EP0491700A1

EP0491700A1 - Nuclear reactor plant

Info

Publication number: EP0491700A1
Application number: EP90908443A
Authority: EP
Inventors: John S. Hewitt; Hani C. Ajus; Terrance J. Jamieson; Antonino F. Oliva; Bruce M. Pearson; William P. Wong
Original assignee: ECS-POWER SYSTEMS Inc
Current assignee: ECS-POWER SYSTEMS Inc
Priority date: 1989-09-15
Filing date: 1990-05-30
Publication date: 1992-07-01
Also published as: CA1333941C; WO1991004559A1

Abstract

Une centrale à réacteur nucléaire comporte un noyau (1), un moyen de commande de la réactivité du noyau (5, 6, 7, 8, 9), un bouclier radiologique (16, 17, 18, 19), ainsi que des conduits d'admission (14) et des conduits de sortie (15) destinés à conduire du liquide de refroidissement dans le noyau et hors de celui-ci. La centrale se caractérise en ce que les orifices (20, 21) sont aménagés dans les conduits d'admission et de sortie afin de conduire du liquide de refroidissement entre un réservoir (30) de liquide de refroidissement de réserve et les conduits d'admission et de sortie par convection naturelle lors d'une défaillance du circuit de refroidissement du réacteur. Un réservoir de désactivation (26) reçoit du liquide de refroidissement provenant des conduits de sortie; il entoure le noyau et constitue un bouclier radiologique supplémentaire. Un réservoir faisant écran (19), contenant des stratifiés (20) absorbant le rayonnement, et du liquide de refroidissement entourent le noyau.A nuclear reactor plant comprises a core (1), a means for controlling the reactivity of the core (5, 6, 7, 8, 9), a radiological shield (16, 17, 18, 19), as well as conduits inlet (14) and outlet conduits (15) for conducting coolant into and out of the core. The unit is characterized in that the orifices (20, 21) are arranged in the inlet and outlet ducts in order to conduct coolant between a reservoir (30) of reserve coolant and the intake ducts and output by natural convection during a failure of the reactor cooling circuit. A deactivation tank (26) receives coolant from the outlet ducts; it surrounds the nucleus and constitutes an additional radiological shield. A shielding tank (19), containing radiation absorbing laminates (20), and coolant surrounds the core.

Description

Description NUCLEAR REACTOR PLANT

Technical Field

This invention relates to a nuclear reactor plant to serve as the heat source component of a power plant for the production of heat or electricity. The reactor plant includes means for cooling the reactor core in the event of impairment of the primary heat transport circuit of the power plant. Under normal operating conditions, the reactor plant serves as a major component in a heat transport circuit which also contains a pump and one or more heat exchangers as other major components of the circuit. Such a circuit is referred to as the primary heat transport circuit of the power plant.

Under a variable set of abnormal operating conditions, including the nominal "shutdown" reactor condition when decay heat continues to be released, the said reactor draws on a supply of reserve coolant contained in a tank which is an integral part of the reactor plant. The consequent exchange of coolant between the reactor core and the reserve coolant tank provides, in the interests of reactor safety, auxiliary cooling to the core of the reactor under conditions of zero or reduced primary flow, loss of heat sink, or other form of impairment of the primary heat transport circuit.

As this auxiliary cooling is sustained in the absence of external power or specific operator action, the reserve coolant tank together with the other plant associated with this action is referred to as the passive cooling system.

The reactor core, the coolant-carrying components of the primary heat transport circuit, and the said passive cooling system, together with the coolant itself, are identified as the primary heat transport system of the said nuclear power plant.

Background Art

Prior art reactors known as the PIUS, described in Canadian Patent 1,173,570 (Hannerz) ; the SECURE, described in Canadian Patent 1,070,860 (Blomstrand et al.); and the TRIGA Power Reactor, described in F.C. Foushee, R.W. Schleicher, G. Schlueterand and J.S. Ya polsky, "Small Triga Power Reactors for District Heating", Nuclear Europe, 12 (1984) pp. 33-35, and R.W. Schleicher, "Triga Power System: A Passive Safe Co-Generation Unit for Electric Power and Low Temperature Heat", Small Reactors for Low Temperature Heat Applications, IAEA-TECDOC-463 (International Atomic Energy Agency, Vienna, 1988) pp. 45-55, are power light water reactors which offer capabilities for defaulting passively to passive cooling under abnormal operating conditions which would otherwise tend towards accident conditions. However, the plant arrangements used are too massive and bulky and lack the features of the present invention which permit application in relatively confined spaces. Moreover, none of the basic principles on which such prior art reactors depend lends itself to dealing with the problems of ship reorientation and motion.

Disclosure of Invention There is provided according to the invention a nuclear reactor plant comprising a core, means for controlling the reactivity of said core, radiological shielding around said core, inlet means to conduct coolant into said core, outlet means to conduct coolant out of said core, a reserve coolant tank; and port means in said inlet means and in said outlet means for conducting coolant between said reserve coolant tank and said inlet means and outlet means by convection during impairment of normal flow of coolant through said inlet and outlet means, and for permitting a through-flow of coolant through said inlet means and outlet means during normal operating conditions.

According to one embodiment, the reactor plant comprises a light-water-cooled reactor core, bracketed by inlet and outlet coolant plena, each coupled to one or more ducts leading away from the core at various orientations. At any given time each duct carries some fraction of the coolant flow through the core.

