ANTHONY W. BANFORD[34] AND RICHARD B. JARVIS

ABSTRACT

This paper discusses the various phases of decommissioning and what they seek to achieve, and discusses some drivers for decommissioning both immediately after operations cease and after a period of time. The scale of the decommissioning challenge in the UK is then outlined and some criteria for the selection of the best decommissioning option for a facility are introduced. Finally, the potential environmental impacts of each of the different decommissioning stages are discussed.

1 Introduction

Nuclear decommissioning can be defined as ‘‘The process whereby a nuclear facility, at the end of its economic life, is taken permanently out of service and its site made available for other purposes’’.1 The process of decommissioning incurs financial costs known as liabilities, both from the decommissioning process itself and the associated waste management and environmental reme­diation. The United Kingdom has played a major role in the development of nuclear power and has a large number of facilities which will ultimately need to be decommissioned.

At low concentrations, sorption to mineral surfaces will be the dominant mechanism retarding radiouclide migration in subsurface environments. However, radionuclides can also be removed from the solution phase through co-precipitation as new mineral phases are formed and, at higher radionuclide/ ligand concentrations, by precipitation. At lower concentrations, co­precipitation will be the more important process.45 For the actinides, hydrolysis can occur for all oxidation states and can lead to precipitation from solution.129 In particular, An(iV) has a strong tendency to hydrolyse.110

In natural waters, key mineral phases for (co-)precipitation will include carbonate and iron (oxy)hydroxide mineral phases. Parkman et al. (1998)72 investigated the interaction of Sr21 with calcite and found that at higher ( > 0.3 mM) concentrations Sr21 was precipitated as strontianite on the calcite surface, perhaps as a result of the existing mineral phase providing nucleation sites and higher localised carbonate concentrations at the calcite surface-water interface. NpO21 has also been reported to co-precipitate with calcite.130 Co-precipitation appeared to be independent of pH, even though pH did affect Np(v) solution speciation. Extended X-ray absorption fine structure (EXAFS) analysis suggested NpO21 was structurally incorporated into the calcite lattice, with Np and the axial O atoms substituting for one Ca21 and two CO3 groups, respectively. Co-precipitation of UO221 with calcite is reported to be limited,130 but natural aged calcite has been found with uranyl incorporated into the lattice131,132 and one study investigating uranium contamination in the Aral Sea suggests co-precipitation with calcite and gypsum is the dominant mechanism for U removal from the solution phase.133 U(vi) can also co-precipitate with iron oxides, with X-ray absorption spectroscopy (XAS) suggesting that it is incorporated into the oxide lattice as uranate (containing U-O single bonds and no axial U-O double bonds) rather than uranyl.134

Since its inception people have asked the question: is nuclear power safe? During the forty years that commercial power plants have operated worldwide, there have been about eight dozen major accidents, three of which -Windscale (1957), Chernobyl (1986) and Fukushima (2011) — were especially serious. At Chernobyl, 31 worker deaths were recorded immediately and it is estimated that there were up to 5900 more in the months and years that followed as a result of radioactivity from the fire in the reactor across Europe. Reactor designs since the accident at Three Mile Island (1979) have been upgraded with new, multilayered and redundant safety systems, developed specifically to avoid such errors. Chernobyl, for example, was built without any containment structure, according to a design that would have been instantly condemned anywhere in the West. Both incidents, however, inspired many changes, leading the IAEA to establish a global network for peer review of sites, designs, operating procedures and creating a collection of ‘‘best industry practices’’. Plant design now stresses passive safety; the reactor shuts down automatically in case of any irregularity.

Nonetheless such guidance proved ineffective in preventing the disaster at Fukushima, where worse case scenarios were discounted in the disaster man­agement plans of the site and as a result sea defences were woefully inadequate. As a result, this led to back up diesel power systems failing due to the sea defences being overwhelmed with the wave from the tsunami and with the final battery power running out, a number of the reactors were left without coolant, leading to a partial meltdown of reactors on the site. As a result Japan has officially admitted the Fukushima nuclear disaster is as bad as Chernobyl and has upgraded the incident to the worst case level 7.

