Emergency core cooling systems in boiling- and pressurized water reactors use strainers. Their purpose is to out foreign material suspended in the cooling water so this material does not reach the reactor. After the 1992 event at Barsebäck nuclear power plant when the strainers of the emergency cooling system was clogged due to fibers in the cooling water, all Swedish nuclear plant increased the surface size of the strainers, to avoid them from being clogged. The risk of accumulation on the strainers is increased pressure drop over strainers, potentially causing cavitation in the emergency cooling pumps and an excessively reduced l. An increase in the surface size of the strainers has the side effect of more fibers passing through the strainers. If the foreign material pass through the strainers, they lose their purpose of reducing the risk of clogging the reactor, which can affect the removal of the residual heat. Examples of foreign materials are debris, chemical compounds, and fibres. In the event of Loss of Coolant Accident caused by a pipe break, insulating material consisting of fibres, if present in the containment, is likely to tear of. In boiling water reactors, the fibres can mix with the water in the condensation pool, and in pressurized water reactors, the fibres can reach the sump. In either case, the fibres mix with the emergency cooling water. Additional loose debris may mix into the water. pH-regulating chemicals, with the purpose of binding the iodine, may form sticky chemical compounds. The foreign material can either remain in the pool or sump, accumulate on the strainer surfaces, or pass through it.
There is further background on the subject under the U.S. Nuclear Regulatory Commission (NRC) Generic Safety Issue (GSI) -191, "Assessment of Debris Accumulation on Pressurized-Water Reactor Sump Performance".
The experiment in this study investigates different strainer surface geometries for different sizes of holes and orifce separation. In addition, different size meshes are investigated to see their effectiveness in retaining fibers.
Fibre formation on strainers depends on the fibre length LFibre with respect to the screen opening (hole) DOpen, and screen hole spacing (solid) SOpen. We focused on the deposition of fibres on the screens classified by their geometry in relation to the fibre length and identified 4 regimes based on LFibre/DOpen, and LFibre/SOpen. Most effective was the retention of fibres in regime I. Surprisingly, also screens in regime II, which hole size DOpen is larger than the fibre length LFibre got covered. The coverage rate was sensitive on the hole spacing SOpen. Findings per regime is summarized as:
Regime I, small holes and small spacing, retained fibres that were longer than the diameter or width of the holes. Fibres bridge the openings with their length.
Regime II, large holes and small spacing, was found collecting fibres on the solid hole spacing which then covered the screen with time. The regime was classified as fbre-solid stapling.
Regime III, large holes and large spacing, extends fibre stapling from regime II to cases where the flow obstruction, i.e. SOpen, was large compared to the fibre length LFibre. The solid part of the screen was covered, but the holes remained open. Further increase in SOpen prevented significant stapling of fibres on the screen. An increase of fibre concentration led to the formation of a fibre mat at low approach velocity. The regime was classified as fibre-screen passage. Fibre mat formation in regime III was followed up seeding with a fraction of longer fibres, which result in regime I retention on the screen. For low approach velocity un, an addition of ca. 1% of the longer fibres which bridged the holes, was enough to retain the smaller fibres on the screen.
Regime IV, small holes and large spacing, was found sensitive to the approach velocity. The screen geometry results to a small open area AOpen, which was 6% for the studied case. For low approach velocity un a nearly, but not full, screen coverage was recorded. Fibres were bridging the openings.
Initial covered holes were found unplugging. Higher approach velocity prevented fibre deposition on the screen.
Fibre mat formation on fuel assembly followed the fibre mat formation on screens. Conclusions on the fibre mat formation from this study were thus direct applicable to fibre mat formation on fuel assemblies. In particular we showed for PLUS7 protective grid a fibre stapling process leading to the coverage with a fibre mat. We suggest including the parameters LFibre/DOpen, and LFibre/SOpen into design consideration of fuel assemblies.
SSM initiated this project in order to increase the understanding of which parameters are important for the accumulation of the fibres on the strainers and if it can be improved. It is desirable that a minimum amount of fibres pass through the strainers since they can cause downstream effects and potentially hamper the emergency core cooling system, the containment spray system, and decrease the heat-transfer in the reactor. The investigation is relevant for the strainers and the fuel assemblies in boiling water reactors and pressurized water reactors.
Need for further research
Extending the current set of experiments to include various fibre lengths and fibre length distributions would be valuable. A number of spacers and protected grid layers were printed but never tested. The screen performs poorly under certain conditions. In one of the experiments, many fibres passed though the strainer holes when their diameter was small compared with the spacing between them. The fibre orientation under different conditions might play a role. Since the holes are small in relation to the screen surface, the flow will go parallel with the surface, and into the holes. The acceleration of the flow might be higher for small holes compared with larger holes. This entrainment effect can potentially explain why the fibres cannot cling to the screen surface, and will instead pass through the strainer holes. Future research can test these hypotheses.
Fibre deposition in compact heat exchangers could also be an area of future research. In most Swedish boiling water reactors, the system for containment spraying include strainers and compact heat exchangers, which cool the spray water. The containment spraying system takes water from the same source as the emergency cooling system, and can experience similar downstream effects.