In the period 1966-9 a programme of HTGR critical experiments covered in a first part measurements of the reactivity worth of special elements in a “clean” essentially homogeneous graphite and fully enriched U core. A second part covered lattice measurements (Doppler coefficients, control-rod worth, reactivity worth of burnable poison rods, flux plots) on a central part of a critical facility which simulated the HTR block fuel geometry.

In the first part the C/235U ratio was varied between 432 and 5000. The analysis of the reactivity coefficient measurements indicated that the reaction rates of core materials can be predicted within the following accuracy:01)

Boron ± 2% 238U + 2 to + 5%

235U ± 2% 236U + 3 to + 5%

233U ± 4% 237Np + 2 to — 10%

232Th + 2 to + 5%

At the time of evaluation the data available were the version I of ENDF/B. Newer cross-section sets such as the versions II and III of ENDF/B do not involve significant changes in the data concerning thermal reactors.

The analysis of these experiments has been repeated at CEA with the APOLLO code.<40)

The control-rod worths measured as part of the lattice measurements have shown discrepancies of the order of 5-10% (the calculated rod worth was in general overestimated).<12)