水の放射線分解と水素 (H2)
東京大学 名誉教授

勝村庸介

背景
原子力では放射線は必ず付随 -> 放射線効果の発現
多くはプロセスに悪影響 -> 放射線効果の研究が重要

本発表の内容 : 水の放射線分解の歴史 他
1. 水の放射線分解と線質効果の発見
Ra による水の放射線分解 : 水素の発生
X 線照射では水素発生が見られない ?
Ra 照射と X 線照射 : 線質効果
2. 10B(n,α)7Li による水分解の水素発生を水素で抑制
PWR 水素注入、水素抑制機構
BNCT、 10B(n,α)7Li による水分解G値
3. 沸騰水の放射線分解: F1事故との関連
沸騰条件下の水素の挙動

水の放射線分解と線質効果の発見
Ra による水の放射線分解 : 水素の発生
X 線照射では水素発生が見られない ?
Ra 照射と X 線照射 : 線質効果

Ra による水の放射線分解 : 水素発生
Ra の発見と分離: キュリー夫妻 (1898)
各種放射線効果(1)各種ガスのイオン化、(2)飽和水蒸気からの霧の発生、(3)ガラスや磁器の着色、(4)食卓塩の着色、(5)紙の劣化、(6)パラフィンや有機物の結晶の低下、など

C. R. Acad. Sci. Paris, 129, 823 (1899)
活性なラジウム塩からの放射線が酸素をオゾンに変換する。
C. R. Acad. Sci. Paris, 132, 768 (1901)
ラジウム塩水溶液からの水素と酸素の発生観測:
Pierre Curie & Andre Debierne.
H2+O + OH- = H2O + OH
H 2- O + H + = H + H 2O
2OH = H2O2

水の放射線分解の最初の観測 !!

X 線照射では水素発生が見られない ?
Ra は高価強力な X 線源の開発と利用 (1920 年代)
Ｘ 線照射よる水分解実験(脱気純水)(1) ガス発生は観測できない
O. Risse; in Z. Phys. Chim., 140A, 133 (1929)(2) 確認報告
H. Fricke and E. R. Brownscombe“Inability of X-rays to Decompose Water”
in Phys. Rev., 44, 240 (1933)

Ra 照射と X 線照射 : 線質効果
1901 Curie & Debierne
Ra塩水溶液の連続的ガス発生
水の分解
実験的な大きな矛盾
1920年代後半
水溶液のX線照射ではガス発生は観測されず。
水の分解はない
異なる放射線により異なった
水の放射線効果: 線質効果
Gvalues

-H2O

e-aq

OH

H

H2O2

H2

HO2

g -ray

4.1

2.7

2.8

0.56

0.68

0.45

~0.01

a -ray

2.65

0.06

0.24

0.21

0.985

1.3

0.22

水素発生を水素添加で抑制する
水素抑制機構
BNCT(Boron Neutron Capture Therapy)
10B(n,α)7Li による水分解

水素発生を水素添加で抑制する
E. J Hart,
W. R. McDonell & S. Gordon

10B(n,α)7Li 反応

No H2 formation
above CHC !!

活用例
n PWR 原子炉 水素添加運転
H3BO3 添加量による水素発生量の水素添加依存性

n 中性子捕捉療法: BNCT
boron neutron capture therapy

H3BO3 濃度増加により水素ガスの発生率は増大する。ところが、これに水素添加すると発生量が抑制され、ある濃度(臨界濃度 CHC) 以上では水素発生は完全に抑えられる。
Int. Conf. on the Peaceful Uses of Atomic Energy, Geneva, 8 - 20 August, 1955

Edwin D. Hart 1910 –1995

Argonne National Laboratory:
1948 ⼊所 (Fricke の弟⼦)、1975 退職
⽔和電⼦の発⾒ (1962)
J. Am. Chem. Soc., 84, 4090-4095 (1962)

⽔和電⼦温度変化 (1967)

