JP3614965B2

JP3614965B2 - Nuclear reactor core

Info

Publication number: JP3614965B2
Application number: JP02149196A
Authority: JP
Inventors: 毅中嶋; 紀之吉田; 久生野際
Original assignee: Toshiba Corp; Toshiba Plant Systems and Services Corp
Current assignee: Toshiba Corp; Toshiba Plant Systems and Services Corp
Priority date: 1996-02-07
Filing date: 1996-02-07
Publication date: 2005-01-26
Anticipated expiration: 2016-02-07
Also published as: JPH09211165A

Description

【０００１】
【発明の属する技術分野】
本発明は、原子炉の熱的制限値や炉停止余裕等の炉心特性を悪化させることなく初装荷炉心の取出燃焼度を高くすることができる原子炉炉心に関する。
【０００２】
【従来の技術】
一般に原子炉は、図１１に示す如く断面十字型の制御棒１１の周囲に４体の燃料集合体１３を装荷して単位格子（以下、セルという。）１５を構成し、これらセル１５を複数個格子状態に配列して炉心を構成している。
【０００３】
ところで、このような原子炉は約１年強、すなわち１サイクル運転毎に燃料集合体の交換が行われるが、この燃料集合体は１〜５サイクルの間燃焼されるように設計されており、燃料交換の際には全燃料集合体のうち１／４〜１／５ずつが交換される。したがって、燃料集合体を１／４ずつ交換して運転した場合、第４サイクル以後は燃焼が最も進んだ燃料集合体、燃焼が中程度進んだ燃料集合体、燃焼が進んでいない燃料集合体、すなわち残留濃縮度が最低、中程度、最高の燃料集合体が装荷された炉心となり、以後は燃料交換毎に残留濃縮度最低の燃料集合体から取り出し、代わりに新燃料集合体を装荷すれば、同じ状態の炉心が維持される。このような炉心を平衡炉心と称している。
【０００４】
しかしながら、原子炉を建設して初めて燃料集合体を装荷した炉心すなわち初装荷炉心においては、全燃料集合体が同じ濃縮度の新燃料集合体であるので、はじめの２〜３サイクルの間は、完全に燃焼していない、つまり残留濃縮度の比較的高い燃料集合体が燃料交換の際に炉心から取り出されることになり、不経済であった。
【０００５】
このような課題を解決すべく、原子炉の炉心装荷方法はこれまで様々な改良が加えられてきた。たとえば初装荷炉心に装荷する燃料集合体を濃縮度の異なる３種類のもの、すなわち高濃縮燃料、中濃縮燃料、低濃縮燃料とし、それらを新燃料、２サイクル目燃料、３サイクル目以上燃料にみたてて平衡炉心を模擬した初装荷炉心を構成する。これは多種類濃縮度炉心と呼ばれている炉心装荷方法である。平衡炉心においては、中性子の漏れが大きく熱中性子束の低い炉心最外周部に残留濃縮度の低い燃料から装荷するが、多種類濃縮度炉心においても、これと同様の考えで炉心最外周部に低濃縮燃料を装荷し、それ以外は高、中、低濃縮燃料を適切に組み合わせた複数のセルで炉心を構成する。このような多種類濃縮度炉心により、第１サイクル終了時に取り出す低濃縮燃料の残留濃縮度は低減され、燃料経済性は向上した。
【０００６】
多種類濃縮度炉心を改良し、第１サイクル終了時に取り出す燃料の残留濃縮度をさらに低減することにより燃料経済性をさらに向上したのが、改良型多種類濃縮度炉心である。これは、特公平５−２７０７５号公報に記載があるように、多種類濃縮度炉心と同様に、濃縮度の異なる３種類の燃料、すなわち高濃縮燃料、中濃縮燃料、低濃縮燃料から構成される炉心構成であるが、炉心最外周には高濃縮燃料を装荷し、原子炉出力調整用の制御棒廻りに配置する４体の燃料集合体で構成するセル（以下、コントロールセルという。）には低濃縮燃料を装荷し、そのほかのセルには高濃縮、中濃縮、低濃縮燃料を適当に装荷して構成することが特徴である。
【０００７】
多種類濃縮度炉心では、熱中性子束が低く燃焼があまり進まない炉心最外周に低濃縮燃料を装荷していたが、改良型多種類濃縮度炉心では、第１サイクル終了後に取り出される運命となっている低濃縮燃料を、炉心最外周ではなくむしろ熱中性子束が高く燃焼が良く進む炉心中心付近に装荷してより燃焼させた方が、取出時における残留濃縮度はますます低くなり燃料経済性はさらに向上する、という原理を利用したものである。同様に、炉心最外周のみならず、最外周から２層目にも高濃縮燃料を装荷すれば、低濃縮燃料は必然的にさらに炉心中心付近に装荷することになり、第１サイクル終了後に取り出す低濃縮燃料の残留濃縮度はますます低くなり、燃料経済性はさらに向上する。
【０００８】
【発明が解決しようとする課題】
初装荷炉心において、取出平均燃焼度を上げて燃料経済性を向上させるには、炉心平均濃縮度を上げる必要があるが、これには以下に示すような問題点がある。
【０００９】
すなわち、炉心平均濃縮度を上げるためには、３種類の濃縮度の異なる燃料のうち、高濃縮燃料の割合を多くするか、低濃縮燃料や中濃縮燃料の濃縮度を増加することが考えられる。ここで、高濃縮燃料は、燃料機械設計上の燃焼度の制限より、取替燃料よりも高い濃縮度とすることは困難であるため、上記方法が有効である。このような方法で炉心平均濃縮度を上げると、初装荷炉心の反応度は大きくなるので、制御棒の挿入本数が増加し、出力分布がさらに非均質になり、炉心断面における径方向の出力ピーキングが増加するという問題を生じる。
【００１０】
また、制御棒の本数の増加に対応して制御棒を挿入するコントロールセルを低濃縮燃料で用意すると、逆に平均濃縮度が低下してしまい、これを補償するために炉心最外周付近に多くの高濃縮燃料を装荷すると、これもまた径方向の出力ピーキングが増加するという問題を生じる。さらに、反応度が高い高濃縮燃料の比率が増えると、炉停止余裕も厳しくなるという問題点がある。
【００１１】
炉心断面における径方向の出力ピーキングが増加すると、燃料集合体の最大線出力密度や最小限界出力比といった原子炉の運転に関する熱的パラメータが悪化し、運転裕度の減少につながる。したがって、炉心断面における径方向の出力分布は平坦であるのが望ましい。