At its end remote from the core, each duct terminates in a special branching device, herein referred to as a port, designed in any one of a number of ways, such that:

(i) under normal operating conditions, with respect to both the inlet and the outlet ducts, virtually all of the coolant flow in a duct originates directly from, or passes directly to, the nearest of the two coolant headers (designated as either the inlet or outlet header, respectively) which connect the said reactor plant as a series component within the primary heat transport circuit, and

(ii) under conditions departing from normal, branch flow in the ports can develop such that the combination of such flows in all of the ports produces rates of net coolant exchange between the core and the reserve coolant tank that are adequate for assuring the safety of the reactor plant.

The specific type, or principle of operation, of the said ports applied within the reactor primary heat transport system is not critical to the nominal functioning of the said nuclear plant. In fact, such ports, any associated auxiliary equipment, and the primary heat transport system that accommodates their effective operation may be specified as necessary to meet the particular reactor application requirements. Such specification would accommodate a prescribed level of safety and include the design approach to safety, as may be required or deemed appropriate in a particular reactor application.

The detail of a suitable type of port or its operation is not included as the subject of the present invention, as such may be devised by those skilled in the art of thermal-hydraulic systems design. However, a preferred method of providing the port function in a manner in keeping with a high standard of reactor safety and with the general objectives relating to the present invention, is through the principle and application of the specific type of ports referred to as hydrodynamic ports. Such hydrodynamic ports and their application in achieving a range of objectives for nuclear reactors form the subject of International application Serial No. , filed concurrently herewith, entitled Nuclear Reactor Cooling System.

In the cases of incorporation of such ports as the hydrodynamic ports, it may be said that the auxiliary cooling system operation not only is sustained by passive means but also is initiated by purely passive means.

The said reactor plant further comprises a reactor vessel, gamma and neutron shields, a primary circuit delay tank and reactivity control mechanisms, all located together with such previously mentioned components as the reactor core, the plena, the ducts, the ports, and the headers within the boundary of the reserve tank and submerged in the bulk of the reserve coolant, which is itself thermally coupled to an ultimate heat sink. All such components are arranged in a highly integrated manner, relative to each other and in accordance with measures taken to restrict (i) space requirements, (ii) total mass of reactor plant, and (iii) radiological dose as determined both at the reactor boundary and in the vicinity of coolant inventory found circulating through the various primary heat transport components external to the reactor plant.

An objective of certain embodiments of the present invention is to achieve a nuclear reactor plant which (i) in the case of marine applications, is sufficiently compact in design to permit its installation as a component of a nuclear plant within small quarters such as are found on board a submarine of significantly smaller dimensions and displacement than are generally characteristic of nuclear powered submarines; (ii) which presents manageable operational radiological fields at its boundaries and in its circulating coolant; and (iii) which accommodates a simplicity of design and operation appropriate to peculiar characteristics required of the plant and its application.

Such characteristics include: (a) a modest plant output,

(b) a very small, non-expert, or non-existent operating staff in attendance; accommodated by virtue of the basic reactor concept and its amenability to computer assisted operation, (c) a high standard of nuclear safety and plant reliability, and (d) a limited space and weight allowance for auxiliary equipment.

A further objective of the invention is that the reactor plant, when installed on an unsteady platform, such as on board a submarine, be capable of sustaining normal operation over a sufficiently wide range of motion.

A still further objective is that the passive cooling system remain functional when plant motion and orientation both extend outside the normal operating range, even when operator intervention and internal and external power sources become unavailable.

A still further objective is that, while accommodating these system operational objectives, the reactor plant is also of a physical make-up that can be embodied in a form and structure which exhibits strength and resilience appropriate to the accelerating motion and displacement that can be anticipated for a submarine platform.

In general terms, these objectives are achieved in certain embodiments of the invention by the following means:

(i) Providing a reactor layout which concentrates, to the degree possible, all radiation sources and shielding mass as close as practicable to the geometric centre of the reactor plant boundary, i.e., central to the outer boundary of the reserve coolant tank; (ii) Providing a highly integrated design approach in which: (a) components perform dual roles, e.g., the coolant contained in the reserve coolant tank and in the delay tank, as well as structures such as the reactor vessel, also serve as neutron and gamma shields; (b) all shields, the reserve coolant tank excluded, recapture within the primary heat transport circuit a significant share of the penetrating radiation energy, which otherwise would escape from the process of conversion to useful heat or electrical energy;