J. Phys. Chem. 71, 2102-2106 (1967)

CO3-

e-aq

HO2 → H + + O2 −+ H2 O2 → O2 + OH− + OH

7.00 × 105 s−1 /5.17 × 1010
16

水素抑制機構1.55 × 105 s−1 /5.71 × 1011–

水素注入で酸化生成物を水に変換

酸化雰囲気から
還元雰囲気へ

水素注入の原理
H2

OH

H2O

H2O

H

H2O2
水素注入で酸化性

H2 詳細は+ OH ®
+ H 2OP.HLertnaisat,
et al. 生成物を水に変換
J. Nucl. Sci. Technol., 51, 1087 (2014)
H + O2 ® HO2

The radiolysis of water is closely linked to the corrosion ofB(n,a) Li reaction as a function of temperature f

distribution of reactants formed at the end of the physico"+"
structural
materials.
astheneutron350
C.
chemical stage[eaq", HWater,, OH", which
Hc, cOH,isHused
,
H
O
,
HO
c/O
c
,
2
2 2
2
2
1
moderator
thereactor coolant, is unavoidably exposed to
The paper is organized as follows. The main featucOc(3P), O(andD), O2,.], which is provided as an output of the"extremeof highC),forpres- simulation approach are given in the next section.
IONLYS conditions
program, is thenusedtemperature
directly as the(!275–325
starting point
31 neutron capture therapy)
nthe中性子捕捉療法:
BNCT(boronsubsequent“nonhomogeneous
chemical”
stage(typically,sure(!7–15 MPa),
and intense
mixed
neutronand b–g and 4 present, respectively, the results of our simulat#
10
from 10!1
to the ms time scale at 25 C). This third stage,
B(n,a)7Li radiolysis of water at neutral pH and of 0.
を含む薬剤をガン組織に選択的に吸収させ、熱中性子との反応でBps
during which the diﬀerent 4radiolytic species
randomly
7Lidiﬀuse
aqueous(高solutionsat 短い飛跡
25 " C and as
a function of temp
高
放射線の
イオンと
イオンでガン組織を照射
LET
He
LET
&
)
at rates determined by their diﬀusion coeﬃcients and react withto 350 " C, and their discussion. Conclusions are dra
one another (or with dissolved solutes, if any) until all tracknal section.
processes are complete, is covered by our IRT program. ThisA brief preliminary report of this work has been
program employs the “independent reaction times” (IRT)
elsewhere.26
method,32–34 a computationally eﬃcient stochastic simulation
technique that is used to simulate reaction times without
4
He イオンhaving to follow the trajectories of the diﬀusing species. The
IRT method relies on the approximation that the reaction time
7
Li イオン
短い飛跡
of each pair of reactants is independent of the presence of otherreactants in the system. Its ability to give accurate
time-放射線
低LET
Paper
dependent chemical yields under diﬀerent irradiation
The entire sequence of events generated in the ra
LET:condi0.23 eV/nm
tions has been well validated by comparison with full randomliquid water by 10B(n,a)7Li recoil ions was modeledights (or step-by-step) Monte Carlo simulations, which do
35,36
Fig.
1 Schemeof thetrajectories
nuclear reaction
resultingfromtheprogram
low-energy Monte Carlo track chemistry simulation code calle
follow
the reactant
in detail.
This
IRT
(<0.5
thermalcaptureby a 10Bthe
atom.
After absorption,
94% IRT. This computer program simulates, in a 3D g
can eV)also
usedneutron
to eﬃciently
describe
reactions
that
occur
高LET
7 be
4
7
Li イオンleave the 7Li ion in He
イオン
its
ﬁrst
excited
state
(
Li*)
which
ofinthethe
reactionsenvironment,
the highly nonhomogeneous distribulk solution during the “homogeneous chemical”
rapidly de-excites
to the ground state by releasing a 478 keV g-ray. For reactive species initially produced by the absorption ostage,31 i.e., in the time domain beyond a7few microseconds. The
the remaining 6% of the reactions, the Li ion is left directly in its
radiation and all of the energetic secondary electronsmodel assumptions and procedures employed to carry out the
ground state resulting in the emission of a 1.78 MeV a-particle and4
イオンchemical reactions
He andthesubsequentdiﬀusion
Monte
Carlo
simulationsoftheradiolysis
ofaqueous0.4
M
74
7
a 1.02 MeV Li ion. Note that the He and Li recoil ions are in opposite
H2SO4 solutions(pH" !
0.46)awaywith fromIONLYS-IRT
have
directions(i.e., at a 180
angle),
the site ofthealreadycompound species. A detailed description of the current version o
7
37,38
イオン
been given.
nucleus,and hence they form one straight track.
at both ambientLi
andelevated temperatures and undeIn the current version of IONLYS-IRT, we used the selfconsistent radiolysis database, including rate constants anddiﬀusioncompiled
Elliot and Bartels.20
Thisjournal iscoeﬃcients,© The Royalrecently
Society ofChemistryby2017
RSC Adv., 2017, 7, 10782–10
This new database provides recommendations for the best