【００１２】
このように、濃縮度の異なる３種類の燃料、すなわち高濃縮、中濃縮、および低濃縮燃料から構成され、コントロールセルには低濃縮燃料を装荷し、かつ炉心外周領域の燃料装荷パターンに関しては、高い燃料経済性を実現するために炉心最外周のみならず最外周から２層目にも高濃縮燃料を装荷した炉心においては、前述したような炉心断面における径方向の出力ピーキングの増加をもたらすために炉心の運転裕度が少なくなるという問題が生じる。
【００１３】
本発明は、かかる事情に対処してなされたもので、炉心の出力ピーキングや炉停止余裕が設計条件を満たしつつ、炉心平均濃縮度を高くして取出平均燃焼度を向上させることができるより経済的な原子炉炉心を提供することを目的とする。
【００１４】
【課題を解決するための手段】
すなわち、請求項１の発明は、濃縮度の低い方から順に低濃縮燃料、中濃縮燃料および高濃縮燃料に分類される濃縮度の異なる３種類の燃料集合体が初装荷される炉心において、十字型の制御棒廻りの燃料集合体４体で構成されるセルを基本単位とし、運転サイクル中出力調整用制御棒が挿入されるコントロールセルを低濃縮燃料４体で構成して、炉心中央領域に１つ置きに格子状態に配列するとともに、前記コントロールセルに面隣接しかつ炉心最外周部を含まない第１セルを中濃縮燃料２体と高濃縮燃料２体で構成し、前記コントロールセルに面隣接せずかつ炉心最外周部を含まない第２セルを低濃縮燃料２体と中濃縮燃料１体と高濃縮燃料１体で構成することを特徴とする。
【００１５】
請求項２の発明は、濃縮度の低い方から順に低濃縮燃料、中濃縮燃料および高濃縮燃料に分類される濃縮度の異なる３種類の燃料集合体が初装荷される炉心において、十字型の制御棒廻りの燃料集合体４体で構成されるセルを基本単位とし、運転サイクル中出力調整用制御棒が挿入されるコントロールセルを低濃縮燃料４体で構成して、炉心中央領域に１つ置きに格子状態に配列するとともに、前記コントロールセルに面隣接しかつ炉心最外周部を含まない第１セルを中濃縮燃料２体と高濃縮燃料２体で構成し、前記コントロールセルに面隣接せずかつ炉心最外周部を含まない第２セルを低濃縮燃料２体と高濃縮燃料２体で構成することを特徴とする。
【００１６】
請求項３の発明は、濃縮度の低い方から順に低濃縮燃料、中濃縮燃料および高濃縮燃料に分類される濃縮度の異なる３種類の燃料集合体が初装荷される炉心において、十字型の制御棒廻りの燃料集合体４体で構成されるセルを基本単位とし、運転サイクル中出力調整用制御棒が挿入されるコントロールセルを低濃縮燃料４体で構成して、炉心中央領域に１つ置きに格子状態に配列するとともに、前記コントロールセルに面隣接しかつ炉心最外周部を含まない第１セルを中濃縮燃料２体と高濃縮燃料２体で構成し、前記コントロールセルに面隣接せずかつ炉心最外周部を含まない第２セルを低濃縮燃料１体と中濃縮燃料２体と高濃縮燃料１体で構成することを特徴とする。
【００１７】
請求項４の発明は、上記炉心において、第１セルおよび第２セルに装荷される高濃縮燃料は、お互い面隣接しないように配置することを特徴とする。
【００１８】
請求項５の発明は、上記炉心において、炉心最外周部、コントロールセル、第１セルおよび第２セルを除いた炉心周辺部に高濃縮燃料を配置することを特徴とする。
【００１９】
請求項６の発明は、上記炉心において、炉心最外周部に高濃縮燃料を配置することを特徴とする。
【００２０】
請求項７の発明は、上記炉心において、炉心最外周部に中濃縮燃料を配置することを特徴とする。
【００２１】
請求項８の発明は、上記炉心において、高濃縮燃料を可燃性毒物の含有量が少ないタイプと可燃性毒物の含有量が多いタイプの２種類のタイプに分け、出力調整用制御棒が第１サイクル初期において挿入されないコントロールセルに面隣接する高濃縮燃料は可燃性毒物の含有量が多いタイプとし、その他の高濃縮燃料は可燃性毒物の含有量が少ないタイプとすることを特徴とする。
【００２２】
本発明においては、原子炉炉心の出力分布を平坦化するために、まず、初装荷（第１サイクル）炉心の長期に渡って余剰反応度を補償するために出力調整用制御棒が挿入されるコントロールセルを、制御棒廻りに４体の低濃縮燃料で構成し、炉心最外周のセルを除く炉心中央領域に、炉心中心部を中心基点として一つ置きに配置する。一方、炉心中央領域の他のセルは、コントロールセルに面隣接する第１セルと、コントロールセルに面隣接せずコントロールセルの対角線上に配列される第２セルに分類される。
【００２３】
ここで、出力分布および炉停止余裕の観点より、制御棒廻りの燃料集合体４体を１単位としたセルを中心として組合わせを考えていくと解りやすい。コントロールセルには、通常定格出力運転中は制御棒が挿入されているため、コントロールセルに挟まれた第１セルは比較的出力は低く、−方、コントロールセルの斜めに位置する第２セルは比較的出力は高い傾向にある。この特性を利用すると、第１セルは第２セルと比べて比較的反応度が高い燃料を装荷しても出力が高すぎることはなく、また、第２セルには第１セルと比べて比較的反応度が低い燃料を装荷して出力が高すぎないように燃料の配置を行うことができる。
【００２４】
本発明は、かかる特性を利用したもので、第１セルには、炉心の濃縮度をできるだけ高めることができるよう、高濃縮燃料２体と中濃縮燃料２体を装荷し、第２セルには、低濃縮燃料２体と中濃縮燃料１体と高濃縮燃料１体の組合わせ、または低濃縮燃料２体と高濃縮燃料２体の組合わせ、あるいは低濃縮燃料１体と中濃縮燃料２体と高濃縮燃料１体の組合わせを装荷する。
【００２５】
これを、コントロールセルと合わせて炉心中央領域に規則的に配置することにより、炉心全体でバランスよい出力分布を得ることができるものである。炉停止余裕も、高濃縮燃料４体を１つのセルに集めなければ満たすことができる。
【００２６】
ところで、初装荷炉心の取出平均燃焼度を高くするには、高濃縮燃料の体数を増加することが最も容易な方法である。本燃料装荷パターンでは、炉心の特性を損なうことなく規則配置的に炉心の平均濃縮度を高くすることができる。
【００２７】
また、この配置の他の長所として、炉心最外周部を除いて比較的均一に燃料を配置しているため、不測の事態として予定より挿入している制御棒の本数が多くなったり少なくなったりした場合でも、炉心配置を変更することなく、制御棒パターンで容易に調整することが可能である。
