**LANSCE Isotope Production Facility---**

**Utility Chase Study**

**Kenneth A. Van Riper**

**White Rock Science Report 99--14**

**February 26, 2000**

![](media/image1.wmf){width="5.998611111111111in"
height="5.998611111111111in"}

Cutaway View of the IPF Tunnel and Utility Chase

# Summary

We made a Monte Carlo transport calculation of beam spill neutron and
photon radiation from the IPF tunnel to the top of a utility chase
between the tunnel and the upper support room. The spill results from a
500 μampere, 100 MeV proton beam striking a stainless steel plug in the
beam line. A series of calculations for different plug positions shows
that the maximum radiation level in the lower chase opening occurs when
the plug is centered on the opening. With the plug in this position, the
dose equivalent rate one foot above the upper chase opening is 11
rem/hour with a relative error of 15%. The model assumed an empty chase
without power cables or other equipment.

# Introduction

The Isotope Production Facility (IPF) project will build a new beam line
on the north side of the existing LANSCE accelerator building at Los
Alamos National Laboratory. The beam line leads through a tunnel to a
target in which materials will be irradiated for radioisotope
production. Cables supplying magnet power run through a utility chase
from the upper support room to the tunnel. Were a beam spill to occur in
the tunnel, the chase provides a path for radiation to reach the upper
room. Special precautions would be required at the top of the chase were
the Dose Equivalent Rate (DER) from such a spill to exceed 25 rem/hour.

# Calculation Models

A previous report (White Rock Science Report 99--11, referred to here as
Report 1) considered various accident scenarios during construction and
operation of the IPF. We omit here items detailed in that report such as
validation of the MCNPX code, material compositions, and dose response
functions.

## MCNPX Code

All calculations were made with the Monte Carlo transport code MCNPX
from LANL group XTM. We used the latest version, MCNPX2.1.5, which was
released on May 21, 1999. MCNPX transports protons, neutrons, photons,
and other particles simultaneously. A number of evaluated data files
provide accurate neutron cross sections for neutron energies up to 150
MeV. The calculations were run on a Silicon Graphics Inc. INDY
workstation running IRIX 6.5. More details can be found in Report 1.

## Materials

The materials used in the model are tuff (Pajarito Plateau soil),
ordinary concrete (4000 psi mix), air, stainless steel (SS--304), and
vacuum. The concrete and tuff compositions are based on those given in
LANL memorandum ESH--12:99--100 from Russ Durrer to Richard Heaton.
Compositions for air, SS--304 stainless steel, and vacuum are taken from
previous models set up by LANSCE personnel. Report 1 contains Tables
detailing the material compositions and the densities at which the
materials were used in the model.

## Dose Response Functions

MCNPX surface (F2) and volume (F4) tallies are stored internally in
units of particles cm^-2^. Dose response functions convert from these
units to a DER in rem/hour.

For neutrons we used values from NCRP--38[^1], also known as
ANSI/ANS-6.1.1-1977 (kindly supplied by Jeffrey Bull). The photon
response function is also from ANSI/ANS-6.1.1-1997 (also supplied by
Bull). Report 1 includes tabulations of the response functions.

## Variance Reduction

We use the technique of importance splitting to concentrate the
population of neutrons and photons in cells that contribute the tallies.
(Splitting was not applied to protons.) We used 12 layers of importances
through the chase. The importances increased by a factor of two from
layer to layer. In the air cells within the chase (except for the
lowermost volume L---see Figure 5), we applied the forced collision
technique to the neutrons and photons. Each particle that enters a cell
undergoes at least one interaction in that cell, enhancing the
probability that some will scatter upwards towards the top of the chase.

We present tables of the DER at various positions through the model. In
addition to the total DER, we also give the neutron and photon
components. The Monte Carlo accuracy is given by the *Relative Error*
(RE). The MCNP manual characterizes tallies with REs of 0.1 or less as
"generally reliable", those with REs between 0.2 and 0.5 as within a
"factor of a few", and any with a larger error as "garbage". The tables
give the results to 3 or 4 significant digits; the results are not
necessarily known to that accuracy.

# Utility Chase Model

The model includes the IPF tunnel, the utility chase, and a 2 foot layer
of tuff around the tunnel and 3 sides of the chase. Figure 1 is a side
view of the tunnel. The left (upstream) edge is a 2 foot section of 12
foot thick shield wall. To the right of the model is the IPF target room
(not included in the model). The top of the chase opening is 9 inches
below the tunnel ceiling. The chase is surrounded on all sides by 9
inches of concrete. To the left of the chase is a bank of conduits
encased in concrete. We modeled this bank together with the left
concrete lining by treating the combination as a 4 foot 9 inch thick
layer of solid concrete. Figure 2 is a cross section through the tunnel
and chase. The model includes 2 feet of air above the upper chase
opening into the support room. Figure 3 is a horizontal cross section
through the rise of the chase.