10B(n,α)7Li による放射線治療

2. Monte Carlo track chemistry
simulations

a well dened LET. The number of individual ion “histories”(usually !2–100, depending on the irradiating ion and energy)

mm. Thus, its contribution to the overall chemical reaction was
neglected in this study.

10B(n,α)7Li による水の放射線分解
e--aq
aq

OH

H

低LET

低LET
低LET
10

10

10

B(n,α)7Li

B(n,α)7Li
低LET

H2O2

B(n,α)7Li

H2 10B(n,α)7Li

低LET

10

B(n,α)7Li○ A. O. Allen, The Radiation
Chemistry of Water and Aqueous
Solutions, D. Van Nostrand Co.,
Princeton, NJ, 1961.
● H. Christensen, Fundamental
aspects of water coolant radiolysis,
SKI Report 2006:16, Swedish Nuclear
Power Inspectorate, Stockholm,
Sweden, April 2006.

G-values (in molecule per 100 eV) for the 10B(n,a)7Li radiolysis of pure, deaerated liquid water as a function of temperature in the range of
25–350 ' C: (a) G(eaq"); (b) G(cOH); (c) G(Hc); (d) G(H2O2); and (e) G(H2). Our simulated results, obtained at 10"7 and 10"6 s, are shown as solid,
blue and red lines, respectively. Symbols are the water decomposition yields induced by the 10B(n,a)7Li reaction estimated by Cohen (ref. 15) at
20 ' C (based on the approximate relationship between LET and G-values given in Fig. 5.3 of Allen (ref. 43), using an average initial LET of 240 eVnm"1) (B) and by Christensen (ref. 19) at 289 ' C (C). The primary (or “escape”) yields for the low-LET (!0.3 eV nm"1) radiolysis of water (ref. 29)
obtained using our previously calculated spur lifetimes between 25–350 ' C (ref. 44) are also given (black dashed lines) for comparison purposes.
Note that all yield curves shown in this ﬁgure were obtained under exactly the same conditions as in ref. 29 as far as the temperature dependences of the diﬀerent parameters intervening in the early physicochemical stage (e.g., the electron thermalization distance – called rth in ref. 29)
Fig. 3

実験に基づく水分解G値の報告はない。

上記は Monte Carlo Simulation による評価である。
詳しくは、M. M. Islam, et al. RSC Advances 7, 10782 (2017)

沸騰水の放射線分解F1事故との関連
沸騰条件下の水素の挙動

沸騰水の放射線分解で用いた装置中のH2 と O2 濃度の挙動変化

Physics and Chemistry 123 (2016) 14–19

mura
tive,
es of

dee of
ume
the
We
gas,
istic
renismrepureocol
peri-exMildure
ded
e.g.,

up is
ssel,

water. In
the expe
O2 in theThese ex
H2O degyield oftemperalonger p
increasecluded t
water. Tprocesse
sufﬁcien
impurity
organic
the evo
troduced
H2 and O
67 Gy/mcycle, thAfter seand O2 imost 2:1
trace am
あるいは
装置下部
1cc
のH
O
注入時
Fig. 2. The “H2Septum(boiling)” and(1)“O2 から(boiling)”
indicatethechange
of
the
H
and
O
2
2
2
2
during t
gas concentrations measured at the upper sampling. Port: septum (2) after injecの、Septum (2) で採取したガス相中濃度の時間変化 concent
tion of 1 mL H2 and O2 gas, respectively, under boiling condition. The “H2 (nondence, mboiling)” indicates the concentration change measured after 1 mL H2 injection
と
は
1.
沸騰下では
H
O
Septum(2)で濃縮
22experienunder non-boiling condition.

