Patent Number: 045267421
Section: summary

TECHNICAL FIELD This invention relates to a nuclear reactor plant of the kind comprising a pool which is filled and pressurized with a neutron-absorbing pool liquid and a reactor vessel within the pool and enclosing a water-filled reactor core provided with cooling channels. The reactor vessel includes an inlet chamber communicating with the pool liquid via a lower shutdown opening and an outlet chamber for cooling water which communicates with the pool liquid via an upper shutdown opening. Heat exchange means are provided, at least one outlet conduit connecting the heat exchange means to the outlet chamber and at least one inlet conduit connecting the inlet chamber to the heat exchange means. A circulating pump in the primary circuit of the reactor provides, during normal operation of the reactor, a substantially constant flow of reactor cooling water through the core which, in addition to being included in the primary coolant circuit of the reactor plant, is included in a shutdown circuit which comprises the inlet chamber, the lower shutdown opening, the pool, the upper shutdown opening and the outlet chamber. The said shutdown circuit has an inherent flow-driving tendency which, when cooling water flows through the reactor core, causes a pressure difference to prevail in the shutdown circuit, this pressure difference being counteracted and balanced by the pressure drop prevailing across the reactor core due to the flow therethrough of reactor cooling water, so that no significant supply of pool liquid to the primary circuit takes place. BACKGROUND ART A nuclear reactor of the above-mentioned kind is disclosed in Canadian Patent Specification No. 1,070,860. In this known reactor, the flow-driving tendency of the shutdown circuit is constituted to a predominant extent by a gas cushion included in the shutdown circuit in the upper part of the reactor vessel. The pressure difference which, during normal operation of the reactor, acts on the shutdown circuit is substantially equal to the static pressure difference between two pool liquid levels, the spacing apart of which is equal to the vertical dimension of the gas cushion. During normal operation of the reactor this pressure difference is substantially equal to the pressure drop across the reactor core. The imbalance required to enable the pressure difference of the shutdown circuit to drive a flow of pool liquid through the core will only occur if a significant change of the lifting pressure of the circulating pump takes place. A reaction to too high a temperature in the reactor cooling water can only be obtained indirectly, namely in the form of a reaction to an increase in hydraulic resistance occurring in the primary circuit due to the production of bubbles of steam. Such an increase in the hydraulic resistance in the primary circuit will reduce the cooling water flow through the core, and thus the pressure drop across the core will be reduced. A condition for sufficient reactor safety is then that a sufficiently large volume of steam bubbles, with a good safety margin, is achieved before the water in the core starts boiling. Since the steam volume in each bubble is reduced with increasing pressure, this condition sets a relatively low limit on the operating pressure at which this known reactor can be used. Calculations have shown that this pressure should be below 10 bar, which means that this known reactor is not well suited for generating steam for steam turbines. Furthermore, it is a disadvantage in this known reactor that the driving pressure difference, which upon an emergency shutdown, for example due to a cut-out of the circulating pump, acts in the shutdown circuit, is increasingly reduced as when the amount of pool liquid supplied to the reactor core increases, and is equal to zero when the amount of pool liquid supplied to the reactor vessel corresponds to the volume of the gas cushion. This means that a predominant part of the flow-driving tendency, which is inherent in the shutdown circuit but latent during normal reactor operation, is not capable of driving liquid through the core for any appreciable part of the time during which the decay power of the core is still relatively great, for example greater than 50% of the maximum decay power. DISCLOSURE OF THE INVENTION The present invention aims to provide a reactor which, like the known reactor, can be shut down in an emergency quite independently of temperature-indicating devices, valves or similar technical means with movable parts, but which, contrary to the known reactor, is suited for generation of steam of a relatively high pressure and, therefore, suited for driving steam turbines. One object of the invention is to increase the safety by making the emergency shutdown occur as a direct reaction to an impermissible increase in the temperature of the reactor coolant, independently of whether the pressure drop across the reactor core is or is not maintained. With a reactor according to the invention, the cooling water density reduction associated with such a temperature increase is sufficient for releasing an emergency shutdown. The release is then due to a disturbance of an existing balance between two pressure differences in a hydraulic system, in which a substantial part of one of these pressure differences is determined by the weight of a cooling water column. A further object of the invention is to obtain the advantage that a substantial part of the pressure difference, which during normal reactor operation acts in the shutdown circuit and balances the pressure drop across the reactor core, shall be maintained after the time at which the reactor power, upon emergency shutdown, has been reduced to decay power and, as long as there is liquid in the pool, result in such a great flow of coolant through the core as to prevent impermissible heating of the core caused by the decay power. Thus, the present invention, like Canadian Patent Specification No. 1,070,860, proposes a solution to a rather specific problem, namely how to arrange a reactor vessel, which is provided with open emergency cooling holes at both ends, below the liquid surface of a pool which is filled with a neutron-absorbing liquid, in such a way that the neutron-absorbing liquid does not flow in through any of the open emergency cooling holes during normal reactor operation. The Canadian Patent Specification provides one solution to this problem, and the present invention provides a completely different solution. According to the present invention, in a nuclear reactor plant of the kind referred to, the portion of the reactor vessel that is filled with cooling water has a vertical extension which constitutes a predominant part of the vertical extension of the reactor vessel, and the inherent flow-driving tendency substantially is accomplished by at least one system for driving a liquid flow with a driving pressure difference which is substantially independent of the volume of liquid driven by the device. In this specification, "predominant" is used in its commonly accepted sense to indicate prevalence over all others in number or magnitude.