(c) design simplifications result, in certain manifestations of the invention, from the use of a single water quality management system. (Such a simplification is possible in an arrangement in which a single water inventory is made common to all components in a set and is allowed to circulate among them at an adequate turnover rate without compromising their principal functions. Examples of components sharing the water inventory include the primary heat transport circuit, the neutron shield tank, the reserve coolant tank, and the reactivity mechanism coolant and lubrication systems) ; and (d) the delay tank is internal to the reserve coolant tank and, therefore, the latter affords some shielding against radiation emitted from the former. Further, the flow pattern in the delay tank is arranged, by the positioning of internal baffles and the inlet and outlet locations, so that the segments of the tank which contain coolant of the highest specific radioactivity during reactor operation are at the side of the reactor core where they least expose operating personnel and sensitive equipment to radiation, (iii) Employing commonly accepted methods for safely managing the excess reactivity required for long fuel burnup life and acceptable demand load- following capability. These may include the use of fuel which contains burnable absorber and, as well, provides a large, prompt negative temperature coefficient of reactivity. In certain embodiments of the invention, provision is also made for reactor regulating absorber rods and fuel-follower reactor shutoff rods, each type of rod being manipulated automatically by a reactivity control mechanism employing current technology with regard to safety and reliability in accordance with signals received from the power plant's control computer of highly reliable design. In other embodiments, where it may be desirable to avoid all direct mechanical means of controlling reactivity, and to effect reactor shutdown automatically without involving the in-core complexity of control mechanisms, the present invention accommodates such avoidance by the addition of dissolved neutron absorber in low concentrations to the coolant of the primary heat transport circuit and in high concentrations to that of the reserve coolant tank, and providing outside the said reactor plant the chemical plant necessary to maintain the concentration difference as necessary. The dominance of the large mass of the reserve coolant at the higher absorber concentration and the incorporation of suitable ports, will result in reactor shutdown immediately following the development of coolant exchange flow

(through the ports) between the two regions of differing concentrations. (iv) Providing a cooling circuit configuration involving the ducts and ports that, even in a space of limited extent, assures sufficient convective cooling at all times and at all reactor orientations with respect to gravity, to dissipate thermal power to the ultimate heat sink as necessary to meet safety requirement. Further, such configuration does not allow the basic thermal performance of the plant to be significantly compromised over a range of operating circumstances including significant random or periodic accelerations of the platform of installation. Moreover, the configuration provides for the transition of the heat transport system from the normal operating mode to the passive decay heat dissipation mode by strictly passive means, if safety design criteria so require. In such an embodiment, the interplay of the passive cooling system with the primary heat transport circuit under nearly-normal operating conditions can contribute in a positive manner to overall plant performance and safety.

Some embodiments of the invention provide a self- shielding/self-contained reactor plant for power production that is accommodated in a small space, i.e., characteristically 3.7 m in diameter, 30 m³ in volume and 70 tonnes in weight. It is anticipated that such reactors may be successfully scaled-up by as much as 10 to 30 times in power, accompanied by an approximate doubling in linear dimensions, while still retaining most of the design and operating features of the basic invention.

Some embodiments also provide a power reactor, sized to operate at the low end of power level range normally associated with nuclear reactors dedicated to power production, and which, on the one hand, delivers its heat energy at a temperature as high as within a few degrees of the saturation temperature of its coolant at normal operating conditions, and, on the other hand, retains the safety of the passive (convective) cooling of a swimming- pool type of research reactor, which is characterized by a large thermal ballast maintained as the massive highly sub- cooled pool water. In the present reactor plant, a massive body of highly sub-cooled reserve coolant is likewise freely available at all times as auxiliary cooling, and particularly during those times when operating conditions depart from normal.

Some embodiments also provide a reactor with a passive cooling system designed to function at orientations departing from normal and under accelerating motion (as when a ship in which such a reactor is installed is subjected to currents and surface waves) , and in which the said cooling system is such that it does not detract from the ability of the reactor to operate efficiently under normal conditions.

Brief Description of the Drawing

The drawing is a vertical cross-section of an embodiment of the reactor plant made according to the invention and shown in normal operation, and at a normal orientation with regard to the direction of gravitational forces. The drawing shows, by means of arrows, a finite amount of exchange flow of coolant between the reactor core 1 and the reserve coolant tank 30, superimposed on the dominant flows for normal operation. The reactor plant configuration has substantially radial symmetry about the vertical centre line indicated.

The numerals of the drawing further identify certain elements of the said reactor plant according to the following list:

1 - reactor core assembly

2 - fuel elements

3 - top grid-plate 4 - bottom grid-plate

5 - regulating rod 6 - regulating rod guide tube

7 - shutoff rod

8 - shutoff rod guide tube

9 - rod drive mechanisms 10 - radial reflector

11 - reactor vessel

12 - inlet plenum

13 - outlet plenum

14 - inlet ducts 15 - outlet ducts

16 - radial gamma shield

17 - lower axial gamma shield

18 - upper axial gamma shield

19 - shielding tank 20 - shielding tank laminates

21 - inlet ports

22 - outlet ports

23 - inlet header network

24 - outlet header network 25 - thermal insulating layers

26 - delay tank

27 - delay tank flow baffles

28 - main inlet conduit

29 - main outlet conduit 30 - water filled reserve coolant tank

31 - control rod mechanism deck

32 - reactor vessel cover

33 - collected gas relief spigot

34 - reserve tank access hatch

Best Mode for Carrying out the Invention

In the drawing, reactor core 1 is built up of a plurality of fuel elements 2 arranged so that water circulating among the elements serves as both moderator and coolant. The preferred selection of fuel type depends on the particular reactor embodiment, but the choice of uranium-zirconium-hydride fuel, or similar fuel consisting of a fissile-fertile nuclide mixture alloyed with metal hydride to yield significant hydrogen moderation within the fuel matrix, provides the advantages of (i) a strongly enhanced prompt negative temperature coefficient of reactivity, (ii) a favourable through-core coolant velocity profile and magnitude for a given mass flow, and (iii) a reduced in-core water volume and, hence, a reduced production rate of radioactive isotopes which are subject to transport throughout the primary heat transport circuit. The fuel elements, or assemblies of such elements as the case may be, are suspended between the upper and lower grid plates 3 and 4 which, in addition to supporting the fuel, provide openings to facilitate the passage of coolant traversing the core component of the primary heat transport circuit. The grid plates are designed also in such a way that they hold the fuel elements or assemblies firmly in place in the face of acceleration imposed externally on the reactor plant, while providing for their insertion, securing, release and removal by remote manipulation during refuelling operations.

