer depends entirely on spray mechanisms and radiation. The heat transfer history of an accident is divided into three parts: - Convective heat transfer, terminated by dryout. - Low radiative heat transfer. - Spray cooling of the cluster. LOCA-MK-2 performs a 1-D transient conduction calculation for a typical pin of each ring; temperature is found as a function of radius from the centre of the fuel. The solution of the conduction equation is by an implicit method and is therefore stable for all sizes of time steps. The variation of neutron flux after the accident is determined from the conventional six precursor group single point kinetics equations

[en] Comparison is made of experimental and calculated reactivity changes when the fuel plates of the VERA assemblies are bunched from 1/8 in. thickness to 1/2 in. In addition, U238 fine structure reaction rates are studied. The method used is that investigated in detail by James and Matthews ('A Perturbation Method for Multigroup Neutron Transport Calculations in Plane Lattices', AEEW - R 219), while the multigroup data sets used are both those of Yiftah, Okrent and Moldauer, and Roach. The method and data give only rough agreement with experimental reactivity changes, with a tendency to over-predict the results. U238 fission rates are in better agreement, though the method predicts too small a variation for the U238 capture rate across a cell. (author)

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[en] This report describes a method for calculating the k_eff of, and the multigroup fluxes in, an infinite plane periodic lattice with a cell of width L that is of the order of a neutron mean free path or smaller. (author)