【００２８】
【発明の実施の形態】
以下、図面に基づいて本発明の実施の形態を説明する。なお、全ての実施の形態において、共通する部分には同一符号を付して説明する。
【００２９】
図１は、本発明の第１の実施の形態の炉心の燃料装荷パターンを示すもので、１、２、３の数字がそれぞれ記されたマスは、濃縮度が異なる３種類の燃料集合体１３を表し、１のマスは低濃縮燃料１３ａ、２のマスは中濃縮燃料１３ｂ、３のマスは高濃縮燃料１３ｃである。なお、全ての実施の形態において、低濃縮燃料１３ａの濃縮度は１．３ｗｔ％、中濃縮燃料１３ｂの濃縮度は２．５ｗｔ％、高濃縮燃料１３ｃの濃縮度は３．７ｗｔ％とした。
【００３０】
図１に示す炉心は、７６４体の燃料集合体１３で構成され、出力調整用制御棒廻りに４体の低濃縮燃料１３ａが装荷されるコントロールセル１５ａを炉心中心部に配し、これを中心として炉心最外周のセルを除く炉心中央領域に１つおきに格子状態に３７個配置している。そしてコントロールセル１５ａの上下左右に隣接しかつ炉心最外周にかからないセルを第１セル１５ｂとして７６個配置し、コントロールセル１５ａに斜めに隣接しかつ炉心最外周にかからないセルを第２セル１５ｃとして３２個配置している。
【００３１】
第１セル１５ｂは、制御棒が挿入されるコントロールセル１５ａに面隣接しており、これにより比較的出力は抑えられるので、炉心の濃縮度をできるだけ高めることができるよう、第１セル１５ｂには２体の高濃縮燃料１３ｃと２体の中濃縮燃料１３ｂを装荷する。
【００３２】
第２セル１５ｃは、第１セル１５ｂと比べて出力が高くなりやすいので、２体の低濃縮燃料１３ａと１体の中濃縮燃料１３ｂと１体の高濃縮燃料１３ｃを装荷する。
【００３３】
なお、第１セル１５ｂ、第２セル１５ｃともに高濃縮燃料１３ｃが互いに隣接して出力ピーキングが高くならないように配置している。この場合、第１セル１５ｂの２体の中濃縮燃料１３ｂと２体の高濃縮燃料１３ｃは第２セル１５ｃのものよりも反応度は低いので、反応度が低いセルに挟まれていなくても、出力ピークが発生することはない。
【００３４】
また、上記３種類のセル１５ａ、１５ｂ、１５ｃを除く炉心周辺領域で、最外周は低濃縮燃料１３ａを配置し、その他は高濃縮燃料１３ｃを配置して，できるだけ炉心平均濃縮度を高くしたものとしている。
【００３５】
図１に示すように、コントロールセル１５ａが配置され比較的出力が高い炉心中央領域では、セルを中心に３種類の濃縮度の燃料を分散して配置することにより、出力分布を平坦にすることができる。
【００３６】
上記３種類の燃料の濃縮度・可燃性毒物の分布は、最終的な詳細３次元評価で運転制限値を満足するように決めればよい。この実施の形態では、特に細かな濃縮度・可燃性毒物の分布については述べない。
【００３７】
なお、この実施の形態の炉心において、７６４体の燃料集合体１３のうち低濃縮燃料１３ａは３０４体、中濃縮燃料１３ｂは１８４体、高濃縮燃料１３ｃは２７６体であり、平均濃縮度は２．４６ｗｔ％となる。
【００３８】
図２は、本発明の第１の実施の形態の変形例を示すもので、第１の実施の形態と比較して、１／４炉心対称性として鏡面対称の燃料装荷パターンを形成している。したがって、１／４対称軸上の第１セル１５ｂにおいて、高濃縮燃料１３ｃが２体隣接することになるが、制御棒が挿入されるコントロールセル１５ａに隣接しているのでピーキング上厳しくなることはない。
【００３９】
図３は、第１の実施の形態の他の変形例を示すもので、８７２体の燃料集合体１３が装荷される炉心に対する適用例を示している。この場合もコントロールセル１５ａは３７個であり、２体の高濃縮燃料１３ｃと２体の中濃縮燃料１３ｂを装荷した第１セル１５ｂがコントロールセル１５ａと面隣接する位置に、２体の低濃縮燃料１３ａと１体の中濃縮燃料１３ｂと１体の高濃縮燃料１３ｃを装荷した第２セル１５ｃがコントロールセル１５ａと面隣接しない位置にそれぞれ配置されている。
【００４０】
このように、コントロールセル１５ａを炉心中央領域に１つおきに格子状態に配置し、それに対して第１セル１５ｂと第２セル１５ｃを規則的に配置する燃料装荷パターンは、炉心の大きさにより制約を受けるものではなく、大型炉心においても適用可能である。
【００４１】
なお、この炉心において、８７２体の燃料集合体１３のうちの低濃縮燃料１３ａは３４４体、中濃縮燃料１３ｂは２２８体、高濃縮燃料１３ｃは３００体であり、平均濃縮度は２．４４ｗｔ％である。
【００４２】
図４は、本発明の第２の実施の形態の炉心の燃料装荷パターンを示すもので、第１の実施の形態の炉心の燃料装荷パターンと比較して、さらに炉心平均濃縮度を高めるため、第２セル１５ｃに低濃縮燃料１３ａ２体と高濃縮燃料１３ｃ２体を装荷している。この場合でも、第１セル１５ｂの中濃縮燃料１３ｂ２体と高濃縮燃料１３ｃ２体よりも反応度は低いので、反応度が低いセルに挟まれていなくても、出力ピークが発生することはない。
【００４３】
これによって、第１の実施の形態と同様に出力分布を平坦にし炉停止余裕に余裕を持った炉心を得ることができる。
【００４４】
また、この実施の形態の炉心において、７６４体の燃料集合体１３のうち低濃縮燃料１３ａは３０４体、中濃縮燃料１３ｂは１５２体、高濃縮燃料１３ｃは３０８体であり、炉心平均濃縮度は第１の実施の形態のものより高く２．５１ｗｔ％となり、さらなる取出燃焼度の増加が期待できる。
【００４５】
図５は、本発明の第３の実施の形態の炉心の燃料装荷パターンを示すもので、第１の実施の形態の炉心の燃料装荷パターンと比較して、第２セル１５ｃに低濃縮燃料１３ａ１体と中濃縮燃料１３ｂ２体と高濃縮燃料１３ｃ１体を装荷している。
【００４６】
この場合でも、第２セル１５ｃは第１のセル１５ｂの中濃縮燃料１３ｂ２体と高濃縮燃料１３ｃ２体よりも反応度は低いので、反応度が低いセルに挟まれていなくても、出力ピークが発生することはない。また、炉心平均濃縮度は第１の実施の形態より高く２．５１ｗｔ％となり、さらなる取出燃焼度の増加が期待できる。
【００４７】
なお、この実施の形態の炉心において、７６４体の燃料集合体１３のうち低濃縮燃料１３ａは２７２体、中濃縮燃料１３ｂは２１６体、高濃縮燃料１３ｃは２７６体であ