The core-adjacent components of the primary heat transport circuit consist of the inlet and outlet plena 12 and 13, and the inlet and outlet ducts 14 and 15. Only four of the ducts are shown in the sectional drawing although, in general, a plurality of at least eight of such ducts arranged with radial symmetry about the axis shown may be preferred for installations on mobile platforms such as those arising in marine applications. Each of the inlet and outlet ducts leads outward from the reactor core and connects with an inlet port 21 or an outlet port 22, respectively.

The ports and their means of control are designed in such a way that, during normal reactor operation, they accommodate predominantly through-flow in which essentially all of the coolant circulating through the core is matched, by means of the said ports, to the flow circulating in the remaining components of the primary heat transport circuit. Such components include the inlet and outlet header networks 23 and 24, the delay tank 26, the main inlet and outlet conduits 28 and 29, as well as the other major components of the primary heat transport circuit located externally to the reactor plant as currently defined. These latter components include the primary coolant circulation pump and the primary heat exchangers, which are parts of the primary heat transport circuit but not parts of the subject reactor plant. It is noted, however, that in some embodiments of the reactor plant such components might be located within the reserve coolant tank.

In addition to providing through-flow, the ports and their means of control are designed so that for a range of off-normal operating conditions, such as those arising during an accidental impairment of operation of the circulation pump or the heat exchangers of the primary heat transport circuit, the ports accommodate lateral flow. Such flow provides for a naturally convected exchange of coolant to be established and maintained between the reactor core 1 and the reserve coolant tank 30, in response to one or more of a wide range of accident scenarios.

The reserve tank 30 is normally maintained full of coolant water maintained at a temperature considerably lower than that of the primary heat transport circuit, by virtue of (i) layers of insulation 25 applied to all of the outer boundaries of those components of the primary heat transport circuit enclosed by the reserve tank, and (ii) provision of facilities, not shown in the drawing, for the removal of heat from the reserve coolant to external heat sinks. Such facilities may be designed by those skilled in the art of thermal hydraulics so that, under normal operating conditions, the heat removal rate is sufficient to maintain the reserve coolant temperature at a stand-by level low enough to provide the necessary thermal ballast to meet anticipated accident scenarios, and, under reactor shutdown conditions, to accomplish the passive removal of decay heat in a manner which avoids possibility of overheating the fuel elements in the absence of further operator intervention. Depending on the circumstances of a particular application, the equipment used to maintain the stand-by temperature may be in common with that for passive decay heat removal, although this would not be a necessity from an operational or safety point of view.

For the reactor in its normal upright position, the natural convection circuit consists of (i) the core 1 in combination with the several outlet ducts 15 acting in parallel with each other to serve as the hot leg of the circuit, (ii) the reserve coolant tank 30 in combination with the several inlet ducts 14 acting in parallel with each other to serve as the cold leg of the circuit, and (iii) the ports themselves, 21 and 22, operating in lateral-flow to complete the various branches of the circuit, thus creating a thermosyphon. All of the said components are designed to assure that, once the convective coolinq paths are established following a safety-related initiating event, adequate cooling may be maintained for an indefinite period without the requirement of further action on the part of human operators, automatic safety systems, or external energy sources. The circuit, so described, constitutes the passive cooling system of the subject reactor operating in its most likely mode, namely, the one in which the reactor remains in its upright orientation.

For other reactor orientations of any extreme, it will be apparent from an examination of the drawing that, so long as lateral flow in either direction remains possible for each port, the multi-directional arrangement of the ducts assures a sufficient variety of convective paths to permit, regardless of orientation, passive core cooling at levels more or less equivalent to that at the normal upright orientation. Thus, it may be said that a novel aspect of the present invention is that it provides for passive cooling to be sustained even if the reactor departs radically from its normal orientation with respect to the direction of gravitation, as in the case of postulated accident conditions on board a ship powered by the reactor. Inlet ports 21 and outlet ports 22 are designed to perform a switching operation from through-flow to lateral- flow mode, and vice-versa, in response to changing operating circumstances. In some embodiments, however, the ports are designed to allow in some circumstances the superposition of through-flow and lateral-flow, for the further enhancement of operational and safety performance. The choice of a port design fulfilling prevailing safety and reliability requirements may be undertaken by those skilled in the art of thermal hydraulic design.

It may be recognized in the course of such a design process, however, that, to fully meet prevailing requirements of safety and reliability in the face of other constraints relating to space allowances, costs, and special operational circumstances, it may be desirable to avoid in the port design any dependency of the passive cooling system on either specific operator action or the presence of so- called actively operated components, with respect not only to its passive cooling function, as described above, but also to its mode of deployment. In a passive cooling system incorporating this further step in passive design, reliance on mechanical response to electrical sensing devices, operator intervention, and sources of external energy would be specifically avoided in the design of the port and any associated control elements. Thus, a passive cooling system which has already the property of being able to continue indefinitely to cool the reactor without need of external action or energy supply, once passive cooling is established, now has, in addition, the quality of being able to initiate such passive cooling without external action or energy. Such a system may be described as possessing passive cooling initiated by passive means. A cooling system matching such a description, including specific aspects of the port design is the subject of the copending patent application referred to above.

