Low Power Refuelling

The Advanced Gas Cooled Reactors (AGRs) in the fleet can be split into two types, those which have safety cases for both on load, and off load refuelling (OLR) and those which only have approval for off load refueling.

The LPR capable AGRs in the fleet are: 

• Hinkley B
• Hunterston B
• Heysham 2
• Torness

The pattern show below of the unit reducing to exchange load then returning to parking load in commonly known as "casting" or "cycling".

These are load drops that occur prior to a nuclear unit starting a refeulling programme. These need to occur in order to get the reactor into a stable position whereby it can reduce load further to carry out the exchange of fuel stringers. 

In order for the station to reduce power the thermal output of the reactor must be reduced by inserting control rods, this process temporarily increases Xenon gas levels. If the levels of the gas gets too high it can cause a loss of control of reactor power and will cause the reactor to trip. The pre-con allows the Xenon gas levels to remain within acceptable limits allowing that station to further reduce load in order to exchange fuel stringers. 

Exchange - This is the generation level when the reactor is at a reduced thermal output in order to allow the fuelling machine to contain the reactor pressure when connected and thus allowing removal of an old fuel stringer and replacment of a new one.

Parking - In order to maximise the number of MWs the station is producing during a refuelling, the station will temporarily increase generation whilst the refuelling machine physically disposes of the old fuel stringer to the buffer store and collects a new stringer ready to complete the next exchange.

ARSOs are only carried out by Hinkley B and Hunterston B.

In order for the reactors to be as effective as possible it is desirable to have a flat profile of reactivity across the reactor. The conentration of fuel elements around the outside of the reactor is less than the centre.

To avoid "hot spots" fuel that has been enriched to a higher level is used in the outside of the reactor. As the level of reactivity decreases they shuffle to different parts of the reactor to maintain a flat reactivity profile at the highest level possible. 

An ARSO results in an extended period at or about exchange load when compared to a straightforward stringer exchange.

Forecasts from station will often include a risk adjustment in the from of a 12 hour hold at the end of the fuel exchange cycles. This is added to the forecast by the Commercial Planner (CP) in different ways:

• an extended period at parking load after the final exchange
• or as an elongated final exchange. The CP may alter the length of this hold period as the LPR progresses. 
• or by adding a small adjustment to each exchange 

1. The shift trader should check with station as soon as possible during the final exchange to ascertain how long they expect to hold at parking load and then update Helios to remove any remaining risk adjustment.

2. The shift trader should look for elongated final exchanged in the forecast and check with the station CCR how long they expect to spend at exchange load and adjust Helios

3. The CCR will not be aware of risk adjustments added to each exchange. The shift trader will have to update Helios based on station communication and MEL submissions after each exchange.

When Asset Ops make an adjustment it will in the form of an extended mid-point. Check with Asset Ops for details of any risk adjustments and to confirm with Asset Ops during Asset handovr. Ensure any adjustments are detailed in the handover for the incoming shift.