Whilst we have seen the development of a much stronger safety culture within the industry over the last 20 years, nuclear power plants have the dis­advantage of being so complex that almost every reactor has experienced some sort of incident or failure over its history, and even if the risk of a true melt­down is low, the impact of such an accident would be very large.125 Nonetheless the safety record of existing nuclear reactors has improved over time as their margins of error have improved and safety regulations have been upgraded. The industry has incorporated research findings on human factors and safety

xlviA problem discussed by the Committee on Climate Change Science and Technology Integration 2009.

culture through groups and organizations such as the IAEA and the World Nuclear Association of Nuclear Operators created after the Chernobyl accident in 1986.126

A scenario envisaged by the authors of the MIT study was an optimistic three-fold increase in the world nuclear fleet capacity by 2050. They concluded, after undertaking a probabalistic risk assessment (PRA), that one would expect four core damage accidents during this time (they based their analysis on current estimates of core damage to occur once in every 10 000 reactor years). They concluded that this was an unacceptably high number — it should be 1 or less, which is the current expected safety level.127 They concluded that a core damage frequency of 1 in 100000 reactor years is a desirable goal, which is a ten-fold reduction from current levels. The designers of the new light water reactors currently being built argue that they already achieve these goals through advanced safety measures and greater use of passive safety mechan — isms. xlv11 It is beholden on regulators to assess these claims to enhanced safety of the latest generation of LWRs during the (pre)licensing phase of reactors.

We do not believe there is a nuclear plant design that is totally risk free. This is due to technical and workforce issues. Safe operation requires effective regulation, management who is committed to safety and a skilled work force. The restruc­turing of electricity sectors around the world has motivated some operators to place profits before safety. Undue solicitude for profits of the licensee has played a large role in explaining the mishaps that have occurred at nuclear power plants. Nuclear power is least safe in environments where complacency and pressure to maximize profits are the greatest. It is of continuing concern as to ‘‘whether nuclear reactor safety goals are compatible with the transition to competitive electricity markets’’.129 Owners and managers of nuclear plants respond that it is econom­ically beneficial to ensure high levels of safety, given the enormous financial costs of accidents. However, well funded regulatory agencies are vital to ensuring plant operators do not neglect safety inspections for continuous plant operation.

However at Fukushima it would appear that a number of regulatory failures and cost cutting exercises by the owner Tokyo Electric Power Company (TEPCO) contributed to the severity of the disaster. As far back as July 2000, ‘‘four ominous unexpected shutdowns occurred, some releasing unacceptable radiation levels, in the plants run by Tokyo Electric Power Company (TEPCO), Japan’s largest uti­lity. In 2001, a whistle-blower triggered disclosures of falsified tests at some of the company’s seventeen plants, and the government forced TEPCO to close some plants’’.172 Moreover, ‘‘in 2002, the company predicted that all of its seventeen plants might have to be shut down for inspection and repairs, because of falsified inspections and concealment of faults found in inspections that the government ordered; some of the faults were potentially catastrophic” (ibid). As a result a top

xlvn‘‘Additional gains may come with the introduction of High-Temperature Gas Reactors (HTGRs). In principle the HTGR may be superior to the LWR in its ability to retain fission products in a loss-of-coolant accident, because of fuel form and because core temperatures can be kept sufficiently low due to low power density design and high heat capacity of the core, if RD&D validates this feature’’.128 The HTGR also has an advantage compared to light water reactors in terms of proliferation resistance.

company official was charged with giving specific orders to hide large cracks in the ‘‘shrouds,’’ or steel casings around the reactor core, in two of the thirteen reactors at which false inspection reports had been filed. According to documents from Tokyo Electric Power (Tepco), the company “repeatedly missed safety checks over a 10-year period up to two weeks before the 11 March disaster, and allowed ura­nium fuel rods to pile up inside the 40-year-old facility’’.173 This exposes the pro­blem of cost cutting initiated by the chief executive, Masataka Shimizu, in that the company opted to save money by storing the spent fuel on site rather than invest in safer storage options.