The inlet header network, 23, consists of a manifold or other piping arrangement which distributes, via the inlet ports 21 operating in the through-flow mode, the pumped flow from the main inlet conduit 28 in equal measure to the several inlet ducts 14. The outlet header network, 24, consists of a manifold or other piping arrangement which collects, via the outlet ports 22 operating in the through- flow mode, pumped flow in equal measure from the several outlet ducts 15 and directs it to the delay tank 26.

The primary purpose of the delay tank 26 is to provide a dwelling place for the short-lived neutron activation products, created in the primary coolant as it flows through the core, to decay to acceptable levels before they are carried in the coolant to primary heat transfer components located outside the reactor plant. Such components include heat exchangers which may have secondary media that are particularly vulnerable to radiation damage or which may be approached occasionally by operators. In the present embodiment, the delay tank is annular in shape, co-axial with the reactor itself, and located at the periphery of the shielding tank 19 with which it shares a common boundary. In this configuration the delay tank is shielded from significant secondary neutron activation, but is itself relatively shielded as viewed from locations outside the reactor plant. As well as performing the usual role of minimizing the direct streaming flow of coolant from inlet to outlet while introducing a minimum of through-tank head loss, the judicious arrangement of baffling plates 27 within the delay tank, affords the opportunity to control the distribution of specific activity within the tank so that sectors of highest activity are the most shielded as viewed from the direction of regions of greatest radio- sensitivity lying outside the reactor plant. Such an arrangement is a significant factor in minimizing total shielding weight required for the plant. The primary heat transport system of the subject reactor, in summary, operates as follows. Under normal operating conditions the cooling water is circulated through the primary heat transport circuit by means of the pump located externally the reactor plant. For such operation, the ports, 21 and 22, remain closed to lateral flow due either to physical boundaries or hydrodynamic effects, depending on the type of port specifically incorporated in the design. Under these conditions, the reactor power is regulated to achieve the desired conditions in the circuit, taking into account the usual plant operation considerations such as the thermal load demanded by the external equipment. In the event that the primary circuit becomes impaired due to a loss of thermal load or loss of pump operation, the ports switch from the through-flow mode to lateral-flow mode and the reactor core is subsequently cooled by natural convection exchange flow between the core and the reserve coolant tank. This passive mode of cooling, which delivers no power to the normal load, can be allowed to continue until the reserve coolant, which was originally maintained at a temperature considerably lower than that of the primary circuit, approaches the boiling temperature of water at the maintained pressure. The total time required for this condition to be reached is considerably lengthened if, in the meantime, the reactor core is placed in the shutdown state. Following reactor shutdown it is possible, by virtue of the reserve tank cooling system, to assume that the reactor will be adequately cooled from a safety point of view for an indefinitely long period. For these reasons and because at long times after shutdown, only the decay heat is produced in the reactor core, such a reactor is said to possess a passive decay heat removal system. It is seen that, so long as the ports remain open to lateral flow, the passive decay heat removal process continues to operate regardless of the orientation of the reactor with respect to gravity.

If, on the other hand, it is in the nature of an accident that the reactor, operating initially at some elevated pressure and a corresponding elevated temperature, undergoes a sudden depressurization due to a break in the primary circuit, a flashing to steam and an accompanying release of energy will occur in the coolant of the primary heat transport circuit. If, however, the ports become immediately open to lateral flow, the major part of the total coolant inventory of the primary heat transport system, having been maintained as the reserve coolant at less than the saturation temperature for atmospheric pressure, has remained in the liquid phase during the depressurization and becomes available for quenching the steam and restoring core cooling to a stable long term passive cooling process. Here too, it is assumed that the reactor core has in the meantime reached a stable shutdown state early in the scenario.

If in either of the above scenarios the ports were of a design such that they switched to the lateral-flow yielding the passive cooling mode without reliance on operator action, automatic mechanical responses to sensors, or external energy, the passive cooling system can be said to have been initiated by passive means. Further, if either of the above scenarios included a reactor shutdown not dependent on operator action, automatic mechanical reaction to sensors, or external energy sources, the reactor may be said to have a passive reactor shutdown system. The preferred embodiment is capable of such a feature through the addition of absorber to the reserve coolant, as well as the necessary plant to manage the absorber content in the primary circuit of the system as the reactor becomes critical and assumes normal operation. This system variant would be seen to be particularly meritorious from a safety point of view if deployed in conjunction with the passively initiated passive cooling system.

For scenarios involving loss of reactor regulation, the reactor relies for its safety on either neutronic feedback, as provided by the prompt negative fuel temperature coefficient, or on secondary effects caused by changes in the coolant condition. Such changes include phase change, or the induction of absorber dissolved in the reserve coolant as mentioned above.

The penetrations of the reserve tank boundary to admit the main inlet and outlet conduits are located high up on the walls of the reserve tank. This location is chosen to minimize the effect of a postulated breakage, in either the conduits or the connected components occurring outside the reserve tank, on the inventory of water in the reserve tank. The appropriate placing of syphon breaks within the system may be of assistance in this regard.