図６は、本発明の第４の実施の形態の炉心の燃料装荷パターンを示すもので、第１の実施の形態と比較して、炉心中央部の炉心構成は同じであるが、炉心最外周部に中濃縮燃料１３ｂを装荷している。
【００４８】
これにより、さらに出力分布を均一化し、熱的制限値や炉停止余裕を満足する炉心を提供することもできる。これは、最外周に低濃縮燃料より反応度が高い中濃縮燃料を配置することにより、最外周の出力を高くすることができるので、相対的に炉心中央部のピーキングを低くすることができることによる。炉心平均濃縮度は、第１の実施の形態のものより高く２．６０ｗｔ％となり、さらなる取出燃焼度の増加が期待できる。
【００４９】
なお、この実施の形態の炉心において、７６４体の燃料集合体１３のうち低濃縮燃料１３ａは２１２体、中濃縮燃料１３ｂは２７６体、高濃縮燃料１３ｃは２７６体である。
【００５０】
図７は、本発明の第４の実施の形態の変形例として、８７２体の燃料集合体１３が装荷される炉心の例を示している。この場合も図６に示す７６４体の装荷炉心と作用、効果は同じである。
【００５１】
なお、この炉心において、８７２体の燃料集合体１３のうちの低濃縮燃料１３ａは２５２体、中濃縮燃料１３ｂは３２０体、高濃縮燃料１３ｃは３００体であり、平均濃縮度は２．５７ｗｔ％となり、図３に示す燃料装荷パターンの炉心の平均濃縮度２．４４ｗｔ％よりも高くなる。
【００５２】
図８は、本発明の第５の実施の形態の炉心の燃料装荷パターンを示すもので、第１の実施の形態および第４の実施の形態と比較して、炉心中央部の炉心構成は同じであるが、炉心最外周部に高濃縮燃料１３ｃを装荷している。
【００５３】
これにより、さらに、出力分布を均一化し、熱的制限値や炉停止余裕を満足する炉心を提供することもできる。これは、最外周に低濃縮燃料や中濃縮燃料より反応度が高い高濃縮燃料を配置することにより、最外周の出力を高くすることができるので、相対的に炉心中央部のピーキングを低くすることができることによる。
【００５４】
また、この実施の形態において、７６４体の燃料集合体１３のうち低濃縮燃料１３ａは２１２体、中濃縮燃料１３ｂは１８４体、高濃縮燃料１３ｃは３６８体であり、炉心平均濃縮度は、第４の実施の形態のものよりさらに高く２．７５ｗｔ％となり、さらなる取出燃焼度の増加が期待できる。
【００５５】
図９は、本発明の第５の実施の形態の変形例として、８７２体の燃料集合体１３が装荷される炉心の例を示すが、この場合も第５の実施の形態の７６４体の装荷炉心と作用、効果は同じである。
【００５６】
なお、この炉心において、８７２体の燃料集合体１３のうちの低濃縮燃料１３ａは２５２体、中濃縮燃料１３ｂは２２８体、高濃縮燃料１３ｃは３９２体であり、平均濃縮度は２．６９ｗｔ％である。
【００５７】
図１０は、第６の発明の実施の形態の炉心の燃料装荷パターンを示すもので、マスの中に４の数字が記された燃料集合体１３は、高濃縮燃料１３ｃと濃縮度は同じであるが可燃性毒物の含有量が多いタイプの第２の高濃縮燃料１３ｄを示している。
【００５８】
図１０に示すように、この実施の形態では、第１の実施の形態と比較して、コントロールセル１５ａ、第１セル１５ｂ、第２セル１５ｃの配置構成は同じであるが、高濃縮燃料に可燃性毒物の含有量の異なる２種類のタイプを用意して、炉心中央部において第１サイクル初期に出力調整用制御棒１７が挿入されないコントロールセル１５ａと面隣接する位置に、可燃性毒物の含有量が少ない高濃縮燃料１３ｃの替りに可燃性毒物の含有量が多い第２の高濃縮燃料１３ｄを装荷する。
【００５９】
これにより、制御棒が挿入されていないセル周辺の出力分布を可燃性毒物の含有量が多いタイプで抑えることができるので、制御棒パターンと組み合わせて出力分布を均一化し、熱的制限値や炉停止余裕を満足する炉心を提供することができる。なお、この実施の形態の炉心の平均濃縮度は、第１の実施の形態と低濃縮燃料、中濃縮燃料、高濃縮燃料の各装荷数は同じため、２．４６ｗｔ％である。
【００６０】
また、このような燃料装荷方法は、上記いずれの実施の形態とも組み合わせて用いることができる。
【００６１】
【発明の効果】
上記したように、本発明によれば、濃縮度の異なる３種類の燃料集合体から構成される原子炉炉心において、炉心内の出力分布を均一化し、熱的制限値や炉停止余裕を満足し、平均濃縮度を増加することができ、ひいては取出平均燃焼度を増加して燃料経済性を向上させることができる。
【図面の簡単な説明】
【図１】本発明の第１の実施の形態の炉心を示す燃料装荷パターン図である。
【図２】本発明の第１の実施の形態の変形例を示す燃料装荷パターン図である。
【図３】本発明の第１の実施の形態の他の変形例を示す燃料装荷パターン図である。
【図４】本発明の第２の実施の形態の炉心を示す燃料装荷パターン図である。
【図５】本発明の第３の実施の形態の炉心を示す燃料装荷パターン図である。
【図６】本発明の第４の実施の形態の炉心を示す燃料装荷パターン図である。
【図７】本発明の第４の実施の形態の変形例を示す燃料装荷パターン図である。
【図８】本発明の第５の実施の形態の炉心を示す燃料装荷パターン図である。
【図９】本発明の第５の実施の形態の変形例を示す燃料装荷パターン図である。
【図１０】本発明の第６の実施の形態の炉心を示す燃料装荷パターン図である。
【図１１】セルを模式的に示す平面図である。
【符号の説明】
１１………制御棒
１３………燃料集合体
１３ａ………低濃縮燃料
１３ｂ………中濃縮燃料
１３ｃ………高濃縮燃料
１３ｄ………第２の高濃縮燃料
１５………セル
１５ａ………コントロールセル
１５ｂ………第１セル
１５ｃ………第２セル
１７………サイクル初期に挿入される出力調整用制御棒
出願人株式会社東芝
同東芝エンジニアリング株式会社
代理人弁理士須山佐一[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a nuclear reactor core capable of increasing the take-off burnup of an initially loaded core without deteriorating core characteristics such as a thermal limit value and a reactor shutdown margin.