H2 and O2 are nearly identical; these values are directly proportional to the amount of gas injected, which enables us to determine the absolute amount of each evolved gas. For comparison,
we injected H2 into the gas phase above non-boiling room-temperature water. The concentration of H2 increased very slowly andreached steady state 20–30 min after the injection. The steady15

40°C
Cooling
water

単位はmm

ソックスレー抽出器

2. 濃縮速度はH2 の方が O2 より速い:
3. 非沸騰下ではガス相で均一分布
4. 沸騰下でも H2O2 は安定に存在

Mw(H2 / O2 / H2O)
= 2 / 32 / 18

沸騰下の H2 と O2 Septum (2) でのガス相濃度測定から、発生ガス量を定量できる。

Fig. 5. The evolution of H2 and O2 gas in boiling 0.5 M NaCl aqueous solution
Fig. 3. The evolution of H2 and O2 gases measured at the upper sampling port as ameasured at upper sampling point as a function of dose under the dose rate offunction of irradiation time under the dose rate of 67 Gy/min. The cycle of irra67 Gy/min. For comparison, the gas evolution data obtained in pure water are also
diation for 15 min and the measurement for 15 min was repeated. The “H2 (boilshown.
ing)” and “O2 (boiling)” are the results measured with boiling water and “H2 (nonboiling)”
18 is the results measured with non-boiling still water.
S. Yang et al. / Radiation Physics and Chemistry 123 (2016) 14–19

沸騰水放射線分解による H2 と O2 発生
3.4. Gas evolution in the presence of bubbling Ar gas

G(H2) = 0.28, G(O2) = 0.13

化学量論的に水素、酸素
ガス発生

Gas evolution measurements were performed while Ar gas was
bubbled through the water at room temperature. This experiment
investigated the effect of the surface area (between the liquid andgas phases) on gas evolution. Ar bubbles were introduced into the
1 次反応
irradiation vessel using a ball ﬁlter;the vessel was ﬂushed with Argas, which was circulated with a diaphragm pump. A typical experimental result is shown in Fig. 6. The concentration of H2 gas
increased linearly, but no O2 evolved. The lack of O2 is probably
due to the contamination of the water sample. Metal tubes were
employed as a component of the gas circulation line; contamination was unavoidable. Furthermore, the G-value of H2 evolution in
H2O, G(H2) ¼0.80, is larger than that, G(H2)¼0.63, in 0.5 M NaClsolution, which is opposite to the ﬁnding under boiling condition.
Further experiment is needed to clarify this contradiction. Nevertheless, this result indicates clearly that increasing the interface
between water and gas enhances the gas evolution, because gas
evolution from the irradiation of still water is very slow and small
as already shown in Fig. 3.

低線量率で
発生増大

(A) 線量率: 67 Gy/min

G(H2) = 0.2, G(O2) = 0.09

n Physics and Chemistry 123 (2016) 14–19

rt as af irra(boil-

2 次反応(B) 線量率: 15 Gy/min

17

Fig. 7. The simulated data of gas evolution of H2 and O2 at G1 and G2 under the
Fig. 4. The evolution of H2 and O2 gases measured at upper sampling point as adose rate of 67 Gy/min for successive measurement for 15 min after irradiation offunctionof dose under different dose rates of 67 and 15 Gy/min at A and B position,
15 min.
線量率:
Gy/minrespectively. It is noted
that67
bothdata measured at A and B positions were plotted
after the second 15-min irradiation. The data of the ﬁrst 15-min irradiation was
0.5
NaCl Simulation curves with the FACSIMILE code
removed due to the effectof M
impurity.
the space above
the
water
(G1), and the space at the gas
"
1
G(H
)
=
0.48,
G(O
) sample= 0.21
2
by using a value of 0.10 sfor kLG12are
also ﬁtted.
pure H2O

