1.1
This test method covers procedures for measuring reaction rates by assaying a fission product (F.P.) from the fission reaction
237
Np(n,f)F.P.
1.2
The reaction is useful for measuring neutrons with energies from approximately 0.7 to 6 MeV and for irradiation times up to 90 years, provided that the analysis methods described in Practice
E261
are followed. If dosimeters are analyzed after irradiation periods longer than 90 years, the information inferred about the fluence during irradiation periods more than 90 years before the end of the irradiation should not be relied upon without supporting data from dosimeters withdrawn earlier.
1.3
Equivalent fission neutron fluence rates as defined in Practice
E261
can be determined.
1.4
Detailed procedures for other fast-neutron detectors are referenced in Practice
E261
.
1.5
The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.6
This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.
1.7
This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
====== Significance And Use ======
5.1
Refer to Practice
E261
for a general discussion of the determination of fast-neutron fluence rate with fission detectors.
5.2
237
Np is available as metal foil, wire, or oxide powder. For further information, see Guide
E844
. It is usually encapsulated in a suitable container to prevent loss of, and contamination by, the
237
Np and its fission products.
4
5.3
One or more fission products can be assayed. Pertinent data for relevant fission products are given in
Table 1
5
and
Table 2
.
(A)
The lightface numbers in parentheses are the magnitude of plus or minus uncertainties in the last digit(s) listed.
(B)
With
137m
Ba (2.552 min) in equilibrium.
(C)
Probability of daughter
140
La decay.
(D)
With
140
La (1.67850 d) in transient equilibrium.
(E)
Primary reference for half-life, gamma energy, and gamma emission probability is Ref
(
1
) when data is available. Note this reference is to the BIPM data that was recommended at the time of the recommended fission yields were set, that is, as of 2009, and not to the latest Vol 8 data that was published in 2016.
(A)
The JEFF-3.1/3.1.1 radioactive decay data and fission yields sub-libraries, JEFF Report 20, OECD 2009, Nuclear Energy Agency
(
2
).
(B)
All yield data given as a %; RC represents a cumulative yield; RI represents an independent yield.
(C)
The neutron energy represents a generic “fast neutron” spectrum and has been characterized in the JEFF 3.1.1 fission yield library as having an average neutron energy of 0.4 MeV.
5.3.1
137
Cs-
137m
Ba is chosen frequently for long irradiations. Radioactive products
134
Cs and
136
Cs may be present, which can interfere with the counting of the 0.661657 MeV
137
Cs-
137m
Ba gamma ray (see Test Methods
E320
).
5.3.2
140
Ba-
140
La is chosen frequently for short irradiations (see Test Method
E393
).
5.3.3
95
Zr can be counted directly, following chemical separation, or with its daughter
95
Nb, using a high-resolution gamma detector system.
5.3.4
144
Ce is a high-yield fission product applicable to 2- to 3-year irradiations.
5.4
It is necessary to surround the
237
Np monitor with a thermal neutron absorber to minimize fission product production from trace quantities of fissionable nuclides in the
237
Np target and from
238
Np and
238
Pu from (n,γ) reactions in the
237
Np material. Assay of
238
Pu and
239
Pu concentration is recommended when a significant contribution is expected.
5.4.1
Fission product production in a light-water reactor by neutron activation products
238
Np and
238
Pu has been calculated to be insignificant (1.2 %), compared to that from
237
Np(n,f), for an irradiation period of 12 years at a fast neutron (
E
> 1 MeV) fluence rate of 1 × 10
11
cm
−2
·s
−1
, provided the
237
Np is shielded from thermal neutrons (see Fig. 2 of Guide
E844
).
5.4.2
Fission product production from photonuclear reactions, that is, (γ,f) reactions, while negligible near-power and research reactor cores, can be large for deep-water penetrations
(
3
).
5.5
This dosimetry reaction is important in the area of reactor retrospective dosimetry
(
4
,
5
)
. Good agreement between neutron fluence measured by
237
Np fission and the
54
Fe(n,p)
54
Mn reaction has been demonstrated
(
6
,
7
).
The reaction
237
Np(n,f) F.P. is useful since it is responsive to a broader range of neutron energies than most threshold detectors.
5.5.1
Fig. 1
shows the energy-dependent cross section for this dosimetry reaction. The figure shows that, while it is not strictly a threshold detector, because of its sensitivity in the greater than 0.1 MeV neutron energy range it can function as a detector with good sensitivity in the fast neutron region. In the fast fission
252
Cf spontaneous fission benchmark field, ~1 % of the
237
Np fission dosimeter response comes from neutrons with an energy less than 0.1 MeV. In the cavity of a fast burst
235
U reactor, ~5 % of the
237
Np ifssion dosimeter response comes from neutrons with an energy less than 0.1 MeV. In the cavity of a well-moderated pool-type research reactor ~50 % of the fission response from the
237
Np(n,f) reaction comes from energies less than 0.1 MeV. The importance of this low neutron energy sensitivity should be determined based on the aplication.
5.6
The
237
Np fission neutron spectrum-averaged cross section in several benchmark neutron fields are given in Table 3 of Practice
E261
. Sources for the latest recommended cross sections are given in Guide
E1018
. In the case of the
237
Np(n,f)F.P. reaction, the recommended cross section source is the Russian Reactor Dosimetry File, RRDF
(
8
)
. This recommended cross section is identical, for energies up to 20 MeV, to what is found in the latest International Atomic Energy (IAEA) International Reactor Dosimetry and Fusion File, IRDFF-1.05
(
9
)
.
Fig. 1
shows a plot of the recommended cross section versus neutron energy for the fast-neutron reaction
237
Np(n,f)F.P.
FIG. 1
RRDF/IRDFF-1.05 Cross Section Versus Energy for the
237
Np(n,f)F.P. Reaction
到馆提醒
收藏跟踪
分享链接
购买正版
分类信息
首页
馆藏资源
舆情信息
标准服务
科研活动
关于我们
️版权归中国标准化研究院所有