[0002]
[Prior art]
In general, in a nuclear reactor, as shown in FIG. 11, four fuel assemblies 13 are loaded around a control rod 11 having a cross-shaped cross section to form a unit cell (hereinafter referred to as a cell) 15, and a plurality of these cells 15 are arranged. The reactor core is configured in a single lattice state.
[0003]
By the way, such a nuclear reactor is a little over a year, that is, the fuel assembly is replaced every cycle operation, and this fuel assembly is designed to burn for 1 to 5 cycles, When changing fuel, 1/4 to 1/5 of the total fuel assembly is changed. Therefore, when the fuel assembly is operated by exchanging by 1/4, the fuel assembly in which combustion is most advanced after the fourth cycle, the fuel assembly in which combustion is advanced moderately, the fuel assembly in which combustion is not advanced, In other words, the reactor core is loaded with the lowest, medium and highest fuel assemblies, and after that, it is removed from the fuel assembly with the lowest residual enrichment every time the fuel is replaced, and a new fuel assembly is loaded instead. The same core is maintained. Such a core is called an equilibrium core.
[0004]
However, in the core in which the fuel assemblies are loaded for the first time after the construction of the nuclear reactor, that is, in the first loading core, all the fuel assemblies are new fuel assemblies having the same enrichment, so during the first two to three cycles, A fuel assembly that was not completely burned, that is, a relatively high residual enrichment, was taken out of the core during fuel exchange, which was uneconomical.
[0005]
In order to solve these problems, various improvements have been made to the reactor core loading method. For example, there are three types of fuel assemblies loaded in the initial loading core with different enrichments: high enriched fuel, medium enriched fuel, and low enriched fuel. A newly loaded core that simulates an equilibrium core is constructed. This is a core loading method called a multi-concentration core. In an equilibrium core, fuel with a low residual enrichment is loaded on the outermost periphery of the core where leakage of neutrons is large and the thermal neutron flux is low, but in the case of multi-concentration cores, the same concept is applied to the outermost periphery of the core. The core is composed of a plurality of cells that are loaded with low-enriched fuel and that are otherwise appropriately combined with high-, medium-, and low-enriched fuel. By such a multi-concentration core, the residual enrichment of the low-concentration fuel taken out at the end of the first cycle is reduced, and the fuel economy is improved.
[0006]
The improved multi-type enrichment core has further improved the fuel economy by improving the multi-type enrichment core and further reducing the residual enrichment of the fuel taken out at the end of the first cycle. As described in Japanese Patent Publication No. 5-27075, this is composed of three types of fuels having different enrichment levels, that is, a highly enriched fuel, a medium enriched fuel and a low enriched fuel, as in the case of a multi-enriched core. This is a reactor core configuration, in which a highly concentrated fuel is loaded on the outermost periphery of the core, and a cell composed of four fuel assemblies (hereinafter referred to as control cells) arranged around control rods for adjusting the reactor power. Is characterized by loading low-enriched fuel and loading other cells with high-enriched, medium-enriched, and low-enriched fuel appropriately.
[0007]
In the multi-enriched core, low-enriched fuel was loaded on the outermost periphery of the core where the thermal neutron flux was low and combustion did not progress much. However, in the improved multi-enriched core, it was destined to be taken out after the first cycle. If the low-enriched fuel is loaded near the core center where the thermal neutron flux is high and the combustion proceeds rather than the outermost periphery of the core, the remaining enrichment at the time of removal becomes lower and the fuel economy Is based on the principle of further improvement. Similarly, if high enriched fuel is loaded not only in the outermost periphery of the core but also in the second layer from the outermost periphery, the low enriched fuel will inevitably be loaded near the core center and taken out after the end of the first cycle. The residual enrichment of the low-enriched fuel will become lower and the fuel economy will be further improved.
[0008]
[Problems to be solved by the invention]
In the initial loading core, it is necessary to increase the average enrichment of the core in order to improve the fuel economy by increasing the take-out average burnup, but this has the following problems.
[0009]
That is, in order to increase the core average enrichment, it is conceivable to increase the proportion of highly enriched fuel among the three types of enriched fuels or increase the enrichment of low enriched fuel and medium enriched fuel. . Here, the highly enriched fuel is difficult to be enriched higher than the replacement fuel due to the restriction of the burnup in the fuel machine design, so the above method is effective. Increasing the core average enrichment in this way increases the reactivity of the initially loaded core, increasing the number of control rods inserted, making the power distribution more non-homogeneous, and radial power peaking in the core cross section. Cause the problem of increased.
[0010]
In addition, if a control cell that inserts control rods is prepared with low enriched fuel in response to the increase in the number of control rods, the average enrichment is conversely reduced. This also causes the problem of increased radial power peaking. Furthermore, if the ratio of highly concentrated fuel with high reactivity increases, there is a problem that the furnace shutdown margin becomes severe.
[0011]
When the power peaking in the radial direction in the core cross section increases, the thermal parameters related to the operation of the reactor, such as the maximum linear power density and the minimum critical power ratio of the fuel assembly, deteriorate, leading to a decrease in operating margin. Therefore, it is desirable that the radial power distribution in the core cross section be flat.
[0012]
In this way, it is composed of three types of fuels with different enrichments, namely high enrichment, medium enrichment, and low enrichment fuels, the control cell is loaded with low enrichment fuel, and the fuel loading pattern in the core peripheral region is as follows: In order to realize high fuel economy, the core loaded with highly concentrated fuel not only in the outermost periphery of the core but also in the second layer from the outermost periphery will increase the output peaking in the radial direction in the core cross section as described above. However, there is a problem that the operating margin of the core is reduced.
[0013]
The present invention has been made in view of such circumstances, and is more economical than the core power peaking and the reactor shutdown margin satisfy the design conditions, and the core average enrichment can be increased to improve the average extraction burnup. It aims to provide an effective nuclear reactor core.
[0014]
[Means for Solving the Problems]
That is, the invention of claim 1 is a cross-section in a core in which three types of fuel assemblies having different enrichments classified into a low enrichment fuel, a medium enrichment fuel, and a highly enriched fuel in order from the lowest enrichment are loaded. A cell composed of four fuel assemblies around the control rods of the mold is a basic unit, and a control cell into which power control rods are inserted during the operation cycle is composed of four low-enriched fuels in the central region of the core. Every other cell is arranged in a lattice state, and the first cell that is adjacent to the control cell and does not include the outermost periphery of the core is composed of two medium-enriched fuels and two highly-enriched fuels. The second cell that is not adjacent and does not include the outermost peripheral part of the core is constituted by two low-enriched fuels, one medium-enriched fuel, and one highly-enriched fuel.