The beam line is a 4 inch outside diameter stainless steel tube with
wall thickness of 0.065 inches. The beam tube interior is vacuum. The
spill is initiated by placing a stainless steel plug in the path of the
pencil proton beam at the tube center. The plug is a cylinder, coaxial
with the beam tube, of 2 inch outside diameter and 2 inches thick, as
shown in Figure 4.

![](media/image2.wmf){width="5.989583333333333in"
height="4.668055555555555in"}

Figure 1---Side view of the IPF tunnel model showing the position of the
utility chase.

# Spill Position

A beam spill may occur anywhere along the beam line. To find the spill
position that will result in the highest DER in the chase, we considered
several spill positions and calculated the DER in the lower chase
opening (volume L in Figure 5). Table 1 gives the results. The distance
is from the center of the chase opening (shown as the plug position in
Figure 1). Negative distances are upstream (to the left in Figure 1).
The most negative distance corresponds to a plug 5 inches from the face
of the shield wall.

The DER in the opening is insensitive to plugs positioned between 7 feet
upstream and 4 feet downstream of the center of the opening. The maximum
rate occurs for a plug centered on the opening. We used that position
for the calculation of radiation up the chase.

![](media/image3.wmf){width="5.6819444444444445in"
height="8.578472222222222in"}

Figure 2---Cross section through the utility chase model.

![](media/image4.wmf){width="3.83125in" height="3.2465277777777777in"}

Figure 3---Horizontal cross section C through the utility chase.

![](media/image5.wmf){width="4.541666666666667in"
height="4.773611111111111in"}

Figure 4---Detail of the cylindrical beam plug.

  ------------ ------------- -------- ------------ --------- ------------ ----------
  **Distance   **Neutron**            **Photon**             **Total**    
  (feet)**                                                                

               **DER         **RE**   **DER        **RE**    **DER        **RE**
               (rem/hr)**             (rem/hr)**             (rem/hr)**   

  -9.42        5.682E+04     0.128    1.217E+03    0.124     5.804E+04    0.125

  -8.42        6.041E+04     0.114    1.271E+03    0.126     6.168E+04    0.112

  -6.65        7.516E+04     0.106    1.615E+03    0.112     7.677E+04    0.104

  -5.00        8.024E+04     0.094    1.703E+03    0.104     8.194E+04    0.092

  -3.36        7.539E+04     0.108    1.842E+03    0.098     7.724E+04    0.105

  -1.72        6.886E+04     0.096    2.112E+03    0.092     7.097E+04    0.093

  0.00         8.146E+04     0.094    1.896E+03    0.097     8.335E+04    0.092

  1.72         7.523E+04     0.093    1.864E+03    0.093     7.710E+04    0.091

  3.36         6.658E+04     0.101    2.040E+03    0.099     6.862E+04    0.098

  5.00         5.783E+04     0.108    2.067E+03    0.096     5.990E+04    0.104
  ------------ ------------- -------- ------------ --------- ------------ ----------

Table 1---DER in the lower chase opening as a function of distance from
the opening.

# Results

## Tally Volumes

We tallied the neutron and photon DER in several in air volumes
throughout and above the chase. We also made a tally on a surface one
foot above the upper chase opening and directly above the chase. Figure
5 shows the tally volumes and surface in a cross section of the chase.
Figure 6 is a horizontal section through the air space above the chase.
The extents of volumes A and B and of surface S are the same as the air
space in the chase. Volume M is the 9 inches of air surrounding those
volumes.

## Dose Equivalent Rates

We followed 20,000,000 incident protons. Table 2 lists the total DER and
the neutron and photon components. The tabulated distance is from the
floor level of the support room to the center of the volume or surface.

![](media/image6.wmf){width="3.5506944444444444in"
height="8.477777777777778in"}

Figure 5---The volumes and surface (S) on and in which tallies were
taken.

![](media/image7.wmf){width="4.007638888888889in"
height="3.1944444444444446in"}

Figure 6---Horizontal section through the tally volumes above the
utility chase.

