The primary coolant receives heat in the reactor core, flows to a region where the neutron flux is low to transfer its heat to the secondary coolant in the intermediate heat exchangers, and then returns to the core. In fixing the layout of this primary coolant circuit two main choices have to be made: whether the heat exchangers and circulating pumps should be in separate vessels from the core or in the same one, and whether the pumps should be located before or after the heat exchangers.

A “pool” reactor is one in which the entire primary circuit is con­tained within a single vessel, as shown in Figure 4.1 A. The core is surrounded by a neutron shield and around this are placed the pumps and heat exchangers. In a “loop” reactor, in the other hand, as shown in Figure 4.1 B, the core is contained in a small vessel with the main neutron shield outside. Hot coolant from the core passes through pipes to the heat exchangers and then back to the core vessel.

The choice between the two schemes is affected by such consid­erations as the design and manufacture of the vessels, the design of the refuelling system, the operating conditions of the pumps, and ease of inspection and maintenance. That the choice is finely balanced is shown by the fact that reactors of both types have been built, but most proposed future reactors are of the pool type.

The main advantages of the pool layout are that the coolant pressure-drop is low and the reactor vessel is very simple in shape without irregularities that might act as locations of high stress. The primary circuit can be arranged so that hot coolant never comes into

contact with the vessel. In a loop reactor parts of the pipework and of the vessel are in contact with hot coolant at temperatures at which thermal creep may be important, while other parts are in contact with cold coolant so that thermal stresses have to be allowed for. In addi­tion there may be the possibility of stress concentrations caused by the pipe branches on the sides of the vessel.

On the other hand a pool reactor vessel is so large that it has to be assembled on site whereas a loop reactor vessel can be made in a factory where the quality of manufacture can be controlled more easily. The roof of a pool reactor vessel is much larger than that of a loop vessel and if advantage is to be taken of the potential simplicity of the vessel itself the pumps and heat exchangers, and possibly the entire core and neutron shield, have to be suspended from it. Moreover part of the underside of the roof is exposed to the temperature of the hot coolant. As a result it is a complex and expensive structure.

A pool reactor has the advantage that there is room within the vessel for a temporary store for irradiated fuel, usually surrounding the neutron shield. Fuel can be transferred from the core to the store without lifting it above the coolant so that no special provision has to be made for cooling it while in transit. It can be left in the store, immersed in coolant, until the fission-product decay power has decayed suffi­ciently to make handling easier when it is removed for reprocessing. A loop reactor vessel is unlikely to have enough room for an irradi­ated fuel store. Irradiated fuel has to be removed from the vessel to a separate store by a machine that is capable of cooling it while it is in transit.

Both pool and loop reactors have pipework or structure operating at the temperature of the hot coolant. The difference is that in a loop reactor the hot pipework is part of the primary coolant containment, and if it should fail radioactive primary coolant could be released. To offset this disadvantage, however, the loop system has the advantage that it may be possible to inspect the high-temperature pipework more easily because it is accessible from outside. It may even be possible to do maintenance work on one coolant loop by closing it off with valves without shutting down the whole reactor. Inspection and mainten­ance of the pipework and structure within a pool reactor vessel are difficult.

Figures 4.2 and 4.3 show a typical arrangement of the components of a pool reactor. In the arrangement shown the core and neutron shield are supported by a strongback attached to the bottom of the vessel while the other main components — the primary pumps and the intermediate heat exchangers — are supported by the roof of the vessel. An alternative arrangement is to hang the core and shield from the roof as well. This has the advantages that the vessel carries only the weight of the sodium it contains and being simply shaped is relatively lightly stressed, and that stresses due to thermal expansion are minimised. Another alternative is to support the vessel and the core from below, but in this case thermal expansion stresses are greater.

9

Above-Core 17 Reactor Roof

Structure 18 Outer

10 Hot Sodium Pool Rotating Shield

11 Cold Sodium Pool 19 Inner

12 Primary Sodium Pump Rotating Shield

13 Pump Motor 20 Fuel-Handling

14 Auxiliary Motor Mechanisms

15 Intermediate Control Rod

Heat Exchanger Mechanisms

16 Secondary Sodium Pipes

Figure 4.2 Arrangement of the primary circuit of a pool reactor.

A typical pool reactor vessel may be about 17 m in diameter and 16 m deep, containing about 2000 tonnes of primary sodium and made of stainless steel about 20 mm thick. It is surrounded by a second “leak jacket” or “guard vessel”, so that even if the main vessel should break the sodium level cannot fall below the top of the core and emergency

Figure 4.3 Plan of the roof of a pool reactor with three secondary circuits.

cooling can be maintained. The space between the vessels provides access for inspection of the main vessel, for example by means of a remotely-controlled vehicle carrying an ultrasonic probe that can examine the welds, particularly those where the stongback is attached.

The inner vessel separates the hot and cold parts of the primary coolant circuit so that all the sodium in contact with the main vessel is at the cooler core inlet temperature. In some cases the inner vessel has a double wall to minimise the transfer of heat between the hot and cold sodium, but an alternative is to shape the lower part of the inner pool so that the sodium in it remains stagnant and acts as an insulating layer.

The reactor roof is a complex structure. Part of its underside faces the surface of the hot pool and has to be protected by thermal insula­tion, usually in the form of thin steel plates, and by a cooling system.

Access through the roof to the core is provided by two rotating plugs mounted eccentrically one within the other. When the reactor is oper­ating they are situated so that the control-rod drive mechanisms moun­ted on the inner plug, and the above-core structure that hangs beneath it, are located centrally over the core. When the reactor is shut down for refuelling the control-rod drives are disconnected, leaving the con­trol absorbers themselves in the core to hold it subcritical. The plugs can then be rotated so that fuel-handling mechanisms mounted on them are positioned above the core and breeder locations as required. The perimeters of the plugs have metal dip seals to prevent leakage of the cover gas. The seals are made of a metal alloy that is solid while the reactor is operating but can be melted when it is shut down to allow the plugs to rotate without breaking the seal.

The above-core structure serves to locate the control-rod drives accurately. It also carries the core outlet instrumentation, consisting of thermocouples to measure the coolant outlet temperature from each subassembly, and in many cases also coolant sampling take-offs that can be used to locate failed fuel (see section 5.2.3).

The vessel for a loop reactor may have inlet and outlet primary coolant connections in the side walls. Some loop reactor designs, how­ever, use a larger vessel so that there is room for the inlet and outlet pipes to pass through the roof and the vessel itself can be simple in form and lightly stressed like a pool reactor vessel. This gives the addi­tional advantage that more space for fuel handling or storage within the vessel may be made available.

The advantages and disadvantages of loop and pool schemes are discussed in detail by Campbell (1973).