[0015]
The invention of claim 2 is a cross-shaped core in which three types of fuel assemblies having different enrichments classified into low enriched fuel, medium enriched fuel, and highly enriched fuel in order from the lowest enrichment are loaded. A cell composed of four fuel assemblies around the control rod is a basic unit, and a control cell into which an output adjustment control rod is inserted during the operation cycle is composed of four low enriched fuels, one in the central region of the core. The first cell that is arranged in a lattice state and that is adjacent to the control cell and does not include the outermost periphery of the core is composed of two medium-enriched fuels and two highly-enriched fuels, and is adjacent to the control cell. The second cell that does not include the outermost periphery of the core is composed of two low-enriched fuels and two highly-enriched fuels.
[0016]
The invention according to claim 3 is a cross-shaped core in which three types of fuel assemblies having different enrichments classified into low enriched fuel, medium enriched fuel, and highly enriched fuel in order from the lowest enrichment are loaded. A cell composed of four fuel assemblies around the control rod is a basic unit, and a control cell into which an output adjustment control rod is inserted during the operation cycle is composed of four low enriched fuels, one in the central region of the core. The first cell that is arranged in a lattice state and that is adjacent to the control cell and does not include the outermost periphery of the core is composed of two medium-enriched fuels and two highly-enriched fuels, and is adjacent to the control cell. The second cell that does not include the outermost periphery of the core is composed of one low-enriched fuel, two medium-enriched fuels, and one highly-enriched fuel.
[0017]
The invention of claim 4 is characterized in that, in the core, the highly concentrated fuels loaded in the first cell and the second cell are arranged so as not to be adjacent to each other.
[0018]
The invention of claim 5 is characterized in that, in the core, highly enriched fuel is disposed in a core peripheral portion excluding the outermost peripheral portion of the core, the control cell, the first cell, and the second cell.
[0019]
The invention of claim 6 is characterized in that, in the core, highly enriched fuel is arranged at the outermost peripheral portion of the core.
[0020]
The invention of claim 7 is characterized in that, in the core, the intermediate concentrated fuel is arranged at the outermost peripheral portion of the core.
[0021]
The invention according to claim 8 divides the highly concentrated fuel into two types, that is, a type with a low content of flammable poisons and a type with a high content of flammable poisons. The highly concentrated fuel adjacent to the control cell that is not inserted at the beginning of the cycle is a type having a high content of combustible poisons, and the other highly concentrated fuel is a type having a low content of combustible poisons.
[0022]
In the present invention, in order to flatten the power distribution of the reactor core, first, a power adjustment control rod is inserted in order to compensate for the excess reactivity over a long period of the initial loading (first cycle) core. The control cells are composed of four low-enriched fuels around the control rods, and are placed every other center around the core center, excluding the outermost peripheral cell. On the other hand, the other cells in the central region of the core are classified into a first cell that is adjacent to the control cell and a second cell that is not adjacent to the control cell and is arranged on the diagonal of the control cell.
[0023]
Here, from the viewpoint of power distribution and furnace stop margin, it is easy to understand when considering a combination centering on a cell having four fuel assemblies around the control rod as one unit. Since the control cell is inserted into the control cell during normal rated output operation, the output of the first cell sandwiched between the control cells is relatively low, and the second cell located diagonally to the control cell is The output tends to be relatively high. Using this characteristic, the output of the first cell is not too high even when a fuel having a relatively high reactivity is loaded compared to the second cell, and the second cell is compared with the first cell. It is possible to arrange the fuel so that the output is not too high by loading the fuel with low target reactivity.
[0024]
The present invention utilizes such characteristics, and the first cell is loaded with two high-enriched fuels and two medium-enriched fuels so that the enrichment of the core can be increased as much as possible. , A combination of two low-enriched fuels and one medium-enriched fuel and one high-enriched fuel, or a combination of two low-enriched fuels and two high-enriched fuels, or one low-enriched fuel and two medium-enriched fuels And a high-enriched fuel combination.
[0025]
By arranging this regularly in the central region of the core together with the control cell, it is possible to obtain a balanced power distribution in the entire core. The furnace shutdown margin can also be satisfied if four highly concentrated fuels are not collected in one cell.
[0026]
By the way, it is the easiest method to increase the number of highly concentrated fuels in order to increase the take-out average burnup of the initially loaded core. In this fuel loading pattern, the average enrichment of the core can be increased regularly without losing the core characteristics.
[0027]
Another advantage of this arrangement is that the fuel is arranged relatively uniformly except for the outermost peripheral part of the core, so that the number of control rods inserted from the schedule may increase or decrease as a contingency. Even in this case, the control rod pattern can be easily adjusted without changing the core arrangement.
[0028]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings. In all the embodiments, common portions will be described with the same reference numerals.
[0029]
FIG. 1 shows the fuel loading pattern of the core according to the first embodiment of the present invention. The masses indicated by numerals 1, 2, and 3 are three types of fuel assemblies 13 having different enrichments. 1 is a low enriched fuel 13a, 2 is a medium enriched fuel 13b, and 3 is a highly enriched fuel 13c. In all the embodiments, the enrichment of the low enrichment fuel 13a is 1.3 wt%, the enrichment of the intermediate enrichment fuel 13b is 2.5 wt%, and the enrichment of the highly enriched fuel 13c is 3.7 wt%. It was.
[0030]
The core shown in FIG. 1 is composed of 764 fuel assemblies 13, and a control cell 15a loaded with four low-concentration fuels 13a around a control rod for power adjustment is arranged in the center of the core. 37 are arranged in a lattice state every other one in the core central region excluding the outermost peripheral cell. Then, 76 cells that are adjacent to the control cell 15a in the upper, lower, left, and right directions and do not extend to the outermost periphery of the core are arranged as first cells 15b, and cells that are adjacent to the control cell 15a and that do not extend to the outermost periphery of the core are set as second cells 15c. It is arranged.
[0031]
Since the first cell 15b is adjacent to the control cell 15a into which the control rod is inserted, and the output is relatively suppressed by this, the first cell 15b includes a first cell 15b so that the concentration of the core can be increased as much as possible. Two highly concentrated fuels 13c and two medium concentrated fuels 13b are loaded.
[0032]
Since the output of the second cell 15c is likely to be higher than that of the first cell 15b, two low-enriched fuels 13a, one medium-enriched fuel 13b, and one highly-enriched fuel 13c are loaded.
