Comparative study on nuclear characteristics of APR1400 between 100% MOX core and UO₂ core

Abbreviations

• APR1400, advance power reactor 1400;
• BA, burnable absorber;
• BOC, beginning of cycle;
• CBC, critical boron concentration;
• CEA, control element assembly;
• EOC, end of cycle;
• EUR, European utility requirement;
• FTC, fuel temperature coefficient;
• KUR, Korean utility requirement;
• LWR, light water reactors;
• MFR, moderator to fuel ratio;
• MOX, mixed oxide;
• MTC, moderator temperature coefficient;
• OMP, optimum moderation point;
• PWR, pressurized water reactor;
• RIA, rod insertion allowance;
• SDM, shutdown margin;
• HFP, hot full power

Keywords

• Mixed oxide (MOX);
• APR1400;
• Shutdown margin

1. Introduction

APR1400 nuclear reactor is designed to generate 3987 MW thermal power with an average volumetric power density of 100.9 W/cm³. It loads 241 fuel assemblies named as PLUS7^TM. This fuel assembly consists of 236 fuel rods containing UO₂ pellets and burnable absorber rods containing Gd₂O₃-UO₂ in a 16 × 16 fuel pin array. The remaining locations are 4 control element assembly (CEA) guide tubes and 1 in-core instrumentation tube for monitoring the neutron flux shape in the reactor core. APR1400 aims a cycle length of 18 months or more.

There are many advantages of using MOX fuel. The primary advantage of plutonium recycle in MOX fuel is reduction in the quantity of partially enriched uranium and reduction of radioactive waste produced from nuclear spent fuel (Graves, 1979). A considerable number of pressurized water reactors are licensed, or a license has been applied to use MOX fuel at levels of up to 30% or more of the reactor core (International Atomic Energy Agency, 2003). Korean Utility Requirements (KUR) states the capability of nuclear design with 30% MOX core. Currently, EUR (European Utility Requirements) requires the capability of nuclear design for 50% MOX core (Utility Requirement, 2012) and APR1400 successfully demonstrated the capability of designing 50% MOX core to get the EUR certification.

MOX (UO₂/PuO₂ mixed-oxide) fuel has been used in Light Water Reactors (LWRs) as a partial substitute for low-enriched UO₂ fuel (Agency, 2006). Plutonium has several isotopes where ²³⁹Pu and ²⁴¹Pu are fissile plutonium, like ²³⁵U. In MOX fuel, Plutonium is mixed with depleted uranium (0.25% ²³⁵U) and has very high resonance absorption in thermal energy region, resulting in neutron spectrum hardening in MOX fuel. It was reported that the spent fuel from Pressurized Water Reactor (PWR) consists of 1.0 w/o plutonium of which two thirds are fissile, i.e. about 50% ²³⁹Pu and 15% ²⁴¹Pu (Fehér et al., 2012). Plutonium content is adjusted to take account of its isotopic composition about 63–70% of fissile plutonium (Provost and Debes, 2006) and fissile plutonium content of 74.05% is used in this study.

The purpose of this study is to analyze nuclear characteristics of MOX fuel assembly, thereby exploring possibility of nuclear design with 100% MOX fuel loading in APR1400 reactor. For the analysis of nuclear characteristics of MOX fuel assembly, CASMO-4 (Inc, 2009) is used to characterize various nuclear design parameters such as k_∞ and MTC of MOX fuel assembly with respect to Moderator to Fuel Ratio (MFR), enrichments, fuel burnups and moderator temperatures.

It was reported that partially MOX loaded core shows similar nuclear characteristics to fully UO₂ loaded core (Graves, 1979). However, MOX fuel show much higher resonance absorption and larger negative MTC than UO₂ fuel so that the most limiting nuclear design requirement in MOX core design emerges from ShutDown Margin (SDM) as MOX fuel loading in a core increases. For SDM calculation, loading patterns for an initial cycle and an equilibrium cycle are developed to figure out reactivity balance for full MOX core and the results are compared with full UO₂ core. The initial cycle and the equilibrium cycle are designed to satisfy some nuclear design requirements: 18 months cycle length, pin peaking factor less than 1.55, negative Moderator Temperature Coefficient (MTC) and Fuel Temperature Coefficient (FTC), and SDM greater than 5500 pcm.

CASMO-4 and SIMULATE-3 code (Inc, 2009) are used to evaluate the nuclear design parameters such as Critical Boron Concentration (CBC), MTC, FTC, pin peaking factor and SDM for both 100 %MOX core and UO₂ core.

2. Impact of MFR on nuclear design parameters of MOX fuel assembly

MFR is a ratio of moderator volume to fuel volume (V_m/V_f). It affects ratio of hydrogen atoms in the moderator to fuel atoms. The conventional definition of MOX fuel weight fraction is the weight fraction of plutonium in the mixture of plutonium and UO₂. In the fuel assembly model shown in Fig. 1 and Table 1, the content of fissile plutonium of MOX fuel is assigned equal to the enrichment of UO₂ fuel in order to clearly compare the effect of MOX fuel on k_∞ with UO₂ fuel. MOX fuel consists of low-enriched UO₂ fuel (0.23%) and plutonium in which the weight percentages of each isotope in the plutonium are 0.86% for ²³⁸Pu, 66.47% for ²³⁹Pu, 20.77% for ²⁴⁰Pu, 7.56% for ²⁴¹Pu, 2.95% for ²⁴²Pu and 1.39% for ²⁴¹Am, respectively.