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Long-Term Heat Load Calculation Methodology for Spent Nuclear Fuel

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Long-Term Heat Load Calculation Methodology for Spent Nuclear Fuel KAERI/TR-3031/2005 Long-Term Heat Load Calculation Methodology for Spent Nuclear Fuel 2005. 7 KOREA ATOMIC ENERGY RESEARCH INSTITUTE KAERI/TR-3031/2005 제 출 문 한국원자력연구소장 귀하 본 보고서를 2005 년도 “건식 재가공 핵연료 노심특성 평가 기술 개발”과제의 기술 보고서로 제출합니다. 제목: Long-Term Heat Load Calculation Methodology for Spent Nuclear Fuel 2005. 7 과제명: 건식 재가공 핵연료 노심특성 평가 기술 개발 주저자: 정 창 준 공저자: 1 KAERI/TR-3031/2005 ABSTRACT This report describes a general methodology for calculations of the short-term and long-term integrated decay heat factors that can be implemented in fuel cycle system dynamic analysis. For the heat load calculation, the decay heat and isotope inventory data are generated for pressurized water reactor (PWR) and Canada deuterium uranium (CANDU) reactor spent fuels. Then the decay heat integral calculations are performed for estimating the long-term heat load for the different isotopes of interest over flexible periods of time. Application of the methodology to the Korean fuel cycle scenarios which include both PWR and CANDU spent fuels is also presented. From the calculation results, it is known that the actinides dominate the long-term heat load, especially Pu and Am isotopes. For the Korean fuel cycle scenario, the PWR spent fuel long-term heat until 1500 yrs with a 5-yr cooling time is estimated to be 9600 MW-yr. The long-term heat of the CANDU reactor spent fuel is much smaller compared with that of the PWR. 2 KAERI/TR-3031/2005 CONTENTS ABSTRACT...............................................................................................................................2 1. Introduction............................................................................................................................6 2. Heat Load Calculation Methodology.....................................................................................8 3. Application to Korean Nuclear Fuel Cycle..........................................................................10 3.1 Korean Fuel Cycle Scenario ........................................................................................10 3.2 Heat Load Calculation Calculations ............................................................................10 3.2.1 PWR Spent Fuel..................................................................................................11 3.2.2 CANDU Spent fuel.............................................................................................11 4. Summary..............................................................................................................................13 References................................................................................................................................14 APPENDIX...........................................................................................................................................27 3 KAERI/TR-3031/2005 LIST OF TABLES Table I Long Term Decay Heat Integrated Parameter for Key Isotopes...........................................15 Table II Half life and decay constant of each isotope ........................................................................16 Table III Example Long-term Decay Heat Factors (GW-yr/g)............................................................17 Table IV Accumulation of spent fuel with time in the Korean fuel cycle scenario.............................18 Table V PWR spent fuel isotope inventories with time after discharge (g/tIHM) .............................19 Table VI CANDU spent fuel isotope inventories with time after discharge (g/tIHM)........................20 4 KAERI/TR-3031/2005 LIST OF FIGURES Fig. 1 Long-term heat load of 1-ton of PWR spent fuel.....................................................................21 Fig. 2 Comparison of heat load contribution of main isotopes (PWR, 5-yr cooling) ........................21 Fig. 3 Comparison of heat load contribution of main isotopes (PWR, 10-yr cooling) ......................22 Fig. 4 Comparison of heat load contribution of main isotopes (PWR, 20-yr cooling) ......................22 Fig. 5 Comparison of total heat load for cooling times (PWR)..........................................................23 Fig. 6 Total heat load at the year of 3500 (10-yr cooling, PWR).......................................................23 Fig. 7 Long-term heat load of 1-ton of CANDU spent fuel...............................................................24 Fig. 8 Comparison of heat load contribution of main isotopes (CANDU, 5-yr cooling)...................24 Fig. 9 Comparison of heat load contribution of main isotopes (CANDU, 10-yr cooling).................25 Fig. 10 Comparison of heat load contribution of main