Operations

One cycle for MYRRHA is 3 months (90 days) of operation followed by 1 month (30 days) for core reshuffling, loading and maintenance. It is foreseen to have 3 such cycles a year. The last cycle is followed by a maintenance period of 90 days instead of the normal 30 days. As such, MYRRHA has a targeted availability of about 70 %. To insure optimal use of the fuel, cycle management (and fuel management which is linked to this) is required.

As a result of the operation of the core, the available amount of fissionable materials in the fuel decreases in time while a number of fission products are produced. This is burn up of the fuel and causes the neutron flux level to decrease in time. Over a single cycle, the source multiplication factor ks drops from 0.9557 to 0.9437 while the effective multiplication factor keff drops from 0.9497 to 0.9361. As a result, the reference core of MYRRHA has lost about 20 % of its initial power during a single cycle. The total flux also drops by about 20 % because it is proportional to the total power.

Another problem posed by burn up is the fact that burn up is not homogeneously distributed over the core. Burn up will always be higher in regions of higher flux (e.g. the assemblies surrounding the spallation target) as the number of fissions is proportional with the flux. This essentially means that flux will decrease more rapidly in zones with higher flux as compared to zones with lower flux (e.g. the assemblies at the outside of the core). We must therefore find a solution to correct both this global and local effect of burn up.

To solve these effects of burn up we must first ensure that the global reactivity level associated with ks is as constant as possible. The first thing we need to do is to provide a reasonable amount of excess reactivity in the fuel (by increasing the enrichment of the fuel). When doing this, we must ensure that this excess amount cannot lead to criticality under any circumstances. This requirement limits the amount of excess reactivity that we can invest. This amount of reactivity can then be compensated by various other means. A first method is the use of burnable absorbers. The absorption of neutrons in the burnable absorber causes the reactivity level to decrease because less neutrons will produce fission. The amount of antireactivity introduced by the burnable absorber decreases in time as the poison is being burned away. Another possibility is the use of voided boxes. We replace the coolant by a low density gas which causes the leakage from the system to increase, which in turn lowers the reactivity level of the system. The antireactivity introduced by these voided boxes will remain constant.

We now use these two sources of antireactivity to compensate the excess reactivity of the fuel as can be seen in the figures below. Because we cannot regulate the burning of the poison, we must make sure that the decrease of antireactivity of the poison is smaller than the decrease of the excess reactivity. In that case, the effective reactivity level will still decrease, but not as fast as in the situation with no poison. After the end of a cycle, a number of voided boxes are removed from the core and the effective reactivity level will increase again. During the shutdown period (which is about 30 days) the core loading pattern can be changed to suit the needs for the following cycle. New elements can be added to the core while the older elements (with high burn up) are removed, introducing an amount of extra excess reactivity.
The reactivity does still vary over the cycle, which still causes the flux to vary over the cycle, but not as strong as without compensation. This is where the proton accelerator comes into play. The flux level is proportional to the proton current provided by the accelerator. When we now slightly increase the current (by 5-10 % over one cycle), we can ensure a constant level of the flux. This repeats itself until all excess reactivity is used up.