+---------+------+---------+-----+---------+-----+---------+-------+
| **D     | **   | **Ne    |     | **P     |     | **      |       |
| istance | Volu | utron** |     | hoton** |     | Total** |       |
| Below   | me** |         |     |         |     |         |       |
| Top of  |      |         |     |         |     |         |       |
| Chase   |      |         |     |         |     |         |       |
| (       |      |         |     |         |     |         |       |
| feet)** |      |         |     |         |     |         |       |
+---------+------+---------+-----+---------+-----+---------+-------+
|         |      | **DER   | **R | **DER   | **R | **DER   | *     |
|         |      | (re     | E** | (re     | E** | (re     | *RE** |
|         |      | m/hr)** |     | m/hr)** |     | m/hr)** |       |
+---------+------+---------+-----+---------+-----+---------+-------+
| -1.50   | > A  | 8.89    | 0.  | 0.605   | 0.  | 9.49    | 0.158 |
|         |      |         | 168 |         | 140 |         |       |
+---------+------+---------+-----+---------+-----+---------+-------+
| -1.00   | >    | 9.99    | 0.  | 0.663   | 0.  | 10.6    | 0.150 |
|         | S^a^ |         | 160 |         | 129 |         |       |
+---------+------+---------+-----+---------+-----+---------+-------+
| -0.50   | > B  | 10.8    | 0.  | 0.749   | 0.  | 11.6    | 0.139 |
|         |      |         | 149 |         | 115 |         |       |
+---------+------+---------+-----+---------+-----+---------+-------+
| 2.00    | > C  | 17.7    | 0.  | 1.48    | 0.  | 19.2    | 0.098 |
|         |      |         | 106 |         | 065 |         |       |
+---------+------+---------+-----+---------+-----+---------+-------+
| 5.75    | > D  | 33.6    | 0.  | 2.88    | 0.  | 36.5    | 0.069 |
|         |      |         | 074 |         | 043 |         |       |
+---------+------+---------+-----+---------+-----+---------+-------+
| 9.25    | > E  | 62.3    | 0.  | 5.50    | 0.  | 67.8    | 0.050 |
|         |      |         | 054 |         | 028 |         |       |
+---------+------+---------+-----+---------+-----+---------+-------+
| 12.75   | > F  | 131\.   | 0.  | 12.1    | 0.  | 143\.   | 0.034 |
|         |      |         | 037 |         | 022 |         |       |
+---------+------+---------+-----+---------+-----+---------+-------+
| 16.25   | > G  | 334\.   | 0.  | 29.1    | 0.  | 363\.   | 0.023 |
|         |      |         | 025 |         | 017 |         |       |
+---------+------+---------+-----+---------+-----+---------+-------+
| 19.00   | > H  | 828\.   | 0.  | 63.7    | 0.  | 892\.   | 0.020 |
|         |      |         | 021 |         | 015 |         |       |
+---------+------+---------+-----+---------+-----+---------+-------+
| 21.62   | > I  | 3,601   | 0.  | 192\.   | 0.  | 3,793   | 0.017 |
|         |      |         | 018 |         | 012 |         |       |
+---------+------+---------+-----+---------+-----+---------+-------+
| 24.50   | > J  | 9,967   | 0.  | 347\.   | 0.  | 10,310  | 0.013 |
|         |      |         | 013 |         | 011 |         |       |
+---------+------+---------+-----+---------+-----+---------+-------+
| 24.50   | > K  | 25,870  | 0.  | 784\.   | 0.  | 26,660  | 0.009 |
|         |      |         | 010 |         | 009 |         |       |
+---------+------+---------+-----+---------+-----+---------+-------+
| 24.50   | > L  | 74,110  | 0.  | 2,051   | 0.  | 76,160  | 0.007 |
|         |      |         | 007 |         | 006 |         |       |
+---------+------+---------+-----+---------+-----+---------+-------+
| -1.00   | >    | 1.25    | 0.  | 0.145   | 0.  | 1.40    | 0.146 |
|         | M^b^ |         | 162 |         | 077 |         |       |
+---------+------+---------+-----+---------+-----+---------+-------+
| -1.00   | >    | 0.0927  | 0.  | 0.0184  | 0.  | 0.111   | 0.124 |
|         | N^c^ |         | 148 |         | 057 |         |       |
+---------+------+---------+-----+---------+-----+---------+-------+

^a^ Surface tally 1 foot above chase top

^b^ Volume tally above concrete walls of chase

^c^ Volume tally above conduit bank and tuff layer around chase

Table 2---DER in air volumes within and above the utility chase.

[^1]: NCRP Scientific Committee 4 on Heavy Particles ,k H. H. Rossi,
    chairman, *Protection against neutron radiation*, NCRP-38, National
    Council on Radiation Protection and Measurements (January 1971).