Yucca Mountain Waste

In 2002, the US Congress and President Bush approved Yucca Mountain, Nevada as a suitable site for the geological disposal of nuclear waste and authorized further developmental work for the design of a repository. This uninhabited arid location about 160 km northwest of Las Vegas is owned by the US government and has been studied by the Department of Energy (DOE) for over 20 years at a cost of $4 billion. The accepted criterion for suitability of this site is generally that any inhabitants in the drainage direction — Amargosa Desert (parallel to Death Valley) — would experience only a minor increase in radiation exposure (about 15 millirem/yr) after 10?000 years. The waste would be placed in separate 5.5 m horizontal ‘drifts’ (or tunnels) 200–500 m below the surface and ∼300 m above the water table. Some 80 km of drifts could hold 70?000 metric tons of waste for a proposed cost of $50 billion, starting as soon as 2010.

Today, Yucca Mountain receives only about 18 cm of rain and snow annually, most of which evaporates. So who would imagine that corrosion might be a lifetime-limiting problem in this arid place? The various types of nuclear waste would be enclosed by a welded cylindrical container with a 5 cm Type 316 Stainless Steel Nuclear Grade (lowC-highN-Fe-18Cr-12Ni-2.5Mo) inner barrier and 2 cm thick ‘super-resistant’ Alloy 22 (Ni-22Cr-13Mo-4Fe-3W) outer barrier. Overhead, a ‘drip-shield’ of a Ti alloy (Ti-0.15Pd) is expected to divert any water seepage through the overlying fractured volcanic tuff from contact with the canisters. Corrosion expert Roger Newman at UMIST in the UK has said he would accept such an arrangement in his own backyard. But one could pose the question as to whether all the high-alloy fabricators in the world could produce enough Alloy 22 plate to meet the demand for this application.

Even this ‘spare-no-expense’ protection system represents a challenge to environment simulation and accelerated testing (a couple of testing decades extrapolated to 10?000 years). For hundreds to thousands of years after the repository is closed, heat from the encapsulated waste (mostly from spent nuclear fuel) would keep the temperatures of the rock and the containers above the boiling point of water. Thus, the protective shields could grow very thin passive oxide films in the absence of any deposited aqueous solution. I have estimated that even (lousy) pure Fe and low alloy steels would suffer about 6.5 μm attack in parabolic scale growth at 250°C in 10?000 years. So the warm initial temperature is advantageous for protection. If, however, after a 1000 or more years the local temperature and rainfall changes to allow water seepage and moisture to contact the cooling Alloy 22, questions about crevice corrosion and stress corrosion cracking (SCC) arise. The potential aging of the alloy to provide a precipitated second phase is also a worry, and incorrect post-welding treatments could contribute to SCC susceptibility and unwanted failures. For this application, the Alloy 22 barrier would only contact a thin aqueous film on top of a preexisting thin protective oxide film formed by air oxidation. In my own opinion, failing some unforeseen mechanical upheaval, this air-formed film is stable and protective against aqueous attack for nearly any period. So the DOE and others continue to test the corrosion resistance of Alloy 22 with the prospect to never detect localized corrosion, even from exaggerated solution chemistries threatened by evaporative concentration.

The safe geological disposal of nuclear waste is certainly not a problem specific to the USA; Japan and France obtain much larger fractions of their electrical power from nuclear plants, with the corresponding generation of unwanted waste. Certainly, environmentalists and Nevada politicians can argue against the Yucca Mountain Project, but the alternative situation is the current dispersion of assorted waste in multiple locations at much less secure facilities scattered all over the country (and the world). It will be interesting how this all turns out, especially since the Yucca site can already be filled with existing waste, and a Yucca II must be identified and researched for the near future. Detailed accounts of the Yucca Mountain Project with opposing views can be found at www.ymp.gov, in National Geographic (July 2002), and in other references [Science (2002) 296, 659 and 2333; Corrosion (2002) 58, 811].