Juhani Hyvärinen

The inherent boron dilution mechanism in pressurized water reactors

Abstract The purpose of this paper is to describe a mechanism that inherently causes boron dilution in pressurized water reactors (PWRs). The phenomenon is due to the fact that boric acid does not markedly dissolve into steam. This is relevant for transient and accident situations in PWRs where decay heat removal is accomplished by coolant vapourization and condensation, which inherently leads to formation of dilute plugs in the primary. In particular, it is found that inherent dilution will be inevitable for a range of small break loss of coolant accidents (SB LOCAs), with maximum amount of total diluted coolant mass exceeding 20 tonnes for a modern 1300 MWe PWR equipped with U-tube steam generators. A simple analysis of dilute plug motion during the late phases of a SB LOCA and core response to boron dilution shows that the damaging potential might extend to widespread fuel failures. Other transients and accidents are also discussed from the point of view of inherent dilution. Some possible remedies to the problem, as well as suggestions for further research, are presented.

Heat transfer characteristics of horizontal steam generators under natural circulation conditions

Abstract This paper deals with the heat transfer characteristics of horizontal steam generators, particularly under natural circulation (decay heat removal) conditions on the primary side. Special emphasis is on the inherent features of horizontal steam generator behaviour. A mathematical model of the horizontal steam generator primary side is developed and qualitative results are obtained analytically. A computer code, called HSG, is developed to solve the model numerically, and its predictions are compared with experimental data. The code is employed to obtain for VVER 440 steam generators quantitative results concerning the dependence of primary-to-secondary heat transfer efficiency on the primary side flow rate, temperature and secondary level. It turns out that the depletion of the secondary inventory leads to an inherent limitation of the decay energy removal in VVER steam generators. The limitation arises as a consequence of the steam generator tube bundle geometry. As an example, it is shown that the grace period associated with pressurizer safety valve opening during a station black-out is 2 21 2 −3 hours instead of the 5–6 hours reported in several earlier studies. (However, the change in core heat-up timing is much less—about 1 h at most.) The heat transfer limitation explains the fact that, in the Greifswald VVER 440 station black-out accident in 1975, the steam generators never boiled dry. In addition, the stability of single-phase natural circulation is discussed and insights on the modelling of horizontal steam generators with general-purpose thermal-hydraulic system codes are also presented.

DEM in Analyses of Nuclear Pebble Bed Reactors

Pebble bed reactors are innovative fission reactors with improved efficiency and unique safety features. These reactors differ from the conventional reactors mainly by their fuel design, which instead of rods, is based on coated fissile particles enclosed inside graphite spheres. The reactor core is a packed bed consisting of these spheres, through which the reactor coolant flows. In the research documented in this paper, DEM code LIGGGHTS is used for various analyses of pebble bed reactors: DEM is used to produce realistic reactor core geometries for subsequent reactor physical, coolant flow and heat transfer analyses. DEM analyses are also performed to investigate various reactor operation aspects, such as the flow of the spherical fuel elements through the cylindrical core geometry and insertion of a control blade into the pebble bed. DEM has been found as a very effective tool for reactor analyses.