[0033]
Note that both the first cell 15b and the second cell 15c are arranged so that the highly concentrated fuel 13c is adjacent to each other so that the output peaking does not increase. In this case, the two medium concentrated fuels 13b and the two highly concentrated fuels 13c in the first cell 15b are less reactive than those in the second cell 15c. The output peak will not occur.
[0034]
Further, in the core peripheral area excluding the three types of cells 15a, 15b and 15c, the outermost periphery is provided with the low enriched fuel 13a, and the others are provided with the highly enriched fuel 13c so that the average enrichment of the core is as high as possible. It is said.
[0035]
As shown in FIG. 1, in the central region of the core where the control cell 15a is arranged and the output is relatively high, the power distribution is flattened by dispersing and arranging three kinds of enriched fuel around the cell. Can do.
[0036]
The concentration of the three types of fuel and the distribution of the flammable poison may be determined so as to satisfy the operation limit value in the final detailed three-dimensional evaluation. This embodiment does not describe a particularly fine concentration / flammable poison distribution.
[0037]
In the core of this embodiment, among the 764 fuel assemblies 13, the low enriched fuel 13a is 304, the intermediate enriched fuel 13b is 184, the highly enriched fuel 13c is 276, and the average enrichment is 2 .46 wt%.
[0038]
FIG. 2 shows a modification of the first embodiment of the present invention. Compared with the first embodiment, a fuel loading pattern having a mirror symmetry as a 1/4 core symmetry is formed. . Accordingly, in the first cell 15b on the 1/4 axis of symmetry, two highly concentrated fuels 13c are adjacent to each other, but since the control cells 15a into which the control rods are inserted are adjacent, the peaking becomes severe. Absent.
[0039]
FIG. 3 shows another modification of the first embodiment, and shows an application example to a core in which 872 fuel assemblies 13 are loaded. Also in this case, the number of control cells 15a is 37, and two low-enriched fuel cells 13c and two medium-enriched fuels 13b loaded with the first cell 15b are located adjacent to the control cell 15a. The second cells 15c loaded with the fuel 13a, one medium-enriched fuel 13b, and one highly-enriched fuel 13c are disposed at positions not adjacent to the control cell 15a.
[0040]
Thus, every other control cell 15a is arranged in a lattice state in the central region of the core, and the fuel loading pattern in which the first cells 15b and the second cells 15c are regularly arranged depends on the size of the core. It is not limited and can be applied to large cores.
[0041]
In this core, among the 872 fuel assemblies 13, the low enriched fuel 13a is 344, the medium enriched fuel 13b is 228, the highly enriched fuel 13c is 300, and the average enrichment is 2.44 wt%. It is.
[0042]
FIG. 4 shows the fuel loading pattern of the core according to the second embodiment of the present invention. In order to further increase the average enrichment of the core as compared with the fuel loading pattern of the core according to the first embodiment, The second cell 15c is loaded with the low-enriched fuel 13a2 body and the high-enriched fuel 13c2 body. Even in this case, the reactivity is lower than that of the medium enriched fuel 13b2 body and the highly enriched fuel 13c2 body of the first cell 15b, so that an output peak does not occur even if it is not sandwiched between cells with low reactivity.
[0043]
As a result, as in the first embodiment, it is possible to obtain a core having a flat power distribution and a sufficient margin for reactor shutdown.
[0044]
Further, in the core of this embodiment, among the 764 fuel assemblies 13, the low enriched fuel 13 a is 304, the intermediate enriched fuel 13 b is 152, the highly enriched fuel 13 c is 308, and the core average enrichment is It is 2.51 wt%, which is higher than that of the first embodiment, and a further increase in the extracted burn-up degree can be expected.
[0045]
FIG. 5 shows a fuel loading pattern of the core according to the third embodiment of the present invention. Compared with the fuel loading pattern of the core according to the first embodiment, the low concentration fuel 13a1 is added to the second cell 15c. Body, medium enriched fuel 13b2 body and highly enriched fuel 13c1 body are loaded.
[0046]
Even in this case, the second cell 15c has a lower reactivity than the medium enriched fuel 13b2 and the highly enriched fuel 13c2 in the first cell 15b. It does not occur. Moreover, the core average enrichment is 2.51 wt%, which is higher than that in the first embodiment, and a further increase in the extracted burn-up degree can be expected.
[0047]
In the core of this embodiment, among the 764 fuel assemblies 13, 272 low enriched fuels 13a, 216 medium enriched fuels 13b, and 276 highly concentrated fuels 13c are shown in FIG. 4 shows the fuel loading pattern of the core of the fourth embodiment of the present invention. Compared with the first embodiment, the core configuration at the center of the core is the same, but the intermediate concentrated fuel 13b is disposed at the outermost periphery of the core. Is loaded.
[0048]
As a result, the power distribution can be further uniformed, and a core that satisfies the thermal limit value and the reactor shutdown margin can be provided. This is because by placing medium enriched fuel having a higher reactivity than the low enriched fuel at the outermost periphery, the output at the outermost periphery can be increased, so that the peaking at the center of the core can be relatively lowered. . The average core enrichment is 2.60 wt%, which is higher than that of the first embodiment, and further increase in the extracted burn-up degree can be expected.
[0049]
In the core of this embodiment, among the 764 fuel assemblies 13, 212 are low enriched fuel 13a, 276 are medium enriched fuel 13b, and 276 are highly enriched fuel 13c.
[0050]
FIG. 7 shows an example of a core in which 872 fuel assemblies 13 are loaded as a modification of the fourth embodiment of the present invention. In this case, the operation and effect are the same as those of the 764 loaded core shown in FIG.
[0051]
In this core, among the 872 fuel assemblies 13, the low enriched fuel 13a is 252, the medium enriched fuel 13b is 320, the highly enriched fuel 13c is 300, and the average enrichment is 2.57 wt%. Thus, the average enrichment of the core of the fuel loading pattern shown in FIG. 3 is higher than 2.44 wt%.
[0052]
FIG. 8 shows the fuel loading pattern of the core according to the fifth embodiment of the present invention. Compared with the first embodiment and the fourth embodiment, the core configuration at the center of the core is the same. However, the highly concentrated fuel 13c is loaded on the outermost periphery of the core.
[0053]
As a result, the power distribution can be made uniform, and a core that satisfies the thermal limit value and the furnace shutdown margin can be provided. This is because the power at the outermost periphery can be increased by arranging highly enriched fuel having a higher reactivity than the low enriched fuel and medium enriched fuel at the outermost periphery, so that the peaking at the center of the core is relatively lowered. By being able to.
[0054]
In this embodiment, among the 764 fuel assemblies 13, the low enriched fuel 13a is 212, the intermediate enriched fuel 13b is 184, the highly enriched fuel 13c is 368, and the core average enrichment is It is further higher than that of the fourth embodiment and becomes 2.75 wt%, and a further increase in the extraction burnup can be expected.
