Application of DYN3D-MSR for transient analyses

Data gained from experiments performed at the MSRE were used for the validation of the numerical codes for simulation of the transient behaviour of Molten Salt Reactors within the EU project MOST. After the validation, selected transient scenarios typical for MSR were simulated using both the 1D and 3D versions of the reactor dynamics code. The following groups of driving events were studied:

  • Pump driven transients
  • Reactivity driven transients
  • Inlet temperature driven transients
  • Slug-flow transients
  • Channel-wise transients

The results of 1D and 3D codes are qualitatively in a good agreement. However, there are some differences caused by 3D effects. Fig. 1 shows the results of the analysis of a prompt reactivity insertion which can be induced e.g. by control rod ejection accident.

MSR reactivity jump

Fig. 1 Results of DYN1D (left) and DYN3D (right) analysis of reactivity jump of different worth

While the power peak is higher in the 1D simulations, the temperature rise is larger in the 3D case. This is caused by the fact, that the negative fuel temperature Doppler feedback to reactivity is more effective taking into account the 3D neutron flux distribution.

Some transients can be adequately modeled only in 3D. These are transients with significantly local effects like hypothetical local fuel channel blockage. Such a flow blockage can be induced e.g. by mechanical damage or plugs of solidified salt. The blockage of 3 central fuel channels in the core of the MSRE by increasing the flow resistance coefficient by 5 orders of magnitudes is assumed. Fig. 2 shows results of this analysis.

MSR local flow blockage

Fig. 2 Behaviour of reactor power and temperatures in the case of local flow blockage in MSRE with U233 and U235 loading

While the reactor power and the average salt and graphite temperatures are decreasing due to negative reactivity feedback, the salt temperature in the blocked channels will raise continuously. Accordingly, a core can be locally damaged, if the local blockage remains un-remedied.