The peculiarities of HTR physics

The calculation of an HTR core presents problems which are rather different from those encountered in the calculation of other thermal reactors. We list here the most important ones, although they have already been partially discussed in the preceding chapters.

The use of graphite as a moderator presents the advantage (in comparison with light water) of a low absorption, but requires a detailed treatment of the scattering law in graphite.

The small parasitic absorption associated with non-fuel components allows a high burn-up with low fuel cycle costs, but this requires a heavy loading of fertile and fissile materials.

The combined effects of the graphite scattering properties, high fuel loading and high temperature produce a rather hard neutron spec’trum, which may strongly deviate from the classical fast 1/E and thermal Maxwellian distribution. However, the ratio of moderator atoms to fuel atoms tends to be considerably higher than in light water reactors (mainly because of the low absorption of graphite) and the fraction of thermal fissions over total fissions is in HTRs larger than in LWRs. It can be seen in Fig. 14.1 that approximatively 30% of the fuel absorptions take place above 1 eV in PWRs, while this figure is only about 15% in HTRs.<3) However, because of the high moderator temperature the peak of the thermal distribution lies at higher energy in HTRs than in LWRs. A comparison of the HTR spectrum with that of other graphite moderated reactors is shown in Fig. 14.2<4) where also 235U and Pu cross-sections are plotted.

The He coolant has no influence on the core reactivity: because of this reason and of the use of a solid moderator, reactivity effects due to changes in core density are negligible.

Because of the rather homogeneous dispersion of fuel in the moderator, the thermal heterogeneity effects are much less pronounced than in other reactors (the thermal self-shielding factors are always very near to unity).

The use of coated particle fuel poses the problem of the treatment of the double heterogeneity, at the macroscopic level of the fuel pins and at the microscopic level of the coated particles. This is particularly important in the calculation of resonance absorption.

Because of the homogeneous dispersion of fuel in the moderator the resonance integral is in HTRs 2 or 3 times higher than in most other reactors.

Because of the hard neutron spectrum the treatment of some low-lying resonances (233U, 135Xe, Pu) becomes particularly important in this reactor type.

The effect of graphite crystal binding and of the low-lying resonances is particularly important in temperature coefficient calculations.

Because of the hard neutron spectrum, particular care must be taken in the calculation of spectrum and power distribution at the boundary between different regions (e. g. core-reflector interface).

HTR fuel has a very high burn-up. The resulting high fission product absorption requires a very detailed treatment of fission product build-up, considering all

important decay and absorption chains. This high burn-up imposes also a detailed treatment of fuel management and its effect on power distribution.

Very characteristic of this reactor type is the treatment of fuel performance limits which are not as simple as fixed melting temperatures which should not be exceeded, but depend upon a combination of fast neutron dose, temperature level and temperature gradient (this last quantity being proportional to the power density).

Particular problems can be related to the different types of fuel cycles. More heterogeneous designs have been considered in the past for low enriched U cycles in order to lower the required enrichment, but recently the same homogeneous design appears to be best suited for both cycles. The presence of Pu in low enriched cycles (or even more in cycles with Pu make-up) may pose the problems of conveniently treating the low-lying Pu resonances.

In general comparing calculational methods for HTRs and for other reactor types one must notice the greater importance of detailed spectrum calculations (it is necessary to take a large number of groups) but the less pronounced thermal heterogeneity effects. With possibly the exception of cases with high Pu loadings, detailed thermal cell calculations are not essential to HTRs. This explains the large use of zero-dimensional spectrum calculation codes in which thermal heterogeneity is simply represented by self-shielding factors.

For the same reason diffusion theory is sufficient for the analysis of most HTR design problems, while transport theory needs only to be used for special problems like calculations of self-shielding, control rods, burnable poison rods, resonance self — shieldings, etc.

Because of the high burn-up, a considerable amount of work has been invested in the development of HTR burn-up codes.

The peculiarities of HTR core dynamics have been treated in Chapter 12; we add here a list of the most important features.

Because of the combination of solid moderator, completely ceramic core, chemically and neutronically inert coolant, high thermal capacity and good fission product retention by the coated particles, core kinetics play a minor role in HTRs. Steam generators can be more affected by transients than the reactor core.

The moderator temperature coefficient can, in some cases, be positive in HTRs because of the effects of the low-lying resonances and of the absence of significant density coefficients so that the overall coefficient, still remaining negative, is smaller than in most liquid moderator reactors. The overall temperature coefficient is in large HTRs of the order of 10”5/°C compared to 10 4/°C for water-moderated reactors and КГ6-КГ7/°С for fast reactors.

The peak temperatures during power excursions are rather insensitive to the value of the temperature coefficient, mostly because of the high thermal capacity of the core.

In HTRs, as in all solid moderated gas-cooled reactors, core dynamics calculations are much easier than in reactors with liquid coolant or moderator.

Because of the high thermal capacity, in the case of transients the higher modes of the flux distribution do not contribute appreciably to the core temperatures, so that all dynamics calculations can be performed in the fundamental mode.

In the case of accident analysis the treatment of space dependence can be more important in thermal calculations than in neutron flux calculations. Space — dependent neutron flux calculations are most important in the analysis of the load following capability of the reactor.

References

1. R. C. Dahlberg, Physics of gas cooled reactors. ANS topical meeting on new developments in reactor physics and shielding, 12-15 Sept. 1972, Kiamesha Lake, NY ANS CONF-720901.

2. J. G. Tyror, J. R. Askew and I. Johnstone, Some problems in the physics of high temperature reactors. ANS topical meeting on new developments in reactor physics and shielding, 12-15 Sept. 1972, Kiamesha Lake, NY ANS-CONF-720901.

3. H. B. Stewart, M. K. Drake and R. C. Traylor, GA-8571, 5 Mar. 1968.

4. J. A. Desoisa, DCPM 18/CEGB 1.

5. K. Friedrich, L. Massimo and E. Vincenti, Space dependent studies of transients in high temperature reactors. Specialist meeting on Reactivity Effects in Large Power Reactors, Ispra, 28-31 Oct. 1970, EUR 4731 f-e.