원자력발전소에서 인출된 사용후핵연료는 핵연료 저장 수조로 이동하여 최소 5년 이상에서 수십 년간 냉각된 후에 저장(storage), 운반(transport) 단계를 넘어 처분(disposal)으로 넘어갈 수 있다. 사용후핵연료에 포함되어있는 핵종이 붕괴되면서 높은 준위의 방사선과 열을 발생시키기 때문이다. 현재 한국의 사용후핵연료 저장 방식은 두 가지로 나뉜다. 원자로에서 인출한 사용후핵연료를 물속에 보관하는 습식저장과, 습식 저장을 거친 뒤 사용후핵연료를 캐니스터 안에 장입하여 공기 중에서 냉각시키는 건식 저장 방식이다. 다만 건식 저장은 1992년부터 중수형 원전인 월성원자력발전소 임시 저장 시설(맥스터)에서만 사용하고 있으므로 대부분의 원전은 습식 저장을 통해 사용후핵연료를 냉각시키고 있는 실정이다. 그러나 습식저장조는 물리적 확장 및 임계 안전성 측면에 있어 한계가 있으므로 더욱 명확한 해결책이 필요하다.
사용후핵연료를 건식 저장 시설에 보관하기 위해서 가장 먼저 수행되어야 할 일은 핵연료집합체의 연소이력을 파악하는 것이다. 사용후핵연료의 방사능은 연소도(burnup)에 크게 의존하기 때문에 안전한 관리, 저장 및 최종 처분을 위해 연소도를 잘 추정해야 한다. 한수원에서 방출 연료의 연소이력에 관하여 기록한 사용후핵연료 특성 자료가 이미 존재하지만, 몇십 년 이전에 발생하였던 모든 사용후핵연료에 대하여 연소 이력이 정확하게 기록되었는지 확신하기는 어렵다. 기존 데이터를 신뢰하는 것은 시간과 비용적 측면에서 많은 절감을 줄 수 있다. 하지만 사용후핵연료와 관련된 주제는 안전성을 제1의 목표로 두어야 하며, 여기에서 파생된 신뢰를 통해 장기적인 보관 문제를 해결해야 하므로 추가적인 연소도 검증이 필요하다. 기존에 사용후핵연료 연소도를 파악하기 위한 방법론은 감마선 방출 핵종인 137Cs의 단일 피크를 이용하는 방법 등이 있었으나 주요 방출 핵종의 이력을 통해 연소 추정을 수행하는 방법론에 관한 연구는 적다.
이 논문에서는 연소도를 역으로 추적할 수 있는 중요한 도구로 사용후핵연료에서 방출되는 중성자와 감마선원의 생성률을 사용할 것을 제안하였다. ORIGEN-ARP를 사용하여 도출된 중요 방사선원에 대한 연소도 의존성을 분석하고, 명확한 상관관계를 제시하였다. 연소 추정 방정식은 PWR 연료에 근거해 총중성자생성률(TNSI: Total Neutron source intensity) 및 연소도(BU: Burnup), 주요 인자인 초기농축도(IE: Initial enrichment), 냉각시간(CT: Cooling time)이 포함된다. 다양한 집합체별 일치율을 확인해본 결과, 99.8%의 정확도를 가진 상관식이 도출되었다. 주요 감마 선원에 대하여서는 , 그리고 , 3가지 핵종에 대한 연소 추정 방정식도 제안하였다.
이 추정 방정식은 연소도 평가의 알고리즘으로 적용되며 특정한 연소 이력을 가진 사용후핵연료를 검출기로 스캔하여 방출된 중성자와 감마선의 에너지를 측정하면 대표 방출 선원을 추적, 역으로 연소도를 도출해낼 수 있다. 따라서 핵연료집합체에서 발생하는 중성자 및 감마선 펄스를 획득하기 위하여 MCNP 코드를 통해 사용후핵연료 집합체와 계측기를 모사하였고, 최종적으로 소스에서 비롯된 에너지 값을 확인하였다. 중성자 검출기는 비례계수관의 일종인 3He(헬륨-3) 챔버를 대상으로 하였다. 감속재인 HDPE로 하우징하여 중성자 생성률에 가장 지배적인 핵종이 계측되기 쉽게 설정하였다. 감마선 측정기법으로는 을 주요 표적으로 삼아 핵연료 내의 총 방사성핵종의 감마선 합을 측정하는 방법이 있으며, CZT 검출기를 사용할 계획이다.
본 연구의 지속적인 수행으로 사용후핵연료 연소도 계측설비의 기술 국산화가 성공적으로 이루어진다면 관련 장비의 해외 기술 의존성에서 벗어나 독자적인 검증 체계 확보가 가능하며, 비용 및 수출입 규제의 감축을 꾀할 수 있다.

Spent nuclear fuel withdrawn from nuclear power plants is transferred to a nuclear fuel storage tank and cooled for at least 5 years to several decades before being transferred for disposal beyond storage and transportation. This is because the nuclides contained in spent nuclear fuel generate high levels of radiation and heat as they decay. Currently, there are two methods of storing spent nuclear fuel in Korea: a wet storage method in which spent nuclear fuel withdrawn from a nuclear reactor is stored in water, and a dry storage method in which spent fuel is charged into a canister after wet storage and cooled in the air. However, since dry storage has been used only at the temporary storage facility (Maxtor) of Wolsong Nuclear Power Plant, a heavy water nuclear power plant, since 1992, most nuclear power plants cool spent nuclear fuel through wet storage.
