N. CERULLO, University of Pisa, Italy, and G. LOMONACO,

University of Genova, Italy

Abstract: This chapter looks at Generation IV nuclear reactors, such as the very high-temperature reactor (VHTR), the supercritical water reactor (SCWR), the molten salt reactor (MSR), the sodium-cooled fast reactor (SFR), the lead-cooled fast reactor (LFR) and the gas-cooled fast reactor (GFR). Reactor designs and fuel cycles are also described.

Key words: Generation IV Initiative, very high-temperature reactor (VHTR), supercritical water reactor (SCWR), molten salt reactor (MSR), sodium-cooled fast reactor (SFR), lead-cooled fast reactor (LFR), gas-cooled fast reactor (GFR).

13.1 Introduction

From the beginning of this century, there has been increased interest in nuclear energy as the only high-capacity source of CO2-free energy available. When compared to fossil fuels, the waste and emissions generated are minimal (Generation IV International Forum, 2009). At the same time, there has been growing pressure to reduce the safety risks posed by plutonium (Pu) stockpiles and nuclear waste material. Given the Three Mile Island, Chernobyl and Fukushima accidents, there is also pressure to improve the safe operation of nuclear power plants (NPPs), although their safety has reached, at present, a relatively high level of reliability.

At present, world wide nuclear energy production is mainly from light water reactors (LWRs) that are fuelled with uranium enriched up to 5%. The discharge burn-up of a nuclear fuel element is limited by both its fissile content and its endurance, i. e. its ability to withstand exposure to a neutron fluence and high temperature. In the case of LWRs, discharge burn-up lies in a range between 30 and 60 GWd/tHM where tHM stands for tons of heavy metal, this being the initial uranium (U) plus Pu content of the fuel (Bende, 1999). This means that a LWR with an output of 1 GWe and an efficiency of 33% must burn, and subsequently discharge, about 10 to 20 tHM per full power year (FPY). An important fraction of the energy output of the fuel comes from transuranic (TRU) elements that are produced in-reactor by neutron capture.

It will be essential to use Pu as an energy source given the limited availability of other energy sources (oil, natural gas and uranium in the case of nuclear power) in the medium-long term (Cerullo et at., 2009; Bomboni et at., 2008b, 2007). However, the presence of long-lived, high-level radiotoxic elements in the waste from LWRs is becoming a more significant issue, especially from a safety point of view. The use of Pu-based fuel (e. g. MOX) in LWRs, even if it is useful as an energy source, does not allow for significant reductions of actinides because ‘new’ actinides are generated within this type of fuel. Furthermore, this choice leads, at the end of the cycle, to a growth in the quantities of minor actinides (MAs), i. e. neptunium, americium and curium, which are long-lived and very dangerous nuclides due to their radiotoxicity (Cerullo et al., 2009, 2005). Because it is not possible, in existing reactors, to entirely avoid their production, it is necessary to provide for their destruction.

The main drawbacks of LWR technology are therefore the limited exploitation of U resources coupled with the high-level long-term radiotoxicity of the final waste (it takes more than 100 000 years to balance the level of mine (LOM), i. e. to reach the radioactivity of the original ore). For LWR spent nuclear fuel (SNF) 96% is of U, Pu (see Table 13.1) and MA (see Table 13.2). The long-term radiotoxicity of the nuclear waste is essentially due to TRUs, which can produce energy by fission directly or by means of transmutation into fissile nuclides. A promising solution is to burn all the heavy metals (HMs), including MAs, as fuel for nuclear reactors. However, burning all HMs is not straightforward due to a range of open technological and neutronic issues (Bomboni et al., 2008b). Some are related to the very strong gamma and neutronic emissions of many MA nuclides as well as to the different dynamic behaviour of cores with a non-negligible MA inventory (Bomboni, 2009). Nevertheless, some reactor designs seem to be particularly

suitable for burning Pu and, to some extent, MAs (Bomboni et al., 2008a). Many of these new designs are addressed in the Generation IV Initiative.