Nuclear safety and security

The safe storage and disposal of radioactive waste (RAW) is of paramount importance, with the goal being to convert nuclear wastes into stable solid forms which can be safely stored before permanent disposal in repositories, frequently not yet designed or built, which would serve the dual function of preventing the waste from entering into the biosphere and supporting non-proliferation. Although the primary disposal objective is to protect mankind from the deleterious effects of radiation, most of the elements are toxic or carcinogenic. Important considerations in the storage and ultimate disposal of wastes include not having concentrations of fissile materials that could pose criticality issues, in addition to keeping them resistant to proliferation.

Many proposals for disposing of RAW have been made but most have been excluded for technical, legislative or environmental reasons. Those that remain viable in the foreseeable future are variations on geological disposal with partitioning and transmutation as potential future technolo­gies (e. g., Bowman and Venneri, 1993; King, 2002; Gonzalez-Romero, 2011).

The option being investigated by most countries is a multi-layer engi­neered approach in which the processed waste, in a passively safe form, is enclosed within a sealed container and placed underground. Much has been written on the selection of suitable sites (e. g., Ewing, 2004; Murphy, 2004) but the enormous timescales for which performance assessments are required undermine their reliability and Ewing (2004) concludes perform­ance assessments should not be used as the sole justification for the reposi­tory’s safety.

Continuing safety arguments and public opposition have meant that to date there is only one functioning underground repository for ILW/TRU, the Waste Isolation Pilot Plant (WIPP) near Carlsbad, NM, USA, which accepts defence-related TRU waste, although there are a number of func­tioning and historical repositories (Rempe, 2007). Several countries have identified sites and are at various stages of developing repositories for the disposal of civil waste, e. g. Finland (expected date 2020), Sweden (2023) and France (2025). The US were developing a repository at Yucca Mountain but, despite spending 23 years and an estimated $13.5 billion, the Adminis­tration cut the funding in February 2010 and withdrew the DOE application for a construction permit from the Nuclear Regulatory Commission. As a result of the current impasse in the nuclear waste programme, the govern­ment is currently looking at alternatives (Levy, 2010; Schaffer, 2011) and set up a commission (Blue Ribbon Commission on America’s Nuclear Future) in January 2010 to conduct a comprehensive review of policies for managing the back end of the nuclear fuel cycle and recommend a new plan. This report (BRC) was submitted in January 2012 and recommends the building of a temporary storage facility concurrent with work on iden­tifying a site for a permanent geological repository.

Direct injection of liquid waste into deep rock formations has been pro­posed (Apps and Tsang, 1996; Tsang and Apps, 2005) and the technology was used by the former Soviet Union to dispose of nearly 50 million m3 at three repositories (Rempe, 2007) in Central European Russia. Disposal of LLW, ILW and HLW at the Krasnoyarsk-26 facility into two aquifers at levels between 180 m and 500 m deep commenced around 1967. An assess­ment of waste migration by Compton et al. (2000) based on official Russian data gathered over many years, concluded that the deep well was function­ing as designed and that there is very little likelihood of the waste reaching the surface in sufficient quantity to exceed the drinking water standards. Whilst this is encouraging for the disposal of LLW and some ILW, longer lived isotopes present in defence wastes may make this option problematic.

The possibility of using boreholes as an alternative to underground repositories for the disposal of solid waste has been proposed on several grounds, increased security being one of them. Originally, the proposals were for boreholes to depths comparable to repositories (e. g., 500-700 m), but proposals for ever deeper borehole disposal have been made. One sug­gestion has been the very deep borehole (Gibb, 1999) in which containers of HLW are placed in a borehole typically 4-6 km deep. The combination of self-heating and geothermal heat would cause either the rock to melt and then encapsulate the waste package (Gibb, 1999) or dissolve the con­tainer and contents (Nirex, 2002). As the HLW decays, the rock cools and solidifies with the waste immobilized. The potential for isotopes to leach out of the rock at some very distant timescale has been addressed by the proposal to perform in-situ sintering of a mixture of waste and other con­stituents which would generate a geochemically stable waste form in equi­librium with the host rock (Ojovan et al., 2004). An alternative to the deep borehole is the self-burial technique (Logan, 1999; Ojovan and Gibb, 2005) in which the radiogenic heat generated by the waste melts the rock and the waste package descends.

Although the safety of disposed waste is of great importance in the pub­lic’s perception of waste disposal, security of the waste, especially weapons grade fissile material, is not significant compared to other concerns such as transport of waste (Solomon et al., 2010). This may be because the quantity of civil fissile material far exceeds the quantity of weapons material. A comparison of the plutonium inventories from the two sources in 2000 suggested nearly 1300 Mt present in the civilian inventory, either in fuel elements or as reprocessed material compared to a military inventory of 255 Mt (Ewing, 2010). This large quantity of civil plutonium forms the basis of the commonly used security standard that was proposed in 1994 by the US National Academy of Science’s Committee of International Security and Arms Control (NAS CISAC). The standard requires that ‘the surplus weapons useable plutonium should be made as inaccessible and unattractive for weapons use as the much larger and growing quantity of plutonium that exists in the spent nuclear fuel from commercial power reactors (Bunn, 1994).

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