Returning, specifically, to a consideration of the neutronic aspects of the reactor core 1, the beryllium reflector 10 plays an important role keeping the basic radial power peaking factor close to unity. To this end, such a reflector is both necessary and particularly effective when used in conjunction with a small hydrogen moderated core. Also in this regard, the preferred use of "burnable" neutron absorber interspersed in the fuel matrix, in order to provide uniform reactivity compensation throughout the core over the fuel burnup lifetime, avoids the severe flux perturbations which would otherwise be caused by necessary movable mechanical absorbers.

As indicated, the presence of both the reflector and the burnable absorber support radial uniformity of the neutron flux, and hence uniformity of power density and coolant conditions throughout the core. This uniformity is a vital requirement in a low-pressure power reactor in which acceptable efficiency of the energy conversion process requires the average core temperature to be close to the maximum core temperature, which, in turn, will have the saturation temperature corresponding to the operating pressure as its extreme upper limit. A consideration of burnable or controllable absorber added to the coolant, as an alternative to the above-mentioned movable or fuel-added absorber, is disregarded in the compact reactor embodiment depicted in the drawing, because of the need to avoid the added plant and operational complications. The regulating absorber rods 5, which are of sufficient number and unit reactivity worth to cover with some redundancy the reactor's operating reactivity margin, are made to travel by rod drive mechanisms 9 within associated guide tubes 5 located in a distributed manner throughout the core. Interspersed among the regulating rods and fuel rods of the core are shutoff rods 7, which are of sufficient number and reactivity worth to cancel under all conceivable operating circumstances the reactor's operating reactivity margin, are made to travel by separate rod drive mechanisms 9 within associated guide tubes 8. Each shutoff rod has an attached fuel rod which, under normal operating conditions with the shutoff rods fully withdrawn, occupies the core lattice site reserved for the receiving of the shutoff rod on shutdown. This arrangement is of particular importance in the preferred embodiment, in order to further support the favourable flux peaking factor mentioned above, and to assure full-complement fuel loading under normal operation in the interests of infrequent refuelling. The rod drive mechanisms specified must exhibit high performance capabilities with regard to reliability, miniaturization (in the case of a particularly compact plant) and the distinctness of operational principles between regulating and shutoff rod functions. In addition to the qualifying remarks already made with regard to the regulating and shutoff rods, other aspects of reactor control normally observed by those skilled in the arts of reactor physics and control are assumed to apply. It is possible to eliminate the need for the shutoff rods and their associated mechanisms, if, for example, the shutoff function is made to follow the onset of passive cooling by means of neutron absorber previously dissolved in the reserve coolant being drawn into the core. Also, the required total reactivity worth of the regulating rods may be reduced by adding and subtracting chemical shim in controlled amounts with respect to the circulating primary coolant, or by reliance on the built-in tendency of the reactor for self regulation to a constant core temperature by virtue of the negative temperature coefficient of reactivity.

The reactor vessel 11 serves as a principal support structure for many components of the reactor plant. Its layout also accommodates the remote disassembly of certain components, including the rod drive mechanisms and the fuel elements or assemblies, for the purpose of servicing and refuelling the reactor. In addition to supporting the core 1 by means of the upper and lower grid plates 3 and 4, the vessel supports the reflector 10, the lower and upper gamma shield inserts 17 and 18, and the control rod mechanism deck 31. It also provides for the coupling of the inlet and outlet ducts 14 and 15 to the inlet and outlet plena 12 and 13, respectively, and for the locating of the radial component of the gamma shield 16. The reactor vessel 11 and the borated water shielding tank, 19, together form a welded unit which is supported within the reserve coolant tank 30 by reactor support structures not shown in the drawing, but which may be specified by those skilled in the art of structural design.

The space enclosed by the shielding tank 19, and not occupied by other reactor components, is filled by reactor shielding material possessing the ideal reactor shielding properties of (i) hydrogen moderation, (ii) relatively radiation-free thermal neutron capture, and (iii) gamma radiation absorption. In the preferred embodiment, this space is filled with a composite made up of thermal neutron and gamma ray absorbing plates interlaminated with water. The plates are composed of a readily available stainless steel alloy, containing boron as a major constituent. An advantage of this shielding design is that, by the circulation of the water component, it facilitates the removal of penetrating radiation heating deposited within the shield during reactor operation. This design also simplifies the problem of managing the liquid shielding material, since the liquid in this case consists of relatively pure water, rather than the more conventional aqueous solution containing neutron absorbing chemicals.

In the preferred embodiment, the water of the shielding tank 19 communicates with the water of the primary heat transport circuit through relatively small openings not shown in the drawing. Under normal operating conditions, a small fraction of the coolant flow returning through the main inlet conduit is diverted into the shielding tank while an equal amount is entrained with the flow emerging from the reactor core and eventually leaving the reactor plant through the main outlet conduit. In the interests of achieving the most efficient recapture of radiation heat deposited in the shielding tank for subsequent conversion to other energy forms in the plant's main energy conversion unit, the magnitude of such flow through the shielding tank is such that the merging core and shielding flows combine at more or less the same temperature. In practice such flows will be relatively small, typically one tenth of the mass flow through the core, and this fact, coupled with the fairly large volume of shield water and the reduced activity of the incoming water due to the delay tank, prevents an unwanted buildup of radioactivity in the shielding tank. Nevertheless, the recapture of the radiation heating of the shield may yield, in the case of a small reactor core, a 10 percent enhancement in the total converted energy output for a given fission rate.

