Advanced HTGR systems

Abstract

G A = Y = NOMENCLATURE ratio of thermal reactor to breeder thermal power ratio of thermal breeder to LWR reactor thermal p o w e r conversion ratio total fissions/thermal fissions in any reactor out-of-pile inventory/in-pile inventory capture/fission ratio fissile rating [MW(t)] per kg fissile fissile out]fissile in ratio in loadings fissions per initial fissile atom burnup MWD per tonne metal enrichment annual fractional industry growth rate fuel life years L = load factor Suffixes B = breeder L = LWR None = HTGR 1. B A C K G R O U N D The prime objective of the H T G R system was, and still is, to provide nuclear heat at a high temperature. This is an objective of great merit, not just for the obvious efficiency advantage to any associated heat engine but because it also offers prospect of applications not otherwise open to use of nuclear heat at all, including association with closed cycle gas turbines and supplying high temperature process heat, for which fuel scarcity and environmental cleanliness considerations are creating increasing demand. While such attractions were in mind from the start, the prudence of dealing with one set of development problems at a time and the desirability of a more immediately needed task were also appreciated. Firm commercial establishment of the H T G R for steam-powered electric generation was in fact the obvious first goal. The most immediately realizable reward for high temperature in this field was the ability to supply the steam conditions already adopted for contemporary fossil plants, and thus to partake of the cost benefits accruing from their extensively evolved and standardized machinery. The required 1000°F steam temperature, however, called for helium temperatures of no more than 1300°-1400°F, the further reduction in boiler surface offered by higher temperatures being more than offset by necessary specialty material cost. The essential point here is that this step far from fully exploited the inherent capabilities of the all-graphite type of core which principally characterizes the HTGR. It did, however, incur the main cost penalty that this form of core construction inevitably involved, which was that it required a fabricated moderator which had to be disposed of with every loading. This consideration accentuated the need for extracting the highest possible energy per unit mass of fuel-graphite elements. The moderator-diluted fuel and graphite structure of H T G R fuel elements already conferred to them ability to survive long burnup (~105 MWD/tonne), but to explo!t this fully it would be necessary to match this with a correspondingly long period of sustained reactivity. This is one reason why the thorium cycle was introduced from the start, because the gain in conversion ratio associated with the high neutron yield available from the 2zzU generated by this cycle greatly aids reactivity maintenance. There was also, of course, a more general motivation to tap the additional fuel resources represented by thorium, proper exploitation of which calls moreover for the high conversion ratios which were available. An important point here is that, in the absence of coexisting high gain fast breeders (the only practically realizable external source of °-~U-), the HTGP, of necessity has to use zasU for its initial fissile inventory. This handicaps initial nuclear performance and introduces dependence on a near fully enriched uranium supply, raising both the cost of separation and the amount of natural fissile supply necessarily discarded in separation plant tails. This places a premium on either attaining a good conversion ratio even in the initial H T G R core or seeking the independent source of z~U that could come from fast reactor breeding blankets. Even starting with z35U, the H T G R could be designed for a conversion ratio approaching 0.85. Such a core would accumulate zmU rapidly and fully enough to very effectively exploit the thorium2z3U cycle after a few refuelings. By contrast, a conversion rato of only 0.6 or so would still entail a perpetual dependence on a highly separated ~asU supply, for at this level the buildup of the zazU content, even with infinitely repeated recycling, would amount to only a fraction of the total fission content.