Abstract: This invention relates to a nuclear reactor plant which includes a nuclear heat source (12), and a cooling system (10). The cooling system (10) includes at least two cooling circuits (26), each of which includes a plurality of coolant chambers (18) each having an inlet (40) and an outlet (42), the coolant chambers (18) being arranged around the nuclear heat source (12). The cooling system (10) further includes pump means (52) for pumping coolant to and from the coolant chambers (18), the volumetric capacity of the coolant chambers (18) being sufficiently large such that, when in a passive mode, the temperature of a water coolant in the coolant chambers (18) will remain below boiling point for at least eight hours. The invention extends to a cooling system, to a method of cooling a nuclear heat source, to a method of constructing a nuclear reactor plant and to a method of operating a nuclear reactor plant.

Abstract: In a nuclear reactor, in-core stability is improved, power density is increased, and an economical natural-circulation reactor (or partial forced-circulation reactor) is achieved. The reactor core has a void reactivity coefficient between −0.07 and −0.03 % &Dgr; k/k/% void fraction. This void reactivity coefficient range is achieved by, for example, the design of the by-pass portion and channel box, the enrichment distribution along the axial direction, the provision of blanket areas, and/or the arrangement of water rods and fuel rods within a channel box.

Abstract: In a nuclear reactor, in-core stability is improved, power density is increased, and an economical natural-circulation reactor (or partial forced-circulation reactor) is achieved. The reactor core has a void reactivity coefficient between −0.07 and −0.03% &Dgr;k/k/% void fraction. This void reactivity coefficient range is achieved by, for example, the design of the by-pass portion and channel box, the enrichment distribution along the axial direction, the provision of blanket areas, and/or the arrangement of water rods and fuel rods within a channel box.

Abstract: To detect core-wide and regional neutron flux oxcillations in a nuclear reactor core induced by thermal-hydraulic instabilities, local power range monitoring (LPRM) strings radially distributed throughout the core and having plural vertically spaced neutron flux detectors are locally assigned to individual oscillation power range monitoring (OPRM) cells radially distributed throughout the core. Groups of OPRM cells are assigned to different OPRM channels based on their geographical positions. Detector signals of the LPRM strings assigned to each OPRM cell are processed pursuant to a unique trip algorithm to detect neutron flux oscillations, and, upon meeting prescribed amplitude and frequency criteria, the assigned OPRM channel is tripped. Suppression of a thermal hydraulic instability is initiated when at least two OPRM channels assigned to geographically adjacent OPRM cells are tripped.

Abstract: A forced-circulation boiling-water reactor includes bypass check valves between a downcomer and a core inlet plenum. When the recirculation pumps are operating at full capacity, there is a maximum pressure differential from the downcomer to the core inlet plenum. This pressure differential keeps the valves closed so that recirculating fluid is constrained to flow through the pumps. When the pumps are not operating, a driving water head in the downcomer forces the valves open, augmenting the flow cross section between the downcomer and the core inlet plenum, enhancing natural circulation. The enhanced natural circulation provides greater core stability during pump shutdown. The valves are selected or adjusted so that they open when the pressure differential falls through a predetermined range to augment diminished pumping capacity with a higher natural circulation flow rate.