Also as a result of these shield cooling arrangements in which different portions of the same body of water serve various functions at different times, the shield tank water will share not only operating pressures and temperatures similar to those of the reactor core, but will undergo continual chemical upgrading by virtue of the main water chemistry plant normally connected to the primary heat transport circuit. The advantages of this arrangement with regard to the elimination of the added weight, volume and complexity of a water management plant explicitly for the maintenance of shielding solution, will be particularly apparent in applications where compactness of plant is essential.

Provisions similar to those described above for coupling the shielding water system to the primary cooling system under normal operating conditions should, in principle, also be present when the reactor is being cooled by the passive cooling system. Under these latter conditions, however, the reactor will have been placed in the shutdown state, during which transition the radiation heating is reduced disproportionately faster than the core power, and it is unlikely that any special cooling provisions will be required. In any case, small passages to facilitate adequate natural convection between the shielding water and the main passive cooling circuit under shutdown conditions, without compromising either of the primary cooling functions, may be readily devised if necessary.

It may be noted that the principle of sharing a single water chemistry plant may be extended, depending primarily on the particular port design adopted, to include the reserve coolant which resides in the reserve coolant tank 30. Such inclusion is particularly appropriate in reactor plants using the hydrodynamic ports referred to above, or other port designs which can allow a small constant exchange of coolant between the circulating coolant and that of the reserve tank. In such cases, the normal residual rate of coolant exchange between the reserve coolant tank and the circulating primary coolant may suffice. It must be borne in mind, however, that such exchange rates are subject, under normal operating conditions, to an imposed upper limit to prevent (i) undue losses of energy from the primary circuit to the reserve tank, and also (ii) the significant build-up of core activation products in the reserve tank that accompanies the exchange of primary and reserve coolants. Layers of thermal insulation 25 are placed, as shown in the drawing, on all surfaces which demarcate the thermal boundaries between the body of reserve coolant itself, and those components of the primary heat transport circuit enclosed within the reserve tank. Clearly, both the shielding tank 19 and the delay tank 26, which share a common hydraulic boundary, may be considered for present purposes as components of the primary circuit, as all components located within them are allowed to rise to general level of the normal operating temperature of the primary circuit and need not be insulated individually. The insulation used may be of the wettable type, but it must be capable of physically withstanding the effects of hot water suddenly depressurizing within the matrix of the insulation and flashing to steam, if the insulation system is to remain intact following a loss of coolant event involving coolant phase change. The materials of the gamma shielding components,

16, 17 and 18, are of a high-Z variety, typically, lead, or tungsten. If necessary to prevent distortion due to overheating, cooling passages may be arranged in these components to permit the circulation of the water from the nearby shielding tank 19. The surfaces of the gamma shielding components, including the cooling passages, are clad with material which is chemically compatible with the shielding tank water. The actual dimensions of the gamma shielding components, including the laminate structures of the shielding tank 19, may be optimised for a particular application by those skilled in the art of radiation shielding design. Also depending on the particular application, auxiliary shielding components, not shown in the drawings, may be added in and around the reactor plant, and in the vicinity of adjacent equipment. Such localized shielding may be necessary to counteract the possible streaming of radiation from the core through the inlet or outlet ducts and ports. Such shielding may be arranged as an integral part of the ducts or ports themselves. The head space above the control rod mechanism deck 31, is likewise filled with water communicating marginally with the main body of reactor coolant. This water, as well as contributing to the general shielding, also may provide cooling and lubrication to the rod drive mechanisms, depending on their particular design. For a reactor designed to operate at temperatures elevated above the operating temperature for the rod drive mechanisms, the thermal insulation must be applied to the control rod mechanism deck, rather than over the upper part of the reactor vessel and its cover 32, as shown. In any case, provision must be made, in the form of suitable passages, not shown, for the immediate equalization of pressures between the head space and the remainder of the reactor vessel, to eliminate any tendency for the position of the regulating or shutoff rods to be altered inadvertently, in the event of pressure transients which may arise during operation. Although provisions for the control of the coolant pressure and gas content is made at points of the primary heat transport circuit lying outside the reactor plant, the collected gas relief spigot 33 appropriately coupled to the gas control system allows the timely removal of any gases accumulating within the reactor vessel.

The reserve coolant tank 30, and the reactor assembly it contains, have low elevation profiles to facilitate installation in a low-profile environment such as on board a small or medium-sized submarine vehicle. In such a marine application, the reserve coolant tank may stand alone within the vehicle structure or it may be an integral part of the submarine structure, namely, the main pressure hull or a secondary inner hull. In a land-based application, the reserve tank could be in the form of an in- ground pool, or could serve as a liner for an in-ground tank structure which serves as a backup container for the reserve tank, offering, at the same time, an opportunity for monitoring for leaks in the reserve tank using methods which are commonly applied in the industry. The arrangement of the reactor plant, while determined largely by thermal hydraulic and compactness considerations, also facilitates refuelling and other aspects of reactor servicing. The reactor core region is accessed by first erecting auxiliary shielding, if necessitated by a small shielding space allowance in the particular installation. The tank is in the form of vertical extension sealed temporarily to the reserve tank 30 and enclosing a large area around the reserve tank access hatch 34. After filling the extension with water of reactor purity, the hatch cover 34 is removed, followed by the reactor vessel cover 32. At this point, the water of the auxiliary shielding tank will have become united with the water of the reactor systems, thus forming a single body of liquid. After assuring that the reactivity control and shutoff mechanisms have placed the core in its least reactive state, the shutoff rods, 5 and 7, are decoupled from their control mechanisms 9. The latter are then removed, leaving the shutoff rods in the core. Following this, the control rod mechanisms deck 31, some boron absorbing laminates, and the upper axial gamma shield 18, which also serves as the upper boundary of the outlet plenum 13, are removed in that order from within the reactor vessel 11. At this point defuelling operations may begin, followed, at the last, by the removal of the control and shutoff absorbers, including the follower fuel rods. Fuelling operations involve the same procedural steps, but in reverse order and with an approach-to-critical procedure before reassembly of the shielding components and reconnection of the reactivity mechanisms.