[0055]
FIG. 9 shows an example of a core in which 872 fuel assemblies 13 are loaded as a modified example of the fifth embodiment of the present invention. In this case as well, the 764 bodies of the fifth embodiment are loaded. The core has the same function and effect.
[0056]
In this core, among the 872 fuel assemblies 13, the low enriched fuel 13a is 252, the intermediate enriched fuel 13b is 228, the highly enriched fuel 13c is 392, and the average enrichment is 2.69 wt%. It is.
[0057]
FIG. 10 shows the fuel loading pattern of the core according to the embodiment of the sixth invention. The fuel assembly 13 in which the numeral 4 is written in the mass has the same enrichment as the highly enriched fuel 13c. The second highly concentrated fuel 13d of a type that has a high content of combustible poisons is shown.
[0058]
As shown in FIG. 10, in this embodiment, the arrangement configuration of the control cell 15a, the first cell 15b, and the second cell 15c is the same as that in the first embodiment, but the highly concentrated fuel is used. Two types with different contents of combustible poisons are prepared, and the combustible poison is contained in the center of the core at the position adjacent to the control cell 15a where the power control rod 17 is not inserted at the beginning of the first cycle. Instead of the high-concentration fuel 13c having a small amount, the second high-concentration fuel 13d having a large content of combustible poison is loaded.
[0059]
As a result, the power distribution around the cell where the control rod is not inserted can be suppressed with a type with a high content of flammable poisons. A core that satisfies the stop margin can be provided. The average enrichment of the core of this embodiment is 2.46 wt% because the number of loaded low-concentrated fuel, medium-enriched fuel, and highly-enriched fuel is the same as in the first embodiment.
[0060]
Such a fuel loading method can be used in combination with any of the above embodiments.
[0061]
【The invention's effect】
As described above, according to the present invention, in the reactor core composed of three types of fuel assemblies having different enrichments, the power distribution in the core is made uniform, and the thermal limit value and the reactor shutdown margin are satisfied. Therefore, it is possible to increase the average enrichment, and thus increase the take-out average burnup, thereby improving the fuel economy.
[Brief description of the drawings]
FIG. 1 is a fuel loading pattern diagram showing a core according to a first embodiment of the present invention.
FIG. 2 is a fuel loading pattern diagram showing a modification of the first embodiment of the present invention.
FIG. 3 is a fuel loading pattern diagram showing another modification of the first embodiment of the present invention.
FIG. 4 is a fuel loading pattern diagram showing a core according to a second embodiment of the present invention.
FIG. 5 is a fuel loading pattern diagram showing a core according to a third embodiment of the present invention.
FIG. 6 is a fuel loading pattern diagram showing a core according to a fourth embodiment of the present invention.
FIG. 7 is a fuel loading pattern diagram showing a modification of the fourth embodiment of the present invention.
FIG. 8 is a fuel loading pattern diagram showing a core according to a fifth embodiment of the present invention.
FIG. 9 is a fuel loading pattern diagram showing a modification of the fifth embodiment of the present invention.
FIG. 10 is a fuel loading pattern diagram showing a core according to a sixth embodiment of the present invention.
FIG. 11 is a plan view schematically showing a cell.
[Explanation of symbols]
11 ... Control rod 13 ... Fuel assembly 13a ... Low enriched fuel 13b ... Middle enriched fuel 13c ... High enriched fuel 13d ... Second highly enriched fuel 15 ...... Cell 15a ………………………………………………………………………………………………… ······················································································ Saichi

Claims

In the core where three types of fuel assemblies with different enrichments classified as low enrichment fuel, medium enrichment fuel and high enrichment fuel are loaded in order from the lowest enrichment, the fuel assembly around the cross-shaped control rod A cell composed of four bodies is a basic unit, and a control cell into which power control rods are inserted during the operation cycle is composed of four low-enriched fuels, and arranged in a lattice state every other center region of the core. In addition, the first cell adjacent to the control cell and not including the outermost peripheral portion of the core is composed of two medium-enriched fuels and two highly enriched fuels, and the outermost peripheral portion of the core is not adjacent to the control cell and A reactor core characterized in that a second cell not included is constituted by two low-enriched fuels, one medium-enriched fuel, and one highly-enriched fuel.

In the core where three types of fuel assemblies with different enrichments classified as low enrichment fuel, medium enrichment fuel and high enrichment fuel are loaded in order from the lowest enrichment, the fuel assembly around the cross-shaped control rod A cell composed of four bodies is a basic unit, and a control cell into which power control rods are inserted during the operation cycle is composed of four low-enriched fuels, and arranged in a lattice state every other center region of the core. In addition, the first cell adjacent to the control cell and not including the outermost peripheral portion of the core is composed of two medium-enriched fuels and two highly enriched fuels, and the outermost peripheral portion of the core is not adjacent to the control cell and A reactor core characterized in that a second cell not included is composed of two low-enriched fuels and two highly-enriched fuels.

In the core where three types of fuel assemblies with different enrichments classified as low enrichment fuel, medium enrichment fuel and high enrichment fuel are loaded in order from the lowest enrichment, the fuel assembly around the cross-shaped control rod A cell composed of four bodies is a basic unit, and a control cell into which power control rods are inserted during the operation cycle is composed of four low-enriched fuels, which are arranged in a lattice state every other core central region. In addition, the first cell adjacent to the control cell and not including the outermost peripheral portion of the core is composed of two medium-enriched fuels and two highly concentrated fuels, and the outermost peripheral portion of the core is not adjacent to the control cell and is not adjacent to the control cell. A reactor core characterized in that the second cell not included is composed of one low-enriched fuel, two medium-enriched fuels, and one highly-enriched fuel.

The core of a nuclear reactor according to any one of claims 1 to 3, wherein the highly concentrated fuels loaded in the first cell and the second cell are arranged so as not to be adjacent to each other.

The core of a nuclear reactor according to any one of claims 1 to 4, wherein the highly enriched fuel is disposed in a peripheral part of the core excluding the outermost peripheral part of the core, the control cell, the first cell, and the second cell. .

6. The nuclear reactor core according to claim 5, wherein the highly concentrated fuel is disposed on the outermost peripheral portion of the core.

6. The nuclear reactor core according to claim 5, wherein the intermediate concentrated fuel is disposed on the outermost peripheral portion of the core.

Highly concentrated fuel is divided into two types, one with low flammable poison content and one with high flammable poison content, and the output control rod is adjacent to the control cell that is not inserted in the first cycle. The core of a nuclear reactor according to any one of claims 1 to 7, wherein the highly concentrated fuel is a type having a high content of flammable poisons, and the other highly concentrated fuel is a type having a low content of flammable poisons. .