isotopes (CANDU, 20-yr cooling).................25 Fig. 11 Comparison of total heat load for cooling times (CANDU)....................................................26 Fig. 12 Total heat load at the year of 3500 for cooling times (10-yr cooling, CANDU) .....................26 5 KAERI/TR-3031/2005 1. Introduction The purpose of this report is to provide a general methodology for calculations of the short-term and long-term integrated decay heat factors that can be implemented in fuel cycle system dynamics codes such as DYMOND [1] and DANESS [2]. The short-term decay heat at shutdown of repository active cooling and integrated long-term decay heat determine the amount of waste that can be emplaced into the repository [3]. In the case of Yucca Mountain (YM) repository, there are limits on the temperatures at the surface of the tunnels (drifts) where the waste will be placed or the temperature midway between those tunnels. Given a certain type of waste, the amount of waste that can be placed per unit of the tunnel length is determined by either one of those decay heat factors. Thus, how much waste we can place in those tunnels, i.e., the repository capacity depends on those factors. Current version of the DYMOND code quantifies the waste long-term integrated decay heat that can affect YM capacity as follows. The cumulative amount of heat generated by spent fuel and/or high-level waste (HLW) between about 100 years and 1500 years after the spent fuel had been discharged from the reactor are quantified by integrating over the individual isotope decay heat over that period. Table I shows the decay heat integral for the different isotopes that are responsible for the majority of the long-term decay heat per unit mass. The DYMOND code tracks the amounts of those isotopes (that are destined to repository) at each point in time and multiples the mass of each isotope by the corresponding factor, to determine the integrated long term decay heat associate with that isotope. The total amount of integrated decay heat is used to compare the repository heat-load for the different scenarios. However, the above factors are based on integrated decay heat between about 100 years and 1500 years after the spent fuel had been discharged from the reactor. Although this period of time corresponds to LWR SF that will be emplaced in YM, it might be different for other type of SF which will be implaced in different repository. For example, a type of fuel or a repository where it is important to consider integrated decay heat between 100 years and 1000 years after discharge leads to an Am 241 factor that is about 15% smaller than the factor in Table 1. This difference in the integrated decay heat factors can be important in certain cases. For example, it can be important for comparing different scenarios that uses different types of fuel, different types of repositories, or 6 KAERI/TR-3031/2005 different shut down times of the repository active cooling. This work describes a simple methodology for estimating the integrated decay heat integrals for the different isotopes of interest over flexible periods of time, which can be easily implemented in system dynamics codes. Application of the methodology to the Korean fuel cycle scenarios, that include both PWR and CANDU spent fuel is also presented. 7 KAERI/TR-3031/2005 2. Heat Load Calculation Methodology The heat load factors are calculated by the following equation; HL = M t1 ⋅ HLF where HL = Heat load [w-yr], Mt1 = Isotope inventory at specific time t1 [g], HLF = integrated heat load factor [w-yr/g]. The HLF can be calculated by HLF = DH i ⋅ DHFi + DH id ⋅ DHFid where DHi = decay heat of isotope i [W/g], DHFi = decay heat factor of isotope i [yr], DHid = decay heat of daughter of isotope i [W/g], DHFid = decay heat factor of daughter of isotope i [yr] The DHFi and DHFid can be calculated by t t 1 − 1 − 2 DHF = e λi − e λi i λ i λd DHFid = [DHFi − DHFd ] λd − λi where λ i = decay constant of isotope i [1/yr], λ d = decay constant of daughter of isotope i [1/yr], t1 = starting time of integration [yr], t2 = termination time of integration [yr]. The basic data, such as half-life and decay constant are shown in Table 2. The above equation is 8 KAERI/TR-3031/2005 easily implemented in system codes, where the data in Tables 2 are stored into the ITHINK arrays, and the times t1 and t2 are inputs, which are provided by the user to determine the period over which the integrated decay heat is calculated. Table 3 shows example results for heat load factors estimates for different periods of times and different isotopes of interest. As mentioned before, the key isotopes in this table are Pu isotopes and Am241, because of their large integrated decay heat factors, and its relatively large compositional fractions compared to other isotopes. Some of the fission products isotopes such as Sr 90,
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