GB2067822A

GB2067822A - Nuclear reactor heat sink

Info

Publication number: GB2067822A
Application number: GB8100986A
Authority: GB
Original assignee: UK Atomic Energy Authority
Current assignee: UK Atomic Energy Authority
Priority date: 1980-01-23
Filing date: 1981-01-13
Publication date: 1981-07-30
Also published as: GB2067822B

Abstract

Natural circulation of coolant in a gas cooled fast reactor in the case of circulator (4) failure is used to provide time for repair or other actions. The natural circulation is directed through a permeable radial shield (2) by open flap valves (8) or fluidic diodes which are normally held shut by coolant pressure from the circulators. In this way, the radial shield acts as a heat sink. <IMAGE>

Description

SPECIFICATION Nuclear reactors This invention relates to nuclear reactors and in particular to gas cooled reactors. An advantage of such reactors whether fast or thermal is that technology used in the construction of existing gas thermal reactors can be used for parts of their construction.

In a gas cooled fast reactor, the reactor core is cooled by heat exchange with boilers by a gaseous medium circulated through the core and boilers by circulators. A fault situation that must be considered in such a reactor is failure of the circulators.

An object of the present invention is to provide a nuclear reactor wherein provision is made for time to be provided for repair measures to be taken in the event of stoppage of the gas circulators.

According to the present invention, in a gas cooled reactor comprising a core cooled by a gaseous medium by transference of heat thereby to a means for generating electricity and a radiation shield around the core, the shield being permeable to the gaseous medium, means is provided for conducting coolant leaving the core through the shield so that the shield acts as a heat sink.

Advantageously, the means for conducting coolant flow comprises a flap arrangement. The means may comprise an arrangement of fluidic diodes.

Preferably, the radial shield becomes a heat sink when sufficient heat is not being transferred to the means for generating electricity. The reactor is conveniently surrounded by a secondary containment building for containing any radioactive material which may be released from the reactor and for protecting the reactor from external events.

An embodiment of the present invention will now be described by way of example only with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic plan view of a gas cooled fast reactor, Figure 2 is an incomplete, diagrammatic vertical section of half of Figure 1, Figure 3 is an incomplete fragmentary view of Figure 1 showing more detail, and Figure 4 is a graph showing coolant parameters associated with the reactor of the preceding figures.

Reference is directed firstly to Figure 1, wherein a reactor core is indicated by 1. A radial radiation shield 2 surrounds a core 1 and four boilers 3 are arranged outside the radial shield. The boilers are each in a separate quadrant around the reactor core. Gas coolant flows through the core 1 and around the boilers 3 to cause heat exchange between the core and the boilers. The gas is circulated by circulators 4, two of which are located in each quadrant, which quadrants are each fed from an independent power supply. A pressure vessel 5 surrounds the boilers, the circulators 4 being set into the wall thereof. A secondary containment vessel 6 surrounds a pressure vessel 5 to provide a secondary containment area 7.

Reference is now made to Figure 2, wherein like reference numerals are used for like parts as in Figure 1. From Figure 2, it can be seen that a flap arrangement comprising a number of flaps 8 is positioned below the radial shield 2. The flaps fall open under gravity in the absence of sufficient flow from the circulators 4 as wili be explained in operation below.

Reference is now directed to Figure 3 which shows the radial shield 2 in more detail. The shield comprises an arrangement of vertically disposed hexagonal steel rods 10, only three of which are shown. The rods are constructed and spaced so that gas can flow through the shield parallel to the axes of the rods, the amount of such flow being dependent upon the positioning of the flaps 8.

Reference is now made to Figure 4, which is a graph with temperature in degrees Celsius plotted as ordinate and time in hours as abscissa. Zero time is taken when the gas circulators stop functioning and the curves 20, 21 and 22 represent performance of fuel cladding in the reactor, at the coolant exit and coolant inlet, respectively.

Operation of the reactor is now described with reference to Figures 1, 2 and 3. Coolant flow lines are indicated by arrows. The circulators 4 direct coolant gas towards the underside of the core from the boilers 3. The coolant gas passes upwardly through the core and absorbs heat therefrom. The heated gas then passes downwardly through the boilers 3 and gives up the absorbed heat to the boilers 3. The radial shield is cooled by gas flowing up the thereby through in the direction of the dotted arrow in Figure 2. The flow through the shield 2 is restricted by the flaps 8 which are held up by the flow of coolant from the circulators 4. The restriction is provided to avoid significant dilution of the main coolant at the exit of the reactor core.

Now suppose that the circulators fail, unlikely as this is in view of their independent power supplies in each quadrant. In such a situation, there is no longer a coolant flow from the circulators to hold the flaps 8 shut consequently, the flaps open under gravity. The flow of coolant gas then becomes reversed in the radial shield as shown by the full line in Figure 2. Flow from the core 1 recirculates through the shield 2 as a result of the different static heads generated by the heat of the core. In practice, a hot front travels in time from the top of the shield to the bottom thus steadily reducing the coolant driving head.

Natural circulation through the radial shield, i.e. its use as a heat sink is limited in time to proportion to its total heat capacity.

Reference to Figure 4 shows a fall and gradual rise of the core temperature with time after the circulators 4 have stopped in curve 20. Curve 21 shows how the coolant temperature leaving the core varies and curve 22 how the coolant inlet temperature varies. The curves show that period of a few hours is provided by the radial shield in operation, wherein action can be taken to render the circulator or circulators functional or use other cooling means.

In the foregoing description, a number of parallel fluidic diodes could have been used instead of flaps, the fluidic diodes then providing a high flow in the normal direction of flow through the radial shield and low impedance in the reverse The entire reactor can be enclosed in a secondary containment building for containing any radio-active material which may be released from the reactor and for protecting the reactor from external events.

From the above description, it can be seen that a gas cooled fast reactor is provided wherein cooling of the reactor core is provided in the event of circulator failure for sufficient time for corrective action to be taken.

Claims

1. A gas cooled reactor comprising a core cooled by a gaseous medium by transference of heat thereby to a means for generating electricity and a radiation shield around the core, the shield being permeable to the gaseous medium, in which means is provided for conducting coolant leaving the core through the shield so that the shield acts as a heat sink.

2. A gas cooled reactor as claimed in Claim 1, in which the means for conducting coolant flow comprises a flap arangement.

3. A gas cooled reactor as claimed in Claim 1, in which the means comprises an arrangement of fluidic diodes.

4. A gas cooled reactor as claimed in any one of the preceding claims, in which the radial shield becomes a heat sink when sufficient heat is not being transferred to the means for generating electricity.

5. A gas cooled reactor as claimed in any one of the preceding claims, in which the reactor is surrounded by a secondary containment building for containing any radioactive material which may be released from the reactor and for protecting the reactor from external events.

6. A gas cooled reactor substantially as herein before described and as shown in the accompanying drawings.