However, this wet storage method is currently facing limitations. According to the spent nuclear fuel storage status for the fourth quarter of 2021 announced by Korea Hydro & Nuclear Power (KHNP) in 2022, So far, the amount of spent nuclear fuel stored in the wet storage tank is 507,748 bundles, which accounts for about 98.1% of the total storage capacity. As a result of predicting the saturation prospect through this amount, the Hanbit nuclear power plant, which is difficult to transfer between units is expected to be saturated first in 2031, Kori nuclear power plant (2031), and Hanul nuclear power plant (2032) is expected to require urgent action against saturation. Since this forecast is an indicator expected at a time when the utilization rate of nuclear power plants was rather low [2], there is a high possibility that saturation will occur sooner. These concerns were realized at the ‘Spent Nuclear Fuel Generation and Saturation Forecast Briefing’ held in February 2023 hosted by the Ministry of Trade, Industry and Energy and supervised by the Korea Radioactive Waste Agency. As a result of recalculating the saturation forecast based on the 10th Basic Plan on Electricity Demand and Supply, using the same methodology for estimating the amount of spent nuclear fuel generated and saturation used by the Review Committee in 2019, the saturation of most nuclear power plant storage facilities has come forward. The saturation forecast for Hanbit Nuclear Power Plant (2030) and Hanul Nuclear Power Plant (2031) has been shortened by one year. The Kori nuclear power plant (2032) was delayed by one year due to the installation of high-density storage racks in the water tank, which is the easiest solution applicable to nuclear power plant sites where early saturation is expected. The Shinwolsong Nuclear Power Plant (2042) decreased by 2 years, and the Saeul Nuclear Power Plant (2037) had no change in the saturation prospect. Some power plants have delayed saturation because economic feasibility has been somewhat secured by densely arranging nuclear fuel bundles through the installation of dense storage, but there are limitations in terms of physical expansion and critical safety, so a clearer solution is needed.
Construction of interim storage facilities and permanent disposal facilities outside nuclear power plants is a promising alternative, but difficulties are encountered during the site selection stage. Therefore, until the site is selected, a dry storage facility that cools the spent nuclear fuel by a natural circulation method using an air can temporarily perform its role. The Ministry of Commerce, Industry and Energy proposed (and adopted) a dry storage facility as a storage facility in the ‘2nd Master Plan for High-level Radioactive Waste Management’. Accordingly, KHNP is promoting the construction of the first light-water reactor-type storage facility at the Kori nuclear power plant..
The first thing to be done in order to store spent nuclear fuel in a dry storage facility is to identify the burnup history of the fuel assembly. Because the radioactivity of spent nuclear fuel is highly dependent on burnup, burnup must be well estimated for safe management, storage, and final disposal. Although there are already data on the characteristics of spent nuclear fuel held by KHNP, it is difficult to be certain that the burnup history of all spent nuclear fuel that occurred several decades ago was accurately recorded. Trusting existing data can save a lot in terms of time and money. However, for topics related to spent nuclear fuel, safety should be the first goal and long-term storage problems should be solved through trust derived from this, so additional burnup verification is absolutely necessary. Existing methods for determining the burnup of spent nuclear fuel include a method using a single peak of 137Cs, a gamma ray emitting nuclide, but there are few studies on the methodology for estimating burnup through the history of major emitting nuclides. In this paper, it is proposed to use the generation rate of neutron and gamma-ray sources emitted from spent nuclear fuel as an important tool to reverse burnup. The dependence of the burnup on the critical radiation sources derived using ORIGEN-ARP was analyzed, and a simple correlation was proposed. The burnup estimation equation includes the total neutron source intensity(TNSI), burnup(BU), initial enrichment (IE), and cooling time(CT), which are major factors, based on the PWR fuel. As a result of checking the concordance rate for each fuel assembly, a correlation equation with 99.8% accuracy was derived. For the main gamma sources, burnup estimation equations for three nuclides, 137Cs, 134Cs, and 154Eu, were also proposed.
This estimation equation is applied as an algorithm for burnup evaluation. by scanning spent nuclear fuel with a specific burnup history with a detector and measuring the energy of emitted neutrons and gamma rays, the representative emission source can be tracked and, conversely, the burnup can be derived. Therefore, in order to obtain neutron and gamma-ray pulses generated from the fuel assembly, the spent fuel assembly and detector were simulated through the MCNP code, and finally, the energy value from the source was confirmed. The neutron detector was aimed at the 3He chamber, which is a kind of proportional counter. It was housed with HDPE, a moderator, to make it easy to measure the 244Cm nuclide, which is the most dominant in the neutron production rate. As a gamma-ray measurement technique, there is a method of measuring the gamma-ray sum of total radionuclides in nuclear fuel with 137Cs as the main target, and a CZT detector is planned to be used.
If the technology localization of the spent nuclear fuel burnup measurement facility is successfully carried out through the continuous performance of this study, It is possible to secure an independent verification system free from dependence on foreign technology for related equipment and to reduce costs and import and export regulations.