The explosion at Unit 4 of the Chernobyl nuclear power station was the worst nuclear accident in history. There are still some uncertainties regarding the exact causes and events leading to the accident, though the key factors are now known. The accident occurred during an experiment to test the behaviour of an electrical system which powered the station in the event of a failure of the main electricity supply. In order to conduct the experiment, the reactor power was reduced which (possibly due to a problem in the operation of the automatic control rods) led to the reactor being in an unstable state39 and operating outside its design parameters.

At 01:23 on the morning of the 26th of April 1986, the experiment began, despite the fact that:

(i) The reactor power output was well below that required by the experi­mental procedure;

(ii) Certain reactor safety systems had been deliberately disabled in order to carry out the experiment; and

(iii) The number of control rods in the reactor was only half the minimum required for its safe operation.

Thirty seconds after the experiment began, the reactor power began to increase rapidly and ten seconds later the operators attempted a full emergency shut down by re-inserting the control rods. The reactor power was now increasing exponentially, leading to a failure in the pressurised cooling water system. Eight seconds later, the reactor exploded (an explosion of steam, not a nuclear explosion) scattering burning core debris over the surrounding area.

Over 100 firemen were called to the scene and they worked with plant personnel to put out many small fires in the reactor building and on the roofs of Unit 4 and the adjacent Unit 3 building. This work exposed the emergency workers to extremely high doses of radiation. During the days after the explosion, heli­copters were used to dump thousands of tonnes of various materials onto the exposed reactor core. These materials included boron, lead, sand and clay to smother the fire, absorb radiation and reduce nuclear reactions in the molten core material. In total, 1800 helicopter flights were made39 at great risk to the pilots. Despite the heroic efforts of firemen, helicopter pilots and many other emergency workers to put out the fire, the reactor continued to burn for ten days.

Some existing radioactive materials not currently designated as wastes due to their potential economic value may be declared as waste in the future and thus may need to be managed through geological disposal.[36] Such items include spent fuel from UK nuclear reactors that is not already contracted for reprocessing; uranium and plutonium stocks from spent fuel reprocessing; uranium from fuel manufacturing; and non standard or ‘‘exotic’’ fuels from experimental and research reactors. These other potential wastes represent a significant increase in the activity for disposal. For example, as a result of past and currently planned reprocessing operations at Sellafield, the UK will have amassed approximately 100 tonnes of separated plutonium. Regardless of their eventual management route, these materials, in combination with the HLW discussed in section 2.1, represent exceptionally challenging wasteforms. For example, plutonium stocks potentially present major challenges for disposal due to both nuclear security and criticality issues. In addition, wastes from any new nuclear build and Ministry of Defence sources are also likely to be managed via geodisposal.1

Once absorbed, radionuclides enter the circulatory system and are distributed into various tissues of the body. In some cases, radionuclides are bio­transformed within tissues and may be present within the animal in more than one form. For instance, 3H occurs as tissue water or organically bound tritium incorporated into the protein and fat of tissues.

Different radionuclides are accumulated in different tissues. For some radionuclides, the site of deposition is determined by the biological role of the corresponding stable element or analogue. The major iodine storage organ in the body is the thyroid and the element is also actively taken up by the mammary gland and transferred into milk. Radiostrontium behaves as a calcium analogue and is therefore accumulated in bone and shell and is also transferred into milk. Radiocaesium is an analogue of potassium and is, therefore, found in all soft tissues. The actinides and rare earth elements are all accumulated in bone. Liver (or hepatopancreas in arthropods and gastropods) and, to a lesser extent, kidneys are common storage tissues for many pollutants including some radionuclides (e. g. actinide elements and heavy metals).