sampling point (G2). The ﬁrst order rate constants, kLG1(H2) andG(H2) = 0.2, G(O2) = 0.09
kLG1(O2), were applied to the expressions for gas transport from L
dosetoatG1
position
The gas yields
wereenhancedby lowerat the A.
liquid/gas
interfaceforclearlyboth H
2 and O2. Both rate
doseconstants
rates, as are
shown
in
Fig.
4.
Apparent
respective
G-values
2adjustable parameters and were assumed to ofbeHtheand sameO2 evolution
weredetermined
to
be
0.2
and
0.09
at
positionA
for simplicity. The ﬁrst order transport from G1 to G2 isand described0.28 and 0.13
at position(H2) andB.kG1G2(O2), and the respective values,
by kG1G2
1.14 ! 10 " 2 and 2.57 ! 10 " 3 s " 1, were derived from the experi3.3. ment
Effectexplainedof NaCl additionin Fig. 2 (for boiling water). Since kLG1 was assumed to be the same for H2 and O2, there is only one adjustableInthe Fukushima
Nuclearwas injected as anparameter.
A valueof 0.10Accident,s " 1 for kseawater
LG1 was determined from theemergency
coolant;
itisinterestingtocomparethe gasevolution
calculations. A typical simulation for the temporalchangein gasin seawater
with
that
in
pure
water.
Thegas
evolutionwasmeaconcentration for the rate constants described above is shown insured
a 0.5HM
NaCl aqueous solution as a model for seawater;
Fig.from
7. Both
2 and O2 increased with irradiation, and the O2 yield
results
are
shown
in
5. O
Surprisingly,
evolutionnearlyFig.
5.
Theevolutionofandboiling
0.5gas
Mdue
NaCl
solution
2than
2 gas
yield
toaqueous
the relatively
was slightly lowerHFig.
half
of intheH2 themeasuredat
uppersampling
point
asafunction
of
dose
under
the
dose
doubledwhen NaClof
wasto pure water.
The G-values
H2of
The steady-state
concentrationof H2ofOrate
slow convection
O2. added
2 was67
Gy/min.
Forcomparison,thegasevolutiondataobtained
in
pure
water
are
also
and O formation were determined to be 0.48 and 0.21, respec-

NaCl 添加によるガス発生増大3.5. Modeling the radiolysis of boiling water

Cl- + OH ® ClOH-

The deterministic chemical kinetics simulation was used toreproduce the experimental results and clarify the mechanism ofgas evolution. Three spatial regions were deﬁned: liquid water (L),Fig. 8. The scheme of H2 and O2 evolution in the radiolysis of boiling water;(a) boiling pure water, and (b) boiling 0.5 M NaCl aqueous solution.

詳しくは、
S. Yang et al., Radiat. Phys. Chem., 123, 14 (2016)
report by Kelm and Bohnert (2004), OH will react towards

アクリル箱を用いた水素注入実験
実験
非沸騰下と沸騰下で，水槽に100 mL の水素を注入した
時の濃度変化を測定
100 L（50 50 40 cm）のアクリル容器

3L
800 W

非沸騰下注入水素の均一化

実験結果
非沸騰下では，上部と底部の，各々①と⑭ のサンプリン
グポイントで12-13分経過すると0.1 % 濃度に到達し，全空間に均一分布した水素濃度に対応している。
一方，沸騰下では，側壁から5 cm 程度はなれた部分でサンプリングした結果，注入30 秒後には2-3 倍，60
分後は2-6倍程度に濃縮していることが判った。 16

鉄箱を用いた水素注入実験
実験
沸騰下で，水槽に100 mL の水素を注入後の濃度変化
を測定

実験結果
サンプリングポイント②, ⑤,⑧での水素濃度の経時変化
初期60分間程度は②,⑤での濃縮が進行するが、それ以降は⑧での濃縮が著しく、均一分布の90倍以上となる。

⑧

⑤

⑧

②
17

まとめ
• 線質効果低LET放射線 G(e-aq, H, OH)> G(H2, H2O2)
高LET放射線 G(e-aq, H, OH)< G(H2, H2O2)• 水素添加効果
連鎖反応で高LET放射線のガス発生を水素添加で抑制• 沸騰水の放射線分解
G(H2) : G(O2) ~ 2 : 1
H2 & O2 ガス発生: 低線量率の方が発生 G 値は大きい
NaCl 水溶液: H2 & O2 ガス発生量増大
蒸気中の H2 挙動: 濃縮の可能性 / 事故時の挙動

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