Embodiments which do not require that the passive cooling circuit be available at all physical orientations of the plant (eg. land-based plants) may have a configuration with as few as one inlet duct and one outlet duct, connecting with the reactor core. These can be offset relative to the core axis in order that ready access to the core for servicing and refuelling is available. In the case of a research reactor, such ready access may facilitate the design and operation of experimental facilities installed in or near the reactor core.

Claims

1. A nuclear reactor plant comprising: (a) a core; (b) means for controlling the reactivity of said core; (c) radiological shielding around said core; (d) inlet means to conduct coolant into said core; (e) outlet means to conduct coolant out of said core; and (f) a reserve coolant tank; characterized by port means in said inlet means and in said outlet means for conducting coolant between said reserve coolant tank and said inlet means and outlet means by convection during impairment of normal flow of coolant through said inlet and outlet means, and for permitting a flow-through of coolant through said inlet means and outlet means during normal operating conditions.

2. A nuclear reactor plant according to claim 1 characterized by a delay tank for receiving coolant from said outlet means.

3. A nuclear reactor plant according to claim 1 or 2 characterized in that said inlet means comprises an inlet header, an inlet plenum adjacent to said core and a plurality of inlet ducts leading from said header to said inlet plenum, and in that said outlet means comprises an outlet header, an outlet plenum and a plurality of ducts leading from said outlet plenum to said outlet header.

4. A nuclear reactor plant according to claim 1, 2 or 3 characterized in that said reserve coolant tank surrounds said core.

5. A nuclear reactor plant according to claim 1, 2 or 3 characterized in that said reserve coolant tank contains coolant with a neutron absorber.

6. A nuclear reactor plant according to claim 1, 2 or 3 characterized in that said shielding comprises gamma shields adjacent to said core, and a shielding tank.

7. A nuclear reactor plant according to claim 6 characterized in that said shielding tank substantially surrounds said core.

8. A nuclear reactor plant according to claim 6 or 7 characterized by a delay tank for receiving coolant from said outlet means, said delay tank being annular in shape and encompassing said shielding tank.

9. A nuclear reactor plant according to claim 8 characterized in that said delay tank provides additional radiological shielding of said core.

10. A nuclear reactor plant according to claim 8 characterized in that said reserve coolant tank surrounds said shielding tank, said delay tank and said inlet means and outlet means.

11. A nuclear reactor plant according to claim 2 characterized in that said reserve coolant tank surrounds said delay tank.

12. A nuclear reactor plant according to claim 3 characterized in that said port means is located in a plurality of said inlet ducts and in a plurality of said outlet ducts.

13. A nuclear reactor plant according to claim 12 characterized in that the change from a through-flow of coolant in said inlet ducts and said outlet ducts to a flow of coolant between said inlet and outlet ducts and said reserve coolant tank is initiated and maintained by natural convection of said coolant.

14. A nuclear reactor plant according to claim 12 characterized in that said inlet ducts and said outlet ducts are configured with substantially radial symmetry about a vertical axis through the center of said core, whereby a convective flow of coolant can be achieved between said reserve coolant tank and said inlet and outlet ducts regardless of the orientation with respect to gravity of said reactor plant.

15. A nuclear reactor plant according to claim 2 characterized in that said delay tank at least partially surrounds said core.

16. A nuclear reactor plant according to claim 15 characterized in that said delay tank is annular in shape and co-axial with said core.

17. A nuclear reactor plant according to claim 2 characterized in that said delay tank includes baffles to control the flow of coolant therethrough.

18. A nuclear reactor plant according to claim 6 characterized in that said shielding tank contains thermal neutron and gamma ray absorbing plates interlaminated with coolant.

19. A nuclear reactor plant according to claim 18 characterized by means for conducting coolant from said inlet means into said shielding tank and from said shielding tank into said outlet means.

20. A nuclear reactor plant according to claim 10 characterized by thermal insulation insulating said shielding tank and said delay tank from said reserve coolant tank.

21. A nuclear reactor plant according to claim 1 characterized in that said inlet means includes an inlet duct and said outlet means includes an outlet duct, said inlet duct and/or said outlet duct being offset from a vertical axis through the centre of said core to facilitate access to said core.

22. A nuclear reactor plant according to claim 3 characterized in that said inlet ducts and/or said outlet ducts are offset from the axis of said core to facilitate access to said core.

23. A nuclear reactor plant according to claim 19 characterized by means for maintaining the quality of coolant circulating through said nuclear reactor plant, and in which the quality of coolant in said shielding tank is maintained by said coolant quality maintaining means as part of said coolant circulating through said nuclear reactor plant.