Failed Fuel. It is more difficult to detect an incipient accident affecting only one subassembly than one affecting the whole core, but it is necessary to do so to prevent the spread of damage (see section 5.4.2). The difficulty arises because there are hundreds of subassemblies in the core. If instruments have to be attached to each, and triplicated, the resulting trip system is very complex. It may also be very cumbersome, especially if instruments have to be fixed to each subassembly after it has been inserted into the core and detached before it is removed.

Thus instruments outside the core that can detect a developing accident anywhere in the core are very desirable. There are two main candidates: failed cladding detectors and boiling detectors.

A failed cladding detection system searches for failed fuel elements by monitoring the coolant or cover gas for radioactive fission products that must have come recently from the fuel, and then locates the subassembly in which the failure has occurred. This can be done in two ways. p or у activity can be sought if the fission products are separated from the coolant, which is already у-active. Alternatively neutrons from delayed neutron precursors can be sought if a sample of the coolant is removed from the neutron flux.

A typical system has four parts. A sample of the main coolant flow from the core is piped away to a point at which the neutron flux is low. There it passes to a vessel surrounded by moderating material and thermal neutron detectors (usually boron trifluoride counters) that detect any delayed neutron precursors in the coolant. This enables a quick response to cladding failures anywhere in the core, provided the sample has been taken from a point where the flow is well mixed so that it is representative of the flow through the whole core.

Secondly samples are taken from the outlet of each subassembly in turn to a delayed neutron monitor. This serves to locate the sub­assembly in which a failure has occurred. Because there are so many subassemblies to sample, location is relatively slow.

Thirdly a sample of the cover gas over the sodium in the reactor ves­sel is taken to a y-spectrometer and a moving-wire p — precipitator. This latter device uses a charged wire to precipitate the daughter products of p-decaying fission products and to transport them to a chamber con­taining a p-detector. The system thus discriminates against a range of activation products that might be present and selects p-active daugh­ters of gaseous p-active fission products (mainly 88Rb produced by p-decay of 88Kr), so that it responds selectively to cladding failures. Further information about the nuclides present is given by the y — spectrometer.

Finally the gaseous fission products are stripped from the loca­tion system coolant sample by a stream of gas that then goes to a p-precipitator. This provides an independent way of locating failed cladding.

Coolant Boiling. Failed fuel detection has the advantages that it is reliable and a direct indication of the release of radioactivity, which is what has to be avoided. Even a bulk detection system is however relatively slow, taking tens of seconds to detect a failed fuel element, mainly because of the time taken to transport the coolant sample to the detector. It is adequate to control the release of radioactivity to the coolant but a faster system can help to minimise damage to the fuel.

If fuel element failure is caused by overheating it may be accompan­ied by boiling of the coolant (which takes place at about 920-940 °C at the pressure in the core). A boiling detector gives a quicker indication that something is amiss than waiting for the fuel to fail and then for the failed fuel detection system to operate. Boiling can be detected by acoustic means, as is suggested by the ease with which boiling is heard in a domestic kettle. It is particularly attractive in a reactor because a small number of detectors are enough to detect boiling anywhere in the core.

The main difficulty is that the reactor itself is quite noisy. Sound is generated by the coolant pumps, by the turbulence of the coolant flow, and by cavitation. Cavitation is particularly awkward because it is a form of boiling (caused by local reductions in pressure at points where the flow of the coolant is accelerated, such as at sharp corners or on the blades of the pump impellers), and it makes a very similar noise to boiling caused by overheating. It may be possible to avoid the difficulty by designing the reactor to keep cavitation to a minimum and to discriminate against other background noises by listening in a frequency range in which boiling generates a lot of noise.

As an alternative to acoustic means boiling can be detected by temperature measurement. In some cases boiling in a subassembly can be detected by means of a thermocouple at the outlet, but there are some circumstances in which detection would not be reliable. A partial blockage to the flow somewhere in the subassembly could be large enough to cause severe overheating in its wake, possibly to the boiling point, but at the same time have a very small effect on the total flow­rate and the mean outlet temperature. This is because the resistance of the subassembly to coolant flow is already high and the additional resistance caused by a blockage is small in comparison and reduces the flow-rate only slightly. The effects of subassembly blockages are discussed in more detail later in section 5.4.1.

It may be possible, however, to detect a local blockage by observing temperature fluctuations at the subassembly outlet. A partial blockage increases the turbulence of the flow and the differences in temperature between different parts of the flow, and so causes increased temper­ature fluctuations, or “temperature noise”, at the outlet, which can